Organic compounds composed of only carbon and hydrogen are called hydrocarbons. Hydrocarbons are two types
(1) Aliphatic Hydrocarbon (Alkanes, Alkenes and Alkynes).
(2) Aromatic Hydrocarbon (Arenes)
(1) Sources of aliphatic hydrocarbon
Mineral oil or crude oil, petroleum [Petra → rock; oleum → oil] is the dark colour oily liquid with offensive odour found at various depths in many regions below the surface of the earth. It is generally found under the rocks of earth’s crust and often floats over salted water.
(i) Alkanes: found 30 to 70% contain upto 40 carbon atom. Alkanes are mostly straight chain but some are branched chain isomers.
(ii) Cycloalkanes: Found 16 to 64% cycloalkanes present in petroleum are; cyclohexane, methyl cyclopentane etc. cycloalkanes rich oil is called asphaltic oil.
(iii) Aromatic hydrocarbon: found 8 to 15% compound present in petroleum are; Benzene, Toluene, Xylene, Naphthalene etc.
(iv) Sulphur, nitrogen and oxygen compound: Sulphur compound present to the extent of 6% include mercaptans [R-SH] and sulphides [R-S-R]. The unpleasant smell of petroleum is due to sulphur compounds. Nitrogenous compounds are pyridines, quinolines and pyrroles. Oxygen compounds present in petroleum are Alcohols, Phenols and resins. Compounds like chlorophyll, haemin are also present in it.
(v) Natural gas: It is a mixture of Methane (80%), Ethane (13%), Propane (3%), Butane (1%), Vapours of low boiling pentanes and hexanes (0.5%) and Nitrogen (1.3%). L.P.G. Contain butanes and pentanes and used as cooking gas. It is highly inflammable. This contain, methane, nitrogen and ethane.
(vi) C.N.G.: When natural gas compressed at very high pressure is called compressed natural gas (CNG). Natural gas has octane rating of 130 it consists, mainly of methane and may contain, small amount of ethane and propane.
(3) Theories of origin of petroleum: Theories must explain the following characteristics associated with petroleum,
Its association with brine (sodium chloride solution). The presence of nitrogen and sulphur compounds in it. The presence of chlorophyll and haemin in it. Its optically active nature. Three important theories are as follows.
(i) Mendeleeff’s carbide theory or inorganic theory
(ii) Engler’s theory or organic theory
(iii) Modern theory
(4) Mining of petroleum: Petroleum deposits occurs at varying depth at different places ranging from 500 to 15000 feet. This is brought to the surface by artificial drilling.
(5) Petroleum refining: Separation of useful fractions by fractional distillation is called petroleum refining.
(i) Treatment with concentrated sulphuric acid: The gasoline or kerosene oil fraction is shaken with sulphuric acid to remove aromatic compounds like thiophene and other sulphur compound with impart offensive odour to gasoline and kerosene and also make them corrosive.
(ii) Doctor sweetening process:
(iii) Treatment with adsorbents: Various fractions are passed over adsorbents like alumina, silica or clay etc, when the undesirable compounds get adsorbed.
(7) Artificial method for manufacture of Petrol or gasoline
(i) Cracking, (ii) Synthesis
(i) Cracking: It is a process in which high boiling fractions consisting of higher hydrocarbons are heated strongly to decompose them into lower hydrocarbons with low boiling points. Cracking is carried out in two different ways.
(a) Liquid phase cracking: In this process, the heavy oil or residual oil is cracked at a high temperature (475 – 530oC) under high pressure (7 to 70 atmospheric pressure). The high pressure keeps the reaction product in liquid state. The conversion is approximately 70% and the resulting petrol has the octane number in the range 65 to 70.
The cracking can be done in presence of some catalysts like silica, zinc oxide, titanium oxide, ferric oxide and alumina. The yields of petrol are generally high when catalyst is used.
(b) Vapour phase cracking: In this process, kerosene oil or gas oil is cracked in vapour phase. The temperature is kept 600 – 800oC and the pressure is about 3.5 to 10.5 atmospheres. The cracking is facilitated by use of a suitable catalyst. The yields are about 70%.
(ii) Synthesis : Two methods are applicable for synthesis.
(a) Bergius process: This method was invented by Bergius in Germany during first world war.
(b) Fischer- tropsch process: The overall yield of this method is slightly higher than Bergius process.
The best catalyst for this process is a mixture of cobalt (100 parts), thoria (5 parts), magnesia (8 parts) and kieselguhr (200 parts).
Characteristics of hydrocarbons
(1) Knocking: The metallic sound during working of an internal combustion engine is termed as knocking.
“The greater the compression greater will be efficiency of engine.” The fuel which has minimum knocking property is always preferred.
The tendency to knock falls off in the following order : Straight chain alkanes > branched chain alkanes > olefins > cyclo alkanes > aromatic hydrocarbons.
(2) Octane number: It is used for measuring the knocking character of fuel used in petrol engine. The octane number of a given sample may be defined as the percentage by volume of iso-octane present in a mixture of isooctane and n-heptane which has the same knocking performance as the fuel itself.
For example: a given sample has the knocking performance equivalent to a mixture containing 60% iso-octane and 40% heptane. The octane number of the gasoline is, therefore, 60.
Presence of following types of compounds increases the octane number of gasoline.
(i) In case of straight chain hydrocarbons octane number decreases with increase in the length of the chain.
(ii) Branching of chain increases the value of octane number
(iii) Introduction of double bond or triple bond increases the value of octane number.
(iv) Cyclic alkanes have relatively higher value of octane number.
(v) The octane number of aromatic hydrocarbons are exceptionally high
(vi) By adding gasoline additives (eg.: TEL)
(3) Antiknock compounds: To reduce the knocking property or to improve the octane number of a fuel certain chemicals are added to it. These are called antiknock compounds. One such compound, which is extensively used, is tetraethyl lead (TEL). TEL is used in the form of following mixture,
TEL = 63%, Ethylene bromide = 26%, Ethylene chloride = 9% and a dye = 2%.
However, there is a disadvantage that the lead is deposited in the engine. To remove the free lead, the ethylene halides are added which combine with lead to form volatile lead halides.
However, use of TEL in petrol is facing a serious problem of Lead pollution, to avoid this a new compound cyclopenta dienyl manganese carbonyl (called as AK-33-X) is used in developed countries as antiknocking compound.
(4) Other methods of improving octane number of hydrocarbon.
(i) Isomerisation [Reforming] : By passing an alkane over AlCl3 at 200oC.
The octane no. of petrol can thus be improved.
* By increasing the proportion of branched chain or cyclic alkanes.
* By addition of aromatic hydrocarbons Benzene, Toluene and Xylene (BTX).
* By addition of methanol or ethanol.
* By additon of tetraethyl lead (C2H5)4 Pb
(5) Cetane number: It is used for grading the diesel oils.
The cetane number of a diesel oil is the percentage of cetane (hexadecane) by volume in a mixture of cetane and α -methyl naphthalene which has the same ignition property as the fuel oil under consideration.
(6) Flash point: The lowest temperature at which an oil gives sufficient vapours to form an explosive mixture with air is referred to as flash point of the oil.
The flash point in India is fixed at 44oC , in France it is fixed at 35oC, and in England at 22.8oC. The flash point of an oil is usually determined by means of “Abel’s apparatus”.
Chemists have prepared some hydrocarbons with octane number even less than zero (e.g., n-nonane has octane number – 45) as well as hydrocarbon with octane number greater than 100 (e.g., 2, 2, 3 trimethyl-butane. has octane number of 124).
(7) Petrochemicals: All such chemicals which are derived from petroleum or natural gas called petrochemicals. Some chemicals which are obtained from petroleum are summarised in table:
“Alkanes are saturated hydrocarbon containing only carbon-carbon single bond in their molecules.”
Alkanes are less reactive so called paraffins; because under normal conditions alkanes do not react with acids, bases, oxidising agents and reducing agent.
General formula : CnH2n+2
Examples are CH4 ,C2H6,C3H8,
(1) General Methods of preparation
(i) By catalytic hydrogenation of alkenes and alkynes (Sabatier and sanderen’s reaction)
* Methane is not prepared by this method
(ii) Birch reduction :
(iii) From alkyl halide
(iv) By Zn-Cu couple :
(v) Wurtz reaction :
* R - Br or RI preferred in this reaction. The net result in this reaction is the formation of even no. of carbon atoms in molecules.
(vi) Frankland’s reaction :
(vii) Corey-house synthesis
* Reaction is suitable for odd number of Alkanes.
(viii) From Grignard reagent
(a) By action of acidic ‘H’ :
(b) By reaction with alkyl halide :
(ix) From carboxylic acids
(a) Laboratory method [Decarboxylation reaction or Duma reaction]
* NaOH and CaO is in the ratio of 3 : 1. (Sodalime)
Hydrocarbons. >> Page - 17
(b) Kolbe’s synthesis :
* Both ionic and free radical mechanism are involved in this reaction.
(c) Reduction of carboxylic acid :
(x) By reduction of alcohols, aldehyde, ketones or acid derivatives
* Aldehyde and ketones when reduced with amalgamated zinc and conc. HCl also yield alkanes.
Clemmenson reduction :
* Aldehydes and ketones (> C = O) can be reduced to hydrocarbon in presence of excess of hydrazine and sodium alkoxide on heating.
Wolff-kishner reduction :
(xi) Hydroboration of alkenes
(a) On treatment with acetic acid
(b) Coupling of alkyl boranes by means of silver nitrate
(2) Physical Properties
(i) Physical state : Alkanes are colourless, odourless and tasteless.
C1 - C4 Gaseous state
C5 - C17 Liquid state [Except neo pentane which is gas]
C18 and above Solid like waxes
(ii) Density : Alkanes are lighter than water.
* Melting points of even > Odd no. of carbon atoms, this is because, the alkanes with even number of carbon atoms have more symmetrical structure and result in closer packing in the crystal structure as compared to alkanes with odd number of carbon atoms.
(3) Chemical properties
(i) Substitution reactions of Alkanes
(a) Halogenation : R - H + X - X R - X + HX
The reactivity of halogen is : F2 > Cl2 > Br2 > I2
* Fluorine can react in dark Cl2 , Br2 require light energy. I2 doesnot show any reaction at room temperature, but on heating it shows iodination.
* Iodination of methane is done in presence of oxidising agent such as HNO3 / HIO3 / HgO which neutralises HI .
* Chlorination of methane :
(ii) Reaction based on free radical mechanism
* Lower alkanes particularly methane, ethane, do not give this reaction.
(a) Complete Oxidation or combustion :
* This is exothermic reaction.
(b) Incomplete combustion or oxidation
This is the industrial method for the manufacture of methyl alcohol.
* Higher alkanes are oxidised to fatty acids in presence of manganese stearate.
(d) Chemical oxidation :
(iv) Thermal decomposition or cracking or pyrolysis or fragmentation
* This reaction is of great importance to petroleum industry.
(v) Isomerisation :
(vi) Aromatisation :
(vii) Step up reaction
(viii) HCN formation
(ix) Chloro sulphonation/Reaction with SO2 + Cl2
This reaction is known as reed’s reaction.
* This is used in the commercial formation of detergent.
Individual members of alkanes
(1) Methane : Known as marsh gas.
(i) Industrial method of preparation : Mathane gas is obtained on a large scale from natural gas by liquefaction. It can also be obtained by the application of following methods,
(a) From carbon monoxide : A mixture of carbonmonoxide and hydrogen is passed over a catalyst containing nickel and carbon at 250ºC when methane is formed.
(b) Bacterial decomposition of cellulose material present in sewage water : This method is being used in England for production of methane.
(c) Synthesis : By striking an electric arc between carbon electrodes in an atmosphere of hydrogen at 1200oC,methane is formed.
By passing a mixture of hydrogen sulphide and carbon disulphide vapour through red hot copper, methane is formed.
(ii) Physical properties
(a) It is a colourless, odourless, tasteless and non-poisonous gas.
(b) It is lighter than air. Its density at NTP is 0.71 g/L.
(c) It is slightly soluble in water but is fairly soluble in ether, alcohol and acetone.
(d) Its melting point is -182 .5ºC and boiling point is -161 .5ºC .
(a) In the manufacture of compounds like methyl alcohol, formaldehyde, methyl chloride, chloroform, carbon tetrachloride, etc.
(b) In the manufacture of hydrogen, used for making ammonia.
(c) In the preparation of carbon black which is used for making printing ink, black paints and as a filler in rubber vulcanisation.
(d) As a fuel and illuminant.
(i) Methods of preparation
(a) Laboratory method of preparation :
(b) Industrial method of preparation :
(ii) Physical properties
(a) It is a colourless, odourless, tasteless and non-poisonous gas.
(b) It is very slightly soluble in water but fairly soluble in alcohol, acetone, ether, etc.
(c) Its density at NTP is 1.34 g/L
(d) It boils at – 89ºC. Its melting point is –172ºC.
(a) As a fuel. (b) For making hexachloroethane which is an artificial camphor.
(3) Interconversion of Alkanes
Ascent of alkane series,
(i) Methane to ethane :
(ii) Butane from ethane :
Descent of alkane series : Use of decarboxylation reaction is made. It is a multistep conversion.
Ethane to methane
These are the acyclic hydrocarbon in which carbon-carbon contain double bond. These are also known as olefins, because lower alkene react with halogens to form oily substances. General formula is CnH2n . Examples, C2H4, C3H6, C4H8.
(1) Preparation methods
(i) From Alkynes :
* Poison’s catalyst such as BaSO4, CaCO3 are used to stop the reaction after the formation of alkene.
(ii) From mono halides :
* If we use alc. NaOH in place of KOH then trans product is formed in majority because of its stability according to saytzeff rule.
(iii) From dihalides
(a) From Gem dihalides
* If we take two different types of gemdihalides then we get three different types of alkenes .
* Above reaction is used in the formation of symmetrical alkenes only.
(b) From vicinal dihalides :
* Alkene is not formed from 1, 3 dihalides. Cycloalkanes are formed by dehalogenation of it.
(iv) By action of NaI on vicinal dihalide :
(v) From alcohols [Laboratory method] :
(vi) Kolbe’s reaction :
(vii) From esters [Pyrolysis of ester] :
(viii) Pyrolysis of quaternary ammonium compounds :
(ix) Action of copper alkyl on vinyl chloride :
(x) By Grignard reagents :
(xi) The wittig reaction :
(xii) From β bromo ether [Boord synthesis]
(2) Physical Properties
(i) Alkenes are colourless and odourless.
(ii) These are insoluble in water and soluble in organic solvents.
(iii) Physical state
C1 - C4 gas
C4 - C16 liquid
> C17 solid wax
(iv) B.P. and M.P. decreases with increasing branches in alkene.
(v) The melting points of cis isomers are lower than trans isomers because cis isomer is less symmetrical than trans. Thus trans packs more tightly in the crystal lattice and hence has a higher melting point.
(vi) The boiling points of cis isomers are higher than trans isomers because cis-alkenes has greater polarity (Dipole moment) than trans one.
(vii) These are lighter than water.
(viii) Dipole moment : Alkenes are weakly polar. The, π -electron’s of the double bond. Can be easily polarized. Therefore, their dipole moments are higher than those of alkanes.
(3) Chemical properties
(i) Francis experiment : According to Francis electrophile first attacks on olefinic bond.
(ii) Reaction with hydrogen :
(iii) Reduction of alkene via hydroboration : Alkene can be converted into alkane by protolysis
Hydroboration : Alkene give addition reaction with diborane which called hydroboration. In this reaction formed trialkylborane, Which is very important and used for synthesis of different organic compound
The overall result of the above reaction appears to be antimarkownikoff’s addition of water to a double bond.
(iv) By treatment with AgNO3 + NaOH : This reaction gives coupling
(v) Birch reduction : This reaction is believed to proceed via anionic free radical mechanism.
* If NBS [N-bromo succinimide] is a reagent used for the specific purpose of brominating alkenes at the allylic position.
* In presence of polar medium alkene form vicinal dihalide with halogen.
Reactivity of halogen is F2 > Cl2 > Br2 > I2
(vii) Reaction with HX [Hydrohalogenation]
(viii) Reaction with hypohalous acids :
* In case of unsymmetrical alkenes markownikoff rule is followed.
(ix) Reaction with sulphuric acid :
* This reaction is used in the seperation of alkene from a gaseous mixture of alkanes and alkenes.
(x) Reaction with nitrosyl chloride
* If hydrogen is attached to the carbon atom of product, the product changes to more stable oxime.
(xi) Oxidation : With alkaline KMnO4 [Bayer’s reagent] : This reaction is used as a test of unsaturation.
(a) Using peroxy acid :
* If per benzoic acid or peroxy acetic acid is used then oxirane are formed.
* Application of ozonolysis : This process is quite useful to locate the position of double bond in an alkene molecule. The double bond is obtained by Joining the carbon atoms of the two carbonyl compounds.
(xv) Oxy – mercuration demercuration : With mercuric acetate (in THF), followed by reduction with NaBH4 / NaOH is also an example of hydration of alkene according to markownikoff’s rule.
(b) Epoxidation by performic acid or perbenzoic acid :
(xviii) Hydroformylation :
* If CO + H2O is taken then respective acid is formed.
(xix) Addition of formaldehyde
* If in polymerisation zeigler- natta catalyst [(R)3 Al + TiCl4] is used then polymerisation is known as zeiglernatta polymerisation.
(xxi) Isomerisation :
CH3 - CH2 - CH2 - CH = CH2 ⇌ CH3 - CH2 - CH = CH - CH3
The mechanism proceeds via carbocation.
(xxii) Addition of HNO3 :
(xxiii) Addition of Acetyl chloride :
(i) For the manufacture of polythene – a plastic material;
(ii) For artificial ripening of fruits;
(iii) As a general anaesthetic;
(iv) As a starting material for a large number of compounds such as glycol, ethyl halides, ethyl alcohol, ethylene oxide, etc;
(v) For making poisonous mustard gas (War gas);
(vi) For making ethylene-oxygen flame.
These are the acyclic hydrocarbons which contain carbon-carbon triple bond are called alkynes. General formula is CnH2n-2. Ex. Ethyne CH CH ; Propyne CH3 - C CH
(1) General methods of preparation
* In reaction with gem dihalide, Alc. KOH is not used for elimination in 2nd step.
* In reaction with vicinal dihalide, if the reactant is 2-butylene chloride then product is 2-butyne as major product.
Preparation of higher alkynes (by metal acetylide)
* Acetylene gives salt with NaNH2 or AgNO3 (ammonical) which react with alkyl halide to give higher alkyne.
(2) Physical properties
(i) Acetylene is a colourless gas. It has a garlic odour. The odour is due to presence of impurities of phosphorous and hydrogen sulphide. However, pure acetylene has pleasant odour.
(ii) It is insoluble in water but highly soluble in acetone and alcohol. Acetylene is transported under high pressure in acetone soaked on porous material packed in steel cylinders.
(iii) Its boiling point is - 84ºC .
(iv) It is lighter than air. It is somewhat poisonous in nature.
(v) It burns with luminous flame and forms explosive mixture with air.
(3) Chemical reactivity of alkynes : C C is less reactive than the carbon-carbon double bond towards electrophilic addition reaction. This is because in alkyne carbon has more S-character so more strongly will be the attraction for π electrons. Alkyne also undergo nucleophilic addition with electron rich reagents. Ex. Addition of water, cyanide, carboxylic acid, alcohols. Nucleophilic addition can be explained on the basis that alkynes form vinylic carbanion which is more stable than alkyl carbanion formed by alkene
(i) Acidity of alkynes : Acetylene and other terminal alkynes (1- alkynes) are weakly acidic in character
The acetylenic hydrogen of alkynes can be replaced by copper (I) and silver (I) ions. They react with ammonical solutions of cuprous chloride and silver nitrate to form the corresponding copper and silver alkynides.
This reaction can be used to distinguish between 2-alkynes and 1-alkynes. 1-alkynes will give this test while 2-alkynes, will not give this test.
Explanation for the acidic character : It explained by sp hybridisation. We know that an electron in s - orbital is more tightly held than in a p -orbital. In sp hybridisation s -character is more (50%) as compared to sp2 (33%) or sp3 (25%), due to large s -character the carbon atom is quite electronegative.
(ii) Reaction with formaldehyde
(4) Chemical properties of acetylene
Oxidative–Hydroboration : Alkynes react with BH3 (in THF) and finally converted into carbonyl compounds.
Thus it is useful for preparing aldehyde from terminal alkyne.
Reduction of Alkyne : Alkynes add on hydrogen in presence of suitable catalysts like finely divided Ni, Pd.
If the triple bond is not present at the end of the carbon chain of the molecule, the alkene formed may be cis and trans depending upon the choice of reducing agents.
With Na / NH3 or Li / NH3 in (liquid ammonia) trans alkene is almost an exclusive product while catalytic reduction at alkyne affords mainly cis alkenes.
Degree of unsaturation : The number of degree of unsaturation in a hydrocarbon is given by , Where n1 is the number of carbon atoms; n2 is the number of hydrogen atoms. For example in C6H12, the degree of unsaturation is
Tests of unsaturation
(a) Baeyer’s reagent : It is 1% KMnO4 solution containing sodium carbonate. It has pink colour. An aqueous solution of the compound, a few drops of Baeyer’s reagent are added, the pink colour of the solution disappears. The decolourisation of pink colour indicates the presence of unsaturation in the compound.
(b) Bromine- carbon tetrachloride test : The compound is dissolved in carbon tetrachloride or chloroform and then a few drops of 5% bromine solution in carbon tetrachloride are added to it, the colour of bromine disappears. It indicates the presence of unsaturation.
(i) Acetylene is used as an illuminant.
(ii) It is used for the production of oxy-acetylene flame. The temperature of the flame is above 3000ºC . Is is employed for cutting and welding of metals.
(iii) Acetylene is used for artificial ripening of fruits.
(iv) It is used as a general anaesthetic under the name naracylene.
(v) Acetylene has synthetic applications. It serves as a starting material for the manufacture of a large variety of substances.
(vi) On electrical decomposition acetylene produces finely divided carbon and hydrogen. Hydrogen is used in airships. C2H2 2C + H2
Separation of alkane, alkene and alkyne
The gaseous mixture is passed through ammonical cuprous chloride solution. The alkyne (acetylene) reacts with Cu2Cl2 and forms a red precipitate. It is filtered. The alkyne or acetylene is recovered by decomposition of the precipitate with an acid.
The remaining gaseous mixture is passed through concentrated H2SO4. Alkene is absorbed. The Hydrogen sulphate derivatives is heated at 170ºC to regenerate ethene.
The methane or ethane is left behind unreacted.
Distinction between alkanes, Alkenes and Alkynes
(1) Methods of preparation
(i) From dihalogen compounds (Freund reaction):
(ii) From alkenes :
(iii) From Aromatic compounds
2) Physical properties
(i) First two members are gases, next three members are liquids and higher ones are solids.
(ii) They are insoluble in water but soluble in alcohol and ether.
(iii) Their boiling points show a gradual increase with increase of molecular mass. Their boiling points are higher than those of isomeric alkenes or corresponding alkanes.
(iv) Their density increase gradually with increase of molecular mass.
(3) Chemical properties : Cycloalkanes behave both like alkenes and alkanes in their chemical properties. All cycloalkanes undergo substitution reaction with halogen in the presence of light (like alkane). All cycloalkane (lower members) undergo addition reaction (ex. Addition of H2 ,HX, X2). Further the tendency of forming addition compounds decreases with increase in size of ring cyclopropane > Cyclobutane > Cyclopentane.
Relative ring opening of ring is explained by Baeyer strain theory.
(i) Addition in spiro cycloalkane : If two cycloalkane fused with one another then addition take place in small ring
Because small ring is more unstable than large ring
Higher cycloalkanes do not give addition due to more stability.
(ii) Free radical substitution with Cl2
(iii) Addition reaction
Carbocyclic compounds with double bonds in the ring are called cycloalkenes. Some of the common cycloalkenes are
Cycloalkenes can be easily obtained by Diels-Alder reaction. These compounds undergo the electrophilic addition reactions which are characteristic of alkenes, while the ring remains intact. Cycloalkenes decolourise the purple colour of dilute cold KMnO4 or red colour of bromine in carbon tetrachloride.
These are hydrocarbon with two carbon-carbon double bonds. Dienes are of three types
(1) Conjugated dienes : Double bonds are seperated by one single bond.
Ex : CH2 = CH - CH = CH2 (1, 3-butadiene)
(2) Cumulative dienes : Double bonds are adjacent to each other.
Ex : CH2 = C = CH2 Propadiene [allene]
(3) Isolated or Non-conjugated : Double bonds are separated by more than one single bond.
Ex : CH2 = CH - CH2 - CH = CH2 (1, 4 pentadiene)
The general formula is CnH2n-2. The predominant member of this class is 1, 3 - butadiene.
(1) Method of preparation
(2) Physical property : 1, 3-butadiene is a gas.
(3) Chemical properties
(i) Addition of halogens:
(iv) Polymerisation :
Diels-alder reaction :
Stability of conjugated dienes : It is explained on the basis of delocalisation of electron cloud between carbon atoms.
The four π electrons of 1, 3-butadiene are delocalised over all the four atoms. This delocalisation of the π electrons makes the molecule more stable.
(v) Ozonolysis :
(1) Source of Arenes
Source of arenes is coal. It contains benzene, xylene, naphthalene etc. Arenes are obtained by destructive distillation of coal.
(2) Distillation of coal
* Coal tar is a mixture of large numbers of arenes.
(3) Distillation of coal tar : Arenes are isolated by fractional distillation of coal tar,
* The residue left after fractional distillation of coal-tar is called pitch.
(4) Isolation of benzene
General characteristics of arenes
(1) All arenes have general formula [CnH2n - 6y] . Where y is number of benzene rings and n is not less than 6.
(2) Arenes are cyclic and planar. They undergo substitution rather than addition reactions.
(3) Aromaticity or aromatic character : The characteristic behaviour of aromatic compounds is called aromaticity. Aromaticity is due to extensive delocalisation of π-electrons in planar ring system. Huckel (1931) explained aromaticity on the basis of following rule.
Huckel rule : For aromaticity the molecule must be planar, cyclic system having delocalised (4n + 2)π electrons where n is an integer equal to 0, 1, 2, 3,------.
Thus, the aromatic compounds have delocalised electron cloud of 2, 6, 10 or 14 π electrons.
Similarly cyclopentadienyl anion or tropylium ion are also aromatic because of containing 6π electrons (n = 1).
Hetrocyclic compounds also have 6π electrons (n = 1).
(4) Antiaromaticity : Planar cyclic conjugated species, less stable than the corresponding acyclic unsaturated species are called antiaromatic. Molecular orbital calculations have shown that such compounds have 4nπ electrons. In fact such cyclic compounds which have 4nπ electrons are called antiaromatic compounds and this characteristic is called antiaromaticity.
Example : 1, 3-Cyclobutadiene, It is extremely unstable antiaromatic compound because it has 4nπ electrons (n = 1) and it is less stable than 1, 3 butadiene by about 83.6 KJ mol–1.
Thus, cyclobutanediene shows two equivalent contributing structures and it has n = 1.
Benzene is the first member of arenes. It was first discovered by Faraday (1825) from whale oil. Mitscherllich (1833) obtained it by distillating benzoic acid with lime. Hofmann (1845) obtained it from coal tar, which is still a commercial source of benzene.
(1) Structure of benzene : Benzene has a special structure, which is although unsaturated even then it generally behave as a saturated compound.
(i) Kekule's structure : According to Kekule, in benzene 6-carbon atoms placed at corner of hexagon and bonded with hydrogen and double bond present at alternate position.
(a) Evidence in favour of Kekule's structure
* Benzene combines with 3 molecules of hydrogen or three molecules of chlorine. It also combines with 3 molecules of ozone to form triozonide. These reactions confirm the presence of three double bonds.
* Studies on magnetic rotation and molecular refraction show the presence of three double bonds and a conjugated system.
* The synthesis of benzene from three molecule of acetylene also favour's Kekule's structure.
* Benzene gives cyclohexane by reduction with hydrogen.
(b) Objections against Kekule's formula
* Unusual stability of benzene.
* According to Kekule, two ortho disubstituted products are possible. But in practice only one ortho disubstituted product is known.
* Heat of hydrogenation of benzene is 49.8 kcal/mole, whereas theoretical value of heat of hydrogenation of benzene is 85.8 kcal/mole. It means resonance energy is 36 kcal/mole.
* C - C bond length in benzene are equal, (although it contains 3 double bonds and 3 single bonds) and are 1.39 Å.
Kekule explained this objection by proposing that double bonds in benzene ring were continuously oscillating between two adjacent positions.
(2) Methods of preparation of benzene
(i) Laboratory method :
(ii) From benzene derivatives
(a) From phenol :
(b) From chlorobenzene :
(c) By first preparing grignard reagent of chlorobenzene and then hydrolysed
(d) From benzene sulphonic acid :
(e) From benzene diazonium chloride :
(f) From acetylene :
* Cyclic polymerisation takes place in this reaction.
(3) Properties of benzene
(i) Physical properties
(a) Benzene is a colourless, mobile and volatile liquid. It's boiling point is 80°C and freezing point is 5.5°C. It has characteristic odour.
(b) It is highly inflammable and burns with sooty flame.
(c) It is lighter than water. It's specific gravity at 20°C is 0.8788.
(d) It is immiscible with water but miscible with organic solvents such as alcohol and ether.
(e) Benzene itself is a good solvent. Fats, resins, rubber, etc. dissolve in it.
(f) It is a non-polar compound and its dipole moment is zero.
(g) It is an extremely poisonous substance. Inhalation of vapours or absorption through skin has a toxic effect.
(ii) Chemical properties : Due to the presence of π electron clouds above and below the plane benzene ring, the ring serves as a source of electrons and is easily attacked by electrophiles (Electron loving reagents). Hence electrophilic substitution reaction are the characteristic reactions of aromatic compounds.
Substitution reactions in benzene are prefered rather than addition are due to the fact that in the former reactions resonance stabilised benzene ring system is retained while the addition reactions lead to the destruction of benzene ring. Principal reactions of benzene can be studied under three heads,
(a) Addition reactions
(b) Substitution reactions
(c) Oxidation reactions
(a) Addition reactions : In which benzene behaves like unsaturated hydrocarbon.
Addition of hydrogen : Benzene reacts with hydrogen in the presence of nickel (or platinum) as catalyst at 150°C under pressure to form cyclohexane.
(b) Substitution reactions :
Nucleophilic substitution :
Unimolecular : Mostly uncommon in aromatic substitution, there is only one example which obtain in benzene diazonium dichloride.
* Elimination-addition mechanism (Benzyne mechanism)
* Electrophilic substitution reaction : Benzene undergoes this reaction because it is an electron rich system due to delocalized π- electrons.
* Free radical aromatic substitution : The aromatic substitution reactions which follow free radical mechanisms are very few and have limited synthetic value. But some typical example of these reactions are:
The mechanism of chlorination of benzene at high temperature is similar to that of the free radical aliphatic substitution
When vapours of benzene and air are passed over vanadium pentoxide at 450 – 500°C, maleic anhydride is obtained.
* Strong oxidising agents converts benzene slowly into CO2 and water on heating.
(d) Reduction :
(iii) Uses : (a) In dry cleaning (b) As a motor fuel when mixed with petrol. (c) As a solvent. (d) In the manufacture of gammexane (As insecticide). (e) In the preparation of nitrobenzene, chlorobenzene, benzene sulphonic acid, aniline, styrene, etc. Many of these are employed for making dyes, drugs, plastics, insecticides, etc.
Directive effect in substituted benzene derivatives
(1) Directive effect in mono substituted benzene derivatives : The substituent already present on the benzene ring directs the incoming substituent to occupy ortho (2 or 6), meta (3 or 5) or para (4) position. This direction depends on the nature of the first substituent and is called directive or the orientation effect.
The substituent already present can increase or decrease the rate of further substitution, i.e., it either activates or deactivates the benzene ring towards further substitution. These effects are called activity effects.
There are two types of substituents which produce directive effect are,
(i) Those which direct the incoming group to ortho- and para-positions simultaneously (Neglecting meta all together).
(ii) Those which direct the incoming group to meta-position only (Neglecting ortho- and para-positions all together).
Theory of ortho – para directing group
The above mechanism is followed when S is -OH, - NH2 ,-Cl, - Br,-I,-OR,-NR2 ,-NHCOR etc.
In methyl or alkyl group, the +I effect of the methyl group or alkyl group initiates the resonance effect.
Thus, methyl or alkyl group directs all electrophiles to ortho and para positions.
Theory of meta directing group : The substituent, S withdraws electrons from ortho and para positions. Thus, m-position becomes a point of relatively high electron density and further substitution by electrophile occurs at meta position. For example, -NO2 group is a meta directing (Electron withdrawing). Its mechanism can be explained as :
All meta-directing groups have either a partial positive charge or a full positive charge on the atom directly attached to the ring.
(2) Directive effect in disubstituted benzene
(i) If the directive effects of two substituents reinforce, then a single product is formed.
Thus, both (CH3, NO2) direct further substitution to the same position (Ortho with respect to CH3).
(ii) If the directing effect of two groups oppose each other strongly activating groups win over deactivating or weakly activating group. The sequence of directing power is
-NH2 > -OH > -OCH3 - > NHCOCH3 > -C6H5 > CH3 > meta directors
(iii) There is normally little substitution when the two groups are meta to each other. Aromatic rings with three adjacent substituents are generally prepared by same other routes.
Toluene, methyl benzene or phenyl methane
Toluene is the simplest homolouge of benzene. It was first obtained by dry distillation of tolubalsam and hence named toluene. It is commercially known as tolual.
(1) Methods of preparation
(i) From benzene [Friedel-craft's reaction] :
* Alkyl halide employed may undergo an isomeric change
* Catalysts can be used in place of anhydrous AlCl3 are,
AlCl3 > SbCl3 > SnCl4 > BF3 > ZnCl2 > HgCl2
(ii) Wurtz fitting reaction :
(iii) Decarboxylation :
(iv) From cresol :
(v) From toluene sulphonic acid :
(viii) Commercial preparation
From coal tar : The main source of commercial production of toluene is the light oil fraction of coal-tar. The light oil fraction is washed with conc. H2SO4 to remove the bases, then with NaOH to remove acidic substances and finally with water. It is subjected to fractional distillation. The vapours collected between 80 -110º C is 90% benzol which contains 70 - 80% benzene and 14 - 24% toluene. 90% benzol is again distilled and the portion distilling between 108 - 110ºC is collected as toluene.
(ix) From n- heptane and methyl cyclohexane
(2) Physical properties
(i) It is a colourless mobile liquid having characteristic aromatic odour.
(ii) It is lighter than water (sp. gr. 0.867 at 20°C).
(iii) It is insoluble in water but miscible with alcohol and ether in all proportions.
(iv) Its vapours are inflammable. It boils at 110°C and freezes at –96°C.
(v) It is a good solvent for many organic compounds.
(vi) It is a weak polar compound having dipole moment 0.4D.
(3) Chemical properties : Toluene shows the behaviour of both an alipatic and an aromatic compound.
(i) Electrophilic substitution reactions : Aromatic character (More reactive than benzene) due to electron releasing nature of methyl group.
(ii) Reactions of side chain
(a) Side chain halogenation :
* Benzyl chloride on hydrolysis with aqueous caustic soda forms benzyl alcohol.
(b) Oxidation :
* With hot acidic KMnO4 :
* With acidic manganese or chromyl chloride (Etards reaction) :
* All alkyl benzenes on oxidation with hot acidic KMnO4 or Na2Cr2O7 form benzoic acid. The length of the side chain does not matter.
(c) Hydrogenation :
(d) Combustion : C6H5CH3 + 9O2 7CO2 + 4H2O
(e) Ozonolysis :
(i) In the manufacture of benzyl chloride, benzal chloride, benzyl alcohol, benzaldehyde, benzoic acid, saccharin, etc.
(ii) In the manufacture of trinitrotoluene (TNT), a highly explosive substance.
(iii) As an industrial solvent and in drycleaning.
(iv) As a petrol substitute.
(v) In the manufacture of certain dyes and drugs.
T.N.T. (Tri-nitro toluene)
Properties : It is pale yellow crystalline solid (M.P. = 81°C).
Uses : * It is used as an explosive in shells, bombs and torpedoes under the name trotyl.
* When mixed with 80% ammonium nitrate it forms the explosive amatol.
* TNT is also used as a mixture of aluminium nitrate, alumina and charcoal under the name ammonal.
T.N.B. (Tri-nitro benzene)
Properties and uses: It is colourless solid (M.P. = 122°C). It is more explosive than T.N.T. and used for making explosive.
The molecular formula, C8H10 represents four isomers
These are produced along with benzene, toluene and ethylbenzene when aromatisation of C6 - C8 fraction of petroleum naphtha is done. The xylenes are isolated from the resulting mixture (BTX) by fractional distillation.
These can be prepared by Wurtz – Fittig reaction. A mixture of bromotoluene and methylbromide is treated with sodium in dry ethereal solution to form the desired xylene.
* These can also be obtained by Friedel – craft's synthesis,
* m-Xylene can be obtained from mesitylene.
Xylenes are colourless liquids having characteristic odour. The boiling points of three isomers are,
o-Xylene = 144°C; m-Xylene = 139°C; p-Xylene = 138°C.
Xylenes undergo electrophilic substitution reactions in the same manner as toluene. Upon oxidation with KMnO4 or K2Cr2O7, Xylenes form corresponding dicarboxylic acids.
Xylenes are used in the manufacture of lacquers and as solvent for rubber. o-Xylene is used for the manufacture of phthalic anhydride.
It can be prepared by the following reactions,
(1) By Wurtz-Fittig reaction :
C6H5 Br + 2Na + BrC2H5 C6H5C2H5 + 2NaBr
(2) By Friedel-craft's reaction :
C6H5H + BrC2H5 C6H5C2H5 + HBr
(3) By catalytic reduction of styrene :
C6H5CH = CH2 + H2 C6H5CH2CH3
(4) By alkyl benzene synthesis :
It undergoes electrophilic substitution reactions in the same way as toluene. When oxidised with dil. HNO3 or alkaline KMnO4 or chromic acid it forms benzoic acid.
It is present in storax balsam and coal-tar in traces.
(i) Dehydrogenation of side chain of ethylbenzene :
(ii) Decarboxylation of cinnamic acid : This is the laboratory preparation and involves heating of cinnamic acid with a small amount of quinol.
(v) Dehydrohalogenation of 1-phenyl-1-chloro ethane : On heating with alcoholic potassium hydroxide, a molecule of hydrogen chloride is eliminated by the chloroderivative.
(2) Properties : It is a colourless liquid, boiling point 145°C. On keeping, it gradually changes into a solid polymer called metastyrene. The polymerisation is rapid in sunlight or when treated with sodium. It shows properties of benzene ring (Electrophilic substitution) and unsaturated side chain (Electrophilic addition). However, the side chain double bond is more susceptible to electrophilic attack as compared to benzene ring.
At lower temperature and pressure, it reacts with hydrogen to produce ethylbenzene and at higher temperature and pressure, it is converted into ethyl cyclohexane.
Halogen acids add to the side chain.
C6H5CH = CH2 + HX C6H5CHXCH3
Preparation of ring substituted styrenes is not done by direct halogenation but through indirect route.
When oxidised under drastic conditions, the side chain is completely oxidised to a carboxyl group.
In presence of peroxides, styrene undergoes free radical polymerisation resulting in the formation of polystyrene – an industrially important plastic.
Co-polymers of styrene with butadiene and other substances are also important since many of them are industrially useful products such as SBR ( A rubber substitute).
It occurs in coal-tar. It is the simplest example of an aromatic hydrocarbon in which two benzene rings are directly linked to each other.
(1) Methods of formation
(i) Fittig reaction : It consists heating of an ethereal solution of bromobenzene with metallic sodium.
(ii) Ullmann biaryl synthesis : Iodobenzene, on heating with copper in a sealed tube, forms biphenyl. The reaction is facilitated if a strong electron withdrawing group is present in ortho or para position.
(iii) Grignard reaction : Phenyl magnesium bromide reacts with bromo benzene in presence of CoCl2 .
(2) Properties : It is a colourless solid, melting point 71°C. It undergoes usual electrophilic substitution reactions.
Since aryl groups are electron withdrawing , they should have deactivating and m-orientating effect. But, it has been experimentally shown that presence of one benzene ring activates the other for electrophilic substitution and directs the incoming group to o- and p- positions. It has been shown that monosubstitution in the bi-phenyl results in the formation of para isomer as the major product.
Another special feature of the biphenyl is the behaviour towards second substitution in a monosubstituted biphenyl. The second substituent invariably enters the unsubstituted ring in the ortho and para position no matter what is the nature of substituent already present.