Halothane used as an anaesthetic during surgery, Chloroquine used in the treatment of Malaria, Chloramphenocol used in the treatment of Typhoid fever, thyroxine hormone produced in our body, CH2Cl2, CHCl3, CCl4, CHI3, Freons, DDT are Halogenated (Alkanes or arenes) Compounds.
PREPARATION:
→ From Alcohols: Only alkylhalides (but not oryl halides) can be prepared from alcohols.
By halogen exchange: In Finkelstein reaction, Iodide of NaI exchanged either by Chloride or by bromide of RX in presence of dry acetone.
In Swarts reaction, Fluoride of AgF or Hg2F2 will exchange either with chloride or with bromide of RX.
By Addition of HX or X2 to alkene
According to Markovnikov's rule, 2-Iodo Propane is formed as major product.
By Replacement of Diazonium group (Sandmeyer's reaction):
In this reaction diazonium group of diazonium salt is replaced by the nucleophiles like Cl−or Br− or CN − in presence of Cu+ ion.
By Electrophilic substitution:
By free radical substitution: By Free Radical chlorination/ Bromination of alkanes gives a complex of isomeric polyhaloalkanes and monohaloalkanes.
CHEMICAL REACTIONS:
Nucleophilic Substitution reactions:
(a) Bimolecular nucleophilic Substitution (SN2):
(b) Unimolecular nucleophilic substitution (SN1):
Elimination reactions: When alkylhalide (having α-halogen and β-hydrogen) is heated with alcoholic KOH, both the α-Halogen and β-Hydrogen are elimination to give alkene.
If more than one S-hydrogen atoms are available in a compound, more than one alkene are formed during elimination. According to Zaiter (Saytzeff), The major product formed during dehydrohalogenation will be alkene with more number of alkyl groups attached to the double bonded carbons.
An alkyl halide having β - hydrogen atoms when reacts with base or nucleophile undergo either elimination or substitution (SN1 or SN2). It depends upon nature of RX, Strength & size of base or nucleophile and reaction conditions. A bulkier nucleophil prefers elimination reaction. A primary alkyl halide prefers SN2, Tertiary alkyl halide prefers SN1, secondary alkyl halide prefers either SN2 or elimination (depending upon the strength of base (nucleophile).
Reaction with Metals:
When RX reacts with Mg in presence of dry ether, RMgX (Grignard reagent)'s formed, which on hydrolysis gives alkane.
When alkyl halides react with sodium in dry ether to give hydrocarbons containing double the number of carbons present in the halide. This reaction is called "Wurtz reaction".
Reactions of Haloarenes:
Nucleophilic Substitution reaction:
→ Replacement of halide by OH - group:
Electrophilic Substitution reactions:
Halogen atom in haloarene is ortho and para directing.
Reaction with Metals:
Wurtz Fitting reaction: The reaction in which alkyl halide reacts with aryl halide in dry ether and sodium to give alkylarene.
Fitting reaction: The reaction in which two aryl groups are joined together when 2 aryl halides reacts with sodium in presence of dry ether.
Polyhalogen Compouds:
CH2Cl2: Dichloromethane is used as a solvent, as a paint remover, as a solvent in the manufacture of drugs, metal cleaning & metal finishing solvent.
CHCl3: Chloroform is used as solvent for fats, alkaloids and Iodine. It is slowly Oxidised by air in presence of sunlight to form a poisonous gas phosgene (COCl2). So CHCl3 is stored in dark amber coloured, tightly closed bottles and filled completely without any gap to avoid phosgene.
CHI3: As iodoform liberates free iodine, used as antiseptic.
CCl4: Carbon tetra chloride is used in the manufacture of refrigerants, propellants in fire extinguishers, in degreasing, chlorofluoro carbons and pharmaceuticals. As it has harmful effects of health and causing depletion of ozone layer, we must think of alternatives.
Freons: Freons are chlorofluoro carbon compounds of methane and ethane. They are highly stable, unreactive, non-toxic, non- corrosive and easily liquifiable gases. Most common freon12 (CCl2F2) is prepared from CCl4. Freons are cheap and used in aerosol propellants, refrigeration and air conditioning. As it causes ozone depletion now a days CFC free compounds are used.
DDT: P, P' dichloro diphenyl trichloro ethane is the first chlorinated organic insecticide. It is non biodegradable compound. As it becomes uneffective towards insects, highly toxic towards fish, not metabolised by animals, stored in fat tissues it was banned in united states in 1973.
