Haloalkanes and haloarenese

• Haloalkanes and haloarenes are compounds containing halogen atoms attached to an alkyl or aryl group.
• Finkelstein reaction involves the treatment of alkyl chlorides or bromides with NaI in dry acetone, resulting in the formation of alkyl iodides.
• Among isomeric dihalobenzenes, the para isomers are high melting as compared to their ortho and meta isomers, due to symmetry of para isomers that fits in crystal lattice better as compared to ortho and meta isomers.
• In haloalkanes, the C-X bond is polar due to the greater electronegativity of halogen atom, resulting in higher melting and boiling points than hydrocarbons of comparable molar mass.
• Haloalkanes react with water to form alcohols, R-X + H2O → R-OH + HX.
• The boiling points of isomeric haloalkanes decrease with increase in branching, as branching increases, the surface area of the molecule decreases, hence the van der forces decrease and hence the boiling point.
• Swarts reaction is used for the preparation of alkyl fluorides, where alkyl chloride or bromide is treated with a metallic fluoride like AgF, Hg2F2, CoF2 or SbF3, resulting in the formation of alkyl fluoride.
• Nucleophilic substitution reactions involve a weak nucleophile being replaced by a strong nucleophile, these reactions can be represented by: R-X + AqNaOH → R-OH + KX.
• Alkyl halides (Chlorides or Bromides) react with sodium alkoxide to form alkyl iodides, R-X + NaI → R-I + NaX.
• The haloalkanes are only very slightly soluble in water, as they cannot form hydrogen bonds with water (except alkyl fluorides).
• Haloalkanes react with sodium alkoxide to give ethers (R-O-R), R-X + R-ONa → R-O-R + KX.
• Chlorobenzene when treated with methyl chloride (CH 3 - Cl) in presence of anhydrous AlCl 3 , we get p - chlorotoluene as the major product.
• Trichloromethane ( Chloroform, CHCl 3 ) is used as a solvent for fats, alkaloids, iodine and other substances.
• When a mixture of alkyl halide and aryl halide is treated with sodium in dry ether, an alkyl arene is formed and this reaction is called Wurtz - Fittig reaction.
• Chlorobenzene when treated with acetyl chloride (CH 3 - CO - Cl) in presence of anhydrous AlCl 3 , we get p - chloroacetophenone as the major product.
• On nitration using Conc HNO 3 and Conc H 2 SO 4 , chlorobenzene gives p - nitrochlorobenzene as the major product.
• Halogen at om has a tendency to withdraw electrons from the benzene ring, making it a deactivating group.
• Tetrachloromethane (Carbon tetrachloride, CCl 4 ) is used in the manufacture of refrigerants and propellants for aerosol cans.
• Haloalkanes react with halogen (Chlorine or bromine) in presence of anhydrous ferric chloride to form o - dichlorobenzene and p - dichlorobenzene.
• Aryl halides when treated with sodium in dry ether, we get diaryls (diphenyls) and this reaction is called Fittig reaction.
• Polyhalogen compounds are carbon compounds containing more than one halogen atom.
• On sulphonation using Conc H 2 SO 4 , chlorobenzene gives p - chlorobenzenesulphonic acid as the major product.
• Electrophilic substitution reactions in haloarenes occur slowly and require more vigorous conditions due to the deactivating effect of halogen at om.
• Dichloromethane (Methylene chloride, CH 2 Cl 2 ) is widely used as a solvent, as a paint remover, as a propellant in aerosols, and as a process solvent in the manufacture of drugs.
• An example is the reaction between CH 3 Cl and hydroxide ion to yield methanol and chloride ion.
• AgCN is mainly covalent and only N is free to donate an electron pair, hence isocyanides are the main product.
• Alkyl halides react with alcoholic KCN to give alkane nitriles or alkyl cyanides (R - CN).
• Reaction with KCN gives alkyl cyanides because KCN is mainly ionic and gives CN - ions in solution.
• This reaction follows a second order kinetics, i.e., the rate depends upon the concentration of both the reactants.
• There are two types of mechanisms: Substitution Nucleophilic bimolecular (S N 2) and Substitution Nucleophilic unimolecular (S N 1).
• Alkyl halides when reduced with lithium aluminium hydride (LiAlH 4 ) to give alkane.
• Alkyl halides react with Silver nitrite (AgNO 2 ) to give nitroalkane (R - NO 2 ).
• Alkyl halides react with Silver salt of carboxylic acid (R - COOAg) to give esters (R - COOR).
• In the transition state, the carbon atom is simultaneously bonded to five atoms and therefore is unstable.
• These two processes take place simultaneously in a single step and no intermediate is formed.
• If ammonia is in excess, only primary amine is formed.
• AgCN is an ambident nucleophile, i.e., both C and N contain lone pair of electrons and can bind to the carbon atom of the alkyl group either through C or through N.
• Alkyl halides react with Potassium nitrite (KNO 2 ) to give alkane nitrite (R - ONO).
• In the Substitution Nucleophilic Bimolecular (S N 2) Mechanism, the incoming nucleophile interacts with alkyl halide causing the carbon - halogen bond to break while forming a new carbon - OH bond.
• As the reaction proceeds, the bond between the nucleophile and the carbon atom starts forming and the bond between carbon atom and leaving group (the halogen atom) weakens.