JEE Chemistry Hydrocarbons Notes Set 08

Preparation of Alkanes

Alkanes (general formula \( C_nH_{2n+2} \)) are made by six main routes.
1. Hydrogenation: an alkene or alkyne adds \( H_2 \) over Ni, Pt or Pd to give the alkane. Benzene is not reduced at room temperature.
2. Clemmensen reduction (Zn-Hg/conc. HCl, acidic) turns \( C=O \) of aldehydes and ketones into \( CH_2 \). Avoid it when alcohol, ether, alkene or alkyne groups must survive.
3. Wolff-Kishner reduction (\( NH_2NH_2 \)/KOH, basic) does the same job in alkaline medium. Avoid it with halides, acid halides, anhydrides and esters.
4. Decarboxylation: a carboxylic acid or its sodium salt heated with soda lime (NaOH + CaO) loses \( CO_2 \) as \( Na_2CO_3 \), giving an alkane with one carbon fewer.
5. Wurtz reaction: \( 2RX + 2Na \xrightarrow{\text{dry ether}} R-R + 2NaX \). It is good for symmetrical alkanes with an even number of carbons, and methane cannot be made this way.
6. Corey-House synthesis: \( RX \xrightarrow{2Li} RLi \xrightarrow{CuI} R_2CuLi \xrightarrow{R'X} R-R' \). Symmetrical and unsymmetrical alkanes can be made; the halide \( R'X \) works best when primary.

Reactions of Alkanes

1. Free radical halogenation (light or high temperature): \( R-H + X_2 \rightarrow R-X + HX \). Halogen reactivity: \( F_2 > Cl_2 > Br_2 > I_2 \). Hydrogen reactivity: \( 3^\circ > 2^\circ > 1^\circ \). Phenylic, vinylic and bridgehead hydrogens are not replaced.
2. Isomerisation: with anhydrous \( AlCl_3 \) and HCl, straight chain alkanes change to branched isomers (for example n-hexane to 2-methylpentane and 3-methylpentane). A quaternary carbon is not produced.

Preparation of Alkenes

1. Partial reduction of alkynes: Lindlar's catalyst (Pd/CaCO3 with quinoline) or Pd/BaSO4 with S gives the cis alkene; Na or Li in liquid \( NH_3 \) gives the trans alkene (not for terminal alkynes).
2. Dehydrohalogenation of alkyl halides with alcoholic KOH. Both H and X leave from adjacent carbons; the more substituted alkene is the major product (Saytzeff rule). Rate: \( 3^\circ > 2^\circ > 1^\circ \).
3. Dehydration of alcohols with conc. \( H_2SO_4 \) (heat) or over \( Al_2O_3 \) vapour phase. Rate: \( 3^\circ > 2^\circ > 1^\circ \); Saytzeff product is major.
4. Dehalogenation of vicinal dihalides with zinc dust.

Reactions of Alkenes

1. Addition of HX follows Markovnikov's rule: the negative part adds to the carbon with fewer hydrogens.
2. Hydration with dilute acid gives the Markovnikov alcohol (propene gives 2-propanol).
3. Halogen addition (Br2 or Cl2 in CCl4) gives a vicinal dihalide.
4. Oxymercuration-demercuration (Hg(OAc)2, H2O, then NaBH4) gives the Markovnikov alcohol without rearrangement.
5. Hydroboration-oxidation (B2H6, then H2O2/OH-) gives the anti-Markovnikov alcohol by syn addition.
6. Peroxide (Kharasch) effect: only HBr adds against Markovnikov's rule in the presence of peroxides, through a free radical chain. HCl and HI are unaffected.

Alkynes: Preparation and Reactions

Preparation: (a) vicinal or geminal dihalides with 2 equivalents of NaNH2 (double dehydrohalogenation); (b) vicinal tetrahalides with zinc dust and alcohol; (c) 2 PhCCl3 with silver powder gives diphenylacetylene.
Reactions: addition of H2 (Ni/Pt/Pd gives alkane; Lindlar gives cis alkene; Na/NH3 gives trans alkene); addition of X2 in CCl4 (two moles give a tetrahalide); addition of HX (two moles give a geminal dihalide, Markovnikov); hydration with HgSO4/dil. H2SO4 gives a ketone (acetylene gives acetaldehyde); hydroboration-oxidation gives an aldehyde from a terminal alkyne.

Benzene

Preparation: (a) cyclic trimerisation of alkynes in a red hot iron tube (three acetylene molecules give benzene; two propyne and one acetylene give xylenes); (b) heating phenol with zinc dust; (c) decarboxylation of sodium benzoate with soda lime.
Electrophilic substitution reactions: nitration (conc. \( HNO_3 \) + conc. \( H_2SO_4 \)), sulphonation (conc. \( H_2SO_4 \), about 80 C, gives benzenesulphonic acid), halogenation (\( Cl_2 \) or \( Br_2 \) with \( AlCl_3 \) or \( FeCl_3 \)), and Friedel-Crafts alkylation (RX, \( AlCl_3 \)) and acylation (RCOCl or anhydride, \( AlCl_3 \)). Toluene with \( Cl_2 \)/light gives benzyl chloride by side chain substitution.