JEE Chemistry Hydrocarbons Notes Set 01

Alkanes: Preparation

1. Hydrogenation of alkenes or alkynes with Ni, Pt or Pd gives alkanes: \( CH_2=CH_2 + H_2 \xrightarrow{Ni} CH_3CH_3 \).
2. Wurtz reaction: \( 2RX + 2Na \xrightarrow{dry\ ether} R-R + 2NaX \). It gives even-carbon alkanes from a single halide; two different halides give a mixture of three alkanes. The order of reactivity of halides is \( RI > RBr > RCl \). Methane cannot be made by this method and tertiary halides give alkenes.
3. Corey-House synthesis: \( RX \xrightarrow{Li} RLi \xrightarrow{CuI} R_2CuLi \xrightarrow{R'X} R-R' \). It gives unsymmetrical alkanes in good yield.
4. Kolbe electrolysis of sodium or potassium salts of carboxylic acids: at the anode \( 2RCOO^- \to R-R + 2CO_2 + 2e^- \), at the cathode hydrogen and \( OH^- \) form, so the solution becomes more alkaline. A mixture of two salts gives three alkanes.
5. Decarboxylation: \( RCOONa + NaOH \xrightarrow{CaO,\ \Delta} RH + Na_2CO_3 \) (soda lime), giving an alkane with one carbon fewer.
6. Reduction of carbonyl compounds: Clemmensen (Zn-Hg/conc. HCl) in acid and Wolff-Kishner (hydrazine, KOH) in base give alkanes; red P and HI reduce alkyl halides, alcohols and acids to alkanes. Grignard reagents with water give alkanes: \( RMgX + H_2O \to RH + Mg(OH)X \).

Alkanes: Properties

1. Boiling points rise with chain length and fall with branching, so n-octane has a higher boiling point than its branched isomers.
2. Halogenation proceeds by a free radical chain mechanism (initiation, propagation, termination) with the reactivity order \( 3^\circ > 2^\circ > 1^\circ \) for hydrogen and \( F_2 > Cl_2 > Br_2 > I_2 \).
3. Complete combustion gives \( CO_2 \) and \( H_2O \); incomplete combustion gives carbon (soot) or CO. Pyrolysis (cracking) breaks higher alkanes into smaller alkanes and alkenes by a free radical mechanism; it is used to make petrol from heavier fractions.
4. Tetraethyl lead (TEL) is an antiknock compound added to petrol.

Alkenes

1. Preparation: dehydrohalogenation of alkyl halides with alcoholic KOH (Saytzeff rule: the more substituted alkene predominates), dehydration of alcohols with conc. \( H_2SO_4 \) at 443 K, and partial hydrogenation of alkynes (Lindlar catalyst gives cis, Na in liquid ammonia gives trans).
2. Addition of HX follows Markovnikov's rule (hydrogen adds to the carbon with more hydrogens) through the more stable carbocation. In the presence of peroxides, HBr adds in the anti-Markovnikov way by a free radical mechanism (not HCl or HI).
3. Hydroboration-oxidation (\( BH_3\text{-THF} \), then \( H_2O_2/OH^- \)) gives the anti-Markovnikov alcohol.
4. Cold dilute alkaline \( KMnO_4 \) (Baeyer's reagent) gives a glycol and the purple colour is lost; hot acidic KMnO4 splits the double bond to give ketones and carboxylic acids (a terminal CH2 becomes CO2).
5. Ozonolysis (\( O_3 \), then \( Zn/H_2O \)) cleaves the double bond to give aldehydes or ketones; it is used to locate the position of the double bond.
6. Heat of hydrogenation is smaller for more substituted and for trans alkenes, since they are more stable.

Alkynes

1. Preparation: calcium carbide with water gives acetylene, \( CaC_2 + 2H_2O \to C_2H_2 + Ca(OH)_2 \); dehydrohalogenation of vicinal dihalides with alcoholic KOH then \( NaNH_2 \); Kolbe electrolysis of potassium maleate or fumarate gives acetylene.
2. Terminal alkynes are weakly acidic (the C-H bond is sp hybridised). They react with sodium or \( NaNH_2 \) to give sodium alkynides, and with ammoniacal silver nitrate (white precipitate) or ammoniacal cuprous chloride (red precipitate). Alkenes and non-terminal alkynes do not react, so this distinguishes terminal alkynes.
3. Hydration with dilute \( H_2SO_4 \) and \( HgSO_4 \) gives a ketone (acetaldehyde from acetylene); hydroboration-oxidation gives an aldehyde from a terminal alkyne. Polymerisation of acetylene through a red hot tube gives benzene.

Benzene and Aromatic Hydrocarbons

1. Preparation: from phenol by zinc dust distillation, from benzoic acid by soda lime, from benzenediazonium chloride with \( H_3PO_2 \), from phenylmagnesium bromide with water, and by trimerisation of acetylene.
2. Benzene undergoes electrophilic substitution (nitration, sulphonation, halogenation, Friedel-Crafts alkylation and acylation) because of its aromatic stability; it does not decolourise bromine water or Baeyer's reagent. In sunlight or UV light it adds chlorine to give benzene hexachloride (BHC, gammexane).
3. Side chains with a benzylic hydrogen are oxidised to benzoic acid by acidic or alkaline KMnO4 whatever the length of the chain; tert-butylbenzene has no benzylic hydrogen and is not oxidised. Etard reaction (CrO2Cl2) and CrO3 in acetic anhydride convert toluene to benzaldehyde.
4. Friedel-Crafts acylation followed by Clemmensen reduction is the standard way to introduce a straight alkyl chain without rearrangement.
5. Benzene is carcinogenic and toxic.