JEE Chemistry Hydrocarbons Notes Set 02

Alkanes: Methods of Preparation

Alkanes: Reactions

Iodination is reversible because HI reduces the alkyl iodide back, so an oxidising agent such as \(\ce{HIO3}\) or \(\ce{HNO3}\) is added to remove HI.

Alkenes: Methods of Preparation

1. Dehydration of an alcohol with conc. \(\ce{H2SO4}\) (follows Saytzeff: the more substituted alkene is major).
2. Dehydrohalogenation of an alkyl halide with alcoholic KOH.
3. Dehalogenation of a vicinal dihalide with Zn dust (also used for a gem or 1,2-dihalide on a longer chain).
4. Partial hydrogenation of an alkyne: Lindlar catalyst or Pd-\(\ce{BaSO4}\) gives cis, Na in liquid \(\ce{NH3}\) gives trans.
5. Kolbe electrolysis of the potassium salt of an unsaturated dicarboxylic acid (succinate gives ethene).
6. Hofmann elimination of a quaternary ammonium hydroxide on heating.
7. Pyrolysis of esters, and cracking of alkanes.

Alkenes: Reactions

Stability of alkenes follows the number of alkyl groups on the double bond (hyperconjugation): tetrasubstituted > trisubstituted > disubstituted > monosubstituted > ethene. Trans is usually more stable than cis. The less stable isomer gives the greater heat of combustion and the greater heat of hydrogenation.

Alkynes: Methods of Preparation

1. Dehydrohalogenation of a vicinal or geminal dihalide with alcoholic KOH or \(\ce{NaNH2}\) (two moles).
2. Dehalogenation of a tetrahalide such as \(\ce{CHBr2-CHBr2}\) or of \(\ce{CHCl3}\) with Ag powder or Zn dust.
3. Kolbe electrolysis of potassium or sodium maleate or fumarate gives ethyne.
4. Calcium carbide with water: \(\ce{CaC2 + 2H2O -> C2H2 + Ca(OH)2}\).
5. Berthelot's process: electric arc between carbon electrodes in hydrogen at about 1200 C.
6. Alkylation of a terminal alkyne: treat with Na (or \(\ce{CH3MgI}\)) to get the acetylide, then add R-X to extend the chain.

Alkynes: Reactions

Aromatic Hydrocarbons: Preparation of Benzene

1. Cyclic polymerisation (trimerisation) of ethyne in a red hot tube.
2. Aromatisation of n-hexane over \(\ce{Cr2O3/Al2O3}\) at 600 C.
3. Distillation of phenol with zinc dust.
4. Decarboxylation of benzoic acid (sodium benzoate with soda lime).
5. Hydrolysis of benzenesulphonic acid with steam.
6. Reduction of benzene diazonium chloride with \(\ce{H3PO2}\) or ethanol.
7. Reduction of phenyl magnesium chloride with water.
8. Wolff-Kishner reduction or Clemmensen reduction of aryl ketones and aldehydes gives alkylbenzenes; Wurtz-Fittig couples an aryl halide with an alkyl halide using Na.

Aromatic Hydrocarbons: Reactions of Benzene

Side Chain Reactions of Alkylbenzenes

1. Hot alkaline or acidic \(\ce{KMnO4}\) or \(\ce{K2Cr2O7/H+}\) oxidises any side chain having a benzylic hydrogen to \(\ce{-COOH}\). Tert-butylbenzene has no benzylic hydrogen and is not oxidised; mesitylene gives trimesic acid.
2. Chromyl chloride (Etard reaction) oxidises toluene to benzaldehyde.
3. Chlorine or NBS with light brominates or chlorinates the benzylic carbon by a radical path.
4. Friedel-Crafts acylation followed by Clemmensen or Wolff-Kishner reduction makes straight chain alkylbenzenes, such as ethylbenzene.

Points to Remember for Reasoning

1. Wurtz coupling needs an sp3 alkyl halide; vinyl and aryl halides do not couple with Na alone.
2. Radical addition of HBr gives the more stable radical; ionic addition gives the more stable carbocation.
3. Lindlar gives cis and Na/liquid ammonia gives trans alkenes.
4. Reductive ozonolysis of an propadiene-type compound gives \(\ce{CO2}\) from the central carbon.
5. Any carbocation can rearrange by a hydride or methyl shift: use oxymercuration or hydroboration to avoid it.
6. Electron donors on a ring speed electrophilic substitution and addition at a benzylic carbocation; acceptors slow it.