JEE Chemistry Hydrocarbons Notes Set 05

Alkenes: Structure and Stability

Alkenes have one C=C double bond and the general formula \( \ce{CnH2n} \) (also called olefins). The double bond carbons are sp2 hybridised, the molecule is planar around the double bond with angles near 120 degrees, and rotation is restricted, giving cis-trans isomers. The C=C bond (134 pm, about 146 kcal/mol) is shorter and stronger than C-C, but the pi part is weak and reactive.

Stability of an alkene rises with the number of alkyl groups on the double bond (hyperconjugation): \( \ce{R2C=CR2} \) > \( \ce{R2C=CHR} \) > \( \ce{RCH=CHR} \) > \( \ce{RCH=CH2} \) > \( \ce{CH2=CH2} \). trans isomers are more stable than cis. Heat of hydrogenation is smaller for a more stable alkene.

Methods of Preparation

Debromination of the meso-2,3-dibromobutane with zinc is an anti elimination and gives trans-but-2-ene, while the (R,R) or (S,S) dibromide gives cis-but-2-ene.

Electrophilic Addition

The pi electrons attack an electrophile to give a carbocation (or a cyclic onium ion), which is then captured by a nucleophile. Reactivity follows the stability of the carbocation, so more substituted alkenes react faster with HX.

1. Markovnikov's rule: when HX adds to an unsymmetrical alkene, hydrogen goes to the carbon that already has more hydrogens, so X ends up on the more substituted carbon. In modern terms, the more stable carbocation is formed. Propene and HBr give 2-bromopropane.
2. Rearrangements are possible: 3-methylbut-1-ene with HCl gives 2-chloro-3-methylbutane and also 2-chloro-2-methylbutane because a hydride shift converts the secondary cation to a tertiary one.
3. Peroxide effect (Kharasch): with peroxides, HBr adds by a radical chain. \( \ce{Br^.} \) adds first to the terminal carbon, giving the more stable secondary radical, so propene gives 1-bromopropane. HCl and HI do not show the effect because one step is endothermic in each case.
4. Francis' observation: in the presence of a halide ion, bromine addition gives mixed products such as \( \ce{BrCH2CH2Cl} \) because the bromonium ion is opened by the other nucleophile. This supports a mechanism in which the electrophile adds first.

Oxidation of Alkenes

To work out an alkene from ozonolysis products, join the two carbonyl carbons with a double bond after removing their oxygen. Two moles of acetone mean \( \ce{(CH3)2C=C(CH3)2} \), an aldehyde and a ketone mean a trisubstituted alkene such as 2-methylbut-2-ene, and formaldehyde appears from a terminal \( \ce{=CH2} \).

Other Reactions

1. Polymerisation: ethene gives polythene, propene polypropene (Ziegler-Natta catalyst, \( \ce{R3Al + TiCl4} \)), and vinyl chloride gives PVC.
2. Combustion: \( \ce{CnH2n} \) + (3n/2) \( \ce{O2} \) gives n \( \ce{CO2} \) + n \( \ce{H2O} \).
3. Addition of acetyl chloride with \( \ce{ZnCl2} \) to isobutene gives a beta-chloroketone.
4. Ethene in the Wacker process (\( \ce{PdCl2/CuCl2} \), water) gives ethanal.
5. Hydroformylation and the Koch reaction add \( \ce{CO} \) to an alkene to make aldehydes or acids.

Dienes

1. Buta-1,3-diene adds one mole of HBr or \( \ce{Br2} \) to give two products. The 1,2 adduct forms faster (kinetic control, low temperature). The 1,4 adduct is more stable and dominates at higher temperature (thermodynamic control, about 40 degrees C and above).
2. Diels-Alder reaction: a conjugated diene reacts with an alkene (dienophile) to form a cyclohexene by a [4+2] cycloaddition. Butadiene with ethene gives cyclohexene.
3. Chloroprene, \( \ce{CH2=C(Cl)-CH=CH2} \), is made from vinylacetylene and \( \ce{HCl} \) and polymerises to neoprene rubber.