Hydrocarbons and Haloalkanes/Haloarenes
Hydrocarbons and Haloalkanes/Haloarenes tests alkane, alkene, alkyne, arene, and halogenated compound reactions, including addition, substitution, elimination, and aromatic electrophilic substitution.
Core rules
- Alkanes undergo free-radical halogenation; reactivity of halogens is Cl2 > Br2, and selectivity is the reverse.
- Alkenes undergo electrophilic addition. Markovnikov addition puts H on the carbon with more H; peroxide effect gives anti-Markovnikov HBr addition.
- Alkynes add twice and can be reduced to alkenes or alkanes depending on catalyst.
- Benzene is aromatic: planar, cyclic, conjugated, with 4n+2 pi electrons. It undergoes electrophilic substitution rather than addition to preserve aromaticity.
- Haloalkanes undergo SN1, SN2, E1, and E2. Haloarenes are much less reactive toward nucleophilic substitution because the C–X bond has partial double-bond character.
Worked example
Dehydrohalogenation of 2-bromobutane with alcoholic KOH gives but-2-ene as the major product and but-1-ene as minor. The base removes a beta hydrogen, and the more substituted alkene is more stable because of hyperconjugation. This is Saytzeff's rule. If a bulky base is used, the less substituted alkene may increase.
Traps
Do not apply Markovnikov's rule to HBr in the presence of peroxides. Do not expect benzene to decolorize bromine water by addition; it needs a catalyst for substitution. Do not treat haloarenes like haloalkanes in SN2 reactions. Do not forget that terminal alkynes have acidic hydrogens.