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SolutionsAllylic Position

1. Allylic positions

Label the allylic positions in the following compounds:

Eight alkenes: a) tetrasubstituted alkene with ethyl, propyl and two methyl groups; b) dimethylcyclopentene; c) diene with two methyl groups; d) tert-butylcyclohexene; e) isopropylmethylcyclohexene; f) terminal alkene with methyl and ethyl; g) methylcycloheptene; h) trimethylcyclopentene
The eight alkenes with a dot on each allylic carbon, that is, on each carbon directly bonded to a carbon of the double bond

An allylic position is an sp3 carbon next to a carbon of the double bond. Its C–H bonds are especially weak because the radical, cation or anion formed when they break is resonance-stabilized.

2. Allylic bromination

Give the product of the reaction of cyclohexene with N-bromosuccinimide (NBS) and light, and explain why 1,2-dibromocyclohexane is not formed.

3-Bromocyclohexene. NBS keeps a very low concentration of Br2, so addition to the double bond is slow and radical substitution at the allylic position, whose radical is resonance-stabilized, predominates.

3. Two products

Allylic bromination of but-1-ene with NBS gives a mixture of 3-bromobut-1-ene and 1-bromobut-2-ene. Why?

The allylic radical CH2=CH–CH•–CH3 has a resonance form •CH2–CH=CH–CH3: the unpaired electron is shared between C1 and C3, and the bromine can bond to either of them.

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