28E
Question
Electrophilic addition of to isoprene (2-methyl-1,3-butadiene) yields the following product mixture:
Of the 1,2-addition products, explain why 3,4-dibromo-3-methyl-1-butene (21%) predominates over 3,4-dibromo-2-methyl-1-butene (3%).
Step-by-Step Solution
VerifiedBecause of the formation of tertiary carbocation.
When pi bond in a double or triple bond containing compounds is broken with the formation of two new sigma bonds, it is an electrophilic addition.
Dienes are the unsaturated compounds containing two double bonds between carbon atoms. Dienes are used in organic synthesis and as monomers in polymer industry. Based on the position of double bonds in the whole molecule, dienes are of three types-
- Cumulated dienes
- Conjugated dienes
- Isolated dienes
Cumulated dienes
When two double bonds are present successively on adjacent carbons, cumulated dienes are constituted. They are also called as allenes.
Conjugated Dienes
Dienes having two conjugated double bonds which are separated by a single bond, are called conjugated dienes.
Isolated Dienes
When double bonds in the molecule are separated by more than one single bond, it is isolated diene.
Dienes do undergo electrophilic addition reactions. In isolated dienes the reaction proceeds the way alkenes undergo electrophilic addition reactions. In case of conjugated dienes, a 1,4-addition product is formed in addition to the Markonikov and Anti-Markonikov products.
When electrophilic addition of to isoprene takes place we get a mixture of 1,2 and 1,4 addition products
We have already discussed that 1,4 addition and 1,2 addition products are formed because of the allylic carbocation .
Now, out of the two 1, 2 products formed here in this reaction 3,4-dibromo-3-methyl-1-butene (21%) is more stable than 3,4-dibromo-2-methyl-1-butene and thus predominates. This can be explained on the basis of the stability of reaction intermediate carbocation formed during the reaction.
If you talk about the 1,2 addition products, two different carbocations are formed:
Out of these two carbocations the one leading to 3,4-dibromo-3-methyl-1-butene is tertiary carbocation while the one leading to 3,4-dibromo-2-methyl-1-butene is secondary carbocation and you know that tertiary carbocations are more stable than the secondary carbocations, thus leading to the formation of 3,4-dibromo-3-methyl-1-butene predominantly.