Q 14-14-51 E - Organic Chemistry (Mcmurry)

Question

Draw the resonance forms that result when the dienes below are protonated. If the resonance forms differ in energy, identify the most stable one.

a) 

b) 

c) 

Step-by-Step Solution

Answer

(a) Only one allylic carbocation is formed

(b) 

(c) 

Protonation reaction

Protonation is the addition of a proton to an atom, molecule or ion. It is different from hydrogenation in that during protonation a change in charge of protonated species occur, while the charge is unaffected during hydrogenation.

It is an acid base reaction where electron in one of the double bond act as a base and proton as an electrophile to generate an allylic carbocation intermediate.

Identify most stable diene compound (a)

The reaction initiated with the electrophilic addition of HCl to 2,4-hexadiene to form allylic carbocation intermediate. Since is 2,4-hexadiene symmetrical diene, protonation of C1-C2 or C3-C4 will create identical allylic carbocations.

Protonation of both C1-C2 and C3-C4 gives secondary allylic carbocation, hence both possible modes yields only the secondary carbocation as represented below:

               Protonation of (a)

Identify most stable diene compound (b)

The reaction initiated with the electrophilic addition of HCl to 1, 4-dimethylcyclohexa-1, 3-diene to form allylic carbocation intermediate. Since1, 4-dimethylcyclohexa-1, 3-diene is symmetrical diene, protonation of C1-C2 or C3-C4 will create different allylic carbocations.

Protonation of both C1-C2 and C3-C4 gives secondary and tertiary allylic carbocation respectively, hence both possible modes yields two allylic carbocation and tertiary carbocation is more stable than secondary carbocation as represented below:

                                  Protonation of (b)

Identify most stable diene compound (c)

The reaction initiated with the electrophilic addition of HCl to 2, 3-dimethylcyclopenta-1, 3-diene to form allylic carbocation intermediate. Since2, 3-dimethylcyclopenta-1, 3-diene is symmetrical diene, protonation of C1-C2 or C3-C4 will create different allylic carbocations.

Protonation of both C1-C2 and C3-C4 gives secondary and tertiary allylic carbocation respectively, hence both possible modes yields twoallylic carbocation and tertiary carbocation is more stable than secondary carbocation as represented below: