Q9P - Organic Chemistry (Mcmurry)

Question

Which of the following dienes have an s-cis conformation, and which have an s-trans conformation? Of the s-trans dienes, which can readily rotate to s-cis?

Step-by-Step Solution

Answer

Diene (a) 1,2,3,4,6,7-hexahydronaphthalene has s-cis confirmation.

Step 1: Diene s-cis and s-trans

Conjugates dienes have s-cis and s-trans conformation, which only concerns double bond, not caring about the other substituent to which it is attached.

 

The s-cis and s-trans diene are as shown below:

                                      s-cis and s-trans

The other substituents attached to the diene, e.g.: -Cl,-R,-OH, do not affect the diene conformation.

Step 2: 1,2,3,4,6,7-hexahydronaphthalene diene

The diene 1,2,3,4,6,7-hexahydronaphthalene has s-cis confirmation about the single bond to undergo a Diel-alder reaction. Carbon 1 and 4 of the 1,2,3,4,6,7 hexahydronaphthalene compound are in s-cis conformation.

Thus, the diene (a) 1,2,3,4,6,7-hexahydronaphthalene is fixed in the correct s-cis geometry and highly reactive toward Diels-Alder cycloaddition reactions.

                                S-cis conformation diene