Question

How can you account for the fact that cis-1,3-pentadiene is much less reactive than trans-1,3-pentadiene in Diels-Alder reaction.

Short answer

Due to steric reasons.

Step-by-step solution

Diels-Alder reaction

Diels-Alder reaction was discovered by the German chemists Otto Diels and Kurt-Alder in 1928. Diels-Alder reaction is an important organic reaction which includes a substituted alkene in the form of a dienophile and a conjugated diene. It proceeds via concerted mechanism. Diels alder reaction can be used to generate 6 membered rings since there is a simultaneous construction of two new Carbon-carbon bonds.

Or

Diels-Alder reaction is a pericyclic reaction. It is a reaction between a conjugated diene ( two double bonds) and a dienophile( a substituted alkene with electron withdrawing group). The reaction is stereospecific i.e., the cis dienophile will give cis stereochemistry in product and trans dienophile will generate product with cis stereochemistry.

Diene and dienophile

Diene is an organic compound which comprises of two conjugated pi bonds (4 pi bonds) while dienophile is a diene loving compound containing 2 pi electrons. When the interaction between them, occurs a transition state without any additional energy barrier, is obtained.

Explanation

When cis 1,3-pentadiene rotates to s-cis conformation, the steric interactions occur between hydrogens of methyl and hydrogens on C-1.So it becomes difficult for cis 1,3 penatdiene to come to s-cis conformation and thus it is less reactive in Diels-Alder reaction.