The reaction of (Z)-butenedioic acid with 1,3-cyclopentadiene gives only one product. Why?

The reaction of (Z)-butenedioic acid with 1,3-cyclopentadiene gives only one product due to a unique reaction called the Diels-Alder reaction. The Diels-Alder reaction is a pericyclic reaction that forms cyclohexene derivatives called adducts. In this case, the reaction involves a diene (1,3-cyclopentadiene) and a dienophile ((Z)-butenedioic acid), resulting in the formation of a six-membered ring.

To understand why only one product is formed, let's delve into the mechanism of the Diels-Alder reaction. The reaction occurs in a concerted manner, meaning that both the diene and dienophile undergo simultaneous bond formation without any intermediates. The diene, with its four π-electrons, acts as a nucleophile while the dienophile, with its four π-electrons, acts as an electrophile.

In the case of (Z)-butenedioic acid and 1,3-cyclopentadiene, the reaction proceeds by forming two new carbon-carbon bonds. The diene, 1,3-cyclopentadiene, aligns its double bonds in a s-cis conformation, allowing for effective overlap with the two π-electrons of the dienophile, (Z)-butenedioic acid.

The reaction proceeds through a transition state where the electrons from the diene move to the empty π-orbitals of the dienophile, leading to the formation of two new σ-bonds. The resulting intermediate quickly undergoes a retro-electrocyclic reaction, converting it into a stable six-membered cyclic structure.

Now, coming back to why only one product is formed, it is a consequence of the regioselectivity of the Diels-Alder reaction. The regioselectivity refers to the preference for the reaction to occur in a specific way, leading to a single product rather than multiple possibilities. In this case, the formation of a six-membered ring is energetically favored compared to larger or smaller ring sizes. Moreover, the stereochemistry of the reactants also influences the regioselectivity.

Hence, due to the concerted mechanism and regioselectivity of the Diels-Alder reaction, (Z)-butenedioic acid and 1,3-cyclopentadiene react to form a single product, a cyclohexene derivative, as predicted by the reaction conditions and principles of organic chemistry.