Chapter 21.4, Problem 29SSC

Interpretation Introduction

Interpretation:

The structure of trans -2-pentene and cis -2-pentene needs to be drawn and the reason for the $\text{80 \% to 20 \%}$ ratio of trans- to cis- isomer needs to be explained.

Concept introduction:

The molecules with same molecular formula and connectivity but differ in spatial arrangements of atoms are said to be geometric isomers of each other.

Answer to Problem 29SSC

In trans- isomer, the bulkier groups are present at farther apart from each other resulting in less stearic hindrance and more stability (lower energy).

Explanation of Solution

Geometric isomerism, or cis-trans, isomerism is most common in alkenes. The two forms exist due to no free rotation about carbon-carbon double bond. It exists only when the double bonded carbon atoms are joined to two different groups or atoms.

In cis- isomer, the two identical groups or atoms are close to each other whereas in trans- isomer two identical groups or atoms are farther apart.

The structures of trans -2-pentene and cis -2-pentene (pent represents 5 carbon chain as a parent chain and position of double bond is at 2) is:

The $\text{80 \% to 20 \%}$ ratio of trans- to cis- isomer are produced from a certain reaction. The trans- product is formed in higher amount because the bulkier groups in trans- isomer are present at farther apart from each other resulting in less stearic hindrance and more stability (lower energy).