Question

Which of the compounds in Exercise 14 exhibit cis-trans isomerism?

Short answer

Compound:

a (3-hexene) exhibits cis-trans isomerism.

b (2,4-heptadiene) exhibits cis-trans isomerism.

c (2-methyl-3-octene) exhibits cis-trans isomerism.

d (4-methyl-1-pentyne) does not exhibit cis-trans isomerism.

Step-by-step solution

Definition of cis-trans isomerism.

Cis–trans isomerism, also known as geometric isomerism or configurational isomerism, is a chemical word that refers to how atoms are arranged inside molecules.

To be the cis-trans isomer of a compound:

It must have a definite axis of rotation. Thus, either a double bond or a cyclic ring must be present.

The root chain must then be joined to two or more different substituents.

The kind of isomer is cis when substituents are on the same side of the double bond or the ring and trans when the substituents are on the opposing sides of the double bond or the ring.

Subpart (a)

The structure of 3-hexene is:

Here, 3-hexene does exhibit cis-trans isomerism, as shown below.

Subpart (b)

The structure of 2,4-heptadiene is:

The compound 2,4-heptadiene does show geometric isomerism, as shown below.

The cis isomer has two H-atoms on the same side, whereas the trans isomer has the two H-atoms on the opposite side of the double bond.

Subpart (c)

The structure of 2-methyl-3-octene is:

Compound 2-methyl-3-octene shows cis-trans isomerism;

Here, the cis isomer has two bulky groups on the same side:

The trans isomer has two bulky groups on the opposite side of the double bond

Subpart (d)

The structure of 4-methyl-1-pentyne is shown below. It does not exhibit cis-trans isomerism as there is only one substituent (R and H) on both the carbon of the unsaturated triple bond.