If we compare the sizes of the halogens, we find that they increase in size from fluorine to iodine. Nevertheless, fluoroethane, chloroethane, bromoethane, and iodoethane all have very similar barriers to rotation about the C—C bond. Can you offer a possible explanation?

………………………………………….barrier to rotation

Compound ....………………………………(kj / mol)

Fluoroethane…………………………………… 13.8

Chloroethane…………………………………… 15.5

Bromoethane…………………………………… 15.5

Iodoethane……………………………………… 13.4