Q.70. - Organic Chemistry

Question

Question: Haloethanes $\left(C{H}_{3}C{H}_{2}X, X=Cl, Br, I\right)$ have similar barriers to rotation (13.4–15.5 kJ/mol) despite the fact that the size of the halogen increases, $\mathit{C}\mathit{l}\mathbf{\to }\mathit{B}\mathit{r}\mathbf{\to }\mathit{I}$ . Offer an explanation.

Step-by-Step Solution

Answer

Answer

Haloethanes have a similar rotational energy barrier because the increase in the size of the halogen atom leads to an increase in the bond length of the carbon-halogen bond.

Hence, the size of the halogen atom does not affect the halogen-hydrogen interaction.

Step 1: Rotational energy barrier

The energy required to overcome or rotate around a bond is termed the rotational energy barrier.

Step 2: Eclipsed structures of chloroethane and iodoethane

The eclipsed projections of chloroethane and iodoethane are as follows:

Eclipsed conformations of chloroethane and iodoethane

Step 3: Similarity in the rotational energy barrier of haloethanes

Haloethanes have a similar rotational energy barrier despite the change in the size of the halogen atoms.

This is because the bond between the carbon and the halogen is  and single bonds are free to rotate. 

Since iodine is larger than chlorine, the carbon-iodine (C-I) bond will be longer than the carbon-chlorine (C-Cl) bond.

As a result, the eclipsing interaction between iodine and the hydrogen atoms is similar in magnitude to that of the hydrogen-chlorine interactions.