Question

Rank in increasing polarity the carbon-nitrogen single, double, and triple bonds. not enough information to compare. triple < double < single all have the same polarity. single < double < triple

Answer 1

I will rewrite this question so the options are presented more clearly:

Rank in increasing polarity the carbon-nitrogen single, double and triple bonds.

a) not enough information to compare
b) triple < double < single
c) all have the same polarity
d) single < double < triple

The answer we are looking for is b) triple < double < single.

We determine bond polarity by comparing the electronegativities of the two atoms involved in the bond, in this case carbon-nitrogen. Nitrogen is more electronegative than carbon, with electronegativities of 3.04 and 2.55, respectively. You may expect that the electronegativities of the atoms remains the same in each bond, but this is not the case.

In the case of a C-N single bond, the carbon atom is sp³ hybridized. In a C=N double bond, the carbon is sp² hybridized. The more s-character an atom has leads to an increase in the electronegativity of the atom since s-orbitals hold the electrons much tighter. By increasing the s-character of the carbon atom from sp³ to sp², we are increasing the electronegativity which leads to a decrease in the difference in electronegativities between carbon and nitrogen. Therefore, a C-N single bond is more polar than a C=N double bond. This trend continues with a C≡N as the carbon now is sp hybridized with even more s-character and becomes more electronegative still, thus decreasing the polarity further.