Expert: Dr. Henry Boyter, Jr. - 8/15/2009

Question
Hi  Dr. Henry,

This is not a homework question. I already know the answer to it but don't understand the logic behind it and i tried searching for it on a few sites on the net but couldn't really get an answer.

The question is as follows:

Out of
CH3Cl , CH2Cl2 , CHCl3, CCl4,
Which compound has the "shortest" C-Cl bond length.

The answer is CCl4 but I don't understand how.
I know perhaps it has got something to do with greater electronegativity of Chlorine but how does that affect the C-Cl bond length and how is it shortest in CCl4??

It would be great if you could give me the correct reasoning to that...

Thanks,
Shikhin

Answer
One way to think about it is this.  Look at each one not as a C-Cl bond but as a RC-Cl bond, where R is H3, ClH2, Cl2H, and Cl3.  Then think electric attraction assuming the electrons reside of the Cl in the bond.

CH3Cl is CH3-Cl  in this case the CH3 is basically neutral so little electrical attraction to Cl.

ClCH2-Cl now because of the Cl pulling electron density from C, the C is more positive than in the first case.

Cl2CH-Cl now with two Cl, the C is getting even more positive so the Cl is attracted even more by the carbon

Cl3C-Cl has the most positive C so the bond gets even shorter.

A large oversimplification, but in general it is correct.