Question #f961a

1 Answer
Apr 26, 2015

Ordinarily, we would predict the order to be C > N > O > F.

jamesash.wpengine.netdna-cdn.com

C is the least electronegative atom.

It has the least tendency to hold on to its electrons, so it is the best electron donor and the strongest base.

So, why is C out of order in your series?

We are comparing #"CH"_3^-# and #"CH"_3"C≡C"^-#.

The difference is in the hybridization of the C atom.

In #"CH"_3^-#, the hybridization is #sp^3#.

In #"CH"_3"C≡C"^-#, the hybridization is #sp#.

www6.miami.edu

An #sp^3# orbital has 25 % #s# character, while an #sp# orbital has 50 % #s# character.

#s# electrons are held more tightly to the nucleus, so an alkyne C atom is less basic than an alkane carbon.

The order of acidities (with #"p"K_"a"# values) is

#"CH"_4# (48) < #"NH"_3# (38) < #"CH"_3"C≡CH"# (25) < #"CH"_3"CH"_2"OH"# (16) < #"HF"# (3)

So the order of basicities is

#"CH"_3^-# > #"NH"_2^-# > #"CH"_3"C≡C"^-# > #"CH"_3"CH"_2"O"^-# > #"F"^-#