Organic Chemistry

Let’s now look at a molecular orbital description of how carbonyl compounds react.
In Section 1.6, which first introduced you to molecular orbital theory, you saw that be-
cause oxygen is more electronegative than carbon, the 2porbital of oxygen contributes
more to the bonding molecular orbital—it is closer to it in energy—and the 2por-
bital of carbon contributes more to the antibonding molecular orbital. (See
Figure 1.8.) This means that the antibonding orbital is largest at the carbon atom,
so that is where the nucleophile’s nonbonding orbital—in which the lone pair re-
sides—overlaps. This allows the greatest amount of orbital overlap, and greater over-
lap means greater stability. When a filled orbital and an empty orbital overlap, the
result is a molecular orbital—in this case, a molecular orbital—that is more stable
than either of the overlapping orbitals (Figure 17.3).

s

p*

p*

p

Section 17.6 Relative Reactivities of Carboxylic Acids and Carboxylic Acid Derivatives 683

C O C O−

Z

Z

a filled

nonbonding

orbital

empty π∗

antibonding

orbital

C Z σ

bonding

orbital

new C

σ bond

Z Energy

C O π∗

antibonding

orbital

C Z σ∗

antibonding

orbital

these orbitals

overlap

nonbonding

orbital of the

lone pair

Figure 17.3
The filled nonbonding orbital containing the nucleophile’s lone pair overlaps the empty
antibonding molecular orbital of the carbonyl group, forming the new bond in the
tetrahedral intermediate.

s

p*

PROBLEM 7

Using the values in Table 17.1, predict the products of the following reactions:

a. c.

b. d.

PROBLEM 8

Is the following statement true or false?

If the newly added group in the tetrahedral intermediate is a stronger base than the group

attached to the acyl group in the reactant, formation of the tetrahedral intermediate is the

rate-limiting step of a nucleophilic acyl substitution reaction.

17.6 Relative Reactivities of Carboxylic Acids

and Carboxylic Acid Derivatives

We have just seen that there are two steps in a nucleophilic acyl substitution reaction:
formation of a tetrahedral intermediate and collapse of the tetrahedral intermediate.
The weaker the base attached to the acyl group, the easier it is for both stepsof the

CH 3 O

+ NaCl

CH 3

C

O

C

O

NaOH

CH 3 Cl

C

O

+

CH 3 Cl Na+

+

CH 3 O−

C

O

C

O

CH 3 OCH 3

C + NaCl

O

pKa