Functional Classes II, Reactions - Organic Chemistry Concepts: An EFL Approach - page 126

the nitrogen. The amide bond is important in many natural products of biologi-
cal importance, such as peptides and proteins. Figure 7.42 shows why the bond-
ing in an amide makes it special.
FIGURE 7.42
The amide bond.
The nitrogen lone pair is easily delocalized into the carbonyl group. The result
of delocalization is best drawn as a resonance hybrid. This gives partial C ] N
character to the amide bond, and a planar sp 2 -hybridized state to the nitrogen.
This stops free rotation around the C-N bond and can cause possible cis/trans
isomers.
Proof of this limited rotation is seen in the fixed conformations in amides.
Amides also have a shorter C-N bond length than amines, ±132 pm compared
with ±147 pm. This delocalization in amides also explains their non-basic char-
acter compared with amines. This is because the lone pair is no longer free to
act as a Lewis base.
7.8.4 Nitriles
In Section 7.5.2 , we saw that nitriles are easily made and can be hydrolyzed to
carboxylic acids. See Figure 7.43 . In Section 7.8.5 , you can see that a nitrile can
be reduced. However, the key feature is that the formation of a nitrile and any
subsequent change adds one carbon to the chain.
FIGURE 7.43
Nitrile chain lengthening sequence.
7.8.5 Reduction
As you can see in Figure 7.44 , reduction is a major reaction of all acyl deriva-
tives. This can be done with metal hydride reagents, such as LiAlH 4 . With acyl
halides, anhydrides, acids, and esters, reduction gives alcohols as the products.
For amides and nitriles, reduction gives amines as the products.