Physical and Chemical Properties

What are some notable properties of the four derivatives of acyl compounds?

1. Acyl chlorides
   • They are very reactive and have practical synthetic purposes.
   • Chlorine makes the carbonyl carbon a very good electrophile that even weak nucleophiles can attack.
   • Chlorine is a good leaving group.
2. Anhydrides
   • They are usually not water soluble (anhydrides commonly react with water).
3. Esters
   • Esters cannot form hydrogen bonds and so they also have lower boiling points.
4. Amides
   • Amides can form hydrogen bonds and therefore have higher boiling points.
     • $\beta$-lactams have significantly more strain since they have only two non-carbonyl carbons.

What are the relative reactivities of acyl compounds?

Figure 51.10: Relative reactivities of acyl compounds.

How can the carbonyl carbon of acyl carbons be modified?

Electron withdrawing groups make the carbonyl carbon more electrophilic, and therefore more susceptible to nucleophilic attack. This phenomenon explains a lot about the relative reactivities of the acyl compounds illustrated above.

What are two differences between amides and amines?

Table 51.1: Two key differences between amides and amines.

	Amides	Amines
pH	Less basic	More basic
e$^{-}$ withdrawing effects	Greater	Less
	(from carbonyl oxygen)

Note: the decreased pH of amides is a result of the greater withdrawing effects from the amide's carbonyl oxygen.

What is the infrared absorptions of carboxylic acids and their derivatives?

Table 51.2: Infrared absorption of carboxylic acids and derivatives.

	Bond Frequency	($cm^{-1}$)
Carboxylic acids	RCOOH	$\sim$1700
Acyl chlorides	RCOCl	$\sim$1800
Acid anhydrides	(RCO)$_{2}$O	$\sim$1800
Esters	RCOORÕ	$\sim$1700
Amides	RCONH$_{2}$	$\sim$1600