Physical and Chemical Properties

What are three physical properties of carboxylic acids?

1. Acidity
   • Typically weak acids.
   • The ability to release hydrogen is increased by the existence of the two electronegative oxygen atoms attached to the carbonyl/hydroxyl carbon. Moreover, the addition of other electronegative groups near the carboxylic group, e.g. chloride, will increase the acidity of the carboxylic acid and is an example of an inductive effect:
     

     Figure 50.3: Inductive effects in carboxylic acids: (a) has the highest acidity (p $K_{a} \approx 0.7$), followed by (b) (p $K_{a} \approx 1.5$), (c) (p $K_{a} \approx 3.0$), and (d) (p $K_{a} \approx 5.0$).

     

2. Boiling point
   • Exceptionally high because of hydrogen bonding. The unique positioning of two oxygens allows for dimeric hydrogen-oxygen associations:
     

     Figure 50.4: Hydrogen bonding in carboxylic acids.

     

3. Solubility
   • Solubility is directly related to polarity and the ability to undergo hydrogen bonding.
   • Lower than corresponding alcohols but the first four carboxylic acids (i.e. methanoic acid to butanoic acid) are soluble in water. As expected, as the carbon chain grows, water solubility decreases.

What role does resonance play in carboxylic acids?

Stabilizing the anionic form:

Figure 50.5: Resonance in carboxylic acids.

What is the infrared absorption of carboxylic acids?

Table 50.1: Infrared absorption of carboxylic acids.

Bond Frequency	($cm^{-1}$)
$RCOOH$	$\sim$1700

How do carbonyl substituents affect reactivity?

Intiuitively, larger substituents cause more steric hindrance and decrease reactivity.