Carboxylic Acids & Esters

Question 1

Physical Properties
- The OH bond in carboxylic acids forms hydrogen bonds. This means they have relatively high MP and BP compared to similar sized alkanes
- The COOH group is very soluble in water due to hydrogen bonding, but
the hydrocarbon chain of a carboxylic acid is non-polar, so doesn’t dissolve in water.
- Whether a carboxylic acid is soluble depends on the length of the carbon chain

Question 2

Short chain carboxylic acids (up to
butanoic acid) dissolve easily in
water because the hydrocarbon
chain part of the molecule is
relatively short

Question 3

The longer a carboxylic acids is the
less soluble it is. This is because the
hydrocarbon chain part of the
molecule is longer, and even though
the COOH dissolves well, this is
cancelled out by the insolubility of
the chain

Question 4

Carboxylic acids have very specific melting points. These can be used to distinguish between different carboxylic acids.

Question 5

Reactivity of Carboxylic Acids
- Carboxylic acids can act as acids and so act as a proton donor.
- All carboxylic acids are weak acids so the following equilibrium lies over to the left:
RCOOH ←→ RCOO-
+ H+
- Carboxylic acids also undergo reactions with alcohols to form esters (see
below)

Question 6

Esters
- These are derivatives of carboxylic acids with pleasant aromas.
- Esters have the general formula RCOOR

Question 7

Naming Esters
- The first part of the name comes from the chain attached to the O with a single bond
-The second part of the name needs to come from the chain which
contains the C=O bond

Question 8

Forming Esters
Esters are formed from a carboxylic acid reacting with an alcohol.
- The reaction uses conc H2SO4 catalyst
- The reaction is reversible
- Water is always the byproduct
- The carboxylic acid and alcohol selected for the reaction will determine the ester produced

Question 9

As the formation of an ester is reversible, esters can be turned back into their constituent carboxylic acid and alcohol via a hydrolysis reaction

Question 10

Hydrolysis of Esters
Hydrolysis means ‘breaking a bond using water’
Esters can be hydrolysed back to their constituent carboxylic acid and alcohol by breaking the ester link
There are two methods of ester hydrolysis:

Question 11

(1) Acid catalysed hydrolysis
Esters can be hydrolysed at room temp using a strong acid catalyst
As this is an equilibrium reaction it is hard to get a high yield of the carboxylic acid and alcohol

Question 12

(2) Base catalysed hydrolysis
Bases such as warm aqueous NaOH can also catalyse the hydrolysis reaction

• Once the carboxylic acid forms it reacts with the NaOH catalyst.
• This forms the salt of the acid
• This increases the yield as it shifts the position of the equilibrium to the right hand side.
• It also means if the carboxylic acid is the desired product this method is
  not appropriate as the salt forms instead.