Chemistry 2213a Fall 2012 Western University
Topic 10a. Carboxylic Acids
A. Structure and Nomenclature
The carboxyl group results from the RCOOH or RCO H 2
connection of a hydroxyl group to aR OH
The functional group retains some properties of a carbonyl group, some of a
hydroxyl group, plus some properties unique to the carboxyl group.
In IUPAC nomenclature, the O O
carboxyl group is always given Br
priority, with its carbon assigned OH OH
as atom #1. The e in the word
alkane is changed to oic acid.propanoic acid 4-bromo-2-butenoic acid
For complicated compounds, we
can use the ending carboxylic acid. COOH
If we use this system, the COOH
carboxylic acid name does not
count as part of the parent
hydrocarbon. trans-1,2-cyclopentanedicarboxylic acid Carboxylic Acids 2
The simplest acids are known by their common names.
HCOOH formic acid
CH COOH acetic acid COOH
HOOC–COOH oxalic acid benzoic acid
If a molecule contains a carbonyl in addition to the
carboxyl group, the carbonyl oxygen is denoted by oxo.
Like carbonyl compounds, Greek letters are often used to indicate substituent
R an -amino acid
2 Carboxylic Acids 3
As seen in Chapter 2, carboxylic acids are acidic. Acids generally taste sour.
Vinegar is 5% acetic acid by volume.
They are very polar compounds that can have extensive
H-bonding and have bp higher than similarly sized alcohols.
CH 3H C2 CH2OH2 bp 118°
CH 3H C2OH bp 141°
In solution, the carboxyl groups of O - O H O R
adjacent molecules hydrogen bond
with each other to form dimers. R + O H R O H O
Carboxylic acids have polar bonds located at one end of the molecule, and it is
this polar end, termed the hydrophillic end, that interacts with water. The other
end of the molecule, represented by the R group, is non-polar (termed the
hydrophobic end) and repels water.
When the R group is small (< 6 carbons), the carboxylic acid is water-soluble.
As the length of the C chain increases, solubility in water goes down. Carboxylic Acids 4
The IR spectrum of an acid has two characteristic, strong peaks: the OH at
2400–3400 cm and the C=O at 1700–1725 cm .
In H NMR, the acidic H of the COOH appears far downfield, between 10–13
ppm. Often this is off of the scale for the plot, so it is “offset” by an amount. Carboxylic Acids 5
1. Reduction to 1° alcohols
The COOH group is quite resistant to 1. LiAlH / ether
reduction. However, it can be reduced 4 OH
to a 1° OH using Li4lH followed bR OH 2. Water R 2
treatment with2H O.
Note that NaB4 is not strong enough of a reducing agent, and that catalytic
hydrogenation does not reduce carboxyl groups. Therefore, selective reduction
can be done by carefully choosing the reducing agent.
O H2/ Pt O
1. LiAlH / ether
4 2. 2 O
OH OH Carboxylic Acids 6
2. Decarboxylation of -keto acids
Carboxylic acids with a -keto group decarboxylate when heated, to eliminate
CO2. (i.e. break C–C bond)
O O O
R OH R CH3
The reaction proceeds through a cyclic conformation, which is ideally arranged
for the facile elimination of CO . The -keto group accepts the electrons from
the broken bond.