The Hydroxy Group

Two groups of organic compounds contain the hydroxy group, -OH: alcohols and carboxylic acids.

Carboxylic Acid - general structure

We primarily use IR and ¹³C-NMR spectra to differentiate between these functional groups. There are also characteristic fragments in Mass Spectrometry in some cases. This blog post will go through how three types of spectra can be used together to determine the presence of an alcohol or carboxylic acid.

IR Spectroscopy

O-H Stretch

Both alcohols and carboxylic acids give a characteristic broad O-H stretch. For an alcohol this is between 3600-3300cm^-1. For a carboxylic acid, this is shifted to between 3400-2500cm^-1. However, we need to be careful jumping to a conclusion based on this evidence alone.

C-O Stretch
Again, we expect to see this present in the IR spectra for both functional groups. They are in very similar positions (1260-1000 cm^-1 vs 1320-1210^-1), so cannot be used to conclusively differentiate between alcohols and carboxylic acids.

C=O Stretch
This is not present in alcohols. This is the key to concluding (from IR) whether the structure is an alcohol or carboxylic acid. If the other two characteristic stretches are present, look for this stretch around 1700cm^-1. Its absence strongly supports the conclusion that the compound is an alcohol. Its presence strongly supports the conclusion that the compound is a carboxylic acid.

Ethanoic Acid

Ethanol

When comparing/contrasting these spectra, note the "shift" in the O-H stretch and C-O stretch. However, if you do not have another spectrum to contrast, these "shifts" are less obvious. The real tell-tale sign is the presence of the peak around 1700cm^-1 in carboxylic acids, and its absence in alcohols.

¹³C-NMR

The presence/absence of a signal between 165-195 ppm is what we look for to confirm whether our compound is an alcohol or carboxylic acid. 

Mass Spectrometry

The following fragments are not always present, or giving strong enough signals to be conclusive. Mass Spectrometry should really only be used to differentiate between possible isomers (that are consistent with the other two spectra). However, they can (in some cases) be used to confirm the functional group:

ALCOHOLS

Alcohols have two characteristic signals:

• The M⁺ fragment - 1. This is due to the removal of a hydrogen atom from the hydroxy group (M⁺ -H).
• The M⁺ fragment - 18. This is due to the removal of H₂O (M⁺ -H₂O).

CARBOXYLIC ACID
For short-chain carboxylic acids, there are three characteristic signals:

• The M⁺ fragment - 17. This is due to the removal of an -OH (M⁺ -OH). This fragment is also often present in alcohols, so be careful relying on this fragment as evidence of the functional group.
• The M⁺ fragment - 45. This is due to the removal of -COOH (M⁺ -COOH)
• A fragment at 45 m/z. This is due to COOH⁺ 

Butanoic Acid - 86 g/mol

PRIMARY ALCOHOLS

Most alcohols will create a fragment that creates a signal at 31 m/z. This is due to a CH₂OH⁺ fragment. In primary alcohols, this signal is very strong (a very tall peak). In secondary and tertiary alcohols it is not so strong.

The following images are from this SOURCE

Primary Alcohol: butan-1-ol

Secondary Alcohol: butan-2-ol

Note that the signal at 31 m/z is much smaller than in the primary alcohol. The M⁺ -H₂O fragment is also much less pronounced.

Tertiary Alcohol: 2 methyl propan-2-ol

NOTE: This is only a section of the spectrum. The M⁺ fragment would be at 74 m/z.

Again, we see that the signal at 31 m/z is much smaller than in the primary alcohol,and that the M⁺ -H₂O fragment is also less pronounced.