Using Infrared Spectra

Mass Spectra and IR

What are three important uses of infrared spectra?

Predicting absorptions in an organic compound, identifying functional groups from absorptions, and deducing structures using wavenumbers and molecular formulae.

What is the fingerprint region of an infrared spectrum?

The region below 1500 cm⁻¹.

Why is the region below 1500 cm⁻¹ called the fingerprint region?

Its complex absorption pattern is slightly different for every molecule, like a fingerprint.

Why is the fingerprint region usually not used to identify individual functional groups?

It contains many overlapping absorptions, mainly from bending vibrations, making individual peaks difficult to interpret.

How can the fingerprint region be used to identify a compound?

The spectrum can be compared with a known reference spectrum; matching fingerprint regions strongly suggest the compounds are the same.

Which part of an IR spectrum should you concentrate on when identifying functional groups?

Approximately 4000–1500 cm⁻¹.

How can you predict the infrared spectrum of a known organic compound?

Identify its functional groups and predict their characteristic absorption ranges using wavenumber data.

What absorptions would you expect for an aldehyde such as propanal?

C–H aldehyde absorptions at 2900–2820 and 2775–2700 cm⁻¹, and a C=O absorption at 1740–1720 cm⁻¹.

What characteristic absorption would you predict for an alcohol?

A broad O–H absorption at 3750–3200 cm⁻¹.

An unknown compound has absorptions at 3675, 2870 and 1735 cm⁻¹. What functional groups are indicated?

O–H from an alcohol, C–H, and C=O from an aldehyde.

An unknown compound has absorptions at 3500–3300 and 3300–2500 cm⁻¹. What functional groups are indicated?

N–H from an amine and O–H from a carboxylic acid.

What type of compound could contain both an amine group and a carboxylic acid group?

An amino acid.

Why should molecular formula information be combined with IR data?

The same absorption may fit several possible structures, so the molecular formula helps determine which structure is possible.

A compound has absorptions at 1730 and 3450 cm⁻¹. What bonds do these suggest?

A C=O bond and an O–H alcohol bond.

A compound has molecular formula C₂H₄O₂ and absorptions at 1730 and 3450 cm⁻¹. What possible structure fits the data?

HOCH₂CHO (2-hydroxyethanal).

Why would C₂H₄O₂ with absorptions at 1730 and 3450 cm⁻¹ not be ethanoic acid, CH₃COOH?

A carboxylic acid would have a broad O–H absorption at 3300–2500 cm⁻¹ rather than the alcohol O–H absorption around 3450 cm⁻¹.

What does an absorption around 1730 cm⁻¹ suggest in this example?

A C=O bond, consistent with an aldehyde.

What does an absorption around 3450 cm⁻¹ suggest?

An O–H bond in an alcohol.

An unknown compound has molecular formula C₃H₆O₂ and shows a broad absorption around 3000 cm⁻¹ and a narrow absorption around 1700 cm⁻¹. What functional group is indicated?

A carboxylic acid group, –COOH.

Why does a broad absorption around 3000 cm⁻¹ suggest a carboxylic acid?

Carboxylic acid O–H bonds produce a broad absorption in the 3300–2500 cm⁻¹ region.

Why does an absorption around 1700 cm⁻¹ support the presence of a carboxylic acid?

The C=O bond of a carboxylic acid absorbs around 1725–1700 cm⁻¹.

An unknown compound has molecular formula C₃H₆O₂ and contains a –COOH group. What structure is suggested?

CH₃CH₂COOH, propanoic acid.

How can IR spectroscopy distinguish between propan-1-ol and propan-2-ol?

It generally cannot easily distinguish them using characteristic functional-group absorptions because both contain the same O–H alcohol group and similar C–H bonds.

Why can two structural isomers have very similar IR spectra?

They may contain the same functional groups and therefore produce absorptions in similar wavenumber ranges.

How could IR spectroscopy show that C₃H₆O has the structure CH₃COCH₃ rather than CH₃CH₂CHO?

Both have a C=O absorption, but an aldehyde would also show characteristic aldehyde C–H absorptions at 2900–2820 and 2775–2700 cm⁻¹. Their absence supports CH₃COCH₃.

What is the key IR evidence for identifying a ketone rather than an aldehyde?

A C=O absorption is present but the characteristic aldehyde C–H absorptions are absent.

Why is absence of an expected IR absorption useful?

It can rule out a functional group or possible structure.

What is a good method for deducing a structure using IR spectroscopy?

Identify major absorptions above 1500 cm⁻¹, match them to functional groups, check for important missing absorptions, then combine this evidence with the molecular formula.