Infrared Spectroscopy

Mass Spectra and IR

What is infrared spectroscopy used for?

To identify functional groups present in organic compounds.

Where is infrared radiation found in the electromagnetic spectrum?

Between visible light and microwaves, with a lower frequency than visible red light.

What happens when molecules absorb infrared radiation?

Bonds in the molecules vibrate.

What are the two types of molecular vibration caused by infrared absorption?

Stretching and bending.

What is stretching?

A vibration in which the bond length increases and decreases.

What is bending?

A vibration in which the bond angle increases and decreases.

What three factors affect the amount of infrared energy absorbed by a bond?

The length of the bond, the strength of the bond and the masses of the atoms involved.

Why can different bonds absorb different frequencies of infrared radiation?

Different bonds have different strengths, lengths and atoms, so they vibrate at different frequencies.

What happens when an organic compound is irradiated with infrared radiation?

Its bonds absorb some frequencies of infrared radiation but transmit others.

What does an infrared spectrum show?

Which frequencies of infrared radiation are absorbed by a compound.

What is usually plotted on the vertical axis of an infrared spectrum?

Transmittance (%).

What does 100% transmittance mean?

All of the infrared radiation is transmitted and none is absorbed.

How are absorptions represented on an infrared spectrum?

As downward dips or troughs in the spectrum.

What does a strong absorption look like on an infrared spectrum?

A deep trough with low transmittance.

What does a weak absorption look like on an infrared spectrum?

A shallow trough with high transmittance.

What does the intensity of an infrared absorption describe?

The amount of infrared radiation absorbed.

What is plotted on the horizontal axis of an infrared spectrum?

Wavenumber in cm⁻¹.

What does wavenumber represent?

The frequency of infrared radiation absorbed by a particular bond.

How does the wavenumber scale usually run on an infrared spectrum?

It decreases from left to right.

What unusual change occurs in the wavenumber scale at 2000 cm⁻¹?

The scale spacing changes.

What is meant by a sharp infrared absorption?

An absorption occurring over a narrow wavenumber range.

What is meant by a broad infrared absorption?

An absorption occurring over a wide wavenumber range.

Why are infrared absorption peaks useful?

Different bonds and functional groups produce characteristic absorptions at particular wavenumbers.

What is the characteristic O–H absorption range for an alcohol?

3750–3200 cm⁻¹.

What is the characteristic N–H absorption range for an amine?

3500–3300 cm⁻¹.

What is the characteristic O–H absorption range for a carboxylic acid?

3300–2500 cm⁻¹.

What is the characteristic C–H absorption range for an alkene?

3095–3010 cm⁻¹.

What is the characteristic C–H absorption range for an alkane?

2962–2853 cm⁻¹.

What is the characteristic C–H absorption range for an aldehyde?

2900–2820 cm⁻¹ and 2775–2700 cm⁻¹.

What is the characteristic C=O absorption range for an aldehyde?

1740–1720 cm⁻¹.

What is the characteristic C=O absorption range for a carboxylic acid?

1725–1700 cm⁻¹.

What is the characteristic C=O absorption range for a ketone?

1720–1700 cm⁻¹.

What is the characteristic C=C absorption range for an alkene?

1669–1645 cm⁻¹.

Which functional groups contain a C=O bond?

Aldehydes, ketones and carboxylic acids.

What does a strong absorption at around 1700 cm⁻¹ suggest?

The presence of a C=O bond.

Why does a C=O absorption alone not necessarily identify the functional group?

Aldehydes, ketones and carboxylic acids can all contain a C=O bond and absorb in a similar region.

How can the absence of an absorption help identify a compound?

A missing characteristic absorption can rule out particular functional groups.

A spectrum has a strong absorption around 1700 cm⁻¹ but no broad absorption at 3300–2500 cm⁻¹. What does this suggest?

A ketone rather than a carboxylic acid.

Why does the absence of a broad 3300–2500 cm⁻¹ absorption rule out a carboxylic acid?

Carboxylic acids contain an O–H bond that produces a broad absorption in this region.

A spectrum contains a broad absorption centred around 2850 cm⁻¹ and a sharp absorption around 1710 cm⁻¹. Which homologous series is suggested?

Carboxylic acids.

Why would a carboxylic acid show a broad absorption around 2850 cm⁻¹?

Its O–H bond absorbs broadly within the 3300–2500 cm⁻¹ region.

Why would a carboxylic acid show an absorption around 1710 cm⁻¹?

Its C=O bond absorbs within the 1725–1700 cm⁻¹ region.

How should you identify a functional group from an infrared spectrum?

Look for characteristic absorptions, compare their wavenumbers with reference data, and also check for expected absorptions that are absent.

What is transmittance?

The amount of radiation transmitted at a particular wavenumber.

What is infrared radiation?

The part of the electromagnetic spectrum with frequencies below those of red visible light.

What is the relationship between absorption and transmittance?

Greater absorption produces lower transmittance.