17.1 Mass Spectrometry

Learning Objectives:1. Describe how mass spectrometry can be

used to identify unknown compounds.

Used to Find Molecular Mass of Organic Compounds

1. Compound enters MS as gas.2. Electron gun ionises sample.3. Positive ions accelerated through MS as beam

of ionised molecules.4. Ions deflected by magnetic field into a

detector.5. Strength of the magnetic field related to m/z

(m/z = mass as charge = +1).6. Detector measures abundance.

Mass Spectrum

Why are there so many peaks?

• molecular ion = ion formed from whole molecule

• The main peak furthest to the right corresponds to the mass of the molecular ion.

• Fragmentation = molecule breaks up during the MS process to form smaller ions

• The pattern can then be used as a fingerprint to identify the molecule.

Mass Spectrum of Pentane

High Resolution MS

• MS usually to nearest whole number.• Can be difficult to differentiate between

different molecules with same similar Mr.• High res MS can measure masses to three or

four decimal places.• More accurate, can be used to differentiate

different molecules of similar masses.

17.2 Intra-Red Spectroscopy

Learning Objectives:1. Describe IR spectroscopy.2. Be able to interpret IR spectra.

All bonds are vibrating

• Each bond has a certain frequency of vibration.

• This frequency corresponds with certain frequencies (wavelengths) of IR radiation.

• Bonds can absorb IR radiation to vibrate more.• The frequencies which are absorbed can be

detected and used to identify what bonds are in a molecule.

Infra-Red Spectrometer

1. Beam of IR radiation with range of frequencies passed through a sample.

2. The sample absorbs certain frequencies.3. Instrument plots the transmittance (intensity

of frequencies which make it through the sample).

4. The absorbed frequencies will be “missing” from the chart.

IR Spectrum

Peaks Correspond to Specific Bonds

FINGERPRINT REGION

• organic molecules have a lot of C-C and C-H bonds within their structure• spectra obtained will have peaks in the 1400 cm-1 to 800 cm-1 range• this is referred to as the “fingerprint” region• the pattern obtained is characteristic of a particular compound the frequency of any absorption is also affected by adjoining atoms or groups.

IR SPECTRUM OF A CARBONYL COMPOUND

• carbonyl compounds show a sharp, strong absorption between 1700 and 1760 cm-1

• this is due to the presence of the C=O bond

IR SPECTRUM OF AN ALCOHOL

• alcohols show a broad absorption between 3200 and 3600 cm-1

• this is due to the presence of the O-H bond

IR SPECTRUM OF A CARBOXYLIC ACID

• carboxylic acids show a broad absorption between 3200 and 3600 cm-1

• this is due to the presence of the O-H bond• they also show a strong absorption around 1700 cm-1

• this is due to the presence of the C=O bond

IR SPECTRUM OF AN ESTER

• esters show a strong absorption between 1750 cm-1 and 1730 cm-1

• this is due to the presence of the C=O bond

WHAT IS IT!

O-H STRETCH

C=O STRETCH

O-H STRETCH

C=O STRETCH

AND

ALCOHOL

ALDEHYDE

CARBOXYLIC ACID

One can tell the difference between alcohols, aldehydes and carboxylic acids by comparison of their spectra.