Organic synthesis

What is the reason for heating under reflux?

To prevent any substances escaping

What does IR radiation do to the covalent bonds in a molecule?

Causes them to vibrate more and absorb energy

How is the IR radiation absorbed by a bond indicated?

Wavenumber (cm^-1)

How does infrared radiation interact with atmospheric gases?

• Sun’s IR radiation passes through atmosphere and is absorbed by the earth

• Some IR radiation is then emitted by the earth

• IR radiation absorbed by atmospheric gases

• Vibrating bonds in these gases re-emit the radiation

• = greenhouse effect

Factors which affect the contribution of gases to the greenhouse effect

• ability of molecule to absorb infrared

• concentration of the gas

• residence time

How is IR radiation used to detect bonds in molecules

They absorb radiation characteristic of the bonds in the molecule

What bonds are infrared spectrometry especially good at identifying?

• alcohol (O-H bond)

• aldehyde or ketone (C=O)

• carboxylic acid (C=O and broad O-H)

How can compounds be identified using the fingerprint region?

Using a computer by comparison to the spectra of known compounds in a spectral database

Applications of infrared spectroscopy

• breathalysers measure ethanol levels

• monitor gases causing air pollution (e.g. CO and hydrocarbons from car emmisions)

Uses of mass spectrometry

• determine molecular mass (by detecting molecular ion peak)

• determine parts of the structure of an organic compound (from fragmentation peaks)

Production of a mass spectrum in a mass spectrometer

1. Sample is vaporised

2. Ionisation - samples are bombarded with electrons = cations produced

3. Acceleration - electric field accelerates cations, focused into a beam

4. Deflection - cations separated according to mass:charge ratio

5. Detection - cations detected and software generates a spectrum

What is a mass:charge ration (m/z)?

As cations are all +1, the m/z gives the molecular mass of the fragment

Features of the mass spectrum

• relative intensity on the y-axis

• most abundant peak is assigned value of 100%

• m/z on the x-axis

• fragment with greatest m/z peak = molecular ion peak

How do fragments occur?

Bonds in the molecular ion break to form fragments

• weaker bonds break more easily

• bonds which produce more stable fragments break more easily

m/z value for CH₃⁺

15

m/z value for CH₃CH₂⁺ or CHO⁺

29

m/z value for CH₃CH₂CH₂⁺ or CH₃CHO⁺

43

m/z value for CH₃CH₂CH₂CH₂⁺

57

m/z value for CH₂OH⁺

31

m/z value for COOH⁺

45

m/z value for C₆H₅⁺

77

m/z value for C₆H₅CO⁺

105

Where does cleavage often occur?

On either side of a carbon attached to an electronegative substituent