organic analysis

Question 1

Mass spectrometry - fragmentation

Answer 1

Molecules are bombarded with high energy e- & ionised, knocking out 1 e- off a molecule leaving it with a 1+ charge and a ‘.’ free radical.

Question 2

Examples of fragmentation of CH3CH2CH2OH+.

Answer 2

Only the fragment with the 1+ charge goes through the mass spec

Question 3

Using mass spectroscopy to find the M, of compounds

Answer 3

The first species formed is called the moleculs ion, [M]+. (note that this is a free radical and has a positive charge.
The signal for [M]+. gives the Mr of the compound - this is the peak with the greatest m/z value.
Some molecules can break down and form smaller fragments which form peaks with lower
m/z ratios

Question 4

High resolution mass spectroscopy

Answer 4

High resolution mass spectrometers measure the m/z values to enough accuracy to find
the molecular formula. Each molecular formula has a different Mr if measured to enough
precision.

Question 5

Shortcoming of High resolution mass spectroscopy

Answer 5

Question 6

Infrared spectroscopy

Answer 6

• All bonds vibrate at a characteristic frequency. The frequency depends on the mass of the atoms in the bond, the bond strength, and the type of bond.
• The frequencies at which they vibrate are in the Infra- red region of the electromagnetic spectrum. If infra-red light is passed through the compound, it will absorb some or all of the light at the frequencies at which its bonds vibrate.
• Rather than using the actual values of the wavelength or frequency, the IR light is measured in wavenumbers [1/wavelength (in cm)] because it gives convenient numbers in the range 4000 - 400 cm-1.

Question 7

Answer 7

Specific frequencies of infrared radiation are absorbed by the molecules which cause particular bonds to vibrate.
As the bonds are different strengths and some of the atoms have different masses, the frequencies absorbed by CO2 are different from SO2.

Question 8

Using the “finger-print” region (500 - 1500 cm-1)

Answer 8

• This part of the spectrum is more complicated and contains many signals, making picking out functional group signals difficult.
• However, this part of the spectrum is unique for every compound, and so can be used as a “fingerprint”. Comparison of the spectrum to that of known compounds can identify it.
• This region can also be used to check if a compound is pure. If a comparison of the spectrum of a sample is made to the spectrum of the pure compound, they should be identical. If there are any extra peaks, they must be due to an impurity.

Question 9

Identifing functional group signals (above 1500 cm-1)

Answer 9

· This part of the spectrum is used to spot
characteristic signals for functional groups
(there are some below 1500 cm-1 but they are usually difficult to identify due to the high number of signals in that region of the spectrum).

Question 10

Identifing functional group signals (above 1500 cm-1) - C-H

Answer 10

Question 11

Identifing functional group signals (above 1500 cm-1) - O-H

Answer 11

Question 12

Identifing functional group signals (above 1500 cm-1) - C=O

Answer 12

Question 13

Identifing functional group signals (above 1500 cm-1) - C=C

Answer 13

Question 14

Absorption of Infra-red and Global Warming

Answer 14

Greenhouse gases like CO2, CH4, H2O all absorb IR radiation.
When these molecules absorb IR their bonds vibrate more - this radiation does not escape the atmosphere and leads to a general increase in kinetic energy of molecules in the atmosphere which can be detected by an increase in temperature.

• Increase in CO2 etc
• Increase in IR absorption
• Increase in kinetic energy of molecules
• Increase in temperature of atmosphere

Question 15

Base peak in mass spec

Answer 15

Highest peak in mass spec - has most abundance