Topic 11.2/11.3 - Spectroscopic identification of organic compounds

Question 1

IHD

Answer 1

Index of hydrogen deficiency. Degree of unsaturation in a molecule. Used to determine the number of rings or multiple bonds in a molecule from a molecular formula.

Question 2

Numbers of IHD

Answer 2

The number of hydrogen pairs that could be added to a compound. (ie benzene could add 8 more hydrogens so has an IHD of 4)

(Essentially how many more hydrogens it would have if it was an alkane)

Question 3

IHD if not hydrocarbon

Answer 3

IHD = (0.5) (2c + 2 - h - x + n)

c = carbon
h = hydrogen 
x = halogen
n = nitrogen

Question 4

Mass spectrometry

Answer 4

When a molecule is ionised, its positive ion is formed. Higher energy leads to fragmentation which then causes peaks. By looking at the Mr of these peaks, you can discover which fragments have been made.

Data booklet contains some values.

Format: peak x is caused by …..⁺

Question 5

Infrared radiation

Answer 5

Causes the molecular vibration due to groups in molecules. These vibrations cause different peaks on the IR spectrum. Lighter atoms - higher frequency, heavier atoms - lower frequency.

Molecules require a dipole for this technique to work, so those that are non-polar will likely not display peaks.

Values are in the data booklet.