14. Organic Analysis

Question 1

What causes bonds to vibrate? And at what rate do they vibrate?

Answer 1

IR energy is absorbed by bonds making them vibrate. The bonds vibrate at the same rate as the IR energy

Question 2

shape of O-H alcohol bond

Answer 2

finger-like

Question 3

shape of O-H bond in carboxylic acids:

Answer 3

like 4 finger (wide)

Question 4

shape of C=O bond

Answer 4

dagger like

Question 5

E.g Why does the spectrum represent a ketone and not a carboxylic acid

Exam Technique: Whenever answering an IR question, may have to state the bonds that aren’t present

Answer 5

There’s a C=O absorption at 180-1750 cm-1 but no O-H peak at 2500-3000cm-1

Question 6

Where is the fingerprint region

Answer 6

below 1500 cm-1

MUST ALWAYS SAY ‘in the fingerprint region below 1500cm-1

Question 7

Example

A person studying the spectra wouldn’t be able to tell the difference between propan-1-ol and propan-2-ol. What should they do?

Answer 7

• both contain -H alcohol functional group so thier main feature is broad O-H peak in the region 3230-355O cm-1
• a computer database can be used on the fingerprint region to identify which alcohol is which

Question 8

How can IR spectroscopy be used to confirm what compound is definately ethyl butanoate?

Answer 8

compare fingerprint region below 1500cm-1 with know spectrum from database, should be exact match

Question 9

Global Warming

Which bonds absorb most IR radiation in atmosphere

Answer 9

• O-H bonds in water, C-O bonds in CO2 and C-H bonds in methane
• IR emitted by earth isn’t allowed to escape back into atmosphere