You have an unknown compound and are told that it is ONE of the two possibilitie

Question

You have an unknown compound and are told that it is ONE of the two possibilities A or B. Describe concisely how you could decide from the IR spectrum of your sample which compound it is. Specify the frequencies of major absorptions and the bonds responsible for them, that would be present or absent if it is A or if it B. (for instance: "3200-3600 O-H present, 1665-1760 C=O absent if it is a; 3200-3600 O-H absent, 1665-1760 C=O present if it is "B", etc.) You have an unknown compound and are A told that it is ONE of the two possibilities A or B. Describe concisely how you could decide from the IR spectrum of your sample which compound it is. Specify the frequencies of major absorptions and the bonds responsible for them, that would be present or absent if it is A or if it B. (for instance: "3200-3600 O-H present, 1665-1760 C=O absent if it is a; 3200-3600 O-H absent, 1665-1760 C=O present if it is B", etc.)

Explanation / Answer

1) at 3500 - 3300 cm-1 medium peak, will indicate presence of amine ( If in case of alcohol, peak will be broad in the same range). Primary amines will produce two N-H stretch absorptions, secondary amines only one, and tetriary none.From the number of peaks at this range we can classify the amines.

In option A primar amine will have 2 medium peak at 3500 - 3300 cm-1 , option B tertiary will not have a peak at 3500 - 3300 cm-1 .

2) A) for carbonyl (C=O) strecthing in IR the characteristic peak is around 1700 cm-1, and for C=C alkene is around 1600 cm-1.

In option A C=O is presents and C=C is absents, hence peak at 1700 cm-1 will be present and 1600 cm-1 will be absents.

In option B C=O is absents and C=C is presents, hence peak at 1700 cm-1 will be absents and 1600 cm-1 will be presents.

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