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Molecular structure deduction - 3 problems - ANSWERS

NMR problem solving with associated molecular data

Original NMR Questions

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 Answer to NMR PROBLEM 1 

nmr problem 1

alkanes structure and naming (c) doc bFrom the data the structure of the molecule fits in with methylpropane

  CH(CH3)3

Empirical formula from % elements

82.76/12 : 17.24/1 = 1 : 2.5 = 2 : 5.0

gives C2H5 (mass = 29)

Molecular formula = C4H10 (since Mr = 29 x 2 = 58 to match the molecular ion peak) 10 protons

infrared spectrum: Peaks in 2850-2950 cm-1 region typical of C-H bonds
From the NMR spectrum in the absence other than alkyl structures:

The peak at 0.9 ppm corresponds to a 'primary' C-H of a methyl group and there must be nine of them i.e. three methyl groups.

The peak at 1.5 ppm corresponds to a tertiary C-H.


 Answer to NMR PROBLEM 2 

nmr problem 2

 

(c) doc b

CH3CH2COOCH3

 

From the data the structure of the molecule is methyl propanoate

Empirical formula from % elements

54.55/12 : 9.09/1 : 36.36/16

4.54 : 9.09 : 2.27 = 2 : 4 : 1 gives C2H4O (mass = 44)

Molecular formula = C4H8O2 (since Mr = 44 x 2 = 88 to match the molecular ion peak) 8 protons

infrared spectrum:

The sharp strong absorption at 1735-1750 cm-1 indicates the carbonyl group C=O (in an ester/aldehyde?)

NO broad absorption band from ~2500-3200 cm-1 suggests NO OH group e.g. in an alcohol or a carboxylic acid.

From the NMR spectrum

The peak at ~1.15 ppm corresponds to a methyl group relatively unaffected by the oxygen atom one more carbon beyond it. The peak at ~2.4 corresponds to a -CH2- group in 'acid' part of an ester. The peak at ~3.75 ppm corresponds to a methyl group of a methyl ester.


 Answer to NMR PROBLEM 3 

nmr problem 3

From the data the structure of the molecule must be

3,3-dimethylbutanoic acid

Empirical formula from % elements

62.07/12 : 10.34/1 : 27.59/16

5.17 : 10.34 : 1.72 = 3 : 6 : 1 gives C3H6O (mass = 58)

molecular formula = C6H12O2 (since Mr = 58 x 2 = 116 to match the molecular ion peak) 12 protons

infrared spectrum:

The strong sharp absorption at 1700-1725 cm-1 is from a carbonyl group in a ketone/carboxylic acid?

The moderate, broad absorption at 2450-3150 cm-1 is from a hydrogen bonded hydroxy group e.g. in alcohols and carboxylic acids.

From the NMR spectrum

The peak at 1.0 ppm corresponds to three methyl groups. The peak at ~2.45 corresponds to a -CH2- group attached to a carboxylic acid/ester group. The peak at 12.0 is likely to be from a hydroxy group.

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