DOC BROWN'S Science-CHEMISTRY HOMEPAGE KS3 SCIENCE QUIZZES and WORKSHEETS (~US grades 6-8)
GCSE SCIENCE help links GCSE ADDITIONAL SCIENCE help links
KS3 BIOLOGY Quizzes KS3 CHEMISTRY Quizzes & Worksheets KS3 PHYSICS Quizzes
KS4 Science GCSE/IGCSE CHEMISTRY NOTES (~US grades 8-10) KS4 Science GCSE/IGCSE CHEMISTRY QUIZZES and WORKSHEETS (~US grades 8-10) ADVANCED LEVEL CHEMISTRY QUIZZES and WORKSHEETS (~US grades 11-12)
Custom Search

 Doc Brown's Chemistry

Molecular structure deduction - 3 problems - ANSWERS

NMR problem solving with associated molecular data

Original NMR Questions

A level Revision notes for GCE Advanced Subsidiary Level AS Advanced Level A2 IB Revise AQA GCE Chemistry OCR GCE Chemistry Edexcel GCE Chemistry Salters Chemistry CIE Chemistry, WJEC GCE AS A2 Chemistry, CCEA/CEA GCE AS A2 Chemistry revising courses for pre-university students (equal to US grade 11 and grade 12 and AP Honours/honors level courses)

KS4 Science GCSE/IGCSE Revision Qualitative Analysis Quiz on chemical test methods for identifying ions, gases and compounds

Qualitative Analysis * Quantitative analysis: acid-base, silver nitrate-chloride, EDTA titrations * Quantitative analysis: Redox titrations


 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.

Advanced Chemistry Page Index and LinksWebsite content copyright Dr W P Brown 2000-2011 All rights reserved on revision notes, puzzles, quizzes, worksheets, x-words etc. * Copying of website material is not permitted * I do not personally endorse the adverts - but they do pay for the site!

Alphabetical Index for Science Pages Content A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

BIG SITE HELP SEARCH Enter several specific words/formula etc.