NMR ( proton NMR & carbon NMR)

Question 1

Give the name and structure of the compound used in the NMR to provide a reference point? [2]

Give four reasons why this compound is used as a reference point? [4]

Answer 1

Tetramethylsilane OR (TMS)    1
           CH3
               I
  CH3--- Si ---CH3      1 
               I
             CH3

all hydrogen’s are equivalent / are in the same environment 1
volatile (so easy to remove from sample) 1
it is inert (won’t react with sample) 1
it is non toxic 1

Question 2

Give an example of a solvent that is suitable for recording NMR. Give one reason for your choice? [3]

Answer 2

CCl4 1
contains no hydrogen’s 1
wont interfere with spectrum readings 1

Question 3

The proton n.m.r of sample Q shows 4 peaks showing the following:
chemical shift = 2.20 2.69 3.40 3.84
splitting pattern = singlet triplet singlet triplet
integration value = 3 2 1 2

What can be deduced about Q from this data? [4]

i) single peak at 2.20
ii) single peak at 3.40
iii) two triplet peaks
iv) Deduce the structure of Q

Answer 3

i) R - C - CH3 1
II
O

ii) O - H 1

iii) CH2CH2 (allow ethyl group) 1
O
II
iv) H - O - CH2CH2 - C - CH3 1

Question 4

The data shows the proton n.m.r spectrum of the compound CH3CHClCOOH, with the following peaks and splitting patterns.

shift = 11.0 ———————

shift = 4.44 ——-

                 - ------------------------
                 - --------------------
                 - ----------

shift = 1.72 ————————————–
——————————–

Name the compound CH3CHClCOOH, and explain the splitting patterns at 1.72 & 4.44? [3]

Predict the splitting pattern that would be seen in the isometric compound ClCH2CH2COOH? [1]

Answer 4

2- chloro propanoic acid 1

(1. 72) doublet, bonded to one adjacent H’s 1
(4. 44) quartet, bonded to three adjacent H’s 1

two triplets 1

Question 5

Use the following data from the spectrum of the compound X ( C7H1203 ) to answer the following

chemical shift = 4.13 2.76 2.57 2.2 1.26
integration ratio = 0.8 0.8 0.8 1.2 1.2

how many different proton environments are shown? [1]

what is ratio of each type of proton present? [1]

what proton type is responsible for the peak at 2.20? [1]

Answer 5

5 peaks 1

2 : 2 : 2 : 3 : 3 ( do not allow multiples ) 1

R - C - CH3 1
II
O

Question 6

Suggest why NMR would not work with an atom of carbon - 12, but will with the isotope carbon - 13? [2]

Answer 6

(C - 12) has an even number of protons and neutrons 1
[allow converse for carbon - 13]

causes nucleus to have (nuclear spin) 1

Question 7

Explain how the addition of an external magnetic field allows a NMR spectrum to be created? [4]

Answer 7

causes nucleus to align with the magnetic field 1
(aligned with field) nuclei have (slightly) lower energy 1
radio waves are absorbed by nuclei 1
causes nuclei to flip to a higher energy level 1

(allow converse for opposing field) *

Question 8

Two isometric cyclic compounds are compared using carbon -13 n.m.r
cyclohexan - 1,3 - diol cyclohexan - 1,4 - diol
OH OH
A I__ B I__
/ \ / \
\__/ \ \__/
OH \OH

Predict the number of peaks in the C n.m.r of each of the compounds shown? [2]
Suggest why the structures represent stereoisomers? [11]

Answer 8

(A) 4 peaks 1
(B) 2 peaks 1

show E / Z isomerism OR geometric isomerism 1

Question 9

The ester compound CH3CH2COOCH2CH3 was analysed using carbon- 13 n.m.r, and showed 4 peaks

Explain why the two methyl groups produced different peaks despite both being bonded to a CH2 group? [1]

Answer 9

are bonded to different atoms 1

each CH3 group feel different amounts of shielding 1

Question 10

The compound methoxyethane ( CH3CH2OCH3 ) is analysed using carbon n.m.r
Draw the displayed formula and identify the name of the two other possible isomers of CH3CH2OCH3? [2]

Deduce which isomer is responsible for the spectrum shown below with two peaks? [1]

Answer 10

H    O-H    H
              I      I        /
      H -- C -- C -- C -- H            propan - 2 - ol    1 
              I      I       \
             H     H      H

             H     H    H
              I      I      I
  H-O -- C -- C -- C -- H           propan - 1 - ol      1
              I      I      I
             H     H    H

propan - 1 - ol 1