PL rst: Modern analytical techniques: high-resolution mass spectrometry; NMR spectroscopy

Question 1

High-resolution mass spectrometry measures the molecular mass of ions to four decimal places.

State one use of this technique which is not shared with regular mass spectrometry.

Answer 1

Differentiating between 2 molecules with the same Mr.

Question 2

C₂H₄, N₂ and CO all have a Mr of 28. Substance X has an accurate Mr of 27.9949.

Identify substance X. The exact relative atomic masses of the relevant elements are given below.

Answer 2

Question 3

Complete the table summarising mass spectrometry and IR spectroscopy.

Answer 3

Question 4

The environment of a carbon atom refers to the atoms or groups it is bonded to.

How many carbon environments are there in propane?

Answer 4

2

Central C is in one environment, and the 2 outer Cs are in the same environment since propane is symmetrical

Question 5

• Draw methylpropan-1-ol.
• Annotate the diagram to show the number of carbon environments it has, and how many carbon atoms are present in each.

Answer 5

Question 6

Read the following explanation of how ¹³C NMR (nuclear magnetic resonance) spectroscopy works.

• Carbon-13 nuclei behave like magnets, so can align with or oppose a magnetic field.
• Supplying the correct frequency causes them to flip from the stabler aligned state to the less stable opposed state.
• Chemical shift is related to the frequency required, relative to a standard (in this case, TMS). Lower shift indicates a higher frequency.
• The environment a ¹³C nucleus is in affects its shift. If a nucleus is bonded to or near to a shifting group (involving electronegative atoms), it experiences more shifting.

Use this information to explain why a ¹³C nucleus bonded to a shifting group experiences more shifting.

Answer 6

• ¹³C nucleus is bonded to an electronegative atom
• Bonding electrons are further from nucleus
• Nucleus experiences less shielding
• Lower frequency is required to flip it (to oppose magnetic field)
• Higher chemical shift

Question 7

Carbon-13 NMR (nuclear magnetic resonance) works on the principle that carbon-13 nuclei are found in different chemical environments.

Describe what can or cannot be deduced from the following features of a ¹³C NMR spectrum (example below):

• % intensity (y-axis)
• Chemical shift, δ (x-axis)
• Number of peaks

Answer 7

% intensity: no information (doesn’t indicate relative number of carbons in each environment)

Chemical shift: type of ¹³C environment

Number of peaks: number of ¹³C environments

Question 8

An isomer of methyl propanoate has the following ¹³C NMR spectrum.

Suggest the structural formula for the compound, explaining your reasoning.

Answer 8

Molecular formula is C₄H₈O₂

4 peaks so 4 carbon environments

There are 4 carbons so each is in a unique environment; therefore asymmetrical

1. Peak around 25 ppm indicates C-C
2. Peak around 60 ppm indicates C-O
3. Peak around 80 ppm indicates another C-O, but more shifted
4. Peak around 205 indicates C=O

There are 2 C-O environments, one being shifted by C=O. Therefore structure is HCOCH₂OCH₂CH₃

Question 9

The ¹³C NMR of a compound with molecular formula C₃H₆O is shown below.

Suggest the structure of the compound.

Answer 9

Possible structures: propanone (CH₃C=OCH₃), 1-hydroxyprop-1-ene (CH₂=CHCH₂OH), propanal (CH₃CH₂CHO).

3 ¹³C environments. Compound contains 3 Cs so is asymmetrical.

• ~10 → C-C
• ~40 → C-C, shifted
• ~200 → C=O

Must be aldehyde or ketone: either propanone or propanal.

Not propanone since it only has 2 ¹³C environments.

So must be propanal, CH₃CH₂CHO.

Question 10

How many peaks would you expect in the ¹³C NMR spectra of the following?

• Ethane
• Propane
• Propan-1-ol
• Propan-2-ol

Answer 10

• 1
• 2
• 3
• 2

Question 11

How many proton environments are there in this molecule?

Answer 11

3

check - but think the 3 methyl groups are same env due to free rotation

Question 12

Proton NMR (nuclear magnetic resonance) works on the principle that ¹H nuclei are found in different environments.

Describe what can or cannot be deduced from the following features of a proton NMR spectrum:

• % intensity (y-axis)
• Chemical shift, δ (x-axis)
• Number of peaks
• Area under a given peak, if labelled

Answer 12

% intensity: no information

Chemical shift: type of proton environment

Number of peaks: number of proton environments

Area under a peak: relative abundance of protons in the environment

Question 13

How many peaks would you expect in the ¹H NMR spectra of the following?

• Ethane
• Propane
• Propan-1-ol
• Propan-2-ol

Answer 13

• 1
• 2
• 4
• 3

Question 14

Determine which of the following is responsible for the low-resolution ¹H NMR spectrum below:

• Propanoic acid
• Methyl ethanoate
• Ethyl methanoate

Answer 14

3 peaks so 3 hydrogen environments.

1. Peak around 1 ppm indicates 3 hydrogens in HC-R environment
2. Peak around 2.5 ppm indicates 2 hydrogens in HC-C=O environment
3. Peak around 12 ppm indicates 1 hydrogen in COOH environment

(1) could apply to any of the three.
(2) and (3) only apply to propanoic acid, so compound is propanoic acid.

Question 15

In high resolution ¹H NMR spectra, splitting patterns may be observed, as shown below.

What can be deduced from the number of sub-peaks in a given cluster?

Answer 15

n + 1 rule

Number of sub-peaks = n + 1, where n = number of hydrogens bonded to adjacent carbon(s) (to carbon to which proton(s) in question is/are bonded).

This rule derives from the fact that n protons bonded to one carbon have n + 1 possible magnetic orientations. These protons affect the chemical shift of those bonded to the adjacent carbon, and can do so in n + 1 different ways, giving rise to n + 1 sub-peaks.

Question 16

State and explain how many ¹H NMR sub-peaks which a proton in a CH₃-OH environment would cause.

Answer 16

• One (singlet).
• Proton is not bonded to a carbon, so n + 1 rule cannot apply (electronegative atoms prevent splitting)

Question 17

State and explain whether there is splitting in the proton NMR spectrum of ethane.

Answer 17

No, since hydrogens on each carbon are in identical environments.

You don’t get splitting if hydrogens are in same environment, because they resonate (flip magnetically) at same time

Question 18

Suggest the name of the compound which produces this proton NMR spectrum.

Answer 18

2 peaks, so 2 proton environments, so must have 2 carbons, or 3 with symmetricality.

• Triplet around 0.9 ppm indicates hydrogens(s) in HC-R environment, with adjacent carbon(s) bonded to 2 hydrogens
• Heptet (7 sub-peaks) around 1.3 ppm indicates hydrogens(s) in HC-R environment, with adjacent carbon(s) bonded to 6 hydrogens

Must be alkane (due to environments).

Heptet must indicate 2 adjacent carbons each bonded to 3 hydrogens, all in same environment.

So must be propane, CH₃CH₂CH₃.

Question 19

The ¹H NMR spectrum of an ester is shown below. Work out the structure of the ester.

Answer 19

3 peaks, so 3 hydrogen environments, in ratio 1:3:6.

• Doublet ~1 ppm → 6 Hs in HC-R environment, with adjacent carbon attached to one hydrogen. So (CH₃)₂-CH
• Singlet ~2 ppm → 3 Hs in HC-C=O environment, with no Hs on adjacent carbon. So H₃C-OC-
• Multiplet ~5 ppm → 1 hydrogen, with adjacent carbon attached to several hydrogens. Cannot be C=CH environment, since this doesn’t fit 1:3:6 ratio, so must be HC-R, but shifted by an electronegative atom

Structure must be CH₃COOCH(CH₃)₂

Question 20

A compound with molecular formula C₄H₈O₂ produces the ¹H NMR spectrum below.

Work out the structure of the compound.

Answer 20

3 peaks, so 3 hydrogen environments, in ratio 2:3:3.

• Triplet around 1 ppm indicates 3 protons in HC-R environment, where adjacent carbon is bonded to 2 hydrogens: so H₃C-CH
• Singlet around 2 ppm indicates 3 protons in HC-C=O environment, with no hydrogens on adjacent carbon (or no adjacent carbon): so H₃C-C=O
• Quartet around 4 ppm indicates 2 hydrogens in HC-O environment, where adjacent carbon is bonded to 3 hydrogens: so H₃C-CH₂-O

So structure must be CH₃COOCH₃CH₃

Question 21

Complete the table summarising NMR spectroscopy.

Answer 21