Chapter 37 Analytical Techniques (HNMR)

Question 1

What does HNMR use?

Answer 1

radiowaves

Question 2

How does HNMR work?

Answer 2

• Protons are positively-charged & generate a magnetic field by spinning.
• When an external magnetic field is applied, one proton aligns & one opposes the magnetic field.

Question 3

Which hydrogen has higher or lower energy?

Answer 3

• Hydrogen aligned with external magnetic field = lower energy
• Hydrogen aligned opposed to external magnetic field - higher energy

Question 4

How do electrons affect HNMR?

Answer 4

• Electrons also spin & generate their own magnetic field.
• Electrons provide shielding effect. Placing an electron between the external magnetic field & the proton, shields the hydrogen.

Question 5

O-H VS C-H

Answer 5

• Electrons in O-H further from hydrogen atom than electrons in C-H.
• The closer the electrons to the hydrogen atom, the more the hydrogen is shielded.
• Energy gap between the electrons is different.

Question 6

What is the environment of a hydrogen atom?

Answer 6

• Distance of electrons from the Hydrogen atom.
• E.g. O-H, C-H, CH2, NH2 are all different environments.

Question 7

What does number of peaks mean in NMR spectroscopy?

Answer 7

• Number of environments

Question 8

What do the heights of peaks mean in NMR spectroscopy?

Answer 8

• Ratio of hydrogen environments in the compound

Question 9

Ethanol HNMR:
- Number of peaks
- Ratio

Answer 9

• 3 peaks
• 3:2:1

Question 10

Cyclopentane HNMR:
- Number of peaks
- Ratio

Answer 10

• 1 peak
• 10

Question 11

Butanol HNMR:
- Number of peaks
- Ratio

Answer 11

• 5 peaks
• 1:2:2:2:3

Question 12

Pentan-2-one HNMR:
- Number of peaks
- Ratio

Answer 12

• 4 peaks
• 3:2:2:3

Question 13

3-chloropentane HNMR:
- Number of peaks
- Ratio

Answer 13

• 3 peaks
• 6:4:1

Question 14

Cyclohexanol HNMR:
- Number of peaks
- Ratio

Answer 14

• 5 peaks
• 1:1:4:4:2

Question 15

What do chemical shifts mean in NMR spectroscopy?

Answer 15

• Information about the type of environment the hydrogen atoms are in / what the hydrogen atoms are bonded to

Question 16

Tetramethylsilane

Answer 16

• (CH3)4-Si
• produces a strong singlet peak (because it has 12 hydrogen atoms)
• maximum shielding
• a reference point
• inert, volatile

Question 17

LHS & RHS of NMR graph?

Answer 17

• LHS: electrons furthest from H atom
• RHS: electrons closest to H atom
• Deshielding increases as you go further left of the graph, chemical shift in ppm increases.

Question 18

1.6 pm

Answer 18

alkyl / alkane

Question 19

2 - 3 ppm

Answer 19

CH3-CO (aldehyde) / CH2

Question 20

3 - 4 ppm

Answer 20

CH2-O / CH3

Question 21

6 - 9 ppm

Answer 21

benzene

Question 22

9 to 10 ppm

Answer 22

aldehydes

Question 23

11 to 12 ppm

Answer 23

COOH

Question 24

How does High Resolution NMR work?

Answer 24

-Think of it as zooming in to the peaks on a low resolution NMR graph
- Each peak on an LNMR graph appears as a cluster of peaks on a HNMR graph

Question 25

Number of peaks at a certain position on a HNMR graph?

Answer 25

• Number of Hydrogens on an adjacent C atom

Question 26

Heights of peaks on a HNMR graph?

Answer 26

• Chemical shift

Question 27

The n+1 rule?

Answer 27

• Number of sub-peaks in a cluster is one more than the number of hydrogens attached to the next door carbon(s)

Question 28

Sample answer.

Answer 28

at 1.0 ppm, CH3-CH2, triplet peak suggests 2H on adjacent C

Question 29

9701/42/M/J/23:
Suggest why CDCl3 is used as a solvent instead of CHCl3 for the HNMR spectrum.

Answer 29

CDCl3 does not give a peak.

Question 30

Why is CDCl3 needed when obtaining a proton NMR spectrum? (3)

Answer 30

• When samples are analysed through NMR spectroscopy, they must be dissolved in a solvent.
• Unlike TMS (which shows one sharp reference peak), a solvent (CDCl3) containing Deuterium (an isotope of hydrogen) can be used instead.
• Deuterium nuclei absorb radio waves in a different region to the protons analysed in organic compounds. Thus, the reference solvent peak will not interfere with those of the sample.

Question 31

How is D2O used to identify O-H & N-H protons by proton exchange?

Answer 31

• Adding D2O ‘removes’ N-H & O-H peaks from the spectrum
• The -OH proton & the -NH proton undergo an exchanging process with a deuterium atom on D2O.
• E.g.: -OH + D2O –> -OD + HOD
• E.g.: -NH-CO- + D2O –> -ND-CO- + HOD