Chapter 20- Analytical techniques

Question 1

What can analytical techniques be used for?

Answer 1

Structure determination, mass determination

Question 2

What is the index of hydrogen deficiency?

Answer 2

It’s the degree of unsaturation. It determines how many rings, double bonds, and triple bonds are present in the compound. It does not give the exact number of rings or double or triple bonds though.

Question 3

Index of hydrogen deficiency=

Answer 3

(H(reference) - H(molecule))/2

Question 4

What is the reference compound for the index of hydrogen deficiency?

Answer 4

A alkane with the same number of carbon atoms as the other molecule. The molecular formula is CnH(2n+2)

Question 5

What are the key stages of mass spectrometry?

Answer 5

Stage 1: Ionisation. The atom or molecule is ionised by knocking one or more electrons off to give a positive ion. Most mass spectrometers work with positive ions.
Stage 2: Acceleration. The ions are accelerated so that they all have the same kinetic energy.
Stage 3: Deflection. The ions are then deflected by a magnetic field according to their masses. The lighter they are, the more they are deflected.
Stage 4: Detection. The beam of ions passing through the machine is detected electrically.

Question 6

How is the relative atomic mass worked out, if the mass spectrum is given in relative abundance?

Answer 6

Suppose you had 123 typical atoms of boron. 23 of these are 10B and 100 are 11B. The total mass of these would be (23 x 10) + (100 x 11) = 1330. The average mass of these 123 atoms would be 1330 / 123 = 10.8 (to 3 significant figures).

Question 7

How is the relative atomic mass worked out, if the mass spectrum is given in percentage abundance?

Answer 7

Suppose you had 100 atoms of zirconium. 51.5 of these are 90 Zr, 11.2 are 91 Zr, 17.1 are 92 Zr, 17.4 are 94 Zr, and 2.8 are 96 Zr. The total mass of these 100 typical atoms would be: (51.5 x 90) + (11.2 x 91) + (17.1 x 92) + (17.4 x 94) + (2.8 x 96) = 9131.8. The average mass of these 100 atoms would be 9131.8 / 100 = 91.3 (to 3 significant figures).

Question 8

What is the molecular ion?

Answer 8

It’s the ion of the molecule being analysed by mass spec. It is also a radical cation because it’s got one unpaired electron and has a positive charge.

Question 9

When molecules interact with infrared radiation, what happens to the covalent bonds?

Answer 9

They get excited to higher vibrational energy levels

Question 10

What does m/z stand for?

Answer 10

Mass/charge

It’s the mass to charge ratio of a molecule

Question 11

What is infrared spectroscopy?

Answer 11

IR spectroscopy is the study of absorption of IR radiation by molecules, by measuring the frequencies of IR radiation absorbed by a substance

Question 12

What is the radiation in the vibrational IR region measured in?

Answer 12

It’s measured in reciprocal cm (cm^-1) and is referred to as its wavenumber

Question 13

What does a bond have to be in order to be infrared active?

Answer 13

The bond has to be polar; the more polar the bond, the stronger its absorption of IR radiation

Question 14

Describe the theory behind IR spectroscopy

Answer 14

Molecules aren’t rigid – the bonds within them can move (vibrate). If these vibrations cause a change in the dipole moment of the molecule, it is said to be IR active. This can be measured in IR spectroscopy

Question 15

What’s used as the reference material in NMR spectroscopy?

Answer 15

Tetramethylsilane (TMS)

Question 16

What signal is assigned to TMS in both H and C NMR?

Answer 16

𝛿=0

Question 17

What does NMR stand for?

Answer 17

Nuclear Magnetic Resonance

Question 18

What form of electromagnetic radiation is used in NMR spectroscopy?

Answer 18

Radio waves are used in NMR spectroscopy

Question 19

How many distinct carbon signals are expected in the 13C-NMR spectrum of 3-methylpentan-3-ol?

Answer 19

Even though there’s 7 carbons, there’s some symmetry in the molecule. Therefore, there should be 4 signals in the spectrum

Question 20

Describe the theory behind NMR spectroscopy

Answer 20

It’s based on the fact that some nuclei have spin. These nuclei either have an odd mass number or an odd atomic number. Because the external magnetic field is in part generated by the movement of electrons in and around atoms close to a specific nucleus, the exact local field experienced by that nucleus will vary depending on where it is in a molecule and what arrangement of neighbouring atoms is present.

Question 21

Why is NMR useful?

Answer 21

The ability to detect and measure signals from such different situations makes NMR an extremely powerful diagnostic tool for investigating the structure of molecules.

Question 22

Why is mass spectrometry useful?

Answer 22

It’s used in the determination of relative atomic masses (Ar), relative molecular masses (Mr), and molecular structure

Question 23

What are the two types of mass spectrometry?

Answer 23

Low resolution- gives M/Z in whole numbers, so you can’t differentiate between molecules with the same mass to 1 d.p.
High resolution- normally measured to 4 d.p., giving better precision to measurements