Topic 7: Modern Analytical Techniques I

Question 1

What are the three main functions of a mass spectrometer?

Answer 1

• Create positive ions from a neutral sample
• Separates ions according to their m/z ratio
• Measures the relative abundances of ions and their relative masses; the information being represented as a mass spectrum

Question 2

How is spectrometry different from spectroscopy?

Answer 2

It is different from spectroscopy because it measures mass rather than the absorption of energy

Question 3

What occurs during stage 1 of mass spectrometry (ionisation)?

Answer 3

The atom/molecule is isonised by knocking one or more electrons off to give a positive ion (by collision with high energy electrons). This is true even for things which would normally expect to form negative ions or never form ions at all (e.g. argon). A vacuum is necessary as the ions produced in the ionisation chamber need a free run through the machine without hitting air molecules.

Question 4

What occurs during stage 2 of mass spectrometry (acceleration)?

Answer 4

The ions are accelerated into a finely focused beam. The ions are accelerated so that they have the same kinetic energy.

Question 5

What occurs during stage 3 of mass spectrometry (deflection)?

Answer 5

Ions are deflected by a magnetic field according to their masses. Deflection occurs on: mass as lighter ions will be more deflected than heavy ones, charge as ions with 2 positive charges are deflected more than those with 1. Ions with the smallest mass/charge ratio are deflected the most and vice versa.

Question 6

What occurs during stage 4 of mass spectrometry (detection)?

Answer 6

Positive ions hit the negative plate and an electron jumps from the metal on to the ion. That leaves a space amongst the electrons and the electrons in the wire shuffle along to fill it. A flow of electrons in the wire is detected as an electric current. The size of the current is proportional to the number of ions.

Question 7

How to identify the molecular ion peak on a mass spectrum?

Answer 7

It is the peak with the highest m/z ratio. The m/z value of the M peak is the Mr value for your molecule. For most organic compounds the M peak is the one with the second highest m/z ratio, the small peak on the right is caused by the isotope C13.

Question 8

Why does the C13 isotope cause the small line (one m/z unit to the right of the main molecular ion peak?

Answer 8

The C13 is a stable isotope of carbon that makes up 1.11% of all carbon atoms. This means that some of the molecular ions have a slightly higher relative molecular mass than that of the normal ion.

Question 9

What occurs during fragmentation?

Answer 9

When excess energy transferred to the molecular ion makes it unstable, breaking them up into smaller pieces. It breaks into another positive ion and an uncharged free radical. The uncharged free radical won’t produce a line on the mass spectrum.

Question 10

Define molecular ion peak:

Answer 10

The molecular ion peak indicates the species formed from the molecule by the loss of one electron.

Question 11

Define fragmentation:

Answer 11

Fragmentation occurs when the molecular ion breaks into smaller pieces: a radical and positive ion.

Question 12

Define base peak:

Answer 12

The base peak indicates the peak with the greatest abundance.

Question 13

What are the fragmentation patterns of aldehydes and ketones?

Answer 13

It’s the cleavage of bonds next to the carbonyl group (C=O). Also the position of the carbonyl group influences the fragmentation pattern because the molecular ion fragments either side of the carbonyl group.

Question 14

Describe the fragmentation patterns of chlorine

Answer 14

Chlorine exists as two isotopes,Cl35 and Cl37. A compound containing one chlorine atom will therefore have two molecular ion peaks due to the two different isotopes it can contain. Cl35 = M+ peak and Cl37 = M+2 peak. The ratio of the peak heights is 3:1 as the relative abundance of Cl35 is 3x greater than that of Cl37.

Question 15

Describe the molecular ion peaks that can occur from a diatomic molecule of chlorine

Answer 15

Cl35 + Cl35 = M+ peak (at m/z 70)
Cl35 + Cl37 = M+2 peak (at m/z 72)
Cl37 + Cl37 = M+4 peak (at m/z 74)

The ratio of heights can be deduced by using the probability of each chlorine atom being Cl35 or Cl37. (Cl35 = 3/4, Cl37 = 1/4)

Question 16

Describe the molecular ion peaks of halogenoalkanes

Answer 16

Using the molecular ion of C2H5Br for example, there are two peaks. One being the isotope of Br79 and the other Br81 isotope. Also because the two isotopes are of similar abundance, the peaks are similar heights.

Question 17

Describe the stability of ions in terms of fragmentation

Answer 17

The most stable ions are less likely to fragment and give tall peaks in a mass spectrum. The stability of carbocations is as follows: primary < secondary < tertiary.

Question 18

How does infrared spectroscopy work?

Answer 18

A pair of atoms joined by a chemical bond is always vibrating. The system behaves like two balls (two atoms) joined by a spring (bond). Stronger bonds vibrate faster (at a higher frequency) and heavier atoms make the bond vibrate slower (at a lower frequency). Every bond has its own unique natural frequency that is in the infrared region of the EM spectrum. When you shine infrared radiation (heat energy) through a sample, the bonds in the sample can absorb energy from the radiation and vibrate more. However, any particular bond can only absorb radiation that has the same frequency as the natural frequency of the bond. Therefore, the radiation that emerges from the sample will be the missing frequencies that correspond to the bonds in the sample.

Question 19

Describe the steps in the infrared spectrometer

Answer 19

• A beam of infrared radiation containing a spread of frequencies is passed through a sample
• The radiation that emerges is missing the frequencies that correspond to the types of bonds found in the sample
• The instrument plots a graph of the intensity of the radiation emerging from the sample, called the transmittance, against the frequency of radiation
• The frequency is expressed as a wavenumber, measured in cm-1

Question 20

When UV is absorbed by a molecule/atom what is the energy used for and what is the spectroscopy technique called?

Answer 20

• Movement of electrons to higher energy levels
• Ultra-violet/ visible spectroscopy

Question 21

When Infrared is absorbed by a molecule/atom what is the energy used for and what is the spectroscopy technique called?

Answer 21

• Vibrate bonds
• Infrared spectroscopy

Question 22

When microwaves is absorbed by a molecule/atom what is the energy used for and what is the spectroscopy technique called?

Answer 22

• rotate molecules
• microwave spectroscopy

Question 23

When radio waves is absorbed by a molecule/atom what is the energy used for and what is the spectroscopy technique called?

Answer 23

• to change nuclear spin
• NMR (Nuclear magnetic resonance) spectroscopy

Question 24

What is a dipole moment?

Answer 24

Is when there are two partial opposite charges, separated by a distance. It’s measured in equation: dipole moment = magnitude of charge x distance between the charges

Question 25

What causes a reading on an IR spectrometer?

Answer 25

If a bond stretches, we are changing the distance between the partial charges and if we change the distance we are changing the dipole moment; this causes an absorption of IR radiation and a peak in the spectrum. Only stretching vibrations that produce a change in the dipole moment will be IR active.

Question 26

Describe the classification of IR bands

Answer 26

IR bands can be classified as strong, medium or weak depending on their relative intensities in the infrared spectrum. A strong band covers most of the y-axis, a medium band falls to about half of the y-axis, and a weak band falls to about one third or less of the y-axis.

Question 27

Stretching vibrations depend on:

Answer 27

- the mass of the atoms on either side of the bond - the bond strength - the bond length

Question 28

Why can't H2 or Cl2 absorb infrared radiation?

Answer 28

Simple, non-polar molecules don't absorb infrared radiation because they don't have a dipole. When the molecule stretches due to a vibration there's no change in the dipole, so there is no absorption of radiation.

Question 29

Why does a C=C IR active?

Answer 29

There must be a change in dipole moment for the stretch to be IR active. To determine whether or not there will be a change in dipole moment, examine the bond that is stretching and look at the two fragments that it's connected to. If these two fragments are equivalent, there will be no net change in dipole moment for the symmetrical stretch and it will be IR-inactive.

Question 30

Define infrared radiation:

Answer 30

Infrared radiation is the part of the EM spectrum with frequencies below that of red light

Question 31

Define stretching:

Answer 31

Stretching occurs when a bond absorbs infrared radiation and uses it to alter the length of the bond.

Question 32

Define transmittance:

Answer 32

The transmittance value in an infrared spectrum represents the amount of radiation absorbed at a particular wavenumber.

Question 33

Define wavenumber:

Answer 33

The wavenumber of an infrared absorption represents the frequency of infrared radiation absorbed by a particular bond in a molecule.

Question 34

Define intensity:

Answer 34

The intensity of an infrared absorption describes the amount of infrared radiation absorbed

Question 35

What is the fingerprint region in an infrared spectrum?

Answer 35

It is caused by the vibration of the molecule as a whole; it is unique for every compound, so by comparison with the IR spectrum for compounds in a data bank we can identify a compound; each different compound produces a different pattern of troughs in this part of the spectrum

Question 36

What are the uses of Infrared spectrometry?

Answer 36

- IR spectroscopy gives information about the presence of absence of key functional groups. - IR can also be used by a chemist to check to see if a reaction has proceeded as planned. For example, if we wished to convert cyclohexanone to cyclohexanol, a quick comparison of the IR spectra of the cyclohexanone and then of the final product would tell us if we had successfully converted the ketone group to an alcohol.

Question 37

What do the y-axis and x-axis represent on an IR spectrum?

Answer 37

y-axis = transmittance x-axis = wavenumber

Question 38

How to calculate wavenumber?

Answer 38

1/wavelength (in cm) = wavenumber

Question 39

What does the IR spectrum of a carboxylic acid show?

Answer 39

- Carboxylic acids show a broad absorption between 3200 and 3600cm-1 due to the presence of the O-H bond - Carboxylic acids also show a strong absorption around 1700cm-1 due to the presence of the C=O bond

Question 40

What does the IR spectrum of an alcohol show?

Answer 40

Alcohols show a broad absorption between 3200 and 3600cm-1 due to the presence of the O-H bond. The breadth of this signal is a consequence of hydrogen bonding between molecules

Question 41

What does the IR spectrum of an ester show?

Answer 41

Esters show a strong absorption between 1750 and 1730cm-1 due to the presence of the C=O bond.

Question 42

How is an O-H bond different in an alcohol than in an acid (as shown on an IR spectrum)

Answer 42

The O-H bond in an alcohol absorbs at a higher wavenumber than in an acid somewhere between 3230-3550cm-1

Question 43

Why do the major constituents of air not absorb infrared radiation?

Answer 43

In order for a vibration to absorb IR radiation, the molecule must change its dipole moment during the vibration. Homonuclear diatomic molecules like N2 and O2 do not have dipole moments. If the molecules undergoes a stretching motion, where spheres represent the two nuclei, there is no change in the dipole moment during the vibrational motion, therefore N2 and O2 do not absorb infrared radiation.`

Question 44

Describe how HCl absorbs Infrared radiation

Answer 44

HCl does have a dipole moment. Stretching the HCl bond leads to a change in the dipole moment. If we stretched the bond so far as to break the bond and produce the two original neutral atoms, there would be no dipole moment. Therefore, as we lengthen the bond in HCl, the dipole moment gets smaller. Because the dipole moment of HCl changes during a stretching vibration HCl absorbs IR radiation.

Question 45

What are peak widths?

Answer 45

Peak widths are related to the strength of the intermolecular bonding. In acids like ethanoic acid, where the strength of the hydrogen bonding is greater, the peak caused by the OH stretch is broad, whereas in alcohols where the H bonding is less significant, the peak caused by the same OH stretch is narrower.

Question 46

Describe the IR spectrum of alkenes on a mass spectrum

Answer 46

Alkenes show sharp, medium bands corresponding to the C=C bond stretching vibration at about 1600-1700cm-1. Some alkenes might also show a band for the =C-H bond stretch appearing at around 3080cm-1.

Question 47

What is the fingerprint region of an IR spectrum?

Answer 47

Is found between 1500-400cm-1. It is used to find the exact match to spectrum of the known compound.

Question 49

What species cannot be oxidised by heating with acidified potassium dichromate (VI)

Answer 49

Tertiary alcohols