Chapter 17: Spectroscopy

Question 1

Describe what happens when an organic compound is placed in the mass spectrometer.

Answer 1

When an organic compound is placed in the mass spectrometer, it loses an electron and forms a positive ion, the molecular ion.

The mass spectrometer detects the mass-to-charge ratio (m/z) of the molecular ion which gives the molecular mass of the compound.

Question 2

What is the molecular ion?

Answer 2

The molecular ion M⁺ is the positive ion formed when a molecule loses an electron.

Question 3

How is the molecular mass of a compound determined from a mass spectrum?

Answer 3

The molecular ion peak (M⁺ peak) has to be located. The molecular ion peak is the clear peak at the highest m/z value on the right-hand side of the mass spectrum. The molecular mass is this m/z value.

Question 4

What is the very small peak one unit after the M⁺ peak?

Answer 4

This is referred to as the M + 1 peak and it exists because 1.1% of carbon is present as the carbon-13 isotope so a small proportion of the organic molecules will contain an atom of carbon-13 and thus have a greater mass than the M⁺ ion by +1.

Question 5

What is fragmentation?

Answer 5

Fragmentation is the process that occurs in a mass spectrometer in which some molecular ions break down into smaller pieces.

Question 6

How do the fragment ions affect the mass spectrum?

Answer 6

The other peaks in a mass spectrum are caused by fragment ions, formed from the breakdown of the molecular ion.

Question 7

Describe the simplest fragmentation of a molecular ion.

Answer 7

The simplest fragmentation breaks a molecular ion into two species – a positively charged fragment ion and a radical.

Any positive ions formed will be detected by the mass spectrometer, but the uncharged radicals are not detected.

Question 8

In the mass spectrum for propan-1-ol, the largest peak has an m/z value of 31. Give the equation that shows the formation of this fragment ion from the molecular ion.

Answer 8

Question 9

Describe the features of this mass spectrum.

Answer 9

Question 10

Give the formulae and corresponding m/z values for common fragment ions.

Answer 10

Question 11

Describe the motion of covalent bonds.

Answer 11

Covalent bonds possess energy and vibrate naturally about a central point, the amount of vibration increasing with increasing temperature. The atoms in molecules are therefore in constant motion.

Question 12

State the effect of infrared radiation on covalent bonds.

Answer 12

Covalent bonds can absorb infrared radiation, which makes them bend or stretch more.

Question 13

Describe two types of vibration in covalent bonds.

Answer 13

The first type of vibration, a stretch, is a rhythmic movement along the line between the atoms so that the distance between the two atomic centres increases and decreases.

The second type of vibration, a bend, results in a changing bond angle.

Question 14

What factors does the amount that a bond stretches or bends depend on?

Answer 14

• the mass of the atoms in the bond – heavier atoms vibrate more slowly than lighter atoms
• the strength of the bond – stronger bonds vibrate faster than weaker bonds

Question 15

1. What frequency range can a particular bond absorb?
2. Why is the wavenumber scale used?

Answer 15

1. Any particular bond can only absorb radiation that has the same frequency as the natural frequency of the bond.
2. The frequency values are very large, so the wavenumber scale is used, which is proportional to frequency.

EXAM NOTE: The vibrations of most bonds are observed in the IR wavenumber range of 200 cm^-1 to 4000 cm^-1.

Question 16

Most of the Sun’s IR radiation passes through the atmosphere to the Earth’s surface, where most of it is absorbed. Some of it is re-emitted from the Earth’s surface in the form of longer-wavelength IR radiation. Describe how this phenomenon is the beginning of the process that causes global warming.

Answer 16

Water vapour, carbon dioxide, and methane (‘greenhouse gases’) absorb this longer-wavelength IR radiation, because it has the same frequency as the natural frequency of their bonds. Eventually, the vibrating bonds in these molecules re-emit this energy as radiation that increases the temperature of the atmosphere close to the Earth’s surface, leading to global warming.

Question 17

How do organic chemists use infrared spectroscopy to analyse organic molecules? Describe the procedure.

Answer 17

1. The sample under investigation is placed inside an IR spectrometer.
2. A beam of IR radiation in the range 200-4000cm^-1 is passed through the sample.
3. The molecule absorbs some of the IR frequencies and the emerging beam of radiation is analysed to identify the frequences that have been absorbed by the sample.
4. The IR spectrometer is usually connected to a computer that plots a graph of transmittance (%) against wavenumber (cm^-1).

Question 18

What do the dips in an IR spectrum graph correlate to?

In your answer, correct the terminology used in the question.

Answer 18

The dips in the graph are called peaks. Each peak is observed at a wavenumber that can be related to a particular bond in the molecule.

Question 19

Describe and explain the fingerprint region in an infrared spectrum. How is it used?

Answer 19

The fingerprint region in an infrared spectrum is seen below 1500cm^-1.

The fingerprint contains unique peaks which can be used to identify the particular molecule under investigation. This can be done by using computer software or comparing the spectrum to booklets of published spectra.

Question 20

Which functional groups are you able to identify in compounds from an IR spectrum?

Answer 20

• O–H group in alcohols
• C=O group in aldehydes, ketones, and carboxylic acids
• COOH group in carboxylic acids.

Question 21

Describe the infrared spectrum of a propan-2-ol.

Answer 21

Question 22

Describe the IR spectrum of an aldehyde or ketone.

Answer 22

Question 23

Describe the IR spectrum of a carboxylic acid.

Answer 23

Question 24

Give the applications of infrared spectroscopy.

Answer 24

Question 25

Describe a typical sequence of identification for an organic molecule.

Answer 25