Chapter 17

Question 1

What happens when a molecule absorbs infrared radiation ?

Answer 1

Causes covalent bonds to vibrate
more and absorb energy

Question 2

What are the three factors that affect the amount of vibration in a bond ?

Answer 2

Bond strength
Bond length
Mass of each atom in a bond

Question 3

What is a mass spectrometer used for ?

Answer 3

The percentage abundances of the isotopes in a sample of an element are determined experimentally using a mass spectrometer

Question 4

How does a mass spectrometer work ?

Answer 4

1) Vaporises and ionises the sample, to form positive ions
2) Accelerates the ions by an electric field, towards a negatively charged plate
3) Allows the ions to drift through the flight tube to become separated according to
mass
4) Detects the ions when they arrive at the detector
5) Feeds a signal into a computer, which produces a graph of the relative abundance of ions against the mass to charge ratio. This graph is called a mass spectrum
6) Identifies isotopes and their atomic masses

Question 5

What is meant by Time of flight mass spectrometry ?

Answer 5

It is used to measure the atomic or molecular mass of different particles ( eg atoms or molecules ) in a sample
1) Gaseous particles are injected into the spectrometer and hit with a high energy beam of electrons to form 1+ ions
2) Particles are accelerated by an electrical or magnetic field
3) Particles hit the detector, lowest mass first, highest mass last
4) Detector read by computer - Vaporises and ionises the sample, to form positive ions- Vaporises and ionises the sample, to form positive ions- the time it takes for an ion on reach the detector is dependent on its speed which is dependent on mass.

Question 6

Mass spectrometry in organic compounds

Answer 6

1) Identify the molecular mass of the compound
2) Give further information about the structure

Question 7

How to determine the molecular mass of an organic compound ?

Answer 7

In the mass spectrometer the molecule loses an electron to form a positive ion.
This ion is called the molecular ion.
The mass spectrometer detects the m/z ratio of this ion. It shows up as the highest mass peak on the spectrum (referred to as the molecular ion peak).
Assuming z=1 its value will be equivalent to the molecular mass of the compound
1.1% of carbon is present as the carbon-13 isotope. Therefore approximately 1.1% of the molecules will have molecular mass one unit greater than the one calculated using ’12’ as the A r of carbon.
A small peak one unit after the M + peak is therefore usually seen. This is referred to as the M + 1 peak.

Question 8

What is fragmentation ?

Answer 8

Before being detected some of the molecular ions break down into smaller fragments.
A molecular ion may fragment in several different ways, and so a spectrum might show lots
of different peaks (and lots of different heights).
Fragmentation produces one charged species and one uncharged species (charges on both sides of the equation must balance!).
The spectrometer is only able to detect the charged species.
Uncharged radical not detected, fragment ion detected

Question 9

How do you work out the structure of an organic compound with given data ?

Answer 9

1) % composition by mass - from this you can calculate an empirical formula
2) an mass spectrum - the M r will then allow you to work out molecular formula ….and
fragment ions will allow you to identify sections of the molecule
3) an infrared (IR) spectrum - this will allow you to identify bonds and functional groups

Question 10

How does infrared spectroscopy work ?

Answer 10

Each bond has a unique vibration frequency in the infrared region of the EM spectrum
Bonds absorb radiation that has the same frequency as their frequency of vibration
Infrared radiation emerged from a sample is missing the frequencies that have been absorbed - this info is used to identify compound’s functional group

Question 11

What do the peaks on an infrared spectrum represent ?

Answer 11

Absorbance of energy from the infrared radiation

Question 12

Can you use an infrared spectrum of an organic compound to identify an alcohol ?

Answer 12

an absorption peak of the O–H bond
3200-3600cm-1 wavenumber

Question 13

Can you use an infrared spectrum of an organic compound to identify an aldehyde or ketone ?

Answer 13

absorption peak of the C=O bond
1630-1820cm-1 wavenumber

Question 14

Can you use an infrared spectrum of an organic compound to identify a carboxylic acid ?

Answer 14

Very broad peak at 2500-3300cm-1 ( O-H group )
Sharp peak at 1680-1750cm-1 ( C=O group )

Question 15

What happens inside a mass spectrometer ?

Answer 15

Organic compound vaporised and passed through spectrometer
Some molecules lose electrons and form molecular ions
Excess energy from ionisation makes bonds vibrate more
Vibration causes bond to weaken
Molecular ion splits by fragementation

Question 16

What is the symbol for molecular ion ?

Answer 16

M +

Question 17

Is fragmentation predictable and why ?

Answer 17

No because it can happen anywhere in the molecule

Question 18

In a mass spectrometry how is a molecular ion represented ?

Answer 18

It is the peak with the highest mass/charge ratio

Question 19

What is the molecular mass of the molecular ions equal to ?

Answer 19

Relative molecular mass of the compound

Question 20

Will the two molecular ion peaks of two isomers of the same compound be same or different ?

Answer 20

Same

Question 21

What is the m/z value of
CH3+
OH-
C2H5+
C3H7+
C4H9+

Answer 21

15
17
29
43
57

Question 22

What are the advantages of using mass spectrometry ?

Answer 22

Cheap
Small quanitites of samples required

Question 23

What is the main disadvantage of using mass spectrometry ?

Answer 23

Sample is completely destroyed

Question 24

How do you interpret an IR spectrum ?

Answer 24

The position of a peak tells us which type of bonds has absorbed energy
Height of peak does not tell us anything
Width can help identify bonds (broad/sharp)

Question 25

Applications of IR spectroscopy

Answer 25

IR spectroscopy can be used to monitor levels of localised pollution from nitrogen monoxide, carbon monoxide, and hydrocarbons. IR-based breathalysers measure the ethanol levels in the captured breath. The C-H peak is the most reliable measure as the O-H peak would also be present from water vapour in the breath.

Question 26

Greenhouse gases and global warming in relation to IR

Answer 26

1) Visible and IR radiation from the Sun passes through the atmosphere to the Earth’s surface, where most of it is absorbed. 2) The Earth warms up and re-emits some of the radiation, but in the form of longer- wavelength IR. 3) These wavelengths can be absorbed by water vapour, carbon dioxide and methane molecules (the three most abundant greenhouses gases present in the atmosphere). 4) Their bonds vibrate more, and energy is re-emitted as radiation that increases the temperature of the atmosphere, leading to global warming. 5) Greenhouse gases (eg carbon dioxide and methane) in the troposphere absorb some of this IR, in the ‘IR window’. 6) Absorption of IR increases the vibrational energy of bonds, the energy is transferred to other molecules by collisions, thus increasing their kinetic energy and raising the temperature. In short: vibrational energy is turned into kinetic energy (= heat energy)

Question 27

How to answer questions putting various bits of data together ?

Answer 27

1) Elemental analysis : use of % composition to determine empirical formula 2) Mass spectra : use of molecular ion peak to determine molecular mass, which together with the empirical formula can be used to determine the molecular formula, fragmentation patterns can help with identifying parts of the molecule. 3) Infrared spectra : absorption peaks help identify types of bonds and functional groups 4) Structure identification