Analysis

Question 1

What are the five stages of mass spectrometry?

Answer 1

Vapourization

Ionisation

Acceleration

Deflection

Detection

Question 2

What does a mass spectrometer measure?

Answer 2

Ion mass/charge

Question 3

Where is the molecular ion peak on a substance’s mass spectrum?

Answer 3

The molecular ion peak is the furthest sizeable peak to the right of a mass spectrum.

Question 4

Why are fragmentation patterns helpful?

Answer 4

They help determine formula and structure.

Question 5

What ways is it possible for the molecular ion CH₃CH₂CH₃^+∙to fragment?

Answer 5

CH₃CH₂∙ CH₃⁺

CH₃CH₂⁺ CH₃∙

Question 6

When a molecule fragments, what determines the likelihood of a particular ion being formed?

Answer 6

That ion’s stability - inductive effects lend stability to ions, so carbons with more groups attached are more likely to form.

Question 7

What ion is usually formed when a carbonyl compound fragments in mass spectrometry? Why is this ion usually formed?

Answer 7

An acylium ion

RCOR^+∙ —-> RCO⁺ + R^∙

Acylium ions exist in a resonant structure in which stabilizes it.

Question 8

When do M+2 peaks occur in a mass spectrum? What is an M+2 peak?

Answer 8

When the compound contains chlorine or bromine (Cl = 35/37, Br = 79/81). An M+2 peak occurs at the m/z value for the molecular peak, +2.

Question 9

In what ratio do chlorine isotopes exist?

Answer 9

3:1 for A_r 35 to 37

Question 10

In what ratio do bromine isotopes exist?

Answer 10

1:1 for A_r 79 to 81.

Question 11

What would an M+4 peak in a mass spectrum suggest about a compound?

Answer 11

That the compound contains two halogen atoms.

Question 12

What are the two types of nuclear magnetic resonance (NMR) spectroscopy?

Answer 12

¹³C NMR and ¹H NMR.

Question 13

How does NMR spectroscopy work?

Answer 13

• An atomic nucleus with an odd number of nucleons has a spin, which creates a magnetic field.
• When an external field is applied across the nuclei, the nuclei’s field either aligns parallel to (low energy) or against (high energy) this field.
• The nuclei in the high-energy state are able to absorb EM radiation and flip to the higher state.
• More nuclei occupy the lower state therefore there is an overall absorption of radiation, producing an NMR absorption spectrum.

Question 14

What factors affect the frequency of radiation that a nucleus will absorb?

Answer 14

Electron shielding, which is further affected by neighbouring atoms and groups.

Question 15

What is the frequency radiation absorbed (chemical shift) in NMR spectroscopy measured relative to?

Answer 15

Tetramethylsilane - TMS.

Question 16

Why is tetramethylsilane used as a base against which all chemical shifts are measured?

Answer 16

Because it produces a single absorption peak - all of its carbon and hydrogen atoms are in the same environment.

It is also non-toxic, won’t react with other substances and volatile (making it easy to remove).

Question 17

What does each seperate peak in ¹H NMR spectroscopy represent? What does the area under each of these peaks represent?

Answer 17

Each peak represents a different hydrogen environment, and the area under each peak represents the relative number of hydrogen atoms in that environment.

Question 18

What does a singlet pattern in ¹H NMR suggest?

Answer 18

That there are no hydrogen atoms on the adjacent carbon.

Question 19

What does a triplet splitting pattern on a ¹H NMR spectrum suggest?

Answer 19

That there are two hydrogen atoms on the adjacent carbom atom.

Question 20

What type of solvent must be used when conducting ¹H NMR spectroscopy? Why is this?

Answer 20

A hydrogen-free solvent, such as one containing deuterium rather than ¹H, or CCl₄, which does not contain any hydrogen. This is because a ¹H-containing solvent would disrupt the spectrum.

Question 21

What is infrared spectroscopy? How does it work?

Answer 21

Inter-atomic bonds absorb different wavelengths of radiation depending on their type and environment. This fact can be used when analysing substances to help establish their constituents.

Question 22

What is chromatography used for?

Answer 22

Seperating the elements of a mixture.

Question 23

Name and explain the two phases of any chromatography process.

Answer 23

The mobile phase: This is either a liquid or gas which is a able to move inside the chromatography apparatus.

The stationary phase: This is either a solid or a liquid held in a solid and it is not able to move inside the chromatography apparatus.

Question 24

Describe how a mixture could be seperated using column chromatography.

Answer 24

The column is packed with an absorbent material such as Al₂O₃, then the mixture is poured into the column along with the mobile phase (solvent), which should be constantly poured through the column. The different elements of the mixture will move with the solvent at different rates according to their solubility and retention and it will be possible to siphon off each element individually at the base of the column.

Question 25

What is retention?

Answer 25

How strongly a substance adsorbs onto the surface of a solid substance.

Question 26

Which elements of a mixture will pass through the column in column chromatography the most quickly?

Answer 26

Those which are most soluble in the selected solvent.

Question 27

/What is column chromatography usually used for?

Answer 27

Purifying organic products and seperating mixtures.

Question 28

What is Gas-Liquid Chromatography used for?

Answer 28

Seperating mixtures of volatile liquids.

Question 29

Describe the apparatus required for Gas-Liquid Chromatography.

Answer 29

A coiled glass tube packed with a viscous liquid (oil) inside an oven, into which there a seperate inlets for the mobile phase (an inert gas) and mixture, as well as an outlet to a detector, which measures the amount of substance received at a given time.

Question 30

How are components differentiated between by Gas-Liquid Chromatography?

Answer 30

By measuring their retention times - the time between the mixture being injected and that particular component arriving at the detector - and comparing them to the retention times of pure substance’s known times.

Question 31

How is the relative amount of substance in a mixture detailed by a GLC chromatogram?

Answer 31

By the area under the peak for that substance.

Question 32

What are the uses of Gas-Liquid Chromatography?