CD cn: Fats & oils; gas-liquid chromatography

Question 1

1. What do fats and oils largely consist of?
2. How are they formed?

Answer 1

1. Triesters (triglycerides) of propane-1,2,3-triol (glycerol) and long-chain carboxylic acids (fatty acids), with varying degrees of unsaturation
2. Condensation / esterification reactions between propane-1,2,3-triol and long-chain carboxylic acids

Question 2

Write an equation, using full structural formulae, to show the formation of a fat or oil.

Answer 2

Question 3

The properties of a given fat or oil depend on its carboxylic acid residues.

Suggest and justify properties of these residues which would give rise to:

1. Hydrophobia (low miscibility with water)
2. Low melting point

Answer 3

Hydrophobia: Long, non-polar hydrocarbon chains → ΔH of breaking water-water H bonds > ΔH of forming water-triester intermolecular bonds

Low melting point: High degree of unsaturation → bent → more space between molecules → weaker intermolecular bonds

Question 4

1. Distinguish between fats and oils.
2. Explain why this difference arises.

Answer 4

1. Fats are solid at room temperature, and oils are liquid
2. Carboxylic acid residues of oils are more unsaturated → bent → more space between molecules → weaker intermolecular bonds

Question 5

Fats and oils are triesters. State the reactants and conditions required for their hydrolysis.

Answer 5

Heat with water (+ acid catalyst) or heat with alkali

Question 6

Write an equation, using full structural formulae, showing the hydrolysis of a generic fat or oil using NaOH.

Answer 6

Question 7

Describe the process of gas-liquid chromatography, including the nature of the mobile and stationary phases.

Answer 7

• Sample injected into inert carrier gas stream (mobile phase)
• Moves through column consisting of high-boiling liquid on porous support (stationary phase)
• Emerging compounds detected + distinguished by retention time

Question 8

Distinguish between the independent and dependent variables in:

• Gas-liquid chromatography
• Thin layer and paper chromatography

Answer 8

Gas-liquid: distance fixed (independent); retention time measured (dependent)

Thin layer / paper: time fixed (independent); distance measured (dependent)

Question 9

Suggest why the liquid which serves as the stationary phase in GLC is high-boiling.

Answer 9

Prevents it from evaporating + contaminating sample.

Question 10

Suggest a carrier gas which could be used in GLC.

Answer 10

Any noble gas, e.g. argon

Question 11

Explain why the column in GLC is kept in a thermostatic oven.

Answer 11

• Temperature must be constant since changes would affect retention times
• Temperature must be known in order to calibrate instrument (with samples of known compounds)

Question 12

• Describe the behaviour of a sample when it is injected into a gas-liquid chromatography column
• Explain how the components in the sample separate

Answer 12

• Gas / mobile phase ⇌ condensed on stationary phase ⇌ dissolved in stationary phase
• Compounds with lower boiling point + lower affinity for stationary phase have shorter retention time, leading to separate peaks

Question 13

How is retention time measured in GLC?

Answer 13

Time of injection to time of maximum peak height.

Question 14

The image shows a gas-liquid chromatogram.

• Which region corresponds to the most volatile compounds?
• What will give a measure of the relative amounts of compounds in the sample?

Answer 14

• Region with lowest retention times
• Areas under peaks (height used as approximation if peaks are sharp)

Question 15

During GLC, the temperature of the column can be increased in a controlled way while the sample passes through.

Suggest the purpose of this.

Answer 15

Allows good resolution without needing to wait ages for all compounds to reach the detector.

• Lower temperature throughout → long wait*
• Higher temperature throughout → low resolution*

Question 16

Answer 16

a) 42% Z; 58% E
b) Same M⁺ peak (but different fragmentation pattern)

Question 17

Shown below are the methyl esters of the E (trans) and Z (cis) isomers of a fatty acid.

• Suggest why the methyl esters are used in the GLC of these fatty acids, rather than the acids themselves.
• Suggest and explain which isomer will have the GLC peak with the longest retention time.

Answer 17

• Methyl esters have methyl group substituted for hydroxyl group → no hydrogen bonding → lower boiling points → allows molecules to vaporise in chromatograph
• E, since it is straighter, so molecules can pack closer together, so stronger intermolecular forces, so has highest boiling point, so will spend more time in stationary phase

Question 18

Fill in the table summarising modern analytical techniques.

Note: unfinished - table is on “Cells”