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The oxidation of propan-1-ol can form propanal and propanoic acid - AQA - A-Level Chemistry - Question 3 - 2018 - Paper 2
Question 3
The oxidation of propan-1-ol can form propanal and propanoic acid. The boiling points of these compounds are shown in Table 1. Table 1 | Compound | Boiling ... show full transcript
Worked Solution & Example Answer:The oxidation of propan-1-ol can form propanal and propanoic acid - AQA - A-Level Chemistry - Question 3 - 2018 - Paper 2
Step 1
Explain, with reference to intermolecular forces, why distillation allows propanal to be separated from the other organic compounds in this reaction mixture.
Answer
Propanal has a lower boiling point (49 °C) compared to propan-1-ol (97 °C) and propanoic acid (141 °C). The separation is due to the difference in the strength of intermolecular forces: propan-1-ol exhibits hydrogen bonding, which requires more energy to break, while propanal has dipole-induced dipole forces, which are weaker. Thus, when the mixture is heated, propanal vaporizes before the other compounds, allowing it to be collected through distillation.
Step 2
Give two ways of maximising the yield of propanal obtained by distillation of the reaction mixture.
Answer
- Maintain the temperature of the reaction mixture below the boiling point of propanal (below 49 °C) to prevent its loss during distillation.
- Collect the distillate as soon as it begins to vaporize to minimize the time propanal is exposed to higher temperatures.
Step 3
Describe how you would carry out a simple test-tube reaction to confirm that the sample of propanal obtained by distillation does not contain any propanoic acid.
Answer
Add sodium carbonate or a small amount of magnesium to a sample of the distilled propanal. The absence of effervescence would confirm that there is no propanoic acid present in the sample since propanoic acid would react with the carbonate to release carbon dioxide.
Step 4
Calculate a value, in kJ mol⁻¹, for the enthalpy of combustion of ethanol in this experiment.
Answer
First, calculate the heat absorbed by the water using the formula:
Where:
- (mass of water)
Thus,
Convert this to kJ:
Now calculate the number of moles of ethanol combusted:
Moles = rac{457}{46} = 9.93 ext{ mmol} = 0.00993 ext{ mol}
The enthalpy change per mole is then:
ext{Enthalpy of combustion} = rac{-0.9467 ext{ kJ}}{0.00993} = -95.4 ext{ kJ mol}^{-1}
This value should be reported to three significant figures: -95.4 kJ mol⁻¹.
Step 5
Name and outline a mechanism for the reaction producing pent-1-ene.
Answer
Name of mechanism: Elimination (E1 or E2).
The mechanism involves the protonation of the hydroxyl group leading to the formation of water and a carbocation intermediate, followed by the elimination of a proton from the adjacent carbon to form the double bond, resulting in the production of pent-1-ene.
Step 6
Name the less polar stereoisomer formed.
Answer
Name: The less polar stereoisomer is typically the trans isomer.
Explanation: This type of stereoisomerism arises due to the spatial arrangement of groups around the double bond. In trans isomers, the substituent groups are on opposite sides of the double bond, reducing steric hindrance and therefore polarity compared to cis isomers.