Organic Chemistry
8 marks · NormalThe table below shows properties and displayed formulae for four organic compounds (A, B, C, D).
| Compound | Displayed Formula | Boiling Point (°C) |
|---|---|---|
| A | H–C–C–H (each C with 2 H, straight chain) | -161 |
| B | H H | | H–C–C–C–H | | H H (each C with correct H count) | -42 |
| C | H H | | H–C=C–H | | H H | -104 |
| D | H H | | H–C–C–O–H | | H H | 78 |
Use the table to answer the questions below.
State the type of molecule represented by compound A. How many carbon and hydrogen atoms are in one molecule?
Identify the compound that belongs to the alcohol homologous series. Give one feature from the displayed formula that confirms this.
For compound B, describe its molecular structure using the displayed formula. How many single covalent bonds are present?
Crude oil is separated into fractions by fractional distillation. Based on the boiling points in the table, which compound would be collected at the top of the fractionating column and which at the bottom? Explain your reasoning.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(i) Ethane; it contains 2 carbon and 6 hydrogen atoms.
(ii) Compound D is an alcohol. It contains an –OH (hydroxyl) group attached to the carbon chain.
(iii) Compound B (propane) has three carbon atoms in a chain with single bonds; there are 8 single covalent bonds.
(iv) Compound A (with the lowest boiling point, -161°C) would be collected at the top of the fractionating column, and compound D (with the highest boiling point, 78°C) at the bottom. Compounds with lower boiling points rise further before condensing; those with higher boiling points condense lower down.
Common mistakes: Confusing positions in fractionating column; Incorrect bond counting
Show mark scheme and worked solution
Marking points
B1 (a(i)) · 1 mark · Correctly identifies ethane; gives both number of C and H atoms.
B2 (a(ii)) · 1 mark · Identifies compound D as alcohol.
B3 (a(ii)) · 1 mark · Gives a valid displayed formula feature (e.g. shows –OH group attached to carbon).
B4 (a(iii)) · 1 mark · Correctly describes three C atoms, all single bonds in chain.
A1 (a(iii)) · 1 mark · Gives correct total single covalent bonds (8).
B5 (a(iv)) · 1 mark · Identifies compounds collected at top and bottom of column from data.
B6 (a(iv)) · 1 mark · Explains that lower boiling points collect at top, higher at bottom.
B7 (a(iv)) · 1 mark · Links boiling points and fractionating column positions with suitable explanation.
Worked solution
a(i) Compound A is ethane. The displayed formula shows two carbon atoms each with 3 and 2 hydrogens and joined by a single bond. So, 2 carbons and 6 hydrogens.
a(ii) Compound D is an alcohol. In the displayed formula, an –OH group (hydroxyl) is attached to a carbon atom, which is characteristic of alcohols.
a(iii) Compound B is propane: three carbon atoms in a chain (C–C–C), each end C has 3 hydrogens, middle C has 2. All bonds are single covalent. Each C–C and C–H is a single bond; there are 2 C–C + 8 C–H = 10. But only the single bonds are asked for, so 8.
a(iv) Compound A (boiling point -161°C) would be collected at the top since it has the lowest boiling point and rises furthest; compound D (78°C) has the highest boiling point and condenses first, collecting at the bottom. In fractional distillation, lower boiling points collect at higher trays and higher boiling points at lower trays.