AS Chemistry: Introduction to Organic Chemistry — Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Introduction to Organic Chemistry revision notes, which covers curly-arrow rules and mechanism types in more depth.

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Section A

1. Define homologous series, functional group and general formula, using an example of each. [3]

2. Give the empirical, molecular, structural, displayed and skeletal formula meanings, briefly (any 3 of 5). [3]

Section B

3. Name each compound: CH₃CH(CH₃)CH₂CH₃, CH₃CH₂COOH, CH₃CHBrCH₃, CH₃COCH₃. [4]

4. C₄H₁₀ has two structural isomers.

(a) Name and describe both. [2] (b) Explain why the branched isomer has a lower boiling point. [3]

5. Define and give one example of each: structural isomerism, chain isomerism, position isomerism, functional group isomerism. [4]

6. Explain the meaning of the terms electrophile, nucleophile, free radical and heterolytic fission, contrasting the last with homolytic fission. [4]

7. Explain why members of a homologous series show a gradual trend in physical properties but similar chemical properties. [3]

8. State the rules for drawing a curly arrow correctly in a reaction mechanism. [4]

9. Hydrogen cyanide, HCN, adds to propanone, CH₃COCH₃, across the C=O bond. Identify the mechanism type, and explain the role of the C=O bond’s polarity and of the cyanide ion in this reaction. [3]

10. Match each substrate to the mechanism it undergoes: (i) an alkane with a halogen and UV light, (ii) an alkene, (iii) a halogenoalkane, (iv) a carbonyl compound (aldehyde or ketone). [4]

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Answers

1. Homologous series — a family of compounds with the same functional group and general formula, each differing from the next by CH₂, and showing a gradual trend in physical properties [1]. Functional group — the atom or group of atoms responsible for the characteristic reactions of the compound [1]. General formula — an algebraic formula representing every member of the series, e.g. C_nH_(2n+2) [1].

2. Empirical — the simplest whole-number ratio of atoms present [1]. Molecular — the actual number of each atom in a molecule. Structural — shows the arrangement of atoms in the molecule using a condensed written form [1]. Displayed — shows every atom and every bond. Skeletal — shows the carbon skeleton as lines, omitting carbon and hydrogen labels [1].

3. 2-methylbutane [1]; propanoic acid [1]; 2-bromopropane [1]; propanone [1] — one mark for each correct name.

4. (a) Butane — an unbranched, straight chain of four carbons [1]. 2-methylpropane — a three-carbon chain with a methyl branch on the middle carbon [1]. (b) The branched isomer is more spherical, with a smaller surface area of contact between molecules [1], so there are fewer points of contact for London (induced dipole) forces [1]; the intermolecular forces are therefore weaker and less energy is needed to separate the molecules, giving a lower boiling point [1].

5. Structural isomerism — same molecular formula, different arrangement of atoms, e.g. butane and 2-methylpropane [1] (an umbrella term covering the three specific types below). Chain — the carbon skeleton differs, e.g. pentane and 2-methylbutane [1]. Position — the functional group is on a different carbon, e.g. propan-1-ol and propan-2-ol [1]. Functional group — a different functional group entirely, e.g. propan-1-ol and methoxyethane [1].

6. Electrophile — an electron-pair acceptor, attracted to regions of high electron density [1]. Nucleophile — an electron-pair donor, attracted to electron-deficient atoms [1]. Free radical — a species with an unpaired electron, formed by homolytic fission [1]. Heterolytic fission — a bond breaks with both electrons going to one atom, forming a positive and a negative ion; this contrasts with homolytic fission, where a bond breaks with one electron going to each atom, forming two free radicals [1].

7. Each member has the same functional group, and it is the functional group that determines chemical reactivity [1]. Physical properties change gradually because each member has one more CH₂ than the last [1], increasing the molecular size and therefore the strength of the London forces, so boiling point rises steadily [1].

8. A full arrow shows a pair of electrons moving; a half-arrow shows just one [1]. It must start from a bond or a lone pair, never from an atom or a charge [1], and end where the electrons go [1]. Charges must balance on both sides of the equation [1].

9. Oxygen is more electronegative than carbon, so the C=O bond is polar, with the carbonyl carbon δ+ and the oxygen δ− [1]. The cyanide ion, CN⁻, acts as a nucleophile, attacking the electron-deficient carbonyl carbon; the π bond breaks and both electrons move onto the oxygen, forming an alkoxide ion that is then protonated [1]. Because a nucleophile adds across a double bond with nothing leaving, this is nucleophilic addition [1].

10. (i) Free radical substitution [1]. (ii) Electrophilic addition [1]. (iii) Nucleophilic substitution [1]. (iv) Nucleophilic addition [1].

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Where marks are usually lost

• Confusing empirical with molecular formula.
• Numbering the carbon chain from the wrong end when naming — always number to give the lowest locants to substituents or the functional group.
• Saying branched alkanes have weaker covalent bonds.
• Defining a nucleophile as “negatively charged” rather than as an electron-pair donor.
• Drawing a curly arrow starting from an atom or a positive charge instead of a bond or a lone pair.
• Assuming a carbonyl compound reacts like an alkene simply because both contain a carbon-to-something double bond — in a C=O bond the carbon is δ+ and attracts a nucleophile, the reverse of the C=C bond in an alkene, which attracts an electrophile.
• Mixing up which of the four mechanisms applies to which substrate — link the substrate type (alkane, alkene, halogenoalkane, carbonyl compound) to the mechanism name every time, rather than guessing.