Free Nomenclature, Structure and Reactivity MCQs with Answers

1. The carbon atom bonded to the halogen in an alkyl halide is attacked by nucleophiles because it carries

• A. a partial positive charge, since the halogen is more electronegative
• B. a partial negative charge
• C. a full negative charge
• D. no charge at all

Explanation: The carbon to halogen bond is polar, with electron density drawn towards the halogen, so the carbon is electron deficient and attracts species with a lone pair. This polarity is the reason alkyl halides are far more reactive than alkanes despite looking similar. The halogen leaves as a stable halide ion, which is why it is described as a good leaving group.

2. The compound (CH3)3CBr is classified as

• A. a primary alkyl halide
• B. a secondary alkyl halide
• C. a tertiary alkyl halide
• D. an aryl halide

Explanation: The carbon carrying the bromine is attached to three other carbon atoms, which makes it tertiary, whereas a primary carbon is attached to one and a secondary to two. This classification matters because tertiary halides react overwhelmingly by the SN1 route while primary halides prefer SN2. An aryl halide would have the halogen attached directly to a benzene ring.

3. Alkyl halides have higher boiling points than the corresponding alkanes because

• A. they contain hydrogen bonds
• B. they are ionic
• C. their molecules are lighter
• D. they are polar and have greater molar mass, so intermolecular forces are stronger

Explanation: The polar carbon to halogen bond adds dipole to dipole attraction on top of the dispersion forces, and the heavy halogen atom increases those dispersion forces as well, so more energy is needed to separate the molecules. Boiling point rises from the fluoride to the iodide for the same alkyl group. Hydrogen bonding is not possible because hydrogen is not bonded to fluorine, oxygen or nitrogen here.

4. The IUPAC name of CH3CH2CHClCH3 is

• A. 1-chlorobutane
• B. 2-chlorobutane
• C. 3-chlorobutane
• D. 2-chloropropane

Explanation: The longest chain has four carbons, giving butane, and numbering from the end that puts the chlorine on the lower numbered carbon makes it position 2 rather than position 3. Naming it 3-chlorobutane means numbering from the wrong end, which is the standard error. The rule is always to give substituents the lowest possible locants.

5. Aryl halides such as chlorobenzene are far less reactive towards nucleophilic substitution than alkyl halides because

• A. the benzene ring repels all nucleophiles equally
• B. chlorine is not electronegative in aromatic compounds
• C. a lone pair on the halogen is delocalised into the ring, giving the carbon to halogen bond partial double bond character
• D. aryl halides are ionic

Explanation: Overlap between the halogen lone pair and the ring pi system shortens and strengthens the bond, so the halogen is a much poorer leaving group and substitution requires extreme conditions of temperature and pressure. The carbon involved is also sp2 hybridised and holds its electrons more tightly. This is why chlorobenzene does not give a precipitate with silver nitrate on warming while chloroethane does.

6. Chlorofluorocarbons are damaging to the ozone layer because in the stratosphere they

• A. dissolve ozone directly
• B. release chlorine radicals under ultraviolet light, and each radical destroys many ozone molecules in a chain reaction
• C. react with oxygen to form carbon dioxide
• D. absorb all incoming ultraviolet radiation harmlessly

Explanation: Ultraviolet light breaks the carbon to chlorine bond homolytically, and the chlorine radical produced converts ozone to oxygen and is then regenerated, so a single radical can destroy thousands of ozone molecules before it is removed. Their very stability in the lower atmosphere is what allows them to reach the stratosphere intact. This is why the Montreal Protocol phased them out in favour of hydrofluorocarbons.

7. IUPAC name of (CH3)3CCH2Br is

• A. 1-bromopentane
• B. 1-bromo-2,2,2-trimethylethane
• C. 1-bromo-2,2-dimethylpropane
• D. 2-bromopentane

Explanation: The longest chain containing the bromine has three carbons, giving propane, with the bromine on carbon 1 and two methyl groups on carbon 2. Counting all five carbons as a straight chain gives the pentane answers, which ignores the branching. Numbering must start from the end nearer the substituent, which puts the bromine at position 1.