Free Chemistry of Hydrocarbons MCQs with Answers

1. Alkanes are relatively unreactive towards most reagents because

• A. their carbon to hydrogen bonds are strong and almost non polar, offering nothing for a nucleophile or electrophile to attack
• B. they contain a double bond
• C. they are ionic compounds
• D. they have very high molar masses

Explanation: Carbon and hydrogen have similar electronegativities, so the bonds carry almost no dipole and there is no region of high or low electron density to attract a reagent. This is why alkanes are called paraffins, meaning little affinity, and why their main reactions are combustion and free radical substitution, both of which need harsh conditions. Alkenes react far more readily because the pi electrons are exposed.

2. The hybridisation of each carbon atom in an alkane such as ethane is

• A. sp
• B. sp2
• C. sp3
• D. unhybridised p

Explanation: Four sigma bonds require four equivalent orbitals, so one s and three p orbitals mix to give four sp3 hybrids pointing to the corners of a tetrahedron with bond angles of 109.5 degrees. Alkenes use sp2 with 120 degree angles and alkynes sp with a linear 180 degree arrangement. Counting the sigma bonds and lone pairs on an atom is the quickest way to assign hybridisation.

3. The chlorination of methane in the presence of ultraviolet light proceeds by

• A. nucleophilic substitution
• B. electrophilic addition
• C. free radical substitution
• D. elimination

Explanation: Ultraviolet light splits the chlorine molecule homolytically into two radicals, which then abstract hydrogen from methane and set up a chain reaction of propagation steps until two radicals combine and terminate it. Because the chain continues, the reaction does not stop at chloromethane but produces a mixture of all four chlorinated products. Substitution is characteristic of saturated compounds, while addition belongs to unsaturated ones.

4. In the free radical substitution of methane, the initiation step is

• A. the reaction of a chlorine radical with methane
• B. the combination of two methyl radicals
• C. the reaction of a methyl radical with chlorine
• D. the homolytic splitting of a chlorine molecule into two chlorine radicals by ultraviolet light

Explanation: Initiation is the step that creates radicals where there were none, and only the chlorine to chlorine bond is weak enough to be broken by ultraviolet light. Propagation steps then consume one radical and produce another, so the chain continues, while termination removes radicals by combining them. Identifying which step is which by counting radicals on each side is the reliable method.

5. Which reaction is characteristic of alkenes?

• A. Free radical substitution
• B. Electrophilic addition
• C. Nucleophilic substitution
• D. Electrophilic substitution

Explanation: The exposed pi electrons of the double bond attract electrophiles, which add across the bond and convert an unsaturated compound into a saturated one, as happens with bromine, hydrogen halides and water. Alkanes undergo free radical substitution and benzene undergoes electrophilic substitution, which preserves its delocalised ring. Addition is favoured because the pi bond is weaker than a sigma bond.

6. The reagent used to distinguish an alkene from an alkane in the laboratory is

• A. sodium metal
• B. silver nitrate solution
• C. bromine water, which is decolourised by the alkene
• D. limewater

Explanation: The orange colour of bromine water disappears as bromine adds across the double bond to form a colourless dibromo compound, and an alkane leaves the colour unchanged in the dark. Acidified potassium permanganate, which loses its purple colour, is the other standard test. Silver nitrate is used to test for halide ions and limewater for carbon dioxide.

7. According to Markovnikov's rule, when hydrogen bromide adds to propene the major product is

• A. 1-bromopropane
• B. 2-bromopropane
• C. 1,2-dibromopropane
• D. propan-2-ol

Explanation: The hydrogen adds to the carbon that already carries more hydrogens, so the bromine ends up on the middle carbon. The underlying reason is that this route passes through the more stable secondary carbocation rather than a primary one, since alkyl groups release electron density and spread the positive charge. In the presence of peroxides the addition becomes a radical process and the anti Markovnikov product dominates instead.

8. The hybridisation of the carbon atoms in ethene and the bond angle about each are

• A. sp3 and 109.5 degrees
• B. sp and 180 degrees
• C. sp2 and 120 degrees
• D. sp2 and 109.5 degrees

Explanation: Each carbon forms three sigma bonds using sp2 hybrids arranged in a plane at 120 degrees, and the remaining unhybridised p orbitals overlap sideways to form the pi bond. That sideways overlap is what prevents rotation and makes geometric isomerism possible. The whole ethene molecule is therefore flat.

9. Ethyne differs from ethene in that ethyne contains

• A. one sigma and two pi bonds between the carbon atoms, with sp hybridisation
• B. two sigma bonds between the carbon atoms
• C. only sigma bonds
• D. three pi bonds between the carbon atoms

Explanation: A triple bond is always one sigma bond plus two pi bonds, formed from two unhybridised p orbitals on each sp hybridised carbon, and the molecule is linear as a result. Ethyne also shows weak acidity, unlike ethene, because the higher s character of the sp orbital holds the bonding electrons closer to carbon. Any multiple bond, however high the order, contains exactly one sigma bond.

10. Ethyne can be prepared in the laboratory by the action of water on

• A. calcium carbonate
• B. calcium oxide
• C. calcium hydroxide
• D. calcium carbide

Explanation: Calcium carbide reacts vigorously with water to give ethyne and calcium hydroxide, which was once used in carbide lamps and is still the classic laboratory preparation. Calcium carbonate with acid would give carbon dioxide instead, which is the tempting confusion between the carbide and the carbonate. The ethyne produced this way smells unpleasant because of phosphine impurities.