Free Chemistry of Hydrocarbons MCQs with Answers

11. Terminal alkynes such as ethyne react with ammoniacal silver nitrate to give a precipitate because

• A. the triple bond adds silver
• B. the hydrogen attached to the triply bonded carbon is weakly acidic and is replaced by silver
• C. silver catalyses polymerisation
• D. alkynes are strong bases

Explanation: The sp hybridised carbon holds its bonding electrons closer, so the attached hydrogen is slightly acidic and can be replaced by a metal to form an acetylide precipitate, which is the standard test distinguishing a terminal alkyne from an internal one. Alkenes and alkanes give no such reaction. The dry precipitates are explosive and must be destroyed with acid.

12. The Kekule structure of benzene failed to explain

• A. the molecular formula C6H6
• B. the ring shape of the molecule
• C. why benzene resists addition reactions and has all its carbon to carbon bonds of equal length
• D. why benzene contains only carbon and hydrogen

Explanation: Alternating single and double bonds would predict two different bond lengths and the ready addition reactions of an alkene, but experiment shows a single intermediate bond length of 0.139 nm and a strong preference for substitution. The modern picture replaces the alternating bonds with a delocalised ring of six pi electrons above and below the plane. That delocalisation is what makes benzene unusually stable.

13. Benzene undergoes electrophilic substitution rather than addition because substitution

• A. preserves the stable delocalised pi system
• B. produces a smaller molecule
• C. requires no catalyst
• D. is always exothermic

Explanation: Addition would destroy the aromatic sextet and lose the large resonance stabilisation, whereas replacing a hydrogen leaves the delocalised ring intact. This is why benzene reacts with bromine only in the presence of a halogen carrier and gives bromobenzene rather than a dibromo addition product. Nitration, sulphonation and Friedel Crafts reactions follow the same pattern.

14. The nitration of benzene is carried out using

• A. dilute nitric acid alone at room temperature
• B. a mixture of concentrated nitric and concentrated sulphuric acids at about 50 to 60 degrees Celsius
• C. sodium nitrate solution
• D. nitrogen dioxide gas in sunlight

Explanation: Sulphuric acid protonates nitric acid so that it loses water and generates the nitronium ion, which is the actual electrophile attacking the ring. The temperature is kept near 55 degrees because higher temperatures give unwanted dinitrobenzene. Nitrobenzene is important industrially as the starting point for aniline and hence for dyes.

15. The general formula of the alkane homologous series is

• A. CnH2n
• B. CnH2n-2
• C. CnH2n+2
• D. CnH2n+1

Explanation: Every carbon in a saturated open chain carries the maximum hydrogen, giving two per carbon plus two extra for the ends of the chain, so hexane is C6H14. CnH2n belongs to alkenes and cycloalkanes and CnH2n-2 to alkynes. CnH2n+1 is not a molecule at all but an alkyl group, such as methyl or ethyl.

16. The boiling points of the alkanes increase with increasing chain length because

• A. the covalent bonds within the molecules become stronger
• B. the molecules become more polar
• C. hydrogen bonding develops between longer chains
• D. the larger surface area increases the dispersion forces between molecules

Explanation: Longer chains touch each other over a greater area, so the temporary induced dipole attractions between molecules add up to more, and more energy is needed to separate them. This is why methane is a gas, hexane a liquid and long chain alkanes waxy solids. Branching lowers the boiling point of an isomer, because a compact molecule presents less contact area.

17. Cracking of long chain alkanes in the petroleum industry is carried out mainly to

• A. produce shorter, more useful alkanes and alkenes for fuel and plastics
• B. remove sulphur from crude oil
• C. convert alkenes into alkanes
• D. increase the molar mass of the products

Explanation: Fractional distillation yields more of the heavy fractions than the market wants and too little petrol, so cracking breaks the long chains into shorter alkanes plus alkenes such as ethene, which are the feedstock for polymers. Catalytic cracking uses zeolites at about 500 degrees Celsius. Removing sulphur is a separate process called desulphurisation.

18. When ethene is passed into cold concentrated sulphuric acid and the product is then diluted with water and warmed, the compound obtained is

• A. ethane
• B. ethanoic acid
• C. ethanol
• D. ethanal

Explanation: Ethene adds sulphuric acid to give ethyl hydrogen sulphate, and hydrolysing that with water releases ethanol and regenerates the acid, which is the industrial route to ethanol from petroleum. The modern direct route passes ethene and steam over a phosphoric acid catalyst. Fermentation of sugar is the alternative, renewable route to the same product.

19. The addition of hydrogen to an alkene in the presence of a nickel catalyst is called

• A. dehydration
• B. hydrogenation
• C. hydrolysis
• D. halogenation

Explanation: Hydrogen adds across the double bond to give the corresponding alkane, and this is the process used to convert liquid vegetable oils into solid fats such as margarine. Nickel at about 150 degrees Celsius is the usual catalyst, though platinum works at lower temperatures. Excessive hydrogenation in food manufacture produces trans fats, which is why the extent of the reaction is controlled.

20. Benzene is described as aromatic because it

• A. has a pleasant smell
• B. contains only carbon and hydrogen
• C. reacts readily by addition
• D. is planar, cyclic and has a delocalised system of six pi electrons obeying Huckel's rule

Explanation: Aromaticity in the modern sense requires a planar cyclic conjugated system containing 4n plus 2 pi electrons, which for benzene means six, and this confers exceptional stability. The word originally referred to the smell of such compounds, which is why the first option is the historical but chemically wrong answer. Aromatic compounds characteristically undergo substitution rather than addition.