NDA (II) 2022 General Ability Test · Question 110 of 150

Correct — B, alkenes. A homologous series is identified by its general formula, and C₄H₈ fits CₙH₂ₙ with n = 4, which is the general formula of the alkenes. The four-carbon member of that series is butene, carrying one carbon-carbon double bond. The alkanes would need CₙH₂ₙ₊₂, which for four carbons is C₄H₁₀, and the alkynes would need CₙH₂ₙ₋₂, which is C₄H₆ — so neither of those formulas matches.

• (a)alkanes — Alkanes are saturated hydrocarbons with the general formula CₙH₂ₙ₊₂. The four-carbon alkane is butane, C₄H₁₀, which has two hydrogen atoms more than the formula given in the question.
• (c)alkynes — Alkynes contain a carbon-carbon triple bond and have the general formula CₙH₂ₙ₋₂. The four-carbon alkyne is butyne, C₄H₆, which has two hydrogen atoms fewer than C₄H₈.
• (d)cycloalkanes — This option is not chemically absurd — cycloalkanes do share the general formula CₙH₂ₙ, and cyclobutane really is C₄H₈. But cycloalkanes are ring compounds, and in the school framework this question is set from, the homologous series a hydrocarbon formula is placed in means one of the three open-chain families. The official UPSC key marks alkenes.

A homologous series is a family of organic compounds sharing the same general formula and the same functional group, in which each member differs from the next by a CH₂ unit. The three basic hydrocarbon series are the alkanes, CₙH₂ₙ₊₂, with only single bonds; the alkenes, CₙH₂ₙ, with one carbon-carbon double bond; and the alkynes, CₙH₂ₙ₋₂, with one triple bond. Placing a compound in its series means testing its molecular formula against these three.

Work backwards from the hydrogen count. For four carbon atoms the three series give C₄H₁₀, C₄H₈ and C₄H₆ in turn, so C₄H₈ lands squarely on the alkenes. There is a real ambiguity worth naming rather than hiding, because option (d) is chemically defensible — cycloalkanes also obey CₙH₂ₙ, so cyclobutane is C₄H₈ as well, and a molecular formula alone cannot tell an open-chain alkene from a ring of the same size. The official key marks alkenes, which reflects the standard convention in which the phrase 'the homologous series' of a hydrocarbon formula refers to the alkane-alkene-alkyne set. When an exam wants the ring compound it says so, by naming it or drawing the structure rather than giving only the formula.

• Alkanes have the general formula CₙH₂ₙ₊₂, alkenes CₙH₂ₙ and alkynes CₙH₂ₙ₋₂.
• C₄H₈ is butene, the four-carbon alkene, and it contains one carbon-carbon double bond.
• The corresponding alkane is butane, C₄H₁₀, and the corresponding alkyne is butyne, C₄H₆.
• Successive members of a homologous series differ by a CH₂ unit, that is by 14 units of molecular mass, and show a gradual gradation in physical properties.
• Cycloalkanes share the alkene general formula CₙH₂ₙ but are ring compounds and are fully saturated.

• Mixing up the three general formulas, especially CₙH₂ₙ and CₙH₂ₙ₊₂.
• Forgetting that cycloalkanes share the alkene formula, so a molecular formula alone does not always fix the family.
• Assuming that more bonds means more hydrogen — unsaturation lowers the hydrogen count.

By giving a molecular formula and asking for the series, or in reverse, naming the series and asking for the formula of a stated member.

• practice — not a real PYQ

  The general formula of the alkyne series is

  • (a)CₙH₂ₙ₊₂
  • (b)CₙH₂ₙ
  • (c)CₙH₂ₙ₋₂
  • (d)CₙH₂ₙ₋₄

  Answer(c) CₙH₂ₙ₋₂ — the triple bond costs four hydrogens relative to the alkane of the same carbon number.

• practice — not a real PYQ

  Two successive members of a homologous series differ by

  • (a)CH₂
  • (b)CH₄
  • (c)C₂H₄
  • (d)CH₃

  Answer(a) CH₂ — a difference of one CH₂ unit, that is 14 units of molecular mass.