260. Esters are __________ substances.
[RRB Group D — 08 Jan 2026, Shift 2]
(A) Sweet-smelling
(B) Musty-smelling
(C) Sour-smelling
(D) Bitter-smelling
Answer: (A) Sweet-smelling
Explanation: Many small, volatile esters possess pleasant, fruity or sweet odours. They are common ingredients in fragrances and flavour formulations. An ester typically contains the –COO– linkage and can form by reacting a carboxylic acid with an alcohol.
259. What type of gas is released when ethanol reacts with sodium?
[RRB Group D — 08 Jan 2026, Shift 1]
(A) Methane
(B) Carbon dioxide
(C) Oxygen
(D) Hydrogen
Answer: (D) Hydrogen
Explanation: Sodium replaces the hydroxyl hydrogen of ethanol, forming sodium ethoxide and liberating hydrogen gas. The observation reflects the presence of the O–H bond in alcohols. It is not the usual reaction of a simple saturated hydrocarbon.
258. Which structural feature contributes to the high reactivity of unsaturated organic compounds?
[RRB Group D — 29 Dec 2025, Shift 3]
(A) Halogen atoms on the carbon framework
(B) Absence of all hydrogen atoms
(C) Incomplete formation of the carbon backbone
(D) Double or triple bonds between carbon atoms
Answer: (D) Double or triple bonds between carbon atoms
Explanation: Unsaturation means a carbon–carbon double or triple bond. The associated pi bonds are susceptible to addition reactions and make such compounds more reactive toward certain reagents than corresponding alkanes. Saturated hydrocarbons contain only carbon–carbon single bonds.
257. Which of the following hydrocarbons contains a branched chain?
[RRB Group D — 27 Dec 2025, Shift 3]
(A) Butane
(B) Ethene
(C) Isobutane
(D) Methane
Answer: (C) Isobutane
Explanation: Isobutane is the common name of 2-methylpropane, a branched structural isomer of butane. Both have the molecular formula C₄H₁₀ but differ in the arrangement of their carbon atoms. Branching can change physical properties such as boiling point.
256. Which of the following represents a straight-chain hydrocarbon?
[RRB Group D — 23 Dec 2025, Shift 2]
(A) Isobutane
(B) Benzene
(C) Cyclopentane
(D) Butane
Answer: (D) Butane
Explanation: Normal butane consists of four consecutive carbon atoms with no side branch. Isobutane has a branched skeleton, while benzene and cyclopentane are ring compounds. Thus butane is the only straight-chain example among the available choices.
255. Why does carbon form covalent bonds in organic compounds?
[RRB Group D — 22 Dec 2025, Shift 1]
(A) It has a complete outer electron shell
(B) It readily forms positive ions
(C) It has four valence electrons and shares electrons to complete its octet
(D) It easily loses all its valence electrons
Answer: (C) It has four valence electrons and shares electrons to complete its octet
Explanation: Carbon possesses four valence electrons. Gaining or losing four electrons completely is energetically unfavourable, so it normally achieves stable bonding by sharing electron pairs. This explains the prevalence of covalent carbon–carbon and carbon–hydrogen bonds in organic substances.
254. Which of the following is a saturated hydrocarbon?
[RRB Group D — 17 Dec 2025, Shift 1]
(A) Butyne
(B) Ethene
(C) Ethyne
(D) Propane
Answer: (D) Propane
Explanation: Propane belongs to the alkane homologous series and has only single carbon–carbon bonds. Butyne and ethyne contain triple bonds, while ethene contains a double bond. Propane therefore has the maximum hydrogen content compatible with its open-chain carbon skeleton.
253. What is the primary role of a functional group in an organic compound?
[RRB Group D — 16 Dec 2025, Shift 2]
(A) Change the chain from straight to branched
(B) Determine the physical state alone
(C) Provide characteristic chemical properties
(D) Increase molecular mass
Answer: (C) Provide characteristic chemical properties
Explanation: A functional group is the atom or group of atoms chiefly responsible for predictable chemical reactions of an organic molecule. Alcohols, aldehydes, ketones and carboxylic acids have different groups and therefore distinct reactivity, even when their carbon frameworks are similar.
252. Which of the following compounds is a saturated hydrocarbon?
[RRB Group D — 14 Dec 2025, Shift 3]
(A) Benzene
(B) Propene
(C) Propyne
(D) Cyclohexane
Answer: (D) Cyclohexane
Explanation: Cyclohexane is a saturated cyclic hydrocarbon whose ring carbons are connected exclusively by single bonds. Propene and propyne have multiple carbon bonds, while benzene has an aromatic delocalised system. Cyclic saturation does not require the open-chain alkane formula.
251. Which of the following is an organic compound?
[RRB Group D — 11 Dec 2025, Shift 3]
(A) NaCl
(B) C₂H₆
(C) CaCO₃
(D) Na₂CO₃
Answer: (B) C₂H₆
Explanation: Ethane, C₂H₆, is a covalently bonded hydrocarbon and a member of the alkane series. Sodium chloride is inorganic, while calcium and sodium carbonates are conventionally classified as inorganic carbon compounds. Carbon content alone does not establish organic classification.
