Question 1 of 5: Family Identification of Four Organic Compounds

Question 1: Family Identification of Four Organic Compounds (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Each part is answered by reading the bonding pattern at the defining atom(s) of the structure — the number and type of bonds at any C=C, the heteroatom substituent pattern, or the ring's bonding — against the standard functional-group definitions.

(a) But-2-ene, CH₃–CH=CH–CH₃ (C₄H₈). Family: alkene. The middle two carbons are joined by one carbon–carbon double bond and every other bond in the molecule is a single bond to hydrogen; there are no heteroatoms. This matches the general saturated-alkene formula C_nH_2n (here C₄H₈) and the defining feature of the alkene family, a planar, sp²-hybridised C=C.

(b) Butan-2-ol, CH₃CH₂CH(OH)CH₃ (C₄H₁₀O). Family: alcohol. A hydroxyl (–OH) group is bonded directly to an sp³ carbon of an otherwise saturated four-carbon chain. Because that carbinol carbon (C2) is itself bonded to two other carbons (C1 and C3) plus the OH and one H, this is specifically a secondary alcohol.

(c) Ethylbenzene, C₆H₅–CH₂CH₃ (C₈H₁₀). Family: aromatic hydrocarbon (arene). The six-membered ring carries three alternating double bonds whose six π electrons are fully delocalised around the ring (a Hückel 4n+2 aromatic sextet with n=1), not three isolated alkenes — the diagnostic sign is the ring's planarity and its refusal to undergo the addition chemistry of an ordinary alkene. The ring bears one ethyl substituent.

(d) Pentan-3-one, CH₃CH₂–C(=O)–CH₂CH₃ (C₅H₁₀O). Family: ketone. The carbonyl (C=O) carbon is bonded to two other carbons (C2 and C4) and to no hydrogen or OH of its own — this is what separates a ketone from an aldehyde (carbonyl carbon bonded to at least one H) or a carboxylic acid (carbonyl carbon bonded to OH).