Gaokao Exam Chemistry: Organic Chemistry Fundamentals 1 — Questions and Answers
Question 1: Which of the following compounds is classified as an ester?
- CH₃COOH
- CH₃OCH₃
- CH₃COOCH₂CH₃ (Correct answer)
- CH₃CH₂OH
Correct answer: CH₃COOCH₂CH₃
CH₃COOCH₂CH₃ (ethyl ethanoate) contains the ester functional group –COO–, formed by the condensation of a carboxylic acid and an alcohol. CH₃COOH is a carboxylic acid, CH₃OCH₃ is an ether, and CH₃CH₂OH is an alcohol.
Question 2: Propene (CH₃CH=CH₂) reacts with HBr. According to Markovnikov's rule, what is the major product?
- CH₃CH₂CH₂Br
- CH₃CHBrCH₃ (Correct answer)
- CH₂BrCH₂CH₃
- CH₃CH₂CHBr₂
Correct answer: CH₃CHBrCH₃
Markovnikov's rule states that in electrophilic addition to an unsymmetrical alkene, H⁺ adds to the carbon bearing the greater number of hydrogen atoms, so Br⁻ adds to the more substituted carbon (C-2), giving 2-bromopropane as the major product.
Question 3: How many structural isomers does the molecular formula C₄H₁₀ represent?
- 1
- 2 (Correct answer)
- 3
- 4
Correct answer: 2
C₄H₁₀ has exactly two structural isomers: n-butane (CH₃CH₂CH₂CH₃), a straight chain, and 2-methylpropane (isobutane, (CH₃)₃CH), a branched chain. No other connectivity is possible for four carbons with the saturated formula.
Question 4: Which reagent can be used in the laboratory to distinguish an alkene from an alkane?
- Dilute hydrochloric acid (HCl)
- Aqueous sodium hydroxide (NaOH)
- Bromine water (Br₂/H₂O) (Correct answer)
- Anhydrous sodium sulfate (Na₂SO₄)
Correct answer: Bromine water (Br₂/H₂O)
Alkenes rapidly decolorise bromine water through electrophilic addition across the C=C double bond, turning the orange-brown solution colourless. Alkanes do not react with bromine water under normal conditions and the colour is retained.
Question 5: Which monomer undergoes addition polymerisation to produce the polymer polyethylene (polythene)?
- Ethanol (CH₃CH₂OH)
- Ethanoic acid (CH₃COOH)
- Ethane (CH₃CH₃)
- Ethylene (CH₂=CH₂) (Correct answer)
Correct answer: Ethylene (CH₂=CH₂)
Addition polymerisation requires a monomer with a C=C double bond. Ethylene (CH₂=CH₂) contains a double bond that opens repeatedly to link thousands of monomer units into the –(CH₂CH₂)ₙ– chain of polyethylene. The other options lack a polymerisable double bond.
Question 6: Ethanol is heated with excess concentrated H₂SO₄ at 170 °C. What is the primary organic product?
- Diethyl ether (CH₃CH₂OCH₂CH₃)
- Ethyl hydrogen sulfate (CH₃CH₂OSO₃H)
- Ethylene (CH₂=CH₂) (Correct answer)
- Acetic acid (CH₃COOH)
Correct answer: Ethylene (CH₂=CH₂)
At 170 °C, concentrated H₂SO₄ promotes intramolecular dehydration (elimination) of ethanol, removing H₂O across the C–C bond to yield ethylene. At the lower temperature of 140 °C, intermolecular dehydration instead produces diethyl ether; temperature is the key distinguishing condition.
Gaokao Exam Chemistry: Organic Chemistry Fundamentals 1
Which of the following compounds is classified as an ester?