Free Alkenes MCQs with Answers

1. Which reaction is characteristic of alkenes?

• A. Free radical substitution
• B. Electrophilic addition
• C. Nucleophilic substitution
• D. Electrophilic substitution

Explanation: The exposed pi electrons of the double bond attract electrophiles, which add across the bond and convert an unsaturated compound into a saturated one, as happens with bromine, hydrogen halides and water. Alkanes undergo free radical substitution and benzene undergoes electrophilic substitution, which preserves its delocalised ring. Addition is favoured because the pi bond is weaker than a sigma bond.

2. The reagent used to distinguish an alkene from an alkane in the laboratory is

• A. sodium metal
• B. silver nitrate solution
• C. bromine water, which is decolourised by the alkene
• D. limewater

Explanation: The orange colour of bromine water disappears as bromine adds across the double bond to form a colourless dibromo compound, and an alkane leaves the colour unchanged in the dark. Acidified potassium permanganate, which loses its purple colour, is the other standard test. Silver nitrate is used to test for halide ions and limewater for carbon dioxide.

3. According to Markovnikov's rule, when hydrogen bromide adds to propene the major product is

• A. 1-bromopropane
• B. 2-bromopropane
• C. 1,2-dibromopropane
• D. propan-2-ol

Explanation: The hydrogen adds to the carbon that already carries more hydrogens, so the bromine ends up on the middle carbon. The underlying reason is that this route passes through the more stable secondary carbocation rather than a primary one, since alkyl groups release electron density and spread the positive charge. In the presence of peroxides the addition becomes a radical process and the anti Markovnikov product dominates instead.

4. The hybridisation of the carbon atoms in ethene and the bond angle about each are

• A. sp3 and 109.5 degrees
• B. sp and 180 degrees
• C. sp2 and 120 degrees
• D. sp2 and 109.5 degrees

Explanation: Each carbon forms three sigma bonds using sp2 hybrids arranged in a plane at 120 degrees, and the remaining unhybridised p orbitals overlap sideways to form the pi bond. That sideways overlap is what prevents rotation and makes geometric isomerism possible. The whole ethene molecule is therefore flat.

5. 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.

6. 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.