JEE Chemistry Hydrocarbons MCQs Set 08

Answer: (d) \( \ce{C7H16} \)

Explanation:
1. An alkane follows \( \ce{CnH2n+2} \)
2. For n = 7 this gives \( \ce{C7H16} \)

Teacher's Note:
1. Recall the key fact or reaction first, then eliminate options that contradict it.
2. Link each answer to the underlying bonding or mechanism rather than memorising it.

Answer: (a) n-Pentane > iso-pentane > neo-pentane

Explanation:
1. Branching reduces surface area and so van der Waals forces
2. The straight chain n-pentane boils highest and the compact neopentane lowest

Teacher's Note:
1. Check the type of reaction involved and what it does to the carbon skeleton.
2. Compare the options one by one against that.

Answer: (a) n-Propyl bromide

Explanation:
1. By the Wurtz reaction, \( \ce{2 CH3CH2CH2Br + 2 Na -> CH3(CH2)4CH3 + 2 NaBr} \)
2. Using one alkyl halide avoids a mixture of alkanes

Teacher's Note:
1. Trends such as boiling point and stability depend on molecular size, branching and substituent effects.
2. Apply the trend directly to rank the options.

Answer: (a) Mg

Explanation:
1. Alkyl halides react with magnesium in dry ether to give \( \ce{RMgX} \)
2. Zn gives Frankland reagents and Li gives organolithiums

Teacher's Note:
1. Recall the key fact or reaction first, then eliminate options that contradict it.
2. Link each answer to the underlying bonding or mechanism rather than memorising it.

Answer: (b) Electrolysing its aqueous solution

Explanation:
1. Kolbe electrolysis of \( \ce{CH3COONa} \) gives \( \ce{CH3-CH3} \) at the anode with \( \ce{CO2} \)
2. Heating with soda lime gives methane instead

Teacher's Note:
1. Check the type of reaction involved and what it does to the carbon skeleton.
2. Compare the options one by one against that.

Answer: (c) 1,2-Dibromoethane

Explanation:
1. Tetraethyl lead leaves lead and lead oxides in the engine
2. 1,2-Dibromoethane converts them into volatile \( \ce{PbBr2} \), which leaves with the exhaust

Teacher's Note:
1. Trends such as boiling point and stability depend on molecular size, branching and substituent effects.
2. Apply the trend directly to rank the options.

Answer: (b) \( \ce{C10-C12} \)

Explanation:
1. Kerosene boils at about 175-275 degrees C
2. It contains hydrocarbons of roughly \( \ce{C10-C12} \)

Teacher's Note:
1. Recall the key fact or reaction first, then eliminate options that contradict it.
2. Link each answer to the underlying bonding or mechanism rather than memorising it.

Answer: (a) Wurtz reaction

Explanation:
1. Two alkyl halides couple on treatment with sodium in dry ether to form a higher alkane, ethane here
2. This is the Wurtz reaction

Teacher's Note:
1. Check the type of reaction involved and what it does to the carbon skeleton.
2. Compare the options one by one against that.

Answer: (d) Butane

Explanation:
1. \( \ce{2 C2H5I + 2 Na -> C2H5-C2H5 + 2 NaI} \)
2. The two ethyl groups join to give butane

Teacher's Note:
1. Trends such as boiling point and stability depend on molecular size, branching and substituent effects.
2. Apply the trend directly to rank the options.

Answer: (c) Isobutane

Explanation:
1. \( \ce{KMnO4} \) oxidises a tertiary C-H bond of an alkane to give the tertiary alcohol, \( \ce{(CH3)3C-OH} \)
2. Isobutane has a tertiary hydrogen, while the others do not react easily

Teacher's Note:
1. Recall the key fact or reaction first, then eliminate options that contradict it.
2. Link each answer to the underlying bonding or mechanism rather than memorising it.

Answer: (d) \( \ce{(CH3)3C^+} \)

Explanation:
1. The tertiary carbocation has the most alkyl groups for +I and hyperconjugation
2. Order: tertiary > secondary > primary > methyl

Teacher's Note:
1. Check the type of reaction involved and what it does to the carbon skeleton.
2. Compare the options one by one against that.

Answer: (a) 2,2-Dimethylpropane

Explanation:
1. Volatility rises as intermolecular forces fall
2. Neopentane is the most compact of the pentanes (b.p. 9.5 degrees C), so it is the most volatile of the options

Teacher's Note:
1. Trends such as boiling point and stability depend on molecular size, branching and substituent effects.
2. Apply the trend directly to rank the options.

Answer: (c) Na/dry ether

Explanation:
1. Alkyl halides couple with sodium in dry ether
2. Water or alcohol would react with sodium, so dryness is essential

Teacher's Note:
1. Recall the key fact or reaction first, then eliminate options that contradict it.
2. Link each answer to the underlying bonding or mechanism rather than memorising it.

Answer: (d) 2,2,4-Trimethylpentane

Explanation:
1. Iso-octane (2,2,4-trimethylpentane) is the reference fuel with an octane number of 100
2. Straight-chain alkanes knock much more and have low octane numbers

Teacher's Note:
1. Check the type of reaction involved and what it does to the carbon skeleton.
2. Compare the options one by one against that.

Answer: (b) Refrigerant

Explanation:
1. Freon-12 is \( \ce{CCl2F2} \)
2. It is non-flammable and easily liquefied, so it was used as a refrigerant

Teacher's Note:
1. Trends such as boiling point and stability depend on molecular size, branching and substituent effects.
2. Apply the trend directly to rank the options.

Answer: (a) 20% normal heptane + 80% iso-octane

Explanation:
1. Octane number is the percentage by volume of iso-octane in a mixture with n-heptane
2. An octane number of 80 means 80% iso-octane and 20% n-heptane

Teacher's Note:
1. Recall the key fact or reaction first, then eliminate options that contradict it.
2. Link each answer to the underlying bonding or mechanism rather than memorising it.

Answer: (b) \( \ce{CH3COCl} \) with \( \ce{CH3MgX} \), then \( \ce{H2O} \)

Explanation:
1. (a) The Grignard reagent is hydrolysed to propane, (c) hydroboration then protonolysis gives propane, (d) HI/P reduces the alcohol to propane
2. (b) gives acetone (\( \ce{CH3COCH3} \))

Teacher's Note:
1. Check the type of reaction involved and what it does to the carbon skeleton.
2. Compare the options one by one against that.

Answer: (d) Tetrahedral

Explanation:
1. Carbon in methane is sp3 hybridised
2. The four C-H bonds point to the corners of a tetrahedron

Teacher's Note:
1. Trends such as boiling point and stability depend on molecular size, branching and substituent effects.
2. Apply the trend directly to rank the options.

Answer: (b) 2,2-Dimethylpropane

Explanation:
1. All twelve hydrogens of neopentane are equivalent
2. Only one monobromo derivative is possible

Teacher's Note:
1. Recall the key fact or reaction first, then eliminate options that contradict it.
2. Link each answer to the underlying bonding or mechanism rather than memorising it.

Answer: (b) 2-Methylpropane

Explanation:
1. \( \ce{AlCl3} \) with HCl or HBr isomerises n-alkanes to branched alkanes
2. Butane gives isobutane (2-methylpropane)

Teacher's Note:
1. Check the type of reaction involved and what it does to the carbon skeleton.
2. Compare the options one by one against that.

Answer: (b) It can be catalytically hydrogenated

Explanation:
1. Ethane has no multiple bond, so it cannot add hydrogen
2. The other statements are correct

Teacher's Note:
1. Trends such as boiling point and stability depend on molecular size, branching and substituent effects.
2. Apply the trend directly to rank the options.

Answer: (a) Distillation of petroleum to give different fractions

Explanation:
1. Refining separates crude oil by fractional distillation into gasoline, kerosene, diesel and so on
2. Cracking and reforming are separate processes

Teacher's Note:
1. Recall the key fact or reaction first, then eliminate options that contradict it.
2. Link each answer to the underlying bonding or mechanism rather than memorising it.

Answer: (b) Ethylene dibromide

Explanation:
1. Ethylene dibromide converts lead into volatile \( \ce{PbBr2} \)
2. The vapour leaves with the exhaust gases

Teacher's Note:
1. Check the type of reaction involved and what it does to the carbon skeleton.
2. Compare the options one by one against that.