JEE Chemistry Hydrocarbons MCQs Set 09

Answer: (a) Branched hydrocarbons

Explanation:
1. Branched alkanes burn smoothly and have a high octane number
2. Straight chains cause knocking

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: (d) \( \ce{CH3CH2CH3} \)

Explanation:
1. Chlorination gives \( \ce{CH3Cl} \), \( \ce{CH2Cl2} \), \( \ce{CHCl3} \) and \( \ce{CCl4} \)
2. Two methyl radicals can combine to give a little ethane in the radical chain, but propane cannot form

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: (c) n-Heptane

Explanation:
1. Boiling point rises with molecular mass and with the length of the chain
2. n-Heptane (98 degrees C) is far above the butanes and neopentane

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) Aluminium carbide

Explanation:
1. \( \ce{Al4C3 + 12 H2O -> 4 Al(OH)3 + 3 CH4} \)
2. Calcium carbide gives acetylene instead

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: (d) \( \ce{C2H6} \)

Explanation:
1. \( \ce{PCl5} \) replaces OH of alcohols and acids and the carbonyl oxygen of aldehydes
2. Alkanes have no such reactive group

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: (c) Hexane

Explanation:
1. Alkanes are saturated and non-polar, so they do not dissolve or react
2. Alkenes, benzene (sulphonation) and aniline (salt formation) do

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) Methane

Explanation:
1. The Grignard reagent is a strong base and removes the acidic OH hydrogen
2. \( \ce{CH3MgBr + C2H5OH -> CH4 + C2H5OMgBr} \)

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) \( \ce{CH3I} \)

Explanation:
1. Reduction of \( \ce{CH3I} \) (Zn/HCl) gives methane
2. Wurtz coupling of \( \ce{CH3I} \) with Na gives ethane

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) Saturated hydrocarbon

Explanation:
1. Paraffin wax is a mixture of higher alkanes, \( \ce{C20-C30} \)
2. It is a saturated hydrocarbon

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

Explanation:
1. The four monochloro products include two chiral ones, 1-chloro-2-methylbutane and 2-chloro-3-methylbutane
2. Each exists as one enantiomeric pair, so there are 2 pairs

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) Aliphatic hydrocarbons

Explanation:
1. Petroleum is mostly a mixture of alkanes and cycloalkanes
2. Aromatic compounds are minor constituents

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) A solvent for fats, oils, varnish and rubber

Explanation:
1. Petroleum ether is a low-boiling mixture of pentanes and hexanes
2. It is a non-polar solvent for fats, oils and rubber, but it is far too volatile to be used as a fuel

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) All the three

Explanation:
1. Coal tar is a source of benzene, toluene, phenol and naphthalene
2. These are raw materials for dyes, drugs and perfumes

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) 109.5 degrees

Explanation:
1. The carbon is sp3 hybridised
2. So the bond angle is the tetrahedral angle, 109.5 degrees

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) Electrolysis

Explanation:
1. This is Kolbe's electrolytic method
2. Two carboxylate ions lose \( \ce{CO2} \) at the anode and the alkyl radicals join to give an alkane

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) Free-radical substitution

Explanation:
1. Light splits \( \ce{X2} \) into radicals which abstract H from the alkane
2. A halogen atom replaces hydrogen by a chain reaction

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) Propane

Explanation:
1. HI/red P reduces \( \ce{-COOH} \) to \( \ce{-CH3} \)
2. \( \ce{CH3CH2COOH + 6HI -> CH3CH2CH3 + 2H2O + 3I2} \)

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) Three alkanes

Explanation:
1. Cross coupling gives pentane (ethyl + propyl)
2. Self coupling gives butane and hexane, so three alkanes form

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) Propane

Explanation:
1. Propane has two kinds of hydrogen, on C-1 and on C-2
2. So it gives 1-bromopropane and 2-bromopropane

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) Alkanes

Explanation:
1. Kerosene is a fraction of petroleum with about 10 to 16 carbon atoms
2. It is mainly alkanes

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) Petroleum ether

Explanation:
1. Fractions come off in order of boiling point
2. Petroleum ether (40-70 degrees C) is the most volatile of the four

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) As an antiknock

Explanation:
1. Iso-octane has an octane number of 100
2. Adding it raises the antiknock quality of the fuel

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) A petroleum additive

Explanation:
1. Tetraethyl lead is added to petrol to raise the octane number
2. It is an antiknock additive

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.