27 practice questions on alkanes from the Hydrocarbons section of the UPSC Prelims syllabus. 27 come with a written explanation. Try the sample set below - the answer stays hidden until you ask for it.
Propane is subjected to monochlorination. How many distinct monochlorination products can be obtained?
Correct answer: B - 2
Monochlorination of propane can yield two distinct products: 1-chloropropane and 2-chloropropane, depending on whether the chlorine atom replaces a hydrogen atom bonded to the primary or secondary carbon.
Which of the following statements is true regarding the Wurtz reaction for alkane preparation?
Correct answer: A - It involves the coupling of haloalkanes in the presence of sodium.
The Wurtz reaction involves the coupling of haloalkanes in the presence of sodium, typically in dry ether, to produce higher alkanes. It is not used to prepare alkanes from lower alkanes or specifically for branched alkanes.
In the free-radical halogenation of methane, what is the first step in the mechanism?
Correct answer: C - Initiation by homolytic cleavage of chlorine molecules.
The first step in the free-radical halogenation of methane is the initiation step, where chlorine molecules undergo homolytic cleavage to form chlorine radicals. Hydrogen abstraction and chloromethane radical formation occur in later steps.
Which of the following is a product of the complete combustion of an alkane?
Correct answer: C - Water and carbon dioxide
The complete combustion of an alkane produces carbon dioxide and water. Carbon monoxide is a product of incomplete combustion, while methane, hydrogen gas, or methane and water are not products of alkane combustion.
In a Newman projection of the staggered conformation of ethane, what is the dihedral (torsional) angle between a front C-H bond and the nearest back C-H bond?
Correct answer: A - $60^\circ$
In the staggered form each rear C-H bond sits exactly midway between two front C-H bonds, which places the nearest pair $60^\circ$ apart. A dihedral angle of $0^\circ$ describes the eclipsed form, where front and back bonds overlap completely. The $120^\circ$ separation is between two bonds on the same carbon, and $180^\circ$ never occurs between C-H bonds in ethane.
At room temperature and ordinary pressure the first four straight-chain alkanes are gases, while pentane and hexane are liquids. Which statement best explains this change of state down the series?
Correct answer: A - Van der Waals forces strengthen as the chain lengthens, so more energy is needed to pull the molecules apart
Alkanes are non-polar, so the only attraction between molecules is the weak van der Waals force, and that force grows with the surface area of the molecule. A longer chain therefore boils higher, and by pentane the boiling point has risen above room temperature. C-C bonds in an alkane are not polar, hydrogen bonding needs hydrogen bound to N, O or F, and boiling separates molecules without breaking any C-H bond.
Assertion (A): Among isomeric alkanes, the one with the most branching has the highest boiling point. Reason (R): Branching makes a molecule more compact and reduces the surface area over which neighbouring molecules can touch.
Correct answer: D - A is false, but R is true.
A is false. More branching lowers the boiling point: $n$-pentane boils at about $309\ \text{K}$ while neopentane boils at about $283\ \text{K}$. R is true and explains why. A branched molecule approaches a sphere, its molecules make less contact with one another, the van der Waals forces between them weaken and less heat is needed to separate them.
Assertion (A): The chlorination of methane is carried out in sunlight or at high temperature. Reason (R): The reaction begins with homolytic fission of the $\text{Cl-Cl}$ bond, which needs energy supplied as light or heat.
Correct answer: A - Both A and R are true, and R is the correct explanation of A
Both statements are true and the reason explains the assertion. Chlorine radicals must exist before any substitution can start, and that initiation step is the homolysis of $\text{Cl}_2$, which absorbs energy from ultraviolet light or heat. Without that energy input no radicals form, which is why methane and chlorine can be kept mixed in the dark without reacting.
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