Chapter 9: Hydrocarbons Short Questions | chemistry Class 11 CBSE

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Chapter 9: Hydrocarbons Short Questions | chemistry Class 11 CBSE | ByteNotes.in

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Question 1

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What is the general formula of alkanes and why are they called paraffins?

The general formula of alkanes is CnH2n+2, representing saturated open-chain hydrocarbons.
Alkanes are called paraffins from the Latin 'parum affinis' meaning 'little affinity', because they are chemically inert under normal conditions.
They do not react with acids, bases, oxidising agents, or reducing agents under ordinary conditions due to the strength of C-C and C-H sigma bonds.

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Question 2

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Explain the term 'torsional strain' as it applies to conformations of ethane.

Torsional strain is the repulsive interaction between electron clouds of adjacent C-H bonds on neighbouring carbon atoms.
It is caused by the tendency of electron clouds to avoid each other due to electrostatic repulsion, and its magnitude depends on the dihedral angle of rotation about the C-C bond.
Torsional strain is maximum in the eclipsed conformation of ethane (where H atoms are as close as possible) and minimum in the staggered conformation, making the latter more stable.

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Question 3

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State Markovnikov's Rule and give one example of its application.

Markovnikov's Rule states that when an unsymmetrical reagent adds to an unsymmetrical alkene, the negative part of the addendum attaches to the carbon atom that has fewer hydrogen atoms.
For example, addition of HBr to propene (CH3-CH=CH2) gives 2-bromopropane (CH3-CHBr-CH3) as the major product, not 1-bromopropane.
This occurs because H+ first adds to the terminal CH2 (with more H atoms) generating the more stable secondary carbocation at C2, which then reacts with Br-.

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Question 4

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What are the conditions required for geometrical isomerism in alkenes?

Geometrical isomerism requires restricted rotation around the C=C double bond, which prevents free interconversion of the two forms.
Each of the two doubly bonded carbon atoms must carry two different atoms or groups; if either carbon carries two identical substituents, geometrical isomers are not possible.
For example, but-2-ene (CH3-CH=CH-CH3) shows cis and trans isomers, while 2-methylpropene does not because one doubly bonded carbon carries two methyl groups.

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Question 5

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What is Wurtz reaction? Give one limitation of this reaction.

Wurtz reaction involves treating alkyl halides with sodium metal in dry ethereal solution to give alkanes with twice the number of carbon atoms as the original halide.
For example, 2 CH3Br + 2Na → CH3-CH3 + 2NaBr; and 2 C2H5Br + 2Na → C2H5-C2H5 + 2NaBr.
The limitation is that when two different alkyl halides are used, a mixture of three different alkanes is obtained, making it unsuitable for the synthesis of alkanes with an odd number of carbon atoms.

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Question 6

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Why are alkynes more acidic than alkenes and alkanes? Explain briefly.

In ethyne, H atoms are bonded to sp hybridised carbon atoms, which have 50% s-character, giving them the highest electronegativity among sp, sp2, and sp3 hybridised carbons.
This high electronegativity causes sp carbons to attract the C-H bond electrons more strongly, making it easier to release H as a proton compared to ethene (sp2) or ethane (sp3).
Consequently, ethyne reacts with sodium metal and sodamide (NaNH2) to liberate H2, a behaviour not shown by alkenes or alkanes, confirming the acidic order: ethyne > ethene > ethane.

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Question 7

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Give two points of difference between staggered and eclipsed conformations of ethane.

In the staggered conformation, H atoms on adjacent carbons are as far apart as possible (dihedral angle = 60°), whereas in the eclipsed conformation, they are as close together as possible (dihedral angle = 0°).
The staggered conformation has minimum electron cloud repulsion, minimum potential energy, and maximum stability; the eclipsed conformation has maximum torsional strain, maximum energy, and minimum stability.
The energy difference between the two extreme conformations is approximately 12.5 kJ mol-1, with the staggered form being more stable and preferred.

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Question 8

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What is decarboxylation? Write the reaction for preparation of methane by this method.

Decarboxylation is the elimination of carbon dioxide from a carboxylic acid or its sodium salt, resulting in an alkane with one fewer carbon atom than the original acid.
Sodium ethanoate (sodium salt of acetic acid) when heated with soda lime (NaOH + CaO) gives methane: CH3COONa + NaOH → CH4 + Na2CO3.
This reaction requires the conditions of heat with calcium oxide acting as a dehydrating agent to drive the reaction forward.

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Question 9

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What is the Kharash effect (peroxide effect)? With which hydrogen halide is it observed?

The Kharash effect (peroxide effect) is the anti-Markovnikov addition of HBr to unsymmetrical alkenes in the presence of peroxides, where the bromine attaches to the carbon with more hydrogen atoms.
It proceeds via a free radical chain mechanism: peroxides generate phenyl radicals which react with HBr to form bromine radicals; these add to the less substituted carbon of the alkene.
This effect is observed only with HBr, not with HCl (H-Cl bond too strong to be cleaved by free radicals) or HI (iodine radicals recombine rather than adding to the double bond).

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Question 10

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Write the three conditions necessary for a compound to be classified as aromatic.

The compound must be planar, allowing all p orbitals to be parallel and capable of lateral overlap for pi electron delocalisation.
There must be complete delocalisation of pi electrons throughout the ring, forming a continuous conjugated cyclic pi system.
The ring must contain (4n + 2) pi electrons where n is a non-negative integer (n = 0, 1, 2, ...). This is known as Hückel's Rule.

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Question 11

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What is pyrolysis of alkanes? How is it used industrially?

Pyrolysis (or cracking) is the thermal decomposition of higher alkanes into smaller alkanes, alkenes, and other products at high temperatures; it is believed to follow a free radical mechanism.
For example, dodecane (a component of kerosene oil) at 973 K with Pt/Pd/Ni catalyst gives heptane and pentene: C12H26 → C7H16 + C5H10 + other products.
Industrially, pyrolysis is used to convert kerosene oil and petrol into oil gas or petrol gas, thereby generating smaller, more useful hydrocarbon fractions from heavier petroleum fractions.

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Question 12

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Explain how the decolourisation of bromine solution can be used to distinguish alkenes from alkanes.

Bromine dissolved in carbon tetrachloride is reddish-orange in colour; when added to an alkene, it immediately decolourises as Br2 adds across the double bond to form a colourless vicinal dihalide.
For example, CH2=CH2 + Br2 → CH2Br-CH2Br (1,2-dibromoethane), which is colourless, causing the bromine solution to lose its colour.
Alkanes do not react with bromine solution under normal conditions (no UV light), so there is no decolourisation, making this a reliable test for the presence of unsaturation.

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Question 13

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What is Baeyer's reagent? How is it used as a test for unsaturation?

Baeyer's reagent is a cold, dilute, aqueous solution of potassium permanganate (KMnO4), which is purple in colour.
When an alkene or alkyne is treated with Baeyer's reagent, the double or triple bond is oxidised: the alkene gives a vicinal glycol (diol), causing decolourisation of the purple KMnO4 solution.
Alkanes do not decolourise Baeyer's reagent under these conditions, so the test distinguishes unsaturated compounds from saturated ones.

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Chapter 9: Hydrocarbons | Short Questions

What is the general formula of alkanes and why are they called paraffins?

Explain the term 'torsional strain' as it applies to conformations of ethane.

State Markovnikov's Rule and give one example of its application.

What are the conditions required for geometrical isomerism in alkenes?

What is Wurtz reaction? Give one limitation of this reaction.

Why are alkynes more acidic than alkenes and alkanes? Explain briefly.

Give two points of difference between staggered and eclipsed conformations of ethane.

What is decarboxylation? Write the reaction for preparation of methane by this method.

What is the Kharash effect (peroxide effect)? With which hydrogen halide is it observed?

Write the three conditions necessary for a compound to be classified as aromatic.

What is pyrolysis of alkanes? How is it used industrially?

Explain how the decolourisation of bromine solution can be used to distinguish alkenes from alkanes.

What is Baeyer's reagent? How is it used as a test for unsaturation?

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