75 free MCQs on Hydrocarbons with worked answers and explanations. The simplest organic compounds made only of carbon and hydrogen. Covers alkanes, alkenes, alkynes, and aromatic compounds like benzene. Learn preparation methods, key reactions, and physical properties.
Take the timed Hydrocarbons chapterwise test →Below are 75 practice questions on Hydrocarbons, sorted Easy → Hard. Tap “Show answer & explanation” under any question to check yourself. Want the full theory first? Read the Hydrocarbons notes.
Going from a single to a triple bond between carbons increases the s-character of hybridisation (sp³→sp²→sp), shortening the bond and locking the geometry, while increasing π-bond reactivity (alkenes and alkynes readily undergo addition reactions that alkanes don't).
Q1.
Decarboxylation of sodium acetate with soda lime produces:
Answer: A. Methane
Why: Soda lime removes CO<sub>2</sub> from CH<sub>3</sub>COONa, and the remaining methyl group gains a hydrogen to give methane (one carbon less than the acid).
Q2.
Which is the simplest alkene capable of showing cis-trans (geometrical) isomerism?
Answer: B. But-2-ene
Why: Geometrical isomerism needs two different groups on each doubly bonded carbon; but-2-ene (CH<sub>3</sub>CH=CHCH<sub>3</sub>) is the smallest alkene satisfying this.
Q3.
Kolbe's electrolysis of an aqueous solution of sodium acetate liberates which gas at the anode?
Answer: C. Ethane
Why: Acetate ions are oxidised to methyl radicals at the anode, which couple to form ethane (CH<sub>3</sub>-CH<sub>3</sub>).
Q4.
Among n-pentane, isopentane and neopentane, which has the lowest boiling point?
Answer: D. Neopentane
Why: More branching makes the molecule more spherical with less surface contact, weakening van der Waals forces, so neopentane boils lowest.
Q5.
According to Markovnikov's rule, when HBr adds to propene the hydrogen atom attaches to:
Answer: A. The carbon bearing more hydrogen atoms
Why: H adds to the carbon already having more hydrogens so that the more stable (secondary) carbocation forms, placing Br on the other carbon.
Q6.
Which hydrocarbon contains only single bonds?
Answer: C. Alkane
Why: Alkanes are saturated hydrocarbons containing only single covalent bonds.
Q7.
What is the general formula of alkenes?
Answer: B. C<sub>n</sub>H<sub>2n</sub>
Why: Alkenes are unsaturated hydrocarbons with one double bond and follow the formula C<sub>n</sub>H<sub>2n</sub>.
Q8.
Which hydrocarbon contains a triple bond?
Answer: C. Alkyne
Why: Alkynes contain at least one carbon-carbon triple bond.
Q9.
What is the simplest alkane?
Answer: B. Methane
Why: Methane is the simplest alkane with the formula CH<sub>4</sub>.
Q10.
Which gas is the major component of natural gas?
Answer: C. Methane
Why: Natural gas mainly consists of methane.
Q11.
Petroleum is primarily a mixture of:
Answer: B. Hydrocarbons
Why: Petroleum contains a complex mixture of hydrocarbons.
Q12.
Which hydrocarbon is unsaturated?
Answer: C. Ethene
Why: Ethene contains a double bond and is an unsaturated hydrocarbon.
Q13.
Combustion of hydrocarbons produces:
Answer: A. CO<sub>2</sub> and H<sub>2</sub>O
Why: Complete combustion of hydrocarbons produces carbon dioxide and water.
Q14.
What is the hybridisation of carbon in methane?
Answer: C. sp<sup>3</sup>
Why: Carbon in methane is sp<sup>3</sup> hybridised because it forms four sigma bonds.
Q15.
Which process breaks large hydrocarbons into smaller molecules?
Answer: B. Cracking
Why: Cracking converts heavy hydrocarbons into smaller useful hydrocarbons.
Q16.
Which hydrocarbon has the formula C<sub>2</sub>H<sub>4</sub>?
Answer: B. Ethene
Why: Ethene is an alkene with the molecular formula C<sub>2</sub>H<sub>4</sub>.
Q17.
Which hydrocarbon gives a sooty flame?
Answer: C. Benzene
Why: Benzene has a high carbon content and burns with a sooty flame.
Q18.
What is the IUPAC name of C<sub>3</sub>H<sub>8</sub>?
Answer: C. Propane
Why: C<sub>3</sub>H<sub>8</sub> is an alkane containing three carbon atoms called propane.
Q19.
Which test identifies unsaturation in hydrocarbons?
Answer: C. Bromine water test
Why: Unsaturated hydrocarbons decolourise bromine water due to addition reactions.
Q20.
How many isomers does butane have?
Answer: B. 2
Why: Butane has two structural isomers: n-butane and isobutane.
Q21.
Which hydrocarbon is aromatic?
Answer: C. Benzene
Why: Benzene is an aromatic hydrocarbon with delocalised pi electrons.
Q22.
What is the bond angle in sp<sup>2</sup> hybridised carbon?
Answer: B. 120 degrees
Why: sp<sup>2</sup> hybridisation results in trigonal planar geometry with 120 degree bond angles.
Q23.
Which hydrocarbon contains only carbon and hydrogen?
Answer: B. Hydrocarbon
Why: Hydrocarbons are compounds made only of carbon and hydrogen atoms.
Q24.
What is the general formula of alkynes?
Answer: C. C<sub>n</sub>H<sub>2n</sub>-2
Why: Alkynes contain one triple bond and follow the formula C<sub>n</sub>H<sub>2n</sub>-2.
Q25.
Which hydrocarbon has the highest boiling point among these?
Answer: D. Butane
Why: Boiling point increases with molecular mass in hydrocarbons.
Q26.
Markovnikov addition of HBr to but-1-ene gives which major product?
Answer: B. 2-bromobutane
Why: Protonation gives the more stable secondary carbocation at C-2, so Br adds there to give 2-bromobutane as the major product.
Q27.
Ozonolysis (reductive) of but-2-ene gives:
Answer: C. Two molecules of ethanal (acetaldehyde)
Why: CH<sub>3</sub>CH=CHCH<sub>3</sub> is cleaved at the double bond; each carbon carries one H and one CH<sub>3</sub>, giving two molecules of ethanal.
Q28.
2-Bromobutane on treatment with alcoholic KOH gives which major alkene?
Answer: D. But-2-ene
Why: Dehydrohalogenation follows Saytzeff's rule, giving the more substituted and more stable alkene, but-2-ene, as the major product.
Q29.
How many structurally isomeric monochlorination products can 2-methylbutane form?
Answer: A. Four
Why: 2-Methylbutane has four distinct types of hydrogen (two equivalent methyls, the tertiary C-H, the CH<sub>2</sub>, and the terminal CH<sub>3</sub>), giving four monochloro isomers.
Q30.
Which reagent converts propyne into propanone (acetone)?
Answer: B. Dilute H<sub>2</sub>SO<sub>4</sub> with HgSO<sub>4</sub>
Why: Markovnikov hydration of propyne (dilute H<sub>2</sub>SO<sub>4</sub>/HgSO<sub>4</sub>) gives an enol that tautomerises to propanone.
Q31.
Which rule predicts the major product during addition of HX to an unsymmetrical alkene?
Answer: B. Markovnikov rule
Why: Markovnikov rule states that hydrogen attaches to the carbon already having more hydrogen atoms.
Q32.
Anti-Markovnikov addition occurs in presence of:
Answer: C. Peroxide
Why: Peroxides initiate free radical addition leading to anti-Markovnikov products.
Q33.
Ozonolysis of ethene produces:
Answer: C. Formaldehyde
Why: Ozonolysis cleaves the double bond in ethene to form two formaldehyde molecules.
Q34.
Hydrogenation of alkenes is carried out in presence of:
Answer: A. Ni catalyst
Why: Nickel catalyst helps addition of hydrogen across double bonds.
Q35.
Which type of reaction is mainly shown by alkenes?
Answer: B. Addition
Why: Alkenes mainly undergo addition reactions because of the reactive double bond.
Q36.
What is formed when ethyne reacts with excess hydrogen?
Answer: B. Ethane
Why: Complete hydrogenation of ethyne produces ethane.
Q37.
Which reagent is used for bromination of benzene?
Answer: B. Br<sub>2</sub> with FeBr<sub>3</sub>
Why: FeBr<sub>3</sub> acts as a Lewis acid catalyst in electrophilic substitution of benzene.
Q38.
What is the hybridisation of carbon atoms in benzene?
Answer: B. sp<sup>2</sup>
Why: Each carbon atom in benzene is sp<sup>2</sup> hybridised.
Q39.
Which hydrocarbon undergoes electrophilic aromatic substitution?
Answer: C. Benzene
Why: Benzene undergoes electrophilic substitution to preserve aromaticity.
Q40.
Friedel-Crafts alkylation requires which catalyst?
Answer: B. AlCl<sub>3</sub>
Why: Anhydrous AlCl<sub>3</sub> acts as a Lewis acid catalyst in Friedel-Crafts reactions.
Q41.
Addition of HBr to propene in presence of peroxide gives:
Answer: A. 1-bromopropane
Why: Peroxide effect leads to anti-Markovnikov addition producing 1-bromopropane.
Q42.
Which intermediate is formed during electrophilic addition to alkenes?
Answer: B. Carbocation
Why: Electrophilic addition generally proceeds through carbocation formation.
Q43.
Which compound gives acetaldehyde on ozonolysis?
Answer: C. But-2-ene
Why: But-2-ene breaks symmetrically to form two molecules of acetaldehyde.
Q44.
What is the major product of nitration of benzene?
Answer: B. Nitrobenzene
Why: Nitration of benzene with concentrated acids produces nitrobenzene.
Q45.
Which reagent converts alkene into alkane?
Answer: B. H<sub>2</sub> with Ni
Why: Hydrogenation with hydrogen gas and Ni catalyst converts alkenes into alkanes.
Q46.
Which hydrocarbon undergoes polymerisation easily?
Answer: B. Ethene
Why: Ethene polymerises due to the reactive double bond.
Q47.
Benzene contains how many pi electrons?
Answer: C. 6
Why: Benzene contains six delocalised pi electrons.
Q48.
Which reaction converts benzene into ethylbenzene?
Answer: B. Friedel-Crafts alkylation
Why: Ethylbenzene is formed by Friedel-Crafts alkylation using ethyl chloride and AlCl<sub>3</sub>.
Q49.
What is the product formed when ethene reacts with bromine water?
Answer: B. 1,2-dibromoethane
Why: Bromine adds across the double bond of ethene forming 1,2-dibromoethane.
Q50.
Which reaction confirms unsaturation in hydrocarbons?
Answer: C. Baeyers test
Why: Baeyers reagent (alkaline KMnO<sub>4</sub>) is decolourised by unsaturated hydrocarbons.
Q51.
An alkene X on reductive ozonolysis gives a mixture of propanone (acetone) and ethanal (acetaldehyde). X is:
Answer: C. 2-Methylbut-2-ene
Why: Cleavage of (CH<sub>3</sub>)<sub>2</sub>C=CHCH<sub>3</sub> gives (CH<sub>3</sub>)<sub>2</sub>C=O (acetone) from one fragment and CH<sub>3</sub>CHO (acetaldehyde) from the other.
Q52.
The peroxide (Kharasch) effect giving anti-Markovnikov addition is observed only with which hydrogen halide?
Answer: D. HBr
Why: Only HBr has the right bond energies for the free-radical chain; HCl and HF bonds are too strong and HI too weak for the propagation steps to succeed.
Q53.
Addition of HCl to 3-methylbut-1-ene gives mainly 2-chloro-2-methylbutane because of:
Answer: A. A 1,2-hydride shift to a tertiary cation
Why: The initial secondary carbocation rearranges by a 1,2-hydride shift to the more stable tertiary carbocation, so Cl adds there giving 2-chloro-2-methylbutane.
Q54.
Which of the following is the most reactive towards electrophilic aromatic substitution?
Answer: B. Toluene
Why: The methyl group is activating (+I and hyperconjugation), enriching the ring, while -NO<sub>2</sub> and -Cl are deactivating, so toluene reacts fastest.
Q55.
Birch reduction of toluene (Na in liquid NH<sub>3</sub> with alcohol) gives mainly:
Answer: C. 1-Methyl-1,4-cyclohexadiene
Why: With an electron-donating substituent the substituted carbon stays part of a double bond, giving the 1,4-diene, 1-methylcyclohexa-1,4-diene.
Q56.
What is the first intermediate formed during electrophilic addition of HBr to propene?
Answer: C. Carbocation
Why: Electrophilic addition begins with protonation of the double bond forming a carbocation intermediate.
Q57.
Which carbocation is formed during Markovnikov addition of HBr to propene?
Answer: C. Secondary carbocation
Why: The more stable secondary carbocation forms during Markovnikov addition to propene.
Q58.
Birch reduction converts benzene into:
Answer: C. 1,4-cyclohexadiene
Why: Birch reduction partially reduces benzene to 1,4-cyclohexadiene.
Q59.
Which reagent is used in Birch reduction?
Answer: A. Na in liquid NH<sub>3</sub>
Why: Birch reduction uses sodium or lithium dissolved in liquid ammonia.
Q60.
Baeyer strain theory explains instability in cyclic compounds due to:
Answer: B. Ring strain
Why: Baeyer strain theory relates instability to deviation of bond angles from the ideal tetrahedral angle.
Q61.
Why are terminal alkynes acidic?
Answer: B. sp hybridised carbon has high s-character
Why: sp hybridised carbon holds electrons more tightly due to high s-character, making the C-H bond more polar and acidic.
Q62.
Which compound can react with sodium metal to release hydrogen gas?
Answer: C. Ethyne
Why: Terminal alkynes like ethyne are acidic enough to react with sodium metal.
Q63.
Which group directs incoming electrophiles to ortho and para positions?
Answer: C. CH<sub>3</sub>
Why: Methyl group is electron donating and activates ortho and para positions of benzene.
Q64.
Which substituent is meta directing?
Answer: C. NO<sub>2</sub>
Why: Nitro group is strongly electron withdrawing and directs substitution to the meta position.
Q65.
What is the major product when benzene reacts with chlorine in presence of FeCl<sub>3</sub>?
Answer: B. Chlorobenzene
Why: Electrophilic substitution of benzene with chlorine produces chlorobenzene.
Q66.
Which intermediate forms during nitration of benzene?
Answer: A. NO<sub>2</sub>+ ion
Why: Nitronium ion NO<sub>2</sub>+ acts as the electrophile during nitration of benzene.
Q67.
Ozonolysis of 2-methylpropene gives:
Answer: B. Acetone and formaldehyde
Why: Breaking the double bond in 2-methylpropene forms acetone and formaldehyde.
Q68.
Cyclopentadienyl anion is aromatic because it contains:
Answer: C. 6 pi electrons
Why: Cyclopentadienyl anion contains 6 pi electrons and follows Huckel's rule.
Q69.
Which hydrocarbon undergoes free radical substitution easily?
Answer: C. Methane
Why: Alkanes like methane undergo free radical substitution in presence of UV light.
Q70.
What is the major product of Wurtz reaction using two molecules of ethyl bromide?
Answer: B. Butane
Why: Wurtz reaction couples two alkyl halides using sodium to form a higher alkane.
Q71.
Which species acts as the electrophile in halogenation of benzene?
Answer: B. Cl+ ion
Why: Lewis acid catalysts like FeCl<sub>3</sub> generate Cl+ electrophile for aromatic substitution.
Q72.
Which compound is antiaromatic?
Answer: B. Cyclobutadiene
Why: Cyclobutadiene contains 4 pi electrons and is antiaromatic.
Q73.
Which alkene gives acetone on ozonolysis?
Answer: B. 2-methylpropene
Why: 2-methylpropene cleaves to form acetone and formaldehyde on ozonolysis.
Q74.
Which conformation of cyclohexane has the highest strain?
Answer: D. Half chair
Why: Half-chair conformation is least stable due to maximum torsional and angle strain.
Q75.
What is the product formed when ethyne reacts with excess bromine?
Answer: B. Tetrabromoethane
Why: Excess bromine adds completely across the triple bond to form tetrabromoethane.