Hydrocarbons - MCQs with Explanations

1. An alkene having molecular formula $C_5H_{10}$, on ozonolysis gives two compounds, acetone and an aldehyde. The alkene is:

• (a) pent-2-ene
• (b) 2-methylbut-2-ene
• (c) 2-methylbut-1-ene
• (d) 3-methylbut-1-ene

Correct Answer: (b)

Explanation: The alkene 2-methylbut-2-ene reacts with $O_3$ followed by $Zn/H_2O$ to produce propanone (acetone) and ethanal.

2. A mixture of three gases, ethane, ethene and ethyne are passed through ammoniacal solution of cuprous chloride. The emergent gas(es) after it was passed through solution contained:

• (a) ethane only
• (b) ethane and ethene
• (c) ethane and ethyne
• (d) ethyne only

Correct Answer: (b)

Explanation: Ethyne has acidic hydrogens and reacts with $Cu_2Cl_2$ to form a precipitate ($Cu-C\equiv C-Cu$). Ethane and ethene remain unreacted and emerge from the solution.

3. An aromatic compound having molecular formula $C_8H_{10}$ (A) on treatment with hot $KMnO_4$ gives (B) which can easily be converted to an anhydride by heating. The compound (A) is:

• (a) toluene
• (b) ethylbenzene
• (c) para-xylene
• (d) ortho-xylene

Correct Answer: (d)

Explanation: Ortho-xylene is oxidized by hot $KMnO_4$ to phthalic acid (B). Phthalic acid easily loses water on heating to form phthalic anhydride.

4. The reaction $CH_2=CH_2 + H_2 \xrightarrow[250-300^\circ C]{Ni} CH_3-CH_3$ is called:

• (a) Wurtz reaction
• (b) Kolbe's reaction
• (c) Sabatier and Senderens reaction
• (d) Carbylamine reaction

Correct Answer: (c)

Explanation: The hydrogenation of alkenes or alkynes in the presence of a Nickel catalyst at elevated temperatures is known as the Sabatier-Senderens reaction.

5. The catalytic hydrogenation is more easier in case of which alkene?

• (a) $R_2C=CH_2$
• (b) $RCH=CH_2$
• (c) $RCH=CHR$
• (d) $R_2C=CR_2$

Correct Answer: (b)

Explanation: Catalytic hydrogenation is a free radical addition process. Alkenes with less steric hindrance and higher heat of hydrogenation are more reactive.

6. In the following structures, which two forms are staggered conformation of ethane?

• (a) 1 and 4
• (b) 2 and 3
• (c) 1 and 2
• (d) 1 and 3

Correct Answer: (c)

Explanation: Structure 1 (Sawhorse projection) and Structure 2 (Newman projection) both represent the staggered conformation of ethane.

7. The number of $\pi$-electrons in cycloheptatrienyl anion is:

• (a) 2
• (b) 3
• (c) 8
• (d) 5

Correct Answer: (c)

Explanation: The cycloheptatrienyl anion has three double bonds (6 $\pi$ electrons) and one lone pair on the anionic carbon (2 $\pi$ electrons), totaling 8.

8. One mole of hydrocarbon (A) reacts with one mole of bromine giving a dibromo compound, $C_5H_{10}Br_2$. Substance (A) on treatment with cold dilute alkaline $KMnO_4$ solution forms a compound $C_5H_{12}O_2$. On ozonolysis (A) gives equimolar quantities of propanone and ethanal. (A) is:

• (a) 1-pentene
• (b) 2-pentene
• (c) 2-methylbut-2-ene
• (d) 3-methylbut-2-ene

Correct Answer: (c)

Explanation: The ozonolysis products (propanone and ethanal) identify the double bond location at the 2-position of 2-methylbut-2-ene.

9. The hydrocarbon which can react with sodium in liquid ammonia is:

• (a) $CH_3CH_2C\equiv CCH_2CH_3$
• (b) $CH_3CH_2CH_2C\equiv CCH_2CH_2CH_3$
• (c) $CH_3CH_2C\equiv CH$
• (d) $CH_3CH=CHCH_3$

Correct Answer: (c)

Explanation: Only terminal alkynes possess an acidic hydrogen that can react with sodium in liquid ammonia.

10. Which of the following alkenes gives these products on ozonolysis: $(CH_3)_2CO$; $CH_3CH_2COOH$; $CH_3-C(CH_3)_2-COOH$?

• (a) (Structure shown in PDF)
• (b) (Structure shown in PDF)
• (c) (Structure shown in PDF)
• (d) (Structure shown in PDF)

Correct Answer: (c)

Explanation: Based on the products of ozonolysis, structure (c) is the only one that cleaves to yield the specific mixture of acetone and carboxylic acids listed.

11. Which of the following exhibits aromaticity by using Huckel rule?

• (a) (4 $\pi$ electrons)
• (b) (6 $\pi$ electrons)
• (c) (12 $\pi$ electrons)
• (d) (4 $\pi$ electrons)

Correct Answer: (b)

Explanation: According to Huckel's rule, a planar cyclic system with $(4n+2)\pi$ electrons is aromatic. For $n=1$, this is 6 electrons, which structure (b) possesses.

12. Which of the following compounds has least boiling point?

• (a) 2-Methylbutane
• (b) n-Hexane
• (c) n-Pentane
• (d) 2,2-Dimethylpropane

Correct Answer: (d)

Explanation: For isomeric alkanes, boiling point decreases with increased branching because the surface area decreases, weakening van der Waals forces.

13. Which of the following is the most stable conformation of 1-phenyl-2-ethylethane?

• (a) (Structure in PDF)
• (b) (Structure in PDF)
• (c) (Structure in PDF)
• (d) (Structure in PDF)

Correct Answer: (d)

Explanation: The anti (staggered) conformation is the most stable as it places the two bulkiest groups as far apart as possible.

14. In the following conversion, identify the reagent 'X': $Vic-dibromide \xrightarrow{X} HC\equiv CH$

• (a) alcoholic KOH
• (b) aqueous KOH
• (c) aqueous KOH followed by $NaNH_2$
• (d) alcoholic KOH followed by $NaNH_2$

Correct Answer: (d)

Explanation: Dehydrohalogenation to an alkyne typically requires a strong base like alcoholic KOH for the first step and the even stronger $NaNH_2$ to remove the second HX molecule.

15. Identify the final product in the sequence: $CH_3C\equiv CH \xrightarrow{(i) NaNH_2, (ii) CH_3CH_2Br} P \xrightarrow{H_2, Lindlar's} Q$

• (a) cis-pent-2-ene
• (b) trans-pent-2-ene
• (c) n-pentane
• (d) 2-methylbut-2-ene

Correct Answer: (a)

Explanation: $P$ is pent-2-yne. Lindlar's catalyst facilitates the partial hydrogenation of alkynes specifically to yield the cis-alkene.

16. Acetylene on reaction with $AsCl_3$ gives:

• (a) (Structure in PDF - Lewisite)
• (b) (Structure in PDF)
• (c) (Structure in PDF)
• (d) (Structure in PDF)

Correct Answer: (a)

Explanation: Acetylene reacts with arsenic trichloride to produce Lewisite, a highly poisonous gas.

17. Total number of cyclic structural as well as stereoisomers possible for a hydrocarbon with molecular formula $C_5H_{10}$ is:

• (a) 6
• (b) 4
• (c) 7
• (d) 5

Correct Answer: (c)

Explanation: There are 7 such isomers including cyclopentane, methylcyclobutane, and various substituted cyclopropanes.

18. Propyne and propene can be distinguished by:

• (a) concentrated $H_2SO_4$
• (b) $Br_2$ in $CCl_4$
• (c) dilute $KMnO_4$
• (d) $AgNO_3$ in ammonia

Correct Answer: (d)

Explanation: Terminal alkynes like propyne react with Tollens' reagent ($AgNO_3$ in ammonia) to form a silver acetylide precipitate, while propene does not.

19. Which of the following will have least hindered rotation about carbon-carbon bond?

• (a) Ethane
• (b) Ethylene
• (c) Acetylene
• (d) Hexachloroethane

Correct Answer: (a)

Explanation: Ethane has single bonds which allow free rotation. Substitution with large atoms like Chlorine (in d) increases the barrier to rotation.

20. Which of the following compound on ozonolysis gives methyl glyoxal and glyoxal only?

• (a) o-Xylene
• (b) p-Xylene, m-Xylene
• (c) Ethyl benzene
• (d) None of these

Correct Answer: (b)

Explanation: Both m- and p-xylene yield a specific mixture containing 2 moles of methyl glyoxal and 1 mole of glyoxal upon ozonolysis.

21. Which one of the following alkenes will show maximum number of hyperconjugation?

• (a) (Structure in PDF)
• (b) (Structure in PDF)
• (c) $(CH_3)_2C=C(CH_3)_2$
• (d) $(CH_3)_2C=CH-CH_3$

Correct Answer: (c)

Explanation: 2,3-Dimethylbut-2-ene has 12 $\alpha$-hydrogen atoms, providing the highest number of hyperconjugative structures.

22. $CH_3-CH=CH-CH_3 + HBr \xrightarrow{CH_3OH} \text{Product}$. The main product is:

• (a) $CH_3-CHBr-CH_2-CH_3$
• (b) $CH_3-CH(OCH_3)-CH_2-CH_3$
• (c) $CH_3-CHBr-CH(OCH_3)-CH_3$
• (d) $CH_3-CHBr-CHOH-CH_3$

Correct Answer: (b)

Explanation: In the presence of excess $CH_3OH$, the intermediate carbocation is attacked by the solvent ($CH_3OH$) rather than $Br^-$, forming an ether.

23. Which among the following compounds are aromatic in nature?

• (a) II, IV, VI
• (b) II, III, V
• (c) II, IV, V
• (d) All are aromatic

Correct Answer: (c)

Explanation: Compounds II, IV, and V satisfy Huckel's rule by being planar, cyclic, and having a conjugated system of $4n+2$ $\pi$ electrons.

24. $HC\equiv CH \xrightarrow[O_3/NaOH]{} X \xrightarrow{Zn/CH_3COOH} Y$. Y is:

• (a) $CH_2OH-CH_2OH$
• (b) $CH_3CH_2OH$
• (c) $CH_3COOH$
• (d) $CH_3OH$

Correct Answer: (a)

Explanation: Ozonolysis followed by reduction of acetylene yields ethylene glycol ($CH_2OH-CH_2OH$).

25. The hydrocarbon which decolourises alkaline $KMnO_4$ solution, but does not give any precipitate with ammonical silver nitrate is:

• (a) benzene
• (b) acetylene
• (c) propyne
• (d) 2-butyne

Correct Answer: (d)

Explanation: 2-Butyne is an internal alkyne; it has a triple bond (reacts with $KMnO_4$) but lacks terminal acidic hydrogens needed for a silver precipitate.

26. $(CH_3)_3C-CH=CH_2 \xrightarrow{(i) Hg(CH_3COO)_2, THF, (ii) NaBH_4 + NaOH} ?$

• (a) $(CH_3)_3C-CHOH-CH_3$
• (b) $(CH_3)_3C-CH_2CH_2OH$
• (c) (Structure in PDF)
• (d) (Structure in PDF)

Correct Answer: (a)

Explanation: Oxymercuration-demercuration yields the Markownikoff addition product (alcohol at the more substituted carbon) without carbon skeleton rearrangement.

27. $CH_2=CH_2 \xrightarrow{HBr} X \xrightarrow{Hydrolysis} Y \xrightarrow{Na_2CO_3, I_2 \text{ excess}} Z$. Z is:

• (a) $C_2H_5I$
• (b) $C_2H_5OH$
• (c) $CHI_3$
• (d) $CH_3CHO$

Correct Answer: (c)

Explanation: $X$ is ethyl bromide, $Y$ is ethanol. Ethanol responds to the iodoform test to produce $CHI_3$.

28. Liquid hydrocarbons can be converted to a mixture of gaseous hydrocarbons by:

• (a) oxidation
• (b) cracking
• (c) distillation
• (d) hydrolysis

Correct Answer: (b)

Explanation: Cracking is the thermal or catalytic decomposition of higher (liquid) alkanes into smaller, gaseous alkanes and alkenes.

29. The reaction of toluene with $Cl_2$ in presence of $FeCl_3$ gives X and reaction in presence of light gives Y. X and Y are:

• (a) $X = $ Benzal chloride, $Y = o$-chlorotoluene
• (b) $X = m$-chlorotoluene, $Y = p$-chlorotoluene
• (c) $X = o/p$-chlorotoluene, $Y = $ Trichloromethyl benzene
• (d) $X = $ Benzyl chloride, $Y = m$-chlorotoluene

Correct Answer: (c)

Explanation: Catalytic chlorination (with $FeCl_3$) occurs on the ring ($o/p$ positions). Photochemical chlorination occurs on the side chain (methyl group).

30. In a set of reactions, ethylbenzene yielded a product D. $Ethylbenzene \xrightarrow{KMnO_4, KOH} A \xrightarrow{Br_2, FeCl_3} B \xrightarrow{C_2H_5OH, H^+} D$. D would be:

• (a) (Structure in PDF)
• (b) (Structure in PDF)
• (c) (Structure in PDF)
• (d) (Structure in PDF)

Correct Answer: (d)

Explanation: Ethylbenzene is oxidized to benzoic acid (A). Bromination gives m-bromobenzoic acid (B), which is then esterified to ethyl m-bromobenzoate (D).

31. Which of the following reacts most readily with concentrated $H_2SO_4$?

• (a) $CH_2=CH_2$
• (b) $CH_3CH=CH_2$
• (c) $(CH_3)_2C=CH_2$
• (d) All of these

Correct Answer: (c)

Explanation: Isobutylene (c) is the most reactive because its protonation yields the most stable tertiary carbocation.

32. $(CH_3)_3C-CH=CH_2 \xrightarrow[H_2O_2, OH^-]{BH_3, THF} A$. The product A is:

• (a) (Structure in PDF)
• (b) (Structure in PDF)
• (c) $(CH_3)_3C-CH_2-CH_2OH$
• (d) (Structure in PDF)

Correct Answer: (c)

Explanation: Hydroboration-oxidation results in the anti-Markownikoff hydration of the alkene, placing the -OH group on the terminal carbon.

33. Which of the following aromatic compounds undergo nitration fastest?

• (a) (Structure in PDF)
• (b) Toluene
• (c) (Structure in PDF)
• (d) (Structure in PDF)

Correct Answer: (b)

Explanation: Toluene is activated most significantly by the hyperconjugative effect of the methyl group, leading to faster electrophilic substitution.

34. Identify B: $C_6H_5-CH=CH_2 \xrightarrow{Br_2} A \xrightarrow[(ii) CH_3-I]{(i) 3.0 \text{ eq. } NaNH_2} B$

• (a) (Structure in PDF)
• (b) (Structure in PDF)
• (c) $C_6H_5-C\equiv C-CH_3$
• (d) (Structure in PDF)

Correct Answer: (c)

Explanation: Styrene bromide (A) undergoes double dehydrohalogenation to form the acetylide ion, which is then methylated to yield 1-phenylpropyne (B).

35. Addition of HBr on two specific pentadiene structures separately gives:

• (a) (Structures listed in PDF)
• (b) (Structures listed in PDF)
• (c) (Structures listed in PDF)
• (d) (Structures listed in PDF)

Correct Answer: (a)

Explanation: HBr adds to the double bond in the first case to form the bromide, but in the second conjugated system, it yields a specific alkadiene.

36. Anti-Markovnikov's addition of HBr is not observed in:

• (a) propene
• (b) 1-butene
• (c) but-2-ene
• (d) pent-2-ene

Correct Answer: (c)

Explanation: The peroxide effect (anti-Markovnikov's addition) is only observable with unsymmetrical alkenes. But-2-ene is symmetrical.

37. The correct IUPAC name for the provided alkene with ethyl and propyl groups is:

• (a) 4-ethyl-3-propylhex-1-ene
• (b) (Other options listed)
• (c) (Other options listed)
• (d) (Other options listed)

Correct Answer: (a)

Explanation: The name is derived by identifying the longest chain containing the double bond and numbering to give the alkene the lowest possible locant.

38. $CH_3CH(C_2H_5)CH=CHCH_3 \xrightarrow{H_3O^+}$ major product of this reaction:

• (a) is optical isomer
• (b) gives white turbidity with HBr immediately
• (c) is dehydrated easily
• (d) all correct

Correct Answer: (d)

Explanation: The major product is an alcohol that satisfies all three properties mentioned.

39. Which of the following represents the correct decreasing order relative reactivity towards an electrophile, $E^+$?

• (a) (Order in PDF)
• (b) (Order in PDF)
• (c) (Order in PDF)
• (d) $p$-xylene > toluene > $p$-nitrotoluene > $p$-dinitrobenzene

Correct Answer: (d)

Explanation: Electron-donating methyl groups activate the ring (p-xylene has two), while electron-withdrawing nitro groups deactivate it.

40. Which of the following gives white precipitate with ammoniacal $AgNO_3$?

• (a) $C_2H_6$
• (b) $C_2H_2$
• (c) $C_2H_4$
• (d) $C_3H_8$

Correct Answer: (b)

Explanation: Acetylene reacts with Tollens' reagent to form insoluble silver acetylide ($AgC\equiv CAg$).

41. Match List I with List II (Tests to distinguish pairs):

• (a) (Match shown in PDF)
• (b) P-2, Q-3, R-4, S-1
• (c) (Match shown in PDF)
• (d) (Match shown in PDF)

Correct Answer: (b)

Explanation: Propyne/Propene (Tollens'), Benzene/Ethene (Baeyer's), Benzene/Hexane (Sooty flame), Benzene/Phenol (Neutral $FeCl_3$).

42. Dehydration of butan-2-ol with conc. $H_2SO_4$ gives Product (I) + Product (II). What is not true?

• (a) Products are position isomers
• (b) Contain same number of $sp^3$ and $sp^2$ carbons
• (c) The yield of the products I and II is same
• (d) Reaction obeys Saytzeff rule

Correct Answer: (c)

Explanation: According to Saytzeff's rule, the more substituted but-2-ene is the major product, so the yields are not equal.

43. Which of the following compounds undergo electrophilic substitution most easily?

• (a) (Structure in PDF)
• (b) (Structure in PDF)
• (c) $N,N$-diethylaniline
• (d) (Structure in PDF)

Correct Answer: (c)

Explanation: The lone pair on Nitrogen in the diethylamino group provides a powerful +M effect, making it the most activating substituent.

44. A hydrocarbon with molecular formula $C_8H_{18}$ gives only one monochloro derivative on chlorination. The compound is:

• (a) n-octane
• (b) iso-octane
• (c) 2,2,4-trimethylpentane
• (d) 2,2,3,3-tetramethylbutane

Correct Answer: (d)

Explanation: In 2,2,3,3-tetramethylbutane, all 18 hydrogens are equivalent, so replacing any one of them yields the same product.

45. A deactivating group in electrophilic substitution reaction:

• (a) deactivates only $o/p$ positions
• (b) deactivates only meta position
• (c) deactivates meta more than $o/p$
• (d) deactivates ortho- and para- positions more than meta- position

Correct Answer: (d)

Explanation: Deactivating groups pull electron density from the entire ring, but the effect is most pronounced at the $o$ and $p$ positions due to resonance.