Chemistry of Hydrocarbons

Chemistry of Hydrocarbons MDCAT MCQs with answers

88 past paper MCQs from Chemistry unit 14, Chemistry of Hydrocarbons, across 13 topics. They are arranged topic by topic; bigger topics show a sample here and link to their full set and to study notes. Tap “Show answer” for the correct option and a short solution.

Practise as a quiz

Free Radical Mechanism

1.Consider the chlorination of methane. The attack of a chlorine free radical on methane to form a methyl free radical occurs in the: (MDCAT 2022)

  1. initiation step
  2. propagation step
  3. termination step
  4. last step
Show answer

Correct answer: (b) propagation step

One radical is consumed and another is produced, which is what keeps the chain going.

2.What is general requirement for free radical substitution reaction of ethane and halogen? (UHS MDCAT 2025)

  1. low pressure
  2. low temperature
  3. high pressure
  4. heat and UV light
Show answer

Correct answer: (d) heat and UV light

Halogenation of an alkane needs heat or ultraviolet light to split the halogen into free radicals.

3.Consider the chlorination of methane to methyl chloride, the attack of chlorine free radical on methane, occurs in which phase? (KMU MDCAT 2025)

  1. Before initiation
  2. Initiation
  3. Propagation
  4. Termination
Show answer

Correct answer: (c) Propagation

A chlorine radical attacking methane is a propagation step: a radical is used and another is made.

All 14 Free Radical Mechanism MCQs → Free Radical Mechanism notes →

Structure and Reactivity of Alkenes

4.The addition reaction of 2-butene with HBr produces: (MDCAT 2023)

  1. 1-bromobutane
  2. 2-bromobutane
  3. 3-bromobutane
  4. 2,3-dibromobutane
Show answer

Correct answer: (b) 2-bromobutane

Either carbon of the double bond is equivalent here, so the bromine ends up on carbon 2.

5.The addition of water to propene in the presence of sulfuric acid produces: (KMU MDCAT 2025)

  1. Propane
  2. Propan-2-ol
  3. Butanol
  4. Ethanol
Show answer

Correct answer: (b) Propan-2-ol

The proton adds to give the more stable secondary carbocation, so the hydroxyl ends on the middle carbon.

6.Addition of HBr to isobutylene mainly gives (KMU MDCAT 2024)

  1. isobutyl bromide
  2. n-butyl bromide
  3. sec-butyl bromide
  4. tert-butyl bromide
Show answer

Correct answer: (d) tert-butyl bromide

By Markovnikov’s rule the bromine goes to the more substituted carbon, giving the tertiary halide.

All 9 Structure and Reactivity of Alkenes MCQs → Structure and Reactivity of Alkenes notes →

Molecular Orbital Treatment of Benzene

7.The three alternate double bonds in benzene are called: (MDCAT 2022)

  1. delocalized bonds
  2. coordinate covalent bonds
  3. fixed bonds
  4. ionic bonds
Show answer

Correct answer: (a) delocalized bonds

The six $p$ electrons spread over all six carbons, so the bonds are delocalized rather than three fixed double bonds.

8.The ratio of sigma to pi electrons in benzene is: (MDCAT 2022)

  1. 2 : 3
  2. 5 : 1
  3. 4 : 1
  4. 1 : 4
Show answer

Correct answer: (c) 4 : 1

Benzene has twelve sigma bonds, so 24 sigma electrons, against 6 pi electrons.

9.Ratio of sigma bonds and pi bonds present in benzene are: (SIBA MDCAT 2025)

  1. 4:1
  2. 1:4
  3. 2:3
  4. 6:1
Show answer

Correct answer: (a) 4:1

Benzene has twelve sigma bonds and three pi bonds, a ratio of four to one.

All 6 Molecular Orbital Treatment of Benzene MCQs → Molecular Orbital Treatment of Benzene notes →

Chemical Reactions of Benzene

10.Alkylbenzene is formed when benzene is treated with an alkyl halide in the presence of anhydrous aluminium chloride. Identify the type of reaction. (MDCAT 2022)

  1. halogenation
  2. Friedel-Crafts acylation
  3. Friedel-Crafts alkylation
  4. sulphonation
Show answer

Correct answer: (c) Friedel-Crafts alkylation

An alkyl group substitutes for a ring hydrogen with the Lewis acid as catalyst.

11.Alkylbenzene is formed when benzene is treated with an alkyl halide in the presence of anhydrous aluminium chloride. Identify the type of reaction. (MDCAT 2023)

  1. halogenation
  2. Friedel-Crafts acylation
  3. Friedel-Crafts alkylation
  4. sulphonation
Show answer

Correct answer: (c) Friedel-Crafts alkylation

The Lewis acid makes a carbocation from the halide, which substitutes into the ring: Friedel-Crafts alkylation.

12.In Friedel-Crafts acylation an acyl group is introduced into the benzene ring in the presence of the catalyst: (MDCAT 2024)

  1. $\mathrm{AlCl_3}$
  2. $\mathrm{H_2SO_4}$
  3. sunlight
  4. $\mathrm{V_2O_5}$
Show answer

Correct answer: (a) $\mathrm{AlCl_3}$

Anhydrous aluminium chloride is the Lewis acid that generates the acylium electrophile.

All 16 Chemical Reactions of Benzene MCQs → Chemical Reactions of Benzene notes →

Effect of Substituents

13.Which of the following is an example of a substituent group which releases electrons to the benzene ring? (MDCAT 2024)

  1. $-\mathrm{CN}$
  2. $-\mathrm{NR_3^+}$
  3. $-\mathrm{NH_2}$
  4. $-\mathrm{NO_2}$
Show answer

Correct answer: (c) $-\mathrm{NH_2}$

The lone pair on nitrogen is donated into the ring, which is why $-\mathrm{NH_2}$ activates it.

14.Which of the following is an example of substituent group which release electrons to benzene ring : (UHS MDCAT 2024)

  1. -CN
  2. -NR3+
  3. -NH2
  4. -NO2
Show answer

Correct answer: (c) -NH2

The lone pair of the amino group is fed into the ring; nitro and cyano groups pull electrons out.

15.The most readily sulphonated compound is: (NUMS MDCAT 2024)

  1. Benzene
  2. Chlorobenzene
  3. Nitrobenzene
  4. Toluene
Show answer

Correct answer: (d) Toluene

The methyl group feeds electrons into the ring and so activates it towards the electrophile.

All 8 Effect of Substituents MCQs → Effect of Substituents notes →

Preparation of Alkynes

16.Identify the method which is used to prepare alkynes. (MDCAT 2023)

  1. dehalogenation of vicinal dihalides in the presence of methanol
  2. dehydration of alcohols in the presence of $\mathrm{Al_2O_3}$ at 340-450$^\circ$C
  3. dehydrohalogenation of vicinal dihalides in the presence of a strong base
  4. electrolysis of carboxylic acid salts
Show answer

Correct answer: (c) dehydrohalogenation of vicinal dihalides in the presence of a strong base

A strong base removes two molecules of hydrogen halide from the vicinal dihalide and leaves a triple bond.

17.Conversion of dihaloalkane into alkyne does not involve (UHS MDCAT 2025)

  1. Addition
  2. Elimination
  3. Base
  4. Heat
Show answer

Correct answer: (a) Addition

A dihaloalkane loses two molecules of hydrogen halide to a strong base on heating, so the step is elimination, not addition.

18.Which of the following reagent is commonly used to prepare alkyne form vicinal dihalide? (SZABMU MDCAT 2025)

  1. H2 and catalyst
  2. KOH/NaNH2
  3. KMnO4
  4. LiAlH4
Show answer

Correct answer: (b) KOH/NaNH2

Two molecules of hydrogen halide are eliminated by alcoholic potash and then sodamide.

Preparation of Alkynes notes →

Preparation of Alkanes

19.The cracking method used to obtain better quality gasoline is: (NUMS MDCAT 2023)

  1. Thermal
  2. Catalytic
  3. Steam
  4. Radiations
Show answer

Correct answer: (b) Catalytic

Catalytic cracking gives branched and aromatic products, which have the higher octane number.

20.Conversion of straight chain hydrocarbons into branched chain is called as __________. (PMC MDCAT 2022)

  1. Reforming
  2. Cracking
  3. lsomerization
  4. Decomposition
Show answer

Correct answer: (a) Reforming

Reforming rearranges straight chains into branched and cyclic ones of higher octane number.

21.Which of the following fraction of petroleum has the lowest boiling point at STP. (PMC MDCAT 2022)

  1. Kerosene oil
  2. Lubricating oil
  3. Gasoline
  4. Heavy Oil
Show answer

Correct answer: (c) Gasoline

The lighter the fraction the lower it boils, and gasoline is the lightest of these.

All 11 Preparation of Alkanes MCQs → Preparation of Alkanes notes →

Acidity of Alkynes

22.Which of the following compounds is more acidic? (MDCAT 2022)

  1. alkane
  2. alkene
  3. alkyne
  4. cycloalkane
Show answer

Correct answer: (c) alkyne

The $sp$ carbon of a terminal alkyne holds its electrons closest, so its hydrogen comes off most readily.

23.Which of the following reagents can be used to distinguish between 1-pentyne and 2-pentyne? (MDCAT 2023)

  1. $\mathrm{AgNO_3+NH_4OH}$
  2. dilute alkaline $\mathrm{KMnO_4}$
  3. $\mathrm{Br_2/CCl_4}$
  4. $\mathrm{K_2Cr_2O_7+H_2SO_4}$
Show answer

Correct answer: (a) $\mathrm{AgNO_3+NH_4OH}$

Only the terminal alkyne has an acidic hydrogen, so only 1-pentyne gives a silver acetylide precipitate.

24.Terminal alkynes show acidic character due to: (UHS MDCAT 2025)

  1. Terminal carbon atoms are sp hybridized.
  2. Terminal carbon atoms are sp2 hybridized.
  3. Terminal carbon atoms are sp3 hybridized.
  4. Terminal carbon atoms show hydrogen bonding.
Show answer

Correct answer: (a) Terminal carbon atoms are sp hybridized.

The sp hybridized carbon holds its electrons close, so the terminal hydrogen leaves as a proton.

All 7 Acidity of Alkynes MCQs → Acidity of Alkynes notes →

Reactions of Alkynes

25.Propanone can be prepared from propyne by: (MDCAT 2023)

  1. passing a mixture of propyne and steam over a catalyst of magnesium at $420^\circ$C
  2. passing a mixture of propyne and ethanol over a catalyst of zinc chromate
  3. boiling propyne with water in the presence of $\mathrm{HgSO_4}$ and $\mathrm{H_2SO_4}$
  4. treating propyne with iodine and NaOH
Show answer

Correct answer: (c) boiling propyne with water in the presence of $\mathrm{HgSO_4}$ and $\mathrm{H_2SO_4}$

Acid-catalysed hydration of the triple bond follows Markovnikov's rule and gives the ketone.

26.Acetylene reacts with hydrogen bromide to form: (NUMS MDCAT 2025)

  1. 2,2-Dibromoethane
  2. 1,1-Dibromoethane
  3. 1,2-Dibromoethane
  4. 1,1,2,2-Tetrabromoethane
Show answer

Correct answer: (b) 1,1-Dibromoethane

Both molecules of hydrogen bromide add by Markovnikov’s rule to the same carbon.

Reactions of Alkynes notes →

Resonance and Resonance Energy

27.Three alternate single and double bonds in benzene are called? (UHS MDCAT 2023)

  1. Conjugate bonds
  2. Coordinate covalent bonds
  3. Fixed bonds
  4. Ionic bonds
Show answer

Correct answer: (a) Conjugate bonds

Alternating single and double bonds make a conjugated system, and the electrons of it are delocalized.

28.Benzene is more stable than hypothetical 1,3,5-cyclohexatriene by: (PMC MDCAT 2022)

  1. 150.5 kJ/mol
  2. 150.5J/mol
  3. 150.5 kJ/molecule
  4. 150.5 J/molecule
Show answer

Correct answer: (a) 150.5 kJ/mol

The resonance energy of benzene is about $150\,\mathrm{kJ\,mol^{-1}}$.

Resonance and Resonance Energy notes →

Substitution versus Addition

29.Under suitable conditions, alkanes cannot undergo which of the following reactions? (MDCAT 2023)

  1. substitution
  2. combustion
  3. hydrogenation
  4. addition
Show answer

Correct answer: (d) addition

An alkane is already saturated, so it has no place to add anything; it reacts by substitution.

30.Generally the decreasing order of reactivity is: (NUMS MDCAT 2025)

  1. Alkane > Alkynes > Alkene
  2. Alkene < Alkynes < Alkanes
  3. Alkene > Alkynes > Alkanes
  4. Alkynes < Alkanes < Alkene
Show answer

Correct answer: (c) Alkene > Alkynes > Alkanes

The loose pi electrons of an alkene are the most available, and the saturated alkane has none.

31.The correct order of reactivity of hydrocarbons is: (SZABMU MDCAT 2023)

  1. Alkanes ˃ Alkynes ˃ Alkenes
  2. Alkenes ˃ Alkanes ˃ Alkynes
  3. Alkynes ˃ Alkenes ˃ Alkanes
  4. Alkenes ˃ Alkynes ˃ Alkanes
Show answer

Correct answer: (d) Alkenes ˃ Alkynes ˃ Alkanes

The loose pi electrons of an alkene are the most available, and the saturated alkane has none.

32.Under suitable conditions alkanes cannot undergo ___________ reactions? (UHS MDCAT 2023)

  1. Substitution
  2. Combustion
  3. Hydrogenation
  4. Addition
Show answer

Correct answer: (d) Addition

An alkane is saturated, so nothing can add to it; it can only substitute or burn.

Substitution versus Addition notes →

Reactivity of Benzene

33.Which one of the following is not a characteristic of benzene? (UHS MDCAT 2025)

  1. Aromaticity
  2. Stability
  3. to follow Huckle rule
  4. High reactivity
Show answer

Correct answer: (d) High reactivity

Delocalization makes benzene unusually stable, so it substitutes rather than reacting readily.

34.Addition of three molecules of chlorine into benzene prove that benzene has (SZABMU MDCAT 2025)

  1. double bonds
  2. non polar
  3. planar
  4. non planer
Show answer

Correct answer: (a) double bonds

Three molecules of chlorine add to it, which is what three double bonds would take.

35.Which of the following is not oxidized by any oxidizing agent? (SZABMU MDCAT 2023)

  1. Benzene
  2. Toulene
  3. Ethylbenzene
  4. Xylene
Show answer

Correct answer: (a) Benzene

Benzene resists oxidation; the side chain of an alkylbenzene is oxidized instead.

All 5 Reactivity of Benzene MCQs → Reactivity of Benzene notes →

IUPAC Nomenclature of Alkynes

36.The IUPAC name of Vinyl acetylene is: (SIBA MDCAT 2025)

  1. But-1-en-3-yne
  2. But-3-en-1-yne
  3. Pent-3-en-1-yne
  4. Pent-2-en-4-yne
Show answer

Correct answer: (a) But-1-en-3-yne

The chain of four carbons carries the double bond at position 1 and the triple bond at position 3.

More MDCAT Chemistry units