250+ TOP MCQs on Aromatic Hydrocarbon and Answers

Chemistry Multiple Choice Questions on “Aromatic Hydrocarbon”.

1. Which of the following hydrocarbon is formed, when benzene reacts with three chlorine molecules under UV condition?
a) Hexachlorobenzene
b) Benzene hexachloride
c) Chlorobenzene
d) Chlorohexane
Answer: b
Clarification: Under ultra-violet light, three chlorine molecules add to benzene to produce benzene hexachloride. It is also known as gammaxene. Its chemical formula is C₆H₆Cl₆ and undergoes addition reaction.

2. Alkylbenzene is formed when benzene is treated with an alkyl halide in the presence of anhydrous aluminum chloride. Identify the type of reaction.
a) Halogenation
b) Friedel-Crafts acylation reaction
c) Friedel-Crafts alkylation reaction
d) Sulphonation
Answer: c
Clarification: Aluminum chloride reacts with methyl chloride to form an electrophile. This electrophile attacks the π electron to form carbocation. As a result, alkylbenzene is formed along with HCl as product.

3. Which of the following is an example of meta directing group?
a) -CHO
b) –NH₂
c) -OCH₃
d) -NHR
Answer: a
Clarification: The groups which direct the incoming group to meta position are called meta directing groups. Some examples are -CHO, -COR, -COOH, -CN. The overall electron density on benzene ring decreases. Hence, they are called deactivating groups.

4. Calculate the number of sigma(σ) and pi(π) bonds present in benzene (C₆H₆).
a) 6 σ and 3 π
b) 3 σ and 6 π
c) 3 σ and 12 π
d) 12 σ and 3 π
Answer: d
Clarification: Sigma bonds are the strongest type of covalent bond, formed by head-on overlapping. In benzene, there are 6(C-C) σ bonds and 6(C-H) σ bonds. Pi bonds are formed by lateral overlapping of atomic orbitals. There are totally 3(C=C) π bonds.

5. Which of the following is used to reduce phenol vapors to benzene?
a) Hydrogen gas
b) Oleum
c) Zinc dust
d) Anhydrous AlCl₃
Answer: c
Clarification: Phenol is reduced to benzene by passing its vapors over heated zinc dust. In this reaction, phenoxide ion and a proton is formed. This proton accepts an electron from zinc and forms ZnO and the phenoxide ion gets converted to benzene.

6. Aromatic hydrocarbons are readily miscible in water and immiscible with organic solvents.
a) True
b) False
Answer: b
Clarification: Aromatic hydrocarbons are non-polar molecules. Nonpolar solvents dissolves nonpolar molecules as ‘like’ dissolves ‘like’. Hence aromatic hydrocarbons are immiscible with water but are readily miscible with organic solvents.

7. Benzene on treatment with excess of chlorine in the presence of anhydrous AlCl₃ , under dark and cold condition yield ‘X’. Identify X.
a) Chlorobenzene
b) Hexachlorobenzene
c) Benzene Hexachloride
d) Acetophenone
Answer: b
Clarification: If excess of electrophilic reagent is used, substitution reaction may take place in which other hydrogen atoms of benzene ring may also be successively replaced by electrophile. Benzene on treatment with excess of chlorine in the presence of anhydrous AlCl₃ can be chlorinated to hexachlorobenzene.

8. Identify the conditions under which hydrogenation of benzene produces cyclohexane.
a) High temperature, High pressure, Nickel catalyst
b) Low temperature, Low pressure, Ziegler catalyst
c) High temperature, Low pressure, Anhydrous AlCl₃
d) Low temperature, Low Pressure, Anhydrous AlCl₃
Answer: a
Clarification: Benzene normally does not undergo addition reactions, but under drastic conditions, i.e. at high temperature (473 – 523K) and high pressure in the presence of nickel catalyst, hydrogenation of benzene gives cyclohexane.

9. Which of the following is an example of ortho and para directing group?
a) -SO₃H
b) -COOR
c) -OCH₃
d) -CN
Answer: c
Clarification: The groups which direct the incoming group to ortho, and para positions are called ortho and para detecting groups. The overall electron density increases at these positions of the ring due to resonance. They are called activating groups. Some examples of activating group are –NH₂, -NHR, OCH₃.

10. Which among the following is not a carcinogen?
a) 1,2-Benzpyrene
b) Ethyl Alcohol
c) 3-Methylcholanthrene
d) 1,2-Benzanthracene
Answer: b
Clarification: Compounds containing more than two benzene rings joined are carcinogenic. They are oxidized in body to form epoxide, which is not excreted readily and finally damage DNA and cause cancer.