Here below some basic MCQ’s about “Haloalkanes and Haloarenes” with answer explained in details for exam practice. Let’s check one by one.
Question 1: Nomenclature
Which of the following is the correct IUPAC name for CH₃-CH₂-CHCl-CH₃?
A) 2-Chlorobutane
B) 3-Chlorobutane
C) 1-Chlorobutane
D) Chloropropane
Answer: A) 2-Chlorobutane
Explanation:
Number the longest carbon chain containing the chlorine atom to give it the lowest possible number. The chlorine is attached to the second carbon of a four-carbon chain, hence the correct name is 2-Chlorobutane.
Question 2: Physical Properties
Which of the following haloalkanes has the highest boiling point?
A) CH₃Cl
B) CH₃CH₂Cl
C) (CH₃)₃CCl
D) CH₂Cl₂
Answer: D) CH₂Cl₂
Explanation:
Boiling points of haloalkanes generally increase with molecular weight and polarity. CH₂Cl₂ (dichloromethane) has more polar C-Cl bonds and can engage in dipole-dipole interactions more effectively than the other options, leading to a higher boiling point.
Question 3: Reactions – Nucleophilic Substitution
Which reagent is most suitable for the conversion of 1-chloropropane to 1-propanol?
A) NaOH (aqueous)
B) NaOH (alcoholic)
C) KOH (aqueous)
D) KOH (alcoholic)
Answer: A) NaOH (aqueous)
Explanation:
Aqueous NaOH promotes SN1 or SN2 nucleophilic substitution to replace the chlorine with a hydroxyl group, forming 1-propanol. Alcoholic NaOH is more likely to act as a base, favoring elimination reactions instead.
Question 4: Reactions – Elimination
When 2-chlorobutane is treated with alcoholic KOH, which major product is formed?
A) 2-Butanol
B) 1-Butene
C) cis-2-Butene
D) trans-2-Butene
Answer: D) trans-2-Butene
Explanation:
Alcoholic KOH favors elimination (E2 mechanism). The more stable alkene (trans-2-butene) is the major product due to less steric hindrance and greater stability compared to cis isomer or 1-butene.
Question 5: Properties – Reactivity
Which haloalkane is least reactive towards nucleophilic substitution?
A) CH₃Cl
B) CH₃CH₂Cl
C) (CH₃)₂CHCl
D) (CH₃)₃CCl
Answer: D) (CH₃)₃CCl
Explanation:
Tertiary haloalkanes like (CH₃)₃CCl are less reactive towards nucleophilic substitution due to steric hindrance, making it difficult for nucleophiles to attack the carbon atom bonded to the leaving group.
Question 6: Nomenclature – Haloarenes
What is the IUPAC name for chlorobenzene?
A) Benzene chloride
B) Chlorobenzene
C) Monochlorobenzene
D) Chlorophenyl
Answer: B) Chlorobenzene
Explanation:
The correct IUPAC name follows the convention of naming the substituent first followed by the parent aromatic ring. Thus, Chlorobenzene is the proper name.
Question 7: Reactions – Haloarenes
Which reaction is commonly used to convert chlorobenzene to nitrobenzene?
A) Electrophilic substitution
B) Nucleophilic substitution
C) Friedel-Crafts acylation
D) Halogenation
Answer: B) Nucleophilic substitution
Explanation:
Chlorobenzene undergoes nucleophilic aromatic substitution when treated with a strong nitronium ion (NO₂⁺) source under appropriate conditions to form nitrobenzene.
Question 8: Physical Properties – Solubility
Which of the following haloalkanes is most soluble in water?
A) CH₃Cl
B) C₂H₅Cl
C) C₃H₇Cl
D) (CH₃)₃CCl
Answer: A) CH₃Cl
Explanation:
Smaller haloalkanes like CH₃Cl have higher solubility in water due to their ability to form hydrogen bonds and have a greater polarity relative to their size. Larger haloalkanes are less soluble due to increased hydrophobic character.
Question 9: Nomenclature – Multiple Substituents
What is the correct IUPAC name for CH₃-CHCl-CHBr-CH₃?
A) 2-Chloro-3-bromobutane
B) 3-Chloro-2-bromobutane
C) 2-Bromo-3-chlorobutane
D) 3-Bromo-2-chlorobutane
Answer: A) 2-Chloro-3-bromobutane
Explanation:
Number the chain to give the substituents the lowest possible numbers. Here, chlorine gets position 2 and bromine position 3, so the name is 2-Chloro-3-bromobutane.
Question 10: Reactions – Dehalogenation
Which reagent is used for the dehydrohalogenation of 2,3-dibromobutane to form an alkene?
A) NaOH
B) KOH
C) Zn/AcOH
D) KI in acetone
Answer: B) KOH
Explanation:
KOH (especially alcoholic KOH) is commonly used for dehydrohalogenation reactions, eliminating hydrogen and halogen atoms to form alkenes. In this case, treating 2,3-dibromobutane with KOH would result in the formation of but-2-ene.
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