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Haloalkanes and Haloarenes — NEET & JEE MCQs with answers
Haloalkanes (alkyl halides) and haloarenes (aryl halides) are formed by replacing hydrogen atom(s) in aliphatic or aromatic hydrocarbons with halogen atom(s); haloalkanes bear the halogen on an sp³ carbon of an alkyl group while haloarenes bear it on an sp² carbon of an aryl group. Halogens are more electronegative than carbon, so the C–X bond is polarised, with carbon partially positive and halogen partially negative; the C–X bond length increases from C–F to C–I while the bond enthalpy decreases. Alkyl halides are prepared from alcohols, alkanes and alkenes, fluorides and iodides are best made by halogen exchange, and these compounds find wide applications in industry and day-to-day life.
Topics in this chapter
- 6.1 Classification of Haloalkanes and Haloarenes
- 6.2 Nomenclature
- 6.3 Nature of C-X Bond
- 6.4 Methods of Preparation of Haloalkanes
- 6.5 Preparation of Haloarenes
- 6.6 Physical Properties of Haloalkanes and Haloarenes
- 6.7 Reactions of Haloalkanes and Haloarenes
- 6.8 Polyhalogen Compounds
Free practice questions
The IUPAC name of $\mathrm{CH_3CHClCH_2Cl}$ is
- A. 1,1-Dichloropropane
- B. 1,2-Dichloropropane
- C. 1,3-Dichloropropane
- D. 2,2-Dichloropropane
Show answer
Answer: B — Option B is correct. The parent chain is propane. Numbering from the $\mathrm{-CH_2Cl}$ end puts chlorines on C1 and C2, giving the locant set {1,2}; numbering from the other end would give {2,3}, which is higher, so 1,2-dichloropropane wins. A 1,1 or 1,3 set does not match the structure, and 2,2 would need both chlorines on the middle carbon.
NCERT Class 12 Chemistry Part-2 · Ch 6 · §6.2 ("In IUPAC system, they are named as dihaloalkanes.")
The transition state of an $S_N2$ reaction is best described as a species which:
- A. is the final product of the substitution
- B. is unstable and cannot be isolated
- C. is a stable intermediate that can be isolated
- D. is a free radical that can be trapped
Show answer
Answer: B — Option B is correct. NCERT describes the transition state as unstable and something that cannot be isolated; the attacking and leaving nucleophiles are only partially attached to the carbon. It is not an isolable intermediate, and it is not the product.
NCERT Class 12 Chemistry Part-2 · Ch 6 · §6.7.1 ("unstable and cannot be isolated")
But-1-ene reacts with hydrogen iodide. The major product is
- A. 1-Iodobutane
- B. 1,2-Diiodobutane
- C. 2-Iodobut-1-ene
- D. 2-Iodobutane
Show answer
Answer: D — Option D. An alkene is converted to the corresponding alkyl halide by hydrogen iodide, and Markovnikov addition places the iodine on C-2, giving 2-iodobutane.
NCERT Class 12 Chemistry Part-2 · Ch 6 · §6.4.2 ("(i) Addition of hydrogen halides: An alkene is converted to")
The carbon-halogen bond of an alkyl halide is polarised because
- A. halogen atoms are less electronegative than carbon
- B. halogen atoms are more electronegative than carbon
- C. the C–X bond has partial double-bond character
- D. the halogen atom is larger than the carbon atom
Show answer
Answer: B — Option B is correct. NCERT opens §6.3 by stating that halogen atoms are more electronegative than carbon, and it is this electronegativity difference that polarises the C–X bond. The halogen is not less electronegative than carbon, the bond is a single covalent bond rather than a double one, and the polarity is not attributed to the halogen's size in this sentence.
NCERT Class 12 Chemistry Part-2 · Ch 6 · §6.3 ("Halogen atoms are more electronegative than carbon, therefore,")
Iodoform was used earlier as an:
- A. Anaesthetic
- B. Insecticide
- C. Aerosol propellant
- D. Antiseptic
Show answer
Answer: D — Option D is correct. The passage begins by noting that it was used earlier as an antiseptic.
NCERT Class 12 Chemistry Part-2 · Ch 6 · §6.8.3 ("It was used earlier as an antiseptic but the antiseptic properties are")
Arrange 1-bromobutane, 1-bromo-2-methylpropane and 2-bromo-2-methylpropane in order of increasing boiling point.
- A. 1-Bromobutane < 1-Bromo-2-methylpropane < 2-Bromo-2-methylpropane
- B. 2-Bromo-2-methylpropane < 1-Bromo-2-methylpropane < 1-Bromobutane
- C. 1-Bromo-2-methylpropane < 1-Bromobutane < 2-Bromo-2-methylpropane
- D. 2-Bromo-2-methylpropane < 1-Bromobutane < 1-Bromo-2-methylpropane
Show answer
Answer: B — Option B is correct. Branching decreases in the order given, and NCERT states that boiling points of isomeric haloalkanes decrease with increase in branching, so the most branched member boils lowest.
NCERT Class 12 Chemistry Part-2 · Ch 6 · §6.6 ("The boiling points of isomeric haloalkanes decrease with increase")
In this route to haloarenes, which group attached to the aromatic ring is replaced by the halogen?
- A. The amino group
- B. The nitro group
- C. The diazonium group
- D. The hydroxyl group
Show answer
Answer: C — Option C. NCERT describes the replacement of the diazonium group by $-\mathrm{Cl}$ or $-\mathrm{Br}$ (or by iodine with potassium iodide).
NCERT Class 12 Chemistry Part-2 · Ch 6 · §6.5 ("replacement of the diazonium group by -Cl or -Br.")
Chloroethane, $\mathrm{CH_3CH_2Cl}$, belongs to which group of the number-based classification?
- A. Monohalogen compounds
- B. Dihalogen compounds
- C. Trihalogen compounds
- D. Tetrahalogen compounds
Show answer
Answer: A — Option A is correct. Chloroethane carries exactly one halogen atom, so it is a monohalogen compound (a monohalocompound). Two halogens would make it dihalogen, three trihalogen and four or more would still fall inside the polyhalogen label.
NCERT Class 12 Chemistry Part-2 · Ch 6 · §6.1.1 ("These may be classified as mono, di, or polyhalogen")
For a tertiary alkyl halide, NCERT says the competition between $S_N1$ and elimination is decided by:
- A. the bond length of the carbon–halogen bond
- B. the stability of the carbocation or the more substituted alkene
- C. the boiling point of the solvent used
- D. the density of the alkyl halide
Show answer
Answer: B — Option B is correct. NCERT states that a tertiary halide gives $S_N1$ or elimination depending upon the stability of the carbocation or the more substituted alkene. Steric approach of the nucleophile is what governs the primary and secondary cases in $S_N2$.
NCERT Class 12 Chemistry Part-2 · Ch 6 · §6.7.1 ("elimination depending upon the stability of carbocation or the more substituted alkene")
Of the eight IUPAC names NCERT gives for the isomers of $\mathrm{C_5H_{11}Br}$, how many begin with '1-Bromo'?
- A. Three
- B. Four
- C. Five
- D. Six
Show answer
Answer: B — Option B is correct. Four names start with 1-bromo: 1-bromopentane, 1-bromo-3-methylbutane, 1-bromo-2-methylbutane and 1-bromo-2,2-dimethylpropane. The remaining four begin with 2-bromo or 3-bromo, so three, five and six are all wrong.
NCERT Class 12 Chemistry Part-2 · Ch 6 · §6.2 ("molecular formula C5H11Br")
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