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Unit 6: Haloalkanes and Haloarenes

6.1 Nomenclature and Nature of C-X Bond

Haloalkanes (Alkyl halides) and Haloarenes (Aryl halides) are obtained by the replacement of one or more hydrogen atoms of an aliphatic or aromatic hydrocarbon by halogen atoms (F, Cl, Br, I).

Nature of C-X bond: Halogen atoms are more electronegative than carbon. As a result, the carbon-halogen bond is polarized. The carbon atom bears a partial positive charge (\(\delta^+\)) and the halogen atom bears a partial negative charge (\(\delta^-\)). Size of halogens increases from F to I, hence the C-X bond length increases from C-F to C-I and bond enthalpy decreases.

6.2 Haloalkanes: Properties and Reactions

Physical Properties:

  • Insoluble in water due to inability to form hydrogen bonds with water molecules.
  • Boiling points increase with the size and mass of the halogen atom (RI > RBr > RCl > RF) due to an increase in magnitude of van der Waals forces. For isomeric haloalkanes, the boiling point decreases with branching (due to reduced surface area).

Chemical Reactions: Nucleophilic Substitution

A nucleophile (electron rich species) attacks the partially positive carbon atom of the C-X bond.

  1. \(S_N2\) Mechanism (Bimolecular Nucleophilic Substitution):

    • Occurs in a single step (concerted mechanism).
    • Rate depends on the concentration of both alkyl halide and nucleophile: \(\text{Rate} = k[RX][Nu^-]\).
    • Attack of nucleophile occurs from the backside.
    • Leads to inversion of configuration (Walden inversion).
    • Order of reactivity: Primary (\(1^\circ\)) > Secondary (\(2^\circ\)) > Tertiary (\(3^\circ\)) due to steric hindrance in bulkier alkyl groups.

  2. \(S_N1\) Mechanism (Unimolecular Nucleophilic Substitution):

    • Occurs in two steps. Step 1: Slow ionization to form a carbocation intermediate. Step 2: Fast attack by the nucleophile.
    • Rate depends only on the concentration of the alkyl halide: \(\text{Rate} = k[RX]\).
    • Leads to racemization if the starting alkyl halide is optically active, because the carbocation intermediate is planar and can be attacked from either side with equal probability.
    • Order of reactivity: Tertiary (\(3^\circ\)) > Secondary (\(2^\circ\)) > Primary (\(1^\circ\)) due to the stability of the carbocation (\(3^\circ > 2^\circ > 1^\circ\)).

Optical Isomerism

  • Chiral carbon: A carbon attached to four different groups.
  • Enantiomers: Non-superimposable mirror image isomers. They rotate plane-polarized light in opposite directions.
  • Dextrorotatory (+ / d): Rotates the plane of polarized light to the right.
  • Laevorotatory (- / l): Rotates the plane of polarized light to the left.
  • Racemic mixture: An equimolar mixture of d- and l- enantiomers, optically inactive due to external compensation.

6.3 Haloarenes: Nature and Substitution

Aryl halides are extremely less reactive towards Nucleophilic Substitution reactions due to:

  1. Resonance effect: The lone pairs on the halogen atom are in conjugation with the (\pi)-electrons of the benzene ring. The C-X bond acquires partial double bond character, making it difficult to cleave.
  2. Difference in hybridization of carbon: The sp² hybridized carbon of the C-X bond in haloarenes is more electronegative than the sp³ carbon in haloalkanes, holding the electron pair of the C-X bond more tightly.

Electrophilic Substitution Reactions

Halogens attached to benzene are deactivating but ortho-para directing. While they withdraw electrons through the -I (inductive) effect, their +R (resonance) effect increases electron density slightly more at the ortho and para positions than at the meta position.

  • Halogenation (with \(X_2/FeX_3\))
  • Nitration (with \(HNO_3 / H_2SO_4\))
  • Sulphonation (with concentrated \(H_2SO_4\))
  • Friedel-Crafts alkylation and acylation

6.4 Environmental Effects of Polyhalogen Compounds

  • Dichloromethane (\(CH_2Cl_2\)): Harms human central nervous system.
  • Trichloromethane (Chloroform, \(CHCl_3\)): Slowly oxidized by air in the presence of light to form poisonous phosgene (\(COCl_2\)). Hence, stored in dark bottles.
  • Tetrachloromethane (Carbon tetrachloride, \(CCl_4\)): Causes liver cancer in humans. O-zone depletion.
  • Iodoform (\(CHI_3\)): Used as an antiseptic due to the liberation of free iodine.
  • Freons (CFCs): E.g., \(CCl_2F_2\). Used as refrigerants. Responsible for severe ozone layer depletion in the stratosphere.
  • DDT: An insecticide. Banned in many countries due to its high toxicity to fish, chemical stability, and fat solubility leading to biological magnification.

Competency-Based Questions (CBQs)

Q1. (CBSE 2022) Among the isomeric alkanes of molecular formula \(C_5H_{12}\), identify the one that on photochemical chlorination yields: (i) A single monochloride. (ii) Three isomeric monochlorides. (iii) Four isomeric monochlorides.


Answer: The possible isomers of pentane (\(C_5H_{12}\)) are:

  1. n-pentane: \(CH_3-CH_2-CH_2-CH_2-CH_3\)
  2. isopentane: \(CH_3-CH(CH_3)-CH_2-CH_3\)
  3. neopentane: \(C(CH_3)_4\)

(i) A single monochloride: Isomer: neopentane (2,2-dimethylpropane). All 12 hydrogen atoms are equivalent. Replacement of any of them yields the same product, 1-chloro-2,2-dimethylpropane.

(ii) Three isomeric monochlorides: Isomer: n-pentane. It has three different sets of equivalent hydrogen atoms (at C1, C2, and C3). Substitution yields 1-chloropentane, 2-chloropentane, and 3-chloropentane.

(iii) Four isomeric monochlorides: Isomer: isopentane (2-methylbutane). It has four different types of hydrogen atoms. Substitution yields 1-chloro-2-methylbutane, 2-chloro-2-methylbutane, 2-chloro-3-methylbutane, and 1-chloro-3-methylbutane.

Q2. (Sample Paper 2023) You are provided with two compounds: 1-bromobutane and 2-bromobutane. One of these undergoes an \(S_N1\) reaction faster, while the other undergoes an \(S_N2\) reaction faster. Identify them and explain your reasoning.


Answer: 1-bromobutane (\(CH_3CH_2CH_2CH_2Br\)) is a primary (\(1^\circ\)) alkyl halide. 2-bromobutane (\(CH_3CH_2CH(Br)CH_3\)) is a secondary (\(2^\circ\)) alkyl halide.

  • \(S_N2\) Reaction: Proceeds via a transition state where nucleophile attacks from the backside. This attack is highly sensitive to steric hindrance. Since a \(1^\circ\) alkyl halide has less steric hindrance around the carbon atom bearing the halogen, 1-bromobutane undergoes \(S_N2\) reaction faster.
  • \(S_N1\) Reaction: Proceeds via the formation of a carbocation intermediate. The stability of the carbocation determines the reaction rate. The secondary carbocation formed by 2-bromobutane is more stable than the primary carbocation formed by 1-bromobutane due to greater +I effect and hyperconjugation. Thus, 2-bromobutane undergoes \(S_N1\) reaction faster.

Q3. (CBSE 2021) Explain why haloarenes are extremely less reactive towards nucleophilic substitution reactions as compared to haloalkanes. Support your answer with resonance structures of chlorobenzene.


Answer: Haloarenes are much less reactive towards nucleophilic substitution compared to haloalkanes due to the following reasons:

  1. Resonance Effect: In haloarenes (e.g., chlorobenzene), the lone pairs of electrons on the halogen atom are in conjugation with the (\pi)-electrons of the benzene ring. This delocalization imparts a partial double bond character to the C-Cl bond, making it shorter and stronger, and difficult to break by a nucleophile.
Cl .. .. .. ↔ Cl⁺ .. .. :- ↔ Cl⁺ .. .. :- ↔ Cl⁺ .. .. :- ↔ Cl .. .. .. Resonance structures of Chlorobenzene
  1. sp² Hybridization representing the C-X bond: In haloarenes, the carbon atom attached to the halogen is sp² hybridized, whereas in haloalkanes it is sp³ hybridized. sp² carbons have more s-character and are more electronegative, thus holding the electron pair of the C-X bond more tightly, reducing the bond length and making it harder to break.
  2. Instability of phenyl cation: If an \(S_N1\) mechanism were to occur, the resulting phenyl cation formed by self-ionization would not be stabilized by resonance.