Haloalkanes and Haloarenes — Questions & Answers

Sample questions from Haloalkanes and Haloarenes

1. Q1. Addition of HBr to propene in the presence of benzoyl peroxide gives mainly:

   • A.1,2-Dibromopropane
   • B.2-Bromopropane
   • C.1-Bromopropane✓
   • D.Propan-2-ol
   Solution

   Peroxides generate bromine radicals, which add to the terminal carbon to give the more stable secondary radical; that radical then abstracts H from HBr, placing Br on C-1. This peroxide (Kharasch) effect is observed only with HBr.

2. Q2. The rate of hydrolysis of 2-chloro-2-phenylpropane is greater than that of 2-chloro-2-methylpropane. If the phenyl ring carries a para nitro group instead, the rate:

   • A.Increases further, because the nitro group withdraws electrons
   • B.Decreases markedly, because the electron-withdrawing nitro group destabilises the positively charged intermediate✓
   • C.Is unchanged, as substituents on the ring do not affect the cation
   • D.Becomes zero
   Solution

   The rate-determining step creates a carbocation whose positive charge is delocalised into the ring. Any group that withdraws electron density makes that already electron-poor system less able to bear the charge, raising the energy of the transition state. A para nitro group is a powerful acceptor by both -I and -M, so the solvolysis slows greatly; electron-donating groups such as -OCH3 would accelerate it.

3. Q3. Benzyl chloride hydrolyses much faster than 1-chlorobutane under SN1 conditions because:

   • A.The benzene ring withdraws electrons inductively, weakening the C-Cl bond
   • B.The C-Cl bond in benzyl chloride has partial double bond character
   • C.The benzyl carbocation is stabilised by resonance delocalisation over the aromatic ring✓
   • D.Benzyl chloride is a tertiary halide
   Solution

   Although benzyl chloride is formally primary, ionisation gives a benzyl cation whose positive charge is delocalised over the ortho and para ring positions by resonance, greatly lowering the activation energy for SN1 relative to an ordinary primary chloride.

4. Q4. What is the FITTIG reaction, and how does it differ from the Wurtz reaction?

   • A.The Fittig reaction couples an alkyl halide with an aryl halide, using potassium rather than sodium
   • B.The Fittig reaction is the aromatic analogue of the Wurtz reaction, coupling two ARYL halides (rather than alkyl halides) using sodium metal in dry ether, to form a biaryl (diphenyl-type) product✓
   • C.The Fittig reaction is identical in every respect to the Wurtz reaction, with no meaningful difference
   • D.The Fittig reaction only works with alkyl halides, never with aryl halides
   Solution

   The FITTIG REACTION is the AROMATIC ANALOGUE of the Wurtz reaction -- it involves treating an ARYL HALIDE (such as chlorobenzene or bromobenzene) with SODIUM metal in dry ETHER solvent, causing TWO aryl halide molecules to COUPLE together, forming a BIARYL product (such as biphenyl, when starting from two molecules of a simple monohalobenzene), with sodium halide as a by-product -- the key difference from the Wurtz reaction is simply the nature of the starting halide: the WURTZ reaction uses ALKYL halides (giving a higher ALKANE product), while the FITTIG reaction uses ARYL halides (giving a BIARYL/aromatic coupling product) -- a related WURTZ-FITTIG reaction uses a MIXTURE of an alkyl halide and an aryl halide together, producing an alkylbenzene (aryl-alkyl coupled) product.

5. Q5. Bromobenzene on treatment with Mg in dry ether forms phenylmagnesium bromide. This Grignard reagent, unlike alkyl Grignards, is prepared from an aryl halide, illustrating that:

   • A.Aryl halides, despite lower reactivity in nucleophilic substitution, still react with Mg metal directly to form aryl Grignard reagents✓
   • B.Haloarenes cannot form any organometallic compound
   • C.Only alkyl halides react with magnesium
   • D.Phenylmagnesium bromide is unstable and cannot be isolated in solution
   Solution

   Aryl halides react directly with magnesium turnings in dry ether to give aryl Grignard reagents (e.g., C6H5MgBr), even though the aryl C-X bond resists nucleophilic substitution.

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