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Chapter 6: Problem 125

What product is formed when \(\mathrm{X}_{2}(\mathrm{X}=\mathrm{Br}, \mathrm{Cl})\) is added to alkenes in the dark? Explain the mechanism of the reaction.

Short Answer

Expert verified
When diatomic halogens, such as Bromine (Br) or Chlorine (Cl), are added to alkenes in the dark, they react through an electrophilic addition mechanism to form a vicinal dihalide. The mechanism involves two key steps: 1) Formation of a cyclic halonium ion, and 2) Attack by the second halogen atom, leading to the formation of the vicinal dihalide product.

Step by step solution

01

Understand the given information

We are given that a diatomic halogen molecule, either Bromine (Br) or Chlorine (Cl), is added to alkenes in the dark.
02

Determine the general product of the reaction

When diatomic halogens (Br2 or Cl2) are added to alkenes in the dark, they react through an electrophilic addition mechanism, resulting in the formation of a vicinal dihalide. For example, when ethene reacts with Br2 in the dark, they form the vicinal dihalide 1,2-dibromoethane.
03

Understanding the general reaction mechanism

The reaction mechanism for the electrophilic addition of halogens (X2, where X=Br, Cl) can be broken down into two key steps: 1. Formation of a cyclic halonium ion. 2. Attack by the second halogen atom and formation of vicinal dihalide.
04

Elaborate on the formation of the cyclic halonium ion

In the first step, the pi electrons in the double bond of the alkene interact with the halogen molecule, forming a bond with one of the halogen atoms and creating a cyclic halonium ion in the process. The electron cloud in the remaining halogen atom becomes negatively charged, resulting in a halide ion (X-).
05

Explain the attack by the second halogen atom and the formation of the vicinal dihalide

In the second step, the negatively charged halide ion (X-) attacks the positively charged cyclic halonium ion at the more substituted carbon (if applicable, depends on the alkene used). This bond formation leads to the opening of the cyclic halonium ion and results in the formation of a vicinal dihalide as the final product. Overall, when a diatomic halogen molecule (Br2 or Cl2) is added to alkenes in the dark, a vicinal dihalide is formed through an electrophilic addition mechanism involving the formation of a cyclic halonium ion and subsequent attack by a halide ion.

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Most popular questions from this chapter

When isopropyl bromide is treated with sodium ethoxide in ethanol, propylene and ethyl isopropyl ether are formed in a \(3: 1\) ratio. If the hexadeuteroisopropy1 bromide, \(\mathrm{CD}_{3} \mathrm{CHBrCD}_{3}\) is used, \(\mathrm{CD}_{3} \mathrm{CH}=\mathrm{CD}_{2}\) and \(\left(\mathrm{CD}_{3}\right)_{2} \mathrm{CHOC}_{2} \mathrm{H}_{5}\) are formed in a ratio of \(1: 2\). Explain.

In the presence of a trace of peroxide or under the influence of ultraviolet light, 1-octene reacts: (a) with \(\mathrm{CHCI}_{3}\) to form \(1,1,1\) -trichlorononane; (b) with \(\mathrm{CHBr}_{3}\) to form \(1,1,3\) -tribromononane; (c) with \(\mathrm{CBrCl}_{3}\) to form \(1,1,1\) -trichloro-3-bromononane; (d) with \(\mathrm{H}-\mathrm{S}-\mathrm{CH}_{2} \mathrm{COOH}\) (thioglycolic acid) to yield \(\mathrm{n}-\mathrm{C}_{8} \mathrm{H}_{17}-\mathrm{S}-\mathrm{CH}_{2} \mathrm{COOH}\)

The dipole moment of 1,2 -dichloroethane is \(1.12 \mathrm{D}\) at room temperature. Show how one could use this dipole moment to calculate the proportions of the possible conformations that are expected to be present.

Write the chemical structures for each compound listed. (a) 1 -Hexene (b) 3 -Methyl-1-butane (c) 2, 4-Hexadiene (d) 1-Iodo-2-methy1-2-pentene (e) 2-Chloro-3-methy1-2-hexane (f) 6,6 -Dibromo-5-methy1-5-ethy1-2,3-heptadiene

In the dark at room temperature, a solution of chlorine in tetrachloroethylene can be kept for long periods with no sign of reaction. When irradiated with ultraviolet light, however, the clorine is rapidly consumed, with the formation of hexachloroethane; many molecules of product are formed for each photon of light absorbed, and the reaction; is slowed down markedly when oxygen is bubbled through the solution. (a) How do you account for the absence of reaction in the dark? (b) Outline all steps in the most likely mechanism for the photochemical reaction. Show how it accounts for the facts, including the effect of oxygen.
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