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Chapter 4: Q 52P (page 236)

When dichloromethane is treated with strong NaOH , an intermediate is generated that reacts like a carbene. Draw the structure of this reactive intermediate, and propose a mechanism for its formation.

Short Answer

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mechanism of the reaction

structure of the reactive intermediate (carbene)

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01

Carbenes

Carbenes may be defined as a neutral divalent chemical species containing a carbon atom consisting six electrons in their valence shell. Out of six electrons, two electrons are unshared. The simplest carbene is methylene CH2 .

02

Mechanism for the formation of reactive intermediate

Dichloromethane when treated with strong sodium hydroxide solution yields an intermediate that reacts more like a carbene.

Deprotonation of dichloromethane with sodium hydroxide takes place in the first step. The second step is the chlorine anion leaving the molecule to give the intermediate.

mechanism of the reaction

structure of the reactive intermediate (carbene)

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Most popular questions from this chapter

Peroxides are often added to free-radical reactions as initiators because the oxygen-oxygen bond cleaves homolytically rather easily. For example, the bond-dissociation enthalpy of O-Obond in hydrogen peroxide (H-O-O-H)is only 213 kJ/mol (51 kcal/mol). Give a mechanism for the hydrogen peroxide- initiated reaction of cyclopentane with chlorine. The BDE for HO-Clis 210 kJ/mol (50kcal/mol).

When healthy, Earth’s stratosphere contains a low concentration of ozone (O3)that absorbs potentially harmful ultraviolet (UV) radiation by the cycle shown at right.

Chlorofluorocarbon refrigerants, such as Freon 12 (CF2Cl2), are stable in the lower atmosphere, but in the stratosphere, they absorb high-energy UV radiation to generate chlorine radicals.

The presence of a small number of chlorine radicals appears to lower ozone concentrations dramatically. The following reactions are all known to be exothermic (except the one requiring light) and to have high-rate constants. Propose two mechanisms to explain how a small number of chlorine radicals can destroy large numbers of ozone molecules. Which of the two mechanisms is more likely when the concentration of chlorine atoms is very small?

The chlorination of pentane gives a mixture of three monochlorinated products.

(a) Draw their structures.

(b) Predict the ratios in which these monochlorination products will be formed, remembering that a chlorine atom abstracts a secondary hydrogen about 4.5 times as fast as it abstracts a primary hydrogen.

Iodination of alkanes using iodine (I2)is usually an unfavorable reaction. (See problem 4-17, for example). Tetraiodomethane (Cl4) can be used as the iodine source for iodination in the presence of a free-radical initiator such as hydrogen peroxide. Propose a mechanism (involving mildly exothermic propagation steps) for the following proposed reaction. Calculate the value of ΔHfor each of the steps in your proposed mechanism.

The following bond-dissociation energies may be helpful:

When exactly 1 mole of methane is mixed with exactly 1 mole of chlorine and light is shone on the mixture, a chlorination reaction occurs. The products are found to contain substantial amounts of di-, tri-, and tetrachloromethane, as well as unreacted methane.

(a) Explain how a mixture is formed from this stoichiometric mixture of reactants, and propose mechanisms for the formation of these compounds from chloromethane.

(b) How would you run this reaction to get a good conversion of methane toCH3Cl? Of methane to CCl4?
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