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CHM 124 Study Guide - Midterm Guide: Methyl Group, Benzene, Chain Termination

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amaranthtoad188
7 Dec 2017
School
URI
Department
Chemistry
Course
CHM 124
Professor
Shahla Yekta

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Document Summary

This reaction between methane and bromine happens in the presence of ultraviolet light - typically sunlight. One of the hydrogen atoms in the methane has been replaced by a bromine atom, so this is a substitution reaction. However, the reaction doesn"t stop there, and all the hydrogens in the methane can in turn be replaced by bromine atoms. If your syllabus wants you to know about the free radical substitution reaction between methane and chlorine as well as this one, don"t waste time trying to learn both mechanisms. You just need to learn one of them. If you are asked for the other one, all you need to do is to write bromine, say, instead of chlorine. In writing the bromine mechanisms on these pages, that"s exactly what i"ve done! If you read both chlorine and bromine versions, you"ll find them boringly repetitive! The chain is initiated (started) by uv light breaking a bromine molecule into free radicals.

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Related Questions

The rates of many atmospheric reactions are accelerated by the absorption of light by one of the reactants. For example, consider the reaction between methane and chlorine to produce methyl chloride and hydrogen chloride:

Reaction 1: CH4(g) + Cl2(g) CH3Cl(g) + HCl(g)

This reaction is very slow in the absence of light. However, Cl2(g) can absorb light to form Cl atoms:

Reaction 2: Cl2(g) + hv 2 Cl(g)

Once the Cl atoms are generated, they can catalyze the reaction of CH4 and Cl2, according to the following proposed mechanism:

Reaction 3: CH4(g) + Cl(g) CH3(g) + HCl(g)
Reaction 4: CH3(g) + Cl2(g) CH3Cl(g) + Cl(g)

The enthalpy changes and activation energies for these two reactions are tabulated as follows:

(a) By using the bond enthalpy for Cl2 (Table 8.4), determine the longest wavelength of light that is energetic enough to cause reaction 2 to occur. In which portion of the electromagnetic spectrum is this light found? (b) By using the data tabulated here, sketch a quantitative energy profile for the catalyzed reaction represented by reactions 3 and 4. (c) By using bond enthalpies, estimate where the reactants, CH4(g) + Cl2(g), should be placed on your diagram in part (b). Use this result to estimate the value of Ea for the reaction CH4(g) + Cl2(g) CH3(g) + HCl(g) + Cl(g). (d) The species Cl(g) and CH3(g) in reactions 3 and 4 are radicals, that is, atoms or molecules with unpaired electrons. Draw a Lewis structure of CH3, and verify that it is a radical. (e) The sequence of reactions 3 and 4 comprises a radical chain mechanism. Why do you think this is called a “chain reaction”? Propose a reaction that will terminate the chain reaction.