6.E: Radical Reactions (Exercises)

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P16.1: Plexiglass is a polymer of methyl methacrylate. Show a mechanism for the first two propagation steps of polymerization (use \(X\cdot\) to denote the radical initiator), and show a structure for the plexiglass polymer. Assume an alkene addition process similar to that shown in the text for polyethylene.

P16.2: In section 16.3 we saw how acrylamide polymerizes to form the polyacrylamide used in PAGE protein gels. Polyacrylamide by itself is not sufficient by itself to form the gel - the long polyacrylamide chains simply slip against each other, like boiled spaghetti. To make a PAGE gel, with pores for the proteins to slip through, we need a crosslinker - something to tie the chains together, forming a three-dimensional web-like structure. Usually, a small amount of bis-acrylamide is added to the acrylamide in the polymerization mixture for this purpose.

Propose a radical mechanism showing how bis- acrylamide might form crosslinks between two polyacrylamide chains.

P16.3: Resveratrol is a natural antioxidant found in red wine (see section 16.5 for the structure).

Draw one resonance structure to illustrate how the resveratrol radical is delocalized by resonance.
Indicate all of the carbons on your structure to which the radical can be delocalized.
Draw an alternate resveratrol radical (one in which a hydrogen atom from one of the other two phenolic groups has been abstracted). To how many carbons can this radical be delocalized?
The curcumin structure is shown in the same figure as that of resveratrol, in section16.5. Draw two resonance contributors of a curcumin radical, one in which the unpaired electron is on a phenolic oxygen, and one in which the unpaired electron is on a ketone oxygen.

P16.4: Draw the radical intermediate species that you would expect to form when each of the compounds below reacts with a radical initiator.

P16.5: Azobis(isobutyronitrile) is a widely used radical initiator which rapidly undergoes homolytic decomposition when heated.

Predict the products of this decomposition reaction, and show a likely mechanism. What is the thermodynamic driving force for homolytic cleavage?

P16.6:

When 2-methylbutane is subjected to chlorine gas and heat, a number of isomeric chloroalkanes with the formula \(C_5H_{11}Cl\) can form. Draw structures for these isomers, and for each draw the alkyl radical intermediate that led to its formation.
In part a), which is the most stable radical intermediate?
In the reaction in part a), the relative abundance of different isomers in the product is not exclusively a reflection of the relative stability of radical intermediates. Explain.

P16.7: We learned in chapter 14 that \(HBr\) will react with alkenes in electrophilic addition reactions with 'Markovnikov' regioselectivity. However, when the starting alkene contains even a small amount of contaminating peroxide (which happens when it is allowed to come into contact with air), a significant amount of 'anti-Markovnikov' product is often observed.

Propose a mechanism for formation of the anti-Markovnikov addition product when 1-butene reacts with \(HBr\) containing a small amount of benzoyl peroxide
Predict the product and propose a mechanism for the addition of ethanethiol to 1-butene in the presence of peroxide.

P16.8: In section 11.5 we learned that aspirin works by blocking the action of an enzyme that catalyzes a key step in the biosynthesis of prostaglandins, a class of biochemical signaling molecules. The enzyme in question, prostaglandin \(H\) synthase (EC 1.14.99.1) catalyzes the reaction via several single-electron steps. First, an iron-bound oxygen radical in the enzyme abstracts a hydrogen atom from arachidonate. The arachidonate radical intermediate then reacts with molecular oxygen to form a five-membered oxygen-containing ring, followed by closure of a cyclopentane ring to yield yet another radical intermediate. (Biochemistry 2002, 41, 15451.)

Propose a mechanism for the steps of the reaction that are shown in this figure.

P16.9: Some redox enzymes use copper to assist in electron transfer steps. One important example is dopamine b-monooxygenase (EC 1.14.1.1), which catalyzes the following reaction:

The following intermediates have been proposed: (see Biochemistry 1994, 33, 226) Silverman p. 222

Draw mechanistic arrows for steps 1-4.