9.P: Radical Reactions (Problems)
( \newcommand{\kernel}{\mathrm{null}\,}\)
P16.1: Plexiglass is a polymer of methyl methacrylate. Show a mechanism for the first two propagation steps of polymerization (use X⋅ 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 C5H11Cl 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.
Contributors
Organic Chemistry With a Biological Emphasis by Tim Soderberg (University of Minnesota, Morris)