CH223HW2solutions

CH223HW2solutions - CH 223-001/003 Spring 2012 Homework 2...

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CH 223-001/003 Spring 2012 Homework 2 SOLUTIONS This HW assignment provides an opportunity to practice breaking seemingly complex questions down into smaller questions you already know the answers to. (Even if you don’t know that you know the answers.) Some general tips for approaching problems like these: Read the problem carefully. This is essential for determining exactly what you are being asked to do (and what you are not being asked to do). Assume you are not being asked about reactions that we have not discussed in lecture. Even if you do not recognize the specific components of a reaction, the product of a reaction tells you what must be true. The product tells you which bonds are formed, and thus which atoms and bonds you need to focus on in your answer. Once you identify a bond you need to make, you probably only know a few ways to make it. (Sometimes only one way.) For any reaction you know to form a given bond, once you remember the mechanism for that reaction most of the details will fall into place. It is often helpful to work a problem backwards as well as forwards.
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Problem 1. Read the problem carefully and look at the product (acetominophen). Note that I tell you how to put an acyl group on the nitrogen atom. This means that if you can get to 4-aminophenol, you already have a reaction for converting this to acetominophen. Thus, the question is really, “How do I make 4- aminophenol from benzene?” not “How do I make acetominophen from benzene?” Compare the structures of benzene and the new target, 4-aminophenol. The final product tells you that you need to make a C-O bond and a C-N bond, starting from benzene. Thinking forward from benzene: You only know one way to make a C-O bond on a benzene ring: nucleophilic aromatic substitution on a nitro halobenzene. You only know one way to make a C-N bond on a benzene ring: electrophilic aromatic nitration. Thinking backwards from 4-aminophenol: You only know one way to get to an NH 2 group on a benzene ring: reduction of a nitro group on a benzene ring. While you could, in principle, reduce an NO 2 group to an NH 2 group at any time, you know you need to have an NO 2 group on the ring to put the OH in place. Thus, reduction of NO 2 to NH 2 must come after the OH group is in place. This means that 4-nitrophenol could be a new target structure. (That is, the question is now simplified to, “How do I make 4-nitrophenol from benzene?”) If I had given you phenol as a starting material, you would be down to the last step: nitration of phenol. But I didn’t, and you aren’t. * This brings you back to the only way you know how to make a C-O bond to a benzene ring: nucleophilic aromatic substitution of a nitro halobenzene with an anionic oxygen nucleophile. In this case, HO is the nucleophile of choice. Now all you need to figure out how to turn benzene into a 4-nitro haloaromatic.
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This note was uploaded on 03/03/2012 for the course CHEM 223 taught by Professor S during the Fall '10 term at N.C. State.

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CH223HW2solutions - CH 223-001/003 Spring 2012 Homework 2...

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