Chemistry 22 Homework Set 5

Chem 22 Homework set 5 End-of-chapter problems from Hornback: Ch 17: 31-38, 39e-l, 40-43, 44 (though some of these are a bit extreme), 49-51, 53, 55, 56, 57a,c-f (also a bit extreme, but not impossible), 61, 63-65, 68. 1. When learning to devise synthetic schemes, students are often tempted to use complex reagents with functional groups that interfere with the chemistry that one is trying to accomplish. Although such problems are sometimes difficult to recognize, a little mechanistic reasoning will often alert you to the potential for trouble. An example relevant to EAS chemistry is the preparation of 1-phenylethanol from benzene. This cannot be made by the pseudo-Friedel-Crafts-like chemistry shown below. OH OH Cl AlCl3 (a) If you tried to do this, what do you think might go wrong? (Hint: the cation that would be formed does something besides EAS and does it very quickly. Think about what else carbocations do.) (b) Actually, the AlCl3 isn't necessary for the 1-chloroethanol to self-destruct. In general, 1haloalcohols are unstable and spit out HX spontaneously. Write a reasonable mechanism for that reaction. (c) Suggest an alternative way to make 1-phenylethanol from benzene. There are many ways to do this, although you have only learned one at this point. (Hint: you can functionalize a benzylic position by using free radical chemistry.) (d) But the best way to do it requires just a wee bit of new chemistry. It’s easy to reduce ketones and aldehydes to alcohols with nucleophilic hydride reagents — remember those? LiAlH4 and NaBH4 are the most common. They add a hydride to the carbonyl C, displacing the π-electrons onto O, which then gets protonated. Propose a route to the alcohol above that uses this hydride reduction chemistry. 2. (a) Propose a mechanism for desulfonation of p-ethoxybenzenesulfonic acid. Show each step clearly using curved arrows, and include all important resonance structures. (b) As mentioned in the text (p 699), electrophiles tend to add to the 1-position of naphthalene rather than the 2-position. Explain by drawing the two cationic intermediates. Propose an explanation more sophisticated than counting resonance structures (think quality, not quantity.) 3. Predict the products of mononitration (with HNO3/H2SO4) of each compound below. Be sure to pay attention to activating/deactivating effects as well as directing effects. (a) biphenyl (Ph–Ph) (b) (c) (d) O O2N N H EtO 4. Phenoxide ion (the conjugate base of phenol) undergoes rapid EAS reactions. For example, it is so strongly activated that it can be brominated without a catalyst, and the reaction cannot be stopped after addition of just one bromine. Write a mechanism for addition of just one bromine, at the para position, and explain why the reaction is so fast. 5. Suggest a synthesis of each of the compounds below starting with benzene. You may use any other compounds or reagents you need. (a) Hint: why won’t FC alkylation won’t work here? Cl (b) Cl (c) p-tert-butylbenzoic acid Chem 22 Spring 2010 Name _________________________________________ HW set 5 30 points; due Wed, Feb 24 at the beginning of class. 1. (a) Electrophiles can add to furan to produce either of the products below. Write an electrophilic aromatic substitution mechanism for formation of each product, including all relevant resonance structures of the cation intermediate, and decide which should be the major product. O E+ O E O or ? E 2. Suggest a mechanism that explains the following reaction. As always, show each step clearly with curved arrows. Please draw all resonance structures of the arenium ion intermediate. OH H2SO4 3. Draw the products of mononitration (with HNO3/H2SO4) of each compound below. (a) O O (b) O (c) O O 4. Suggest two different ways to make styrene (Ph–CH=CH2) from benzene. Addition of the vinyl group by Friedel-Crafts chemistry is not one of them.