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188 Chapter 9 FURTHER REACTIONS OF ALCOHOLS AND THE CHEMISTRY OF ETHERS (c) Everything is fine except for the leaving group. As the conjugate base of a rather weak acid, it will be a moderately strong base and therefore not a very good leaving group. However, just as proto- nation of oxygen in alcohols leads to a better leaving group (water), protonation of N5 by acid in before nucleophilic displacement should lead to a better leaving group (FH₄ itself) for the reaction in this problem: H N + RSH + N H CH₃ H N CH₃ -H+ + methionine N H 71. (a) Reaction is an process. The S of methionine displaces triphosphate from the CH₂ group in ATP. Reaction 2 is an process. The N of norepinephrine displaces S-adenosyl homocysteine from a CH₃ making the key bond in adrenaline. The ATP makes the second reaction possible by turning everything that's attached to the CH₂ of methionine into one great big leaving group. In other words, S-adenosyl methionine is a fancy biological equivalent of (b) No. SN2 reaction on the CH₃ doesn't occur because the leaving group (essentially RS⁻) is not a good one. (c) Simple! React it with one equivalent of (Actually, it's not quite simple-care is required to prevent extensive reaction of the adrenaline which is still nucleophilic, with additional CH₃I.) 72. (a) To make an oxacyclopropane from 2-bromocyclohexanol, the molecule must be able to adapt a geometry that allows the necessary "internal SN2" backside displacement to occur. This geometry requires that the alkoxide and Br be anti to each other, which is possible only from the trans isomer of the starting material. H :0: Flip Can react Br H Br Reactive conformation (trans, diaxial) o- Compare this cis with reactive trans; no good. Br