Lecture 16 Wednesday February 18 2015 11 35 AM Properties Nucleophile acid Better nucleophile base Preparation Hydration of alkenes problems with carbocations and their rearrangement Oxymercuration demercuration No carbocation intermediates and therefore no rearrangement For export Page 1 Lecture 16 Wednesday February 18 2015 11 36 AM The addition of water is to the more substituted end of the intermediate Why Water is a weak nucleophile and it needs an incentive from the electrophile Both carbons in the 3 membered ring are partially positive charged The charged is distributed between Hg and the two carbons However the more substituted carbon is more positively charged simply because it can handle the charge better So path b is preferred and we get OH at the more substituted end of the double bond Hydroboration oxidation sequence Substitution of alkyl halides Additions to carbonyls OH to the less substituted end and no rearrangement SN2 with these as long as allylic and benzylic are primary and secondary For export Page 2 Lecture 16 Wednesday February 18 2015 11 38 AM Reductions of carbonyls Sodium borohydride Lithium aluminum hydride Selective reduction of carbonyls in the presence of alkenes Selective reduction of carbonyls in the presence of alkenes Alkenes are not electrophilic and do not react with H For export Page 3 Lecture 16 Wednesday February 18 2015 11 39 AM Metal catalyst and hydrogen either reduce both groups like in the case of platinum catalyst or selectively reduce just the double bond Reduction of carboxylic acids Strong hydride reagents like LiAlH4 but not NaBH4 reduce carboxylic acids and their derivatives to alcohols Addition of organometallic reagents with epoxides Another way to make alcohols is addition of organometallic reagents to epoxides Grignard is a strong electrophile and both carbons of the epoxide 3 membered ring are sufficiently electrophilic The reaction occurs selectively on the less substituted carbon because that one is more sterically accessible Retrosynthesis of the product shows that we can make it two ways path a starting from a carbonyl compound and Grignard and path b starting from epoxide and a Grignard For export Page 4
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