Acidity alkenes

Two reactions of the non-aromatic 4,4-disubstituted pyrazolones are worthy of mention. Carpino discovered that 4,4-dihalogenopyrazolones (365) and 4-substituted 4-halogenopyrazolones (366) when treated with bases yield a, 8-alkynic and -alkenic acids, respectively (66JOC2867). The reaction proceeds through an oxopyrazolenine (2,3-diazacyc-lopentadienone (367) (B-74M140408). A modification of the experimental procedure transforms (365) into bimanes (368) (82JOC214), which are formed from (367 R = X),... 

Addition of alcohols to alkenes acid catalysed. Preparation of ethers... 

Another approach to alkene isomerization would be to use a catalyst. Base catalysis is of no use as there are no acidic protons in the alkene. Acid catalysis can work (Chapter 19) if a carbocation is formed by protonation of the alkene. 

Unsaturated acyliutn ions generated from alkenic acids or anhydrides react with alkenes to produce cy-clopentenones (equation 4i).88.i78-i8i with cycloheptene the major products arise from ring contraction. Again, it is unclear whether these reactions proceed via direct cyclization of (76) or a Nazarov cycliza-tion. 

However, the protonolysis of alkenylmercurials derived from the solvomercuration of alkynes is known as a method for preparing stereoisomerically pure alkenes. Acidic deuterolysis provides also a convenient route to labelled organic substrates. For instance, in a study on the regio- and stereo-selectivity of the acetoxymercuration of alkynes, protodemercuration with acetic acid of a 3 1 mixture of the vi-... 

Unsaturated acylium ions generated from alkenic acids or anhydrides react with alkenes to produce cy-clopentenones (equation with cycloheptene the major products arise from ring contraction. 

Nazarov and Related Cationic Cyclizations 63.7A Alkenic Acid Chlorides and Bromides... 

We can gain a general understanding of the mechanism of hydrogen halide addition to alkenes by extending some of the principles of reaction mechanisms introduced earlier. In Section 5.12 we pointed out that carbocations are the conjugate acids of alkenes. Acid-base reactions are reversible processes. An alkene, therefore, can accept a proton from a hydrogen halide to form a carbocation. 

Hopanoids (alkenes, acids, ketones and alcohols) are ubiquitous components of sediments and soils, and are more widespread than higher plant derived triterpenoids. Polyhydroxy-bacteriohopanes are major precursors, yielding ketones and carboxylic acids during early diagenesis. The latter are the chief products, with... 

Alkanes Alkenes Acids and esters Alcohols and ketones O Alkyl halides ... 

Octyne and 2-octyne also give more than 95% anti addition under these conditions. The reactions are formulated as concerted Ad S processes, involving bromide attack on an alkene-acid complex. 

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