Chapter 5 Mechanism Descriptions

Chemistry 261 with Grutzner at Purdue University

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By: Alex Swithers
Textbook:

Study Guide and Solutions Manual to Accompany Organic Chemistry
Created: 2009-11-23
Size: 12 flashcards
Views: 23

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Acid Catalyzed Hydration of Alkenes

Markovnikov?s rule is followed in hydration of alkenes. Therefore, in the alcohol product, the hydroxyl group is located upon the more highly substituted carbon. Watch for rearrangement of the carbocation intermediate, if methyl or hydride shift is probable. Note that this reaction is the reverse of acid catalyzed dehydration of alcohols.

Electrophilic Addition of HX to Alkenes

Because tertiary and secondary carbocations are more stable than primary carbocations, Markovnikov addition is observed in the electrophilic addition of HX to alkenes, so the product formed is the one with the halogen substituent upon the more highly substituted carbon. Also, rearrangement (hydride or methyl shift to form a more stable carbocation) might occur, typical of reactions that have a carbocation intermediate. Remember that electrophilic addition will not be observed in the presence of peroxides. Peroxides initiate anti-Markovnikov addition via free-radical addition. An interesting fact about electrophilic addition of HX to alkenes, is that the more acidic the hydrogen halide, the more electrophilic it will be. HF, for example, only a weak acid, does not react.

Electrophilic Addition of Halogens to Alkenes

In analysis of the addition of halogen to an alkene, the anti stereospecificity of the dihalide product serves as evidence that the mechanism occurs via a cyclic halonium ion intermediate. For problem solving, this anti stereospecificity is especially pertinent in the cases of addition to cyclic alkenes or where the product carbons are chiral.

Halohydrin Formation

In aqueous solution, electrophilic addition of halogen results in the formation of halohydrin. Water performs the ring opening instead of halide ion, which opens the ring in non aqueous halogenation. Water addition is preferential for the more highly substituted carbon, which receives a bit more of the distributed positive charge in the halonium ion than the other carbon.

Free Radical Addition of HX to Alkenes

In the presence of a peroxide initiator, hydrogen halide adds to alkene via an anti-Markovnikov, free-radical mechanism. The carbanion radical product of the first propagation step will be more stable if the carbon with the lone electron, the radical carbon, is highly substituted. For this reason, the halogen atom binds to the less substituted carbon, in other words, anti-Markovnikov addition.

Catalytic Hydrogenation of Alkenes

Hydrogenation of alkenes occurs in the presence of a metal catalyst, a syn addition process. The two hydrogen atoms add to the same face of the double bond. Furthermore, if one side is more hindered than the other, addition is stereoselective for the less hindered side. Hydrogenation is exothermic, and relative heats of hydrogenation can be used to infer the relative stability of double bonds in different contexts. The more stable the double bond, the lower the heat of hydrogenation will be.

Oxidation of Alkenes to Vicinal Diols

Both of the above oxidation mechanisms proceed by syn addition. To accomplish anti hydroxylation of alkenes, the method to employ is hydrolysis of epoxides. As shown above, in basic conditions, oxidation of alkene with potassium permanganate results in formation of vicinal diol, but with acidic or neutral conditions, complete cleavage occurs to produce carboxylic acids or ketones (or carbon dioxide).

Oxidative Cleavage of Alkenes

In basic conditions, treatment of alkene with potassium permanganate forms a vicinal diol. Oxidative cleavage by permanganate in neutral or acidic conditions, however, leads to cleavage to form carbonyl compounds by means of the same cyclic manganate ester intermediate. Where ozonolysis, another method of oxidative cleavage of alkenes, produces an aldehyde or formaldehyde, cleavage with permanganate produces a carboxylic acid or carbon dioxide respectively.

Ozonolysis of Alkenes

If aldehyde product is desired over carboxylic acid, ozonolysis is preferred in cleavage of alkenes over potassium permanganate cleavage. Also, with terminal alkenes, the end carbon leads to formaldehyde molecule. Cleavage of such alkenes by potassium permanganate forms carbon dioxide.

Allylic Halogenation

At high temperatures chlorine and bromine react with alkenes larger than ethylene by free-radical substitution. (At low temperatures, electrophilic addition to produce vicinal dihalides occurs). Halogenation is selective for two particular carbons due to the resonance stabilization that can occur with an allylic radical intermediate.

Oxymercuration-Demercuration

Oxymercuration-demercuration is usually preferred over acidic hydration to form Markovnikov alcohols from alkenes. The reaction begins with electrophilic approach onto the alkene by mercuric acetate to form a mercuric acetate carbocation derivative. The mercuric acetate moeity stabilizes the carbocation, so oxymercuration-demercuration is not nearly as susceptible to rearrangement as ordinary acid-catalized hydration. Following addition of mercuric acetate, hydration then occurs followed by a demercuration step with sodium borohydride.

Hydroboration of Alkenes

Hydroboration-oxidation of an alkene forms an alcohol by means of a mechanism leading to anti-Markovnikov regioselectivity. Furthermore, the geometry of the activated complex produces a syn addition product. Note as a point of general interest that borane derives its electrophilicity from a vacant p orbital.

About this deck

By: Alex Swithers
Textbook:

Organic Chemistry

Study Guide and Solutions Manual to Accompany Organic Chemistry
Created: 2009-11-23
Size: 12 flashcards
Views: 23

About StudyBlue

“I have been getting MUCH better grades on all my tests for school. Flash cards, notes, and quizzes are great on here. Thanks!”
Kathy