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Student%20Notes%20-%20Section%2017 - Section 17: Reaction...

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Section 17: Reaction at the α -Position to a Carbonyl Group 255 Section 17: Reaction at α -Position to Carbonyl Group 17.1 Introduction 17.1.1 Tautomerization A carbonyl compound with a hydrogen atom attached to a carbon atom - to the carbonyl group may exist in one of two tautomeric forms: the keto- tautomer, or the enol form. ± R H R' R" O R R" R' OH keto- for me n o l f o rm These two structures are rapidly interconvertible (a defining quality of tautomers), but in the majority of cases, the keto- tautomer is the dominant component of the equilibrium for ketones, and even more dominant for the carboxylic acid derivatives. OO H : 1 H 1x10 -4 : 1 1x10 -7 Note that tautomeric forms differ in the placement of their atoms: they are not resonance contributors, but distinct equilibrium species. Keto-enol tautomerization may be catalyzed by both acids and bases.
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Section 17: Reaction at the α -Position to a Carbonyl Group 256 17.1.2 Reaction of Enols: -Halogenation. Aldehydes and ketones may be brominated at the -position simply by treatment of the carbonyl compound with X 2 in the presence of an acid catalyst. Cl 2 and Br 2 are commonly used with acetic acid as catalyst. O Cl 2 HCl/H 2 O O Cl O Br 2 CH 3 COOH O Br The mechanism of the reaction proceeds via an enol intermediate (the formation of which is the rate determining step), and involves attack of an electrophile (recall from halogenation of alkenes, that X 2 is sufficiently polarizable that it may react as an electrophile). The electrophile may also be D + from D 3 O + .
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Section 17: Reaction at the α -Position to a Carbonyl Group 257 As indicated earlier, bromination of esters and amides does not occur under these conditions because these compounds enolize to a much lesser extent that aldehydes and ketones. However, acyl halides may be brominated under some conditions: carboxylic acids may therefore be a-brominated using the Hell-Volhard-Zelinskii reaction: HO O 1. Br 2 /PBr 3 2. H 2 O H R R' HO O Br R R' Note that this reaction may only proceed via the enol of the acyl bromide: reaction of carboxylic acids directly with bromine does not lead to -bromination. 17.1.3 Acidity of Carbonyl Compounds at the -Site A comparison of the acidity of acetone with ethane indicates that the presence of a carbonyl group (relative to a methyl group) makes the hydrogen atoms at the a-carbon considerably more acidic: ethane has a pK a around 60, whereas acetone has a pK a of about 19.3. Carbonyl compounds can be relatively easily deprotonated to yield enolate anions. These anions are resonance stabilized, but unlike examples we have seen before; there is a clearly dominant resonance contributor.
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Section 17: Reaction at the α -Position to a Carbonyl Group 258 Charge density calculations show that although there is a significant amount of negative charge density at the -carbon, the bulk of the charge density resides near the oxygen atom – this is not unexpected.
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This note was uploaded on 10/28/2010 for the course CHEM 235 taught by Professor Dr.poole during the Spring '10 term at Ball State.

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Student%20Notes%20-%20Section%2017 - Section 17: Reaction...

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