Carbonyl bonding mechanism

Mechanisms of aldehyde oxidation are not firmly established, but there seem to be at least two main types—a free-radical mechanism and an ionic one. In the free-radical process, the aldehydic hydrogen is abstracted to leave an acyl radical, which obtains OH from the oxidizing agent. In the ionic process, the first step is addition of a species OZ to the carbonyl bond to give 16 in alkaline solution and 17 in acid or neutral solution. The aldehydic hydrogen of 16 or 17 is then lost as a proton to a base, while Z leaves with its electron pair. 

Olefination Reactions Involving Phosphonium Ylides. The synthetic potential of phosphonium ylides was developed initially by G. Wittig and his associates at the University of Heidelberg. The reaction of a phosphonium ylide with an aldehyde or ketone introduces a carbon-carbon double bond in place of the carbonyl bond. The mechanism originally proposed involves an addition of the nucleophilic ylide carbon to the carbonyl group to form a dipolar intermediate (a betaine), followed by elimination of a phosphine oxide. The elimination is presumed to occur after formation of a four-membered oxaphosphetane intermediate. An alternative mechanism proposes direct formation of the oxaphosphetane by a cycloaddition reaction.236 There have been several computational studies that find the oxaphosphetane structure to be an intermediate.237 Oxaphosphetane intermediates have been observed by NMR studies at low temperature.238 Betaine intermediates have been observed only under special conditions that retard the cyclization and elimination steps.239... 

Although the reaction of ketones and other carbonyl compounds with electrophiles such as bromine leads to substitution rather than addition, the mechanism of the reaction is closely related to electrophilic additions to alkenes. An enol, enolate, or enolate equivalent derived from the carbonyl compound is the nucleophile, and the electrophilic attack by the halogen is analogous to that on alkenes. The reaction is completed by restoration of the carbonyl bond, rather than by addition of a nucleophile. The acid- and base-catalyzed halogenation of ketones, which is discussed briefly in Section 6.4 of Part A, provide the most-studied examples of the reaction from a mechanistic perspective. 

It seems that phosphines add directly to the carbonyl bond, but the presence of chains not terminated by phosphorus indicates that there is a second initiation mechanism and/ or termination-transfer processes. 

Using a simpler approach which elaborated upon earlier proposals 42), Cotton and Kraihanzel 39, 41, 95) assumed that the carbonyl oscillators could be factored out both electronically and mechanically from all other molecular vibrations, and that only electronic coupling among the individual carbonyl bonds need be considered. In addition, they introduced relationships, similar in origin to those proposed by Jones 80, 82), in order to reduce the number of carbonyl bond stretching force constants. Because of its simplicity and qualitative success, the Cotton-Kraihanzel method (CKM) has been applied widely, but it has also been the subject of some controversy 22, 24, 40, 43-45, 78, 79, 83). What is beyond dispute, however, is that its use has brought to the attention of the chemist the variations in, and possible... 

When acid catalysts are employed, in the absence of nickel carbonyl, the mechanism involves initial attack by a proton, followed by attack of the resulting carbocation on carbon monoxide to give an acyl cation, which, with water, gives the product, 159. Markovnikov s rule is followed, and carbon skeleton rearrangements and double-bond isomerizations (prior to attack by CO) are frequent. 

These arguments point to a mechanism in which the rate-determining step is abstraction of the first a hydrogen to form a Ti-allyl surface species. The next step is the reversible formation of a a bond between a catalyst oxygen atom and either of the terminal carbon allyl atoms. The second hydrogen abstraction, which is facilitated by the presence of the C-O bond, follows with formation of the carbonyl bond and desorption of the product acrolein. The next question to be addressed is the source of oxygen, which appears in the product acrolein. 

---