Reactions of Ketones and Aldehydes Nucleophilic Addition

A nucleophilic addition reaction to an aldehyde or ketone. The nucleophile approaches the carbonyl group from an angle of approximately 75° to the plane ofthesp orbitals, the carbonyl carbon rehybridizes from sp to sp, and an alkoxide ion is formed. Protonation by addition of acid then gives an alcohol. 

I An electron pair from the nucleophile adds to the electrophilic carbon of the carbonyl group, pushing an electron pair from the C=0 bond onto oxygen and giving an alkoxide ion intermediate. The carbonyl carbon rehybridizes from sp to sp.  

I Protonation of the alkoxide anion intermediate gives the neutral alcohol addition product. 

The nucleophile can be either negatively charged ( Nu ) or neutral ( Nu). If it s neutral, however, it usually carries a hydrogen atom that can subsequently be eliminated, Nu-H. For example  

One further comparison aromatic aldehydes, such as benzaldehyde, are less reactive in nucleophilic addition reactions than aliphatic aldehydes because the electron-donating resonance effect of the aromatic ring makes the carbonyl group less electrophilic. Comparing electrostatic potential maps of formaldehyde and benzaldehyde, for example, shows that the carbonyl carbon atom is less positive (less blue) in the aromatic aldehyde. 

Electronically, aldehydes are more reactive than ketones because of the greater polarization of aldehyde carbonyl groups. To see this polarity difference, recall the stability order of carbocations (Section 6.9). A primary carbocation is higher in energ and thus more reactive than a secondary carbocation because 

Problem 19.6 p-Nltrobenzaldehyde is more reactive toward nucleophilic additions than p-raethoxy- 

Aldehyde oxidations occur through intermediate 1,1-diols, or hydrates, which are formed by a reversible nucleophilic addition of water to the carbonyl group. Even though formed to only a small extent at equilibrium, the hydrate reacts like any typical primary or secondary alcohol and is rapidly oxidized to a carbonyl compound. 

CHAPTER 14 ALDEHYDES AND ketones NUCLEOPHILIC ADDITION REACTIONS 

CHAPTER 19 B Aldehydes and Ketones Nucleophilic Addition Reactions 

Two general reaction pathways following addition of a nucleophile to a ketone or aldehyde. The top pathway leads to an alcohol product the bottom pathway leads to a product with a C=Nu double bond. 

In the remainder of this chapter, well look at specific examples nucleophilic addition reactions. In so doing, well be concerned both wl the reversibility of a given reaction and with the acid or base catalysis of that reaction. Some nucleophilic addition reactions take place reversibl) and some do not. Some occur without catalysis, but many others require acid or base to proceed. 

Treatment of a ketone or aldehyde with cyanide ion ( CsN), followed by protonation of the tetrahedral alkoxide ion intermediate, gives a cyanohydrin. Show structure of tlie cyaiiohydriu obtained from acetone. 

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Nucleophilic Addition Reactions of Aldehydes and Ketones (Chapter 19)... 

Nucleophilic addition reactions of aldehydes and ketones (a) Addition of hydride alcohols (Section 19.7)... 

A summary of nucleophilic addition reactions of aldehydes and ketones. 

The nucleophilic addition reactions of aldehydes and ketones occurring at the carbonyl carbon atom that we have studied so far are summarized below. In Chapters 18 and 19 we shall see other examples. 

The mechanism of a nucleophilic addition reaction of aldehydes and ketones under both basic and acidic conditions, (a) Under basic conditions, a negatively charged nucleophile adds to the carbonyl group to give an alkoxide ion intermediate, which is subsequently protonated. (b) Under acidic conditions, protonation of the carbonyl group occurs first, followed by addition of a neutral nucleophile and subsequent deprotonation. 

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