Nucleophilic Addition to the Carbon-Oxygen Double Bond

Aldehydes and ketones are especially susceptible to nucleophilic addition because of the structural features that we discussed in Section 12.1 and which are shown below. 

Nucleophihc addition to the carbon-oxygen double bond occurs, therefore, in either of 

In this type of addition the nucleophile uses its electron pair to form a bond to the carbonyl carbon atom. As this happens the electron pair of the carbon-oxygen it bond shifts out to the electronegative carbonyl oxygen atom and the hybridization state of both the carbon and the oxygen changes from sp to sp. The important aspect of this step is the ability of the carbonyl oxygen atom to accommodate the electron pair of the carbon-oxygen double bond. 

In this step the nucleophile forms a bond to the carbon by donating an electron pair to the top or bottom face of the carbonyl group [path (a) or (b)]. An electron pair shifts out to the oxygen. 

In the second step the alkoxide oxygen, because it is strongly basic, removes a proton from H—Nu or some other acid. 

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The main reactions of the carbonyl group are nucleophilic additions to the carbon-oxygen double bond. As shown below, this addition consists of adding a nucleophile and a hydrogen across the carbon-oxygen double bond. 

NUCLEOPHILIC ADDITION TO THE CARBON-OXYGEN DOUBLE BOND... 

A Reversibility of Nucleophilic Additions to the Carbon-Oxygen Double Bond... 

The reaction of an aldehyde or ketone with a Grignard reagent (Section 12.8) is a nucleophilic addition to the carbon—oxygen double bond, (a) What is the nucleophile (b) The magnesium portion of the Grignard reagent plays an important part in this reaction. What is its function (c) What product is formed initially (d) What product forms when water is added ... 

FIGURE 18 7 Nucleophilic addition to a p unsaturated aldehydes and ketones may take place either in a 1 2 or 1 4 manner Direct addition (1 2) occurs faster than conjugate addition (1 4) but gives a less stable product The product of 1 4 addition retains the carbon-oxygen double bond which is in general stronger than a carbon-carbon double bond... 

What is the mechanism of the ionic addition of these milder reagents to the carbon-oxygen double bond Two pathways can be formulated nucleophilic addition-protonation and electrophilic protonation-addition. The first, which begins with nucleophilic attack, takes place under neutral or, more commonly, basic conditions. As the (frequently anionic) nucleophile approaches the electrophilic carbon, the carbon rehybridizes and the electron pair of the tt bond moves over to the oxygen, thereby producing an alkoxide ion. Subsequent protonation, usually from a protic solvent such as water or alcohol, yields the final addition product... 

The most common reaction of aldehydes and ketones is the nucleophilic addition reaction, in which a nucleophile, Nu , adds to the electrophilic carbon of the carbonyl group. Since the nucleophile uses an electron pair to form a new bond to carbon, two electrons from the carbon-oxygen double bond must move toward the electronegative oxygen atom to give an alkoxide anion. The carbonyl carbon rehybridizes from sp2 to sp3 during the reaction, and the alkoxide ion product therefore has tetrahedral geometry. 

The description of the carbon-oxygen double bond is analogous, but in addition to the cr-bonds there are unshared pairs of electrons on oxygen so that two excited states are possible, n-n and n-n. For n-n excitation the resultant half-vacant orbital on oxygen should possess electrophilic reactivity, and the electron rich -system should have nucleophilic characteristics. 62>... 

As we saw in Part III of A Preview of Carbonyl Compounds, the most gejb eral reaction of aldehydes and ketones is the nucleophilic addition reaction. A nucleophile, Nu , attacks the electrophilic C=0 carbon atom from a direction approximately 45 to the plane of the carbonyl group. At the same time, rehybridization of the carbonyl carbon from sp to sp occurs, an electron pair from the carbon-oxygen double bond moves toward the electronegative oxygen atom, and a tetrahedral alkoxide ion intermediate is produced (Figure 19.1). 

The addition of a negative ion to a carbon-carbon double or triple bonds leads to a carbanion. The addition of a negative ion to a carbon-oxygen double bond does not give a carbanion, since the negative charge resides on the oxygen. Carbanions are also formed when a nucleophile adds to an a,p-unsaturated compounds (Eq. (3.7)) ... 

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