Nucleophilic addition to the carbonyl group

Molecular orbitals explain the reactivity of the carbonyl group 

You met nucleophilic addition to a carbonyl group on p. 114 and 119, where we showed you how cyanide reacts with acetone to give an alcohol. As a reminder, here is the reaction again, with its mechanism. 

The reaction has two steps nucleophilic addition of cyanide, followed by protonation of the anion. In fact, this is a general feature of all nucleophilic additions to carbonyl groups. 

We will frequently use a device like this, showing a reaction scheme with a mechanism for the same reaction looping round underneath. The reagents and conditions next to the arrow across the top will tell you how you might carry out the reaction, and the pathway shown underneath will tell you how it actually works. 

The addition step is more important, and it forms a new C-C a bond at the expense of the C=0 7t bond. The protonation step makes the overall reaction addition of HCN across the C-O Jt bond. 

Why does cyanide, in common with many other nucieophiies, attack the carbonyl group And why does it attack the carbon atom of the carbonyl group To answer these questions we need to look in detail at the structure of carbonyl compounds in general and the orbitals of the C=0 group in particular. 

Identifying th functional groups In a Explaining thei ctfvity of theC=0 - -.f wt acUpipmt ftottbl i  

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Aldehydes and Ketones Nucleophilic Addition to the Carbonyl Group... 

ALDEHYDES AND KETONES NUCLEOPHILIC ADDITION TO THE CARBONYL GROUP... 

The mechanism for formation of benzaldehyde diethyl acetal which proceeds m two stages is presented m Figure 17 9 The first stage (steps 1-3) involves formation of a hemiacetal m the second stage (steps 4-7) the hemiacetal is converted to the acetal Nucleophilic addition to the carbonyl group characterizes the first stage carbocation chemistry the second The key carbocation intermediate is stabilized by electron release from oxygen... 

Section 17 14 Nucleophilic addition to the carbonyl group is stereoselective When one direction of approach to the carbonyl group is less hindered than the other the nucleophile normally attacks at the less hindered face... 

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