As we have seen in this chapter, carbon-carbon double bonds are electron-rich regions and are attacked by electrophiles (for example, HBr); they are not attacked by nucleophiles (for example, diethylamine). However, when the carbon-carbon double bond has a carbonyl group adjacent to it, the double bond reacts readily with nucleophiles by nucleophilic addition (Section \(19.8\) ). Account for the fact that nucleophiles add to a carbon-carbon double bond adjacent to a carbonyl group, and account for the regiochemistry of the reaction.

When exploring the concept of electrophilic attack on double bonds, it's essential to understand the nature of these bonds in organic molecules. Double bonds, consisting of one sigma ((σ)) and one pi ((π)) bond, are electron-rich sites within a molecule.

The electron density of a carbon-carbon double bond naturally attracts electrophiles – species that seek out electrons to form new chemical bonds. This attraction to an electron-rich area is why compounds such as HBr add across double bonds in a process called electrophilic addition. The electrophile attacks the less hindered carbon atom, gaining electrons and thus forming a new bond.

Importance of Electrophile Stability

As a general rule, the stability of the electrophile plays a substantial role in the reactivity towards the double bond. More stable electrophiles will react more readily, which ties into the concept of carbocation stability when considering reaction intermediates in some electrophilic addition reactions.

Understanding these principles is fundamental when approaching the reactivity patterns of different types of double bonds, particularly when additional functional groups influence these interactions.

The presence of a carbonyl group ((C=O)) adjacent to a carbon-carbon double bond dramatically influences the double bond's reactivity towards nucleophiles. A carbonyl group is composed of a carbon atom double-bonded to an oxygen atom, which is significantly more electronegative than carbon.

This difference in electronegativity causes the sharing of electrons in the double bond to be unequal, and as a result, the carbon atom in the carbonyl group becomes electron-poor, creating what is known as a polar carbonyl compound.

Electron Withdrawal and Polarization

Due to the adjacent position of the double bond to the carbonyl group, there is a withdrawal of electron density from the double bond towards the carbonyl carbon. This effect creates a polarization of the double bond, enhancing its ability to react with nucleophiles, which are species with available electrons that seek out positive or partially positive centers in molecules. The altered electron density makes the double bond behave more like an electrophilic center than it otherwise would without the influence of the carbonyl group.

Regiochemistry refers to the orientation and position where reagents add to a molecule. In the context of nucleophilic addition to carbon-carbon double bonds, particularly when carbonyl groups are in proximity, the regiochemistry becomes predictable based on the polarization effects previously discussed.

The regioselectivity in nucleophilic addition is influenced mainly by the electrophilic character of the carbon atoms within the double bond. Generally, the nucleophile will add to the carbon atom most deficient in electron density—the carbon directly connected to the carbonyl group. This process is known as 1,2-addition.

Product Formation and Stability

The nucleophile's addition to this carbon leads to the formation of a new carbon-nucleophile bond while the oxygen in the carbonyl group can stabilize the intermediate by donating electron density back into the molecule. The result is typically a stable tetrahedral product with the nucleophile and oxygen on adjacent carbons.

It is this understanding of regiochemistry that enables chemists to predict where nucleophiles will add in complex molecules, providing valuable insights into the design of chemical syntheses and the mechanisms by which they occur.