Question

As we have seen in this chapter, carbon-carbon double bonds are electron-rich regions that are attacked by electrophiles (e.g., HBr); they are not attacked by nucleophiles (e.g., diethylamine).

Short answer

Answer: Carbon-carbon double bonds are electron-rich regions due to the presence of π electrons. Electrophiles are attracted to electron-rich sites, making carbon-carbon double bonds likely targets for electrophilic reactions. On the other hand, nucleophiles are also electron-rich and prefer to react with electron-deficient species, making them unlikely to directly interact with carbon-carbon double bonds.

Step-by-step solution

Understanding Electrophiles and Nucleophiles

An electrophile is a species that is electron deficient and has a strong attraction towards electron-rich regions or nucleophile. They tend to "seek" electrons, usually accepting electron pairs from a nucleophile. On the other hand, a nucleophile is a species that is electron-rich and has a strong attraction towards electron-deficient regions, such as electrophiles. Nucleophiles tend to donate electron pairs during a reaction.

Reactivity of Carbon-Carbon Double Bonds

Carbon-carbon double bonds (C=C) have a high electron density due to the presence of π electrons. These π electrons are less tightly bound to the carbon atoms and are more available for chemical reactions. Therefore, carbon-carbon double bonds are electron-rich regions, making them potential targets for electrophiles.

Reaction with Electrophiles - HBr Example

An example of an electrophile is HBr (hydrogen bromide). When it reacts with a carbon-carbon double bond, HBr donates its proton (H+) to one of the carbon atoms in the double bond, creating a carbocation intermediate. The negatively charged bromide ion (Br-) then attacks the carbocation, forming a new bond with the carbon atom. This results in the formation of a new, more stable alkyl halide product. This is considered an electrophilic addition reaction.

Carbon-Carbon Double Bonds and Nucleophiles

Due to the high electron density present in carbon-carbon double bonds, they are not easily attacked by nucleophiles like diethylamine. Nucleophiles are attracted to electron-deficient sites, and the carbon-carbon double bond is electron-rich. These species are therefore unlikely to interact with each other in a direct reaction. In summary, carbon-carbon double bonds are electron-rich regions due to the presence of π electrons. These electron-rich regions are more likely to be attacked by electrophiles, like HBr, that are attracted to electron-rich sites. They are not targets for nucleophiles, like diethylamine, which are also electron-rich and prefer to react with electron-deficient species.