Question

Which carbonyl compound among the following is most reactive towards nucleophilic addition? (a) CCC(=O)CC (b) CC(=O)CCl (c) CC(=O)CCCl (d)

Short answer

(a) Pentanone (b) Chloroacetone (c) 2-Chloropropanone (d) 2-Chloro-4-heptanone Answer: (c) 2-Chloropropanone is the most reactive towards nucleophilic addition.

Step-by-step solution

Identify the carbonyl compounds

We are given four different carbonyl compounds whose reactivity towards nucleophilic addition we need to compare. They are: (a) CCC(=O)CC, which is pentanone (b) CC(=O)CCl, which is chloroacetone (c) CC(=O)CCCl, which is 2-chloropropanone (d) CCCCCC(=O)CCCl, which is 2-chloro-4-heptanone

Examine the substituents' electron-donating or electron-withdrawing properties

Compare the groups present on the carbonyl carbon in each compound. In general: - Electron-donating groups make the carbonyl carbon less electrophilic and less reactive towards nucleophilic attack. - Electron-withdrawing groups make the carbonyl carbon more electrophilic and more reactive towards nucleophilic attack. For the given compounds: (a) Pentanone has two alkyl groups attached to the carbonyl carbon, which have electron-donating properties. (b) Chloroacetone has one alkyl group and one chlorine atom. Chlorine has electron-withdrawing properties. (c) 2-Chloropropanone has an alkyl group and a chlorine atom, just like chloroacetone. (d) 2-Chloro-4-heptanone has three alkyl groups and one chlorine atom.

Compare the reactivity of the carbonyl compounds based on electron-donating or electron-withdrawing substituents

Based on the analysis in Step 2, we see that: - Pentanone (a) has two electron-donating alkyl groups, making it less reactive towards nucleophilic addition. - Chloroacetone (b) has one electron-donating alkyl group and one electron-withdrawing chlorine atom. - 2-Chloropropanone (c) also has one electron-donating alkyl group and one electron-withdrawing chlorine atom. - 2-Chloro-4-heptanone (d) has three electron-donating alkyl groups and one electron-withdrawing chlorine atom, making it less reactive like pentanone (a).

Determine the most reactive compound towards nucleophilic addition

Comparing the electron-donating and electron-withdrawing groups in the four carbonyl compounds, chloroacetone (b) and 2-chloropropanone (c) are likely to be more reactive than the other two because they have both an electron-donating alkyl group and an electron-withdrawing chlorine atom. Among chloroacetone (b) and 2-chloropropanone (c), 2-chloropropanone (c) has the chlorine atom closer to the carbonyl group, which would make the electron-withdrawing effect stronger. This would increase the electrophilicity of the carbonyl carbon in 2-chloropropanone (c) compared to that in chloroacetone (b). Therefore, the carbonyl compound most reactive towards nucleophilic addition among the given options is 2-chloropropanone (c).

Key concepts

Carbonyl Compounds

Carbonyl compounds, a significant classification within organic chemistry, consist of a carbon atom double-bonded to an oxygen atom, symbolized as C=O. This group is the defining feature of aldehydes and ketones, where aldehydes have at least one hydrogen atom connected to the carbonyl carbon, while ketones have two alkyl or aryl groups attached.

Understanding the reactivity of carbonyl compounds is fundamental in organic synthesis. Reactivity towards nucleophilic addition, a typical reaction involving carbonyl compounds, varies based on the substituents attached to the carbonyl carbon.

• Alkyl groups, commonly found in ketones such as pentanone, are electron donating and reduce the electrophilicity of the carbonyl carbon.
• Electron-withdrawing groups, such as halogens in chloroacetone or 2-chloropropanone, enhance the electrophilicity of the carbonyl carbon.

Through these variations, chemists can predict and manipulate the outcomes of reactions involving carbonyl compounds.

Electron-Withdrawing Groups

Electron-withdrawing groups are substituents that pull electron density away from the rest of the molecule, owing to their electronegativity or the resonance effect. When attached to a carbonyl compound, they profoundly influence its reactivity.

Consider the role of chlorine in chloroacetone and 2-chloropropanone:

• Chlorine is highly electronegative, drawing electron density towards itself and away from the carbonyl carbon, making this carbon more electrophilic.
• 2-Chloropropanone, with chlorine closer to the carbonyl group than in chloroacetone, demonstrates a stronger electron-withdrawing effect due to the proximity influencing the carbonyl carbon's partial positive charge.

This concept is crucial when evaluating the reactivity of different carbonyl compounds towards nucleophilic attacks, as seen in the solved exercise.

Electrophilicity

Electrophilicity refers to the ability of a compound to attract and accommodate a pair of electrons from a nucleophile, or an electron-rich species. In carbonyl chemistry, the electrophilic center is typically the carbon atom of the carbonyl group.

The presence of electron-withdrawing groups increases the partial positive charge on the carbonyl carbon, enhancing its electrophilicity and susceptibility to attack by nucleophiles. Conversely, electron-donating groups decrease electrophilicity. An understanding of these dynamics allows for the prediction of reactivity patterns in organic compounds.

• In the given exercise, the electron-withdrawing chlorines in chloroacetone and 2-chloropropanone increase their reactivity towards nucleophiles compared to pentanone and 2-chloro-4-heptanone, which are less electrophilic due to the electron-donating alkyl groups.
• Since nucleophiles seek electrophilic centers, chloroacetone and 2-chloropropanone are more reactive, with the latter being the most reactive due to the stronger electron-withdrawing influence of the adjacent chlorine.

The comparison of electrophilicity in carbonyl compounds aligns directly with their ranking in nucleophilic addition reactivity, as well-illustrated by the exercise in question.