Question

The increasing order of rate of HCN addition to the following compounds is (1) HCHO (2) \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\) (3) \(\mathrm{PhCOCH}_{3}\) (4) PhCOPh (A) \(4<2<3<1\) (B) \(4<3<2<1\) (C) \(3<4<2<1\) (D) \(1<2<3<4\)

Short answer

The correct increasing order of rate of HCN addition to the given compounds is option (B): \(4<3<2<1\).

Step-by-step solution

Understand Nucleophilic Addition

Hydrocyanic acid (HCN) forms a nucleophilic addition reaction with carbonyl compounds. The carbon of the cyanide ion (CN-) acts as the nucleophile, attacking the carbonyl carbon and pushing the carbonyl's pi electrons up to the oxygen atom. This reaction results in the formation of a new C-C bond.

Identify Attached Groups

Each given compound bears a carbonyl group with two attachments, which differ in nature from compound to compound: In compound 1 (HCHO, formaldehyde) the attachments are two hydrogen atoms. In compound 2 (\(\mathrm{CH}_{3} \mathrm{COCH}_{3}\), acetone) they are two methyl groups. In compound 3 (\(\mathrm{PhCOCH}_{3}\), acetophenone) there's one phenyl group and one methyl group. In compound 4 (PhCOPh, benzophenone) two phenyl groups are attached to the carbonyl group.

Effect of Attached Groups on Reactivity

An important factor influencing the reactivity of carbonyl compounds toward nucleophile addition is the nature of the attached groups. The addition of nucleophilic HCN follows the "carbocation stability rule", where the carbocation intermediate formed in the transitional state will be more stable if the attached groups can donate electron density towards the carbonyl carbon. In the order of atoms/groups that donate electrons, we understand that the attached groups can be classified as: H < CH3 < Ph, where Ph is a phenyl group and CH3 is a methyl group. Hydrogen is the least capable of donating electrons towards the carbonyl carbon, while the phenyl group is the most capable.

Determine Increasing Order of Reactivity

Based on the aforementioned electron donation order (H < CH3 < Ph), we understand that the more electron-donating the attached groups are, the faster the addition reaction will be. Hence, the given compounds can be ordered in terms of increasing reactivity as follows: HCHO (compound 1) < CH3COCH3 (compound 2) < PhCOPh (compound 4) < PhCOCH3 (compound 3). To summarise, the correct increasing order of rate of HCN addition to the given compounds is option (B): 4<3<2<1.

Key concepts

Nucleophilic Addition Reactions

Nucleophilic addition reactions represent a fundamental transformation in organic chemistry, especially when it comes to compounds containing a carbonyl group. In these reactions, a nucleophile, which is a species that is electron-rich and seeking a positive center, attacks an electrophilic carbon atom of the carbonyl group. The carbonyl carbon is electrophilic because it is bonded to an oxygen atom, which is highly electronegative and pulls electron density away from the carbon, making the carbon a prime target for nucleophiles.

Key Steps in Nucleophilic Addition to Carbonyl Compounds

• Nucleophile Approach: The nucleophile (in this case, cyanide ion from HCN) approaches the positive center, the carbon in the carbonyl group.
• Nucleophilic Attack: The cyanide ion donates a pair of electrons to the carbonyl carbon, forming a new bond.
• Protonation: A hydrogen ion (from another HCN molecule or solvent) attaches to the negatively charged oxygen, which resulted from the double bond electrons shifting to oxygen.

This sequence converts the carbonyl group into an alcohol. In the context of the exercise, understanding these steps aids in realizing why HCN can add across different carbonyl compounds at varying rates.

Carbonyl Group Reactivity

The reactivity of carbonyl groups in nucleophilic addition depends heavily on the nature of their attached substituents. Carbonyl compounds, such as aldehydes and ketones, can exhibit different reactivities based on what is connected to the carbonyl carbon. Substituents that are electron-withdrawing will decrease the reactivity of the carbonyl group to nucleophilic attack by making the carbon less positive, while electron-donating substituents have the opposite effect.

For example, in aldehydes like formaldehyde (HCHO), the carbonyl carbon is more electrophilic because there are only hydrogens attached, which don't donate electrons. In contrast, ketones have alkyl groups attached which push electron density towards the carbonyl, making it less electrophilic and thus less reactive to nucleophiles. The situation is even more pronounced in compounds with aromatic rings, such as phenyl groups, which are even better electron donors due to resonance stabilization.

Electron Donating Groups

Electron donating groups (EDGs) play a critical role in the chemical reactivity of molecules. They are atoms or groups of atoms that donate electron density towards other parts of the molecule, often through resonance or inductive effects. In the carbonyl compounds discussed in our exercise, the presence of different EDGs can explain the varying rates of the HCN addition reaction.

Effects of Various Electron Donating Groups

• Hydrogens: Attached hydrogens, like in formaldehyde, offer no resonance stabilization and are the least capable of donating electron density.
• Alkyl Groups: Alkyl groups such as methyl (CH3) can donate electrons through an inductive effect but do not offer resonance.
• Aromatic Rings: Phenyl groups provide a significant electron donation due to resonance stabilization, making the carbon of the carbonyl more nucleophilic, favoring the addition reaction.

In summary, the electron-donating ability of attached groups can be observed in increasing order from hydrogen to alkyl to aromatic groups, thus affecting the rate of nucleophilic addition reactions correspondingly.