Question

On what basis can you account for the fact that \(\mathrm{HCN}\) adds to the carbonyl group of 3 -butenal and to the double bond of 3 -buten-2-one? Would you expect the carbonyl or the double-bond addition product of \(\mathrm{HCN}\) to 3 -buten- 2 -one to be more thermodynamically favorable? Give your reasoning.

Short answer

HCN adds to the carbonyl group in 3-butenal and 3-buten-2-one, with the addition to the carbonyl group of 3-buten-2-one being more favorable due to the stability of the resulting cyanohydrin.

Step-by-step solution

Understanding the Reactants

Let's break down the compounds. 3-butenal has a carbonyl group at the end of the molecule that is more electrophilic and susceptible to nucleophilic attack, like by HCN (hydrogen cyanide). However, in 3-buten-2-one, there is both a carbonyl group and an alkene, making it interesting as both sites can react with HCN.

Assessing Nucleophilic Addition Sites

Hydrogen cyanide typically adds to the most electrophilic carbon. In 3-butenal, the carbonyl group is more electrophilic than the alkenyl (double-bonded) carbon, so HCN selectively adds to the carbonyl carbon. However, in 3-buten-2-one, both the carbonyl and the double bond can potentially react, but the carbonyl carbon is still more electrophilic than the double bond.

Evaluating Thermodynamic Favorability

The addition of HCN to a carbonyl group results in a stable cyanohydrin. In the context of 3-buten-2-one, the carbonyl addition product of HCN is typically more stable due to the resonance and electronic factors that stabilize the tetrahedral intermediate formed from the carbonyl addition.

Conclusion and Reasoning

Considering the electrophilicity and stability of formed products, the carbonyl addition product of HCN is likely more thermodynamically favorable with 3-buten-2-one. This is due to the stabilized intermediate and stronger bond formation observed in the addition to the carbonyl group as compared to the alkene.

Key concepts

Carbonyl Compounds

Carbonyl compounds are a class of organic molecules characterized by a carbon atom double-bonded to an oxygen atom, known as the carbonyl group (C=O). This functional group is prominent in molecules such as aldehydes, ketones, carboxylic acids, esters, and amides. The carbon-oxygen double bond is polarized because oxygen is more electronegative than carbon.
This results in a partial positive charge on the carbon atom, making it susceptible to attack by nucleophiles.
The nature of the carbonyl group allows it to partake in various types of chemical reactions, most notably nucleophilic additions. In these reactions, the carbon atom in the carbonyl group serves as the electrophilic center. This electrophilicity is crucial for reactions like the formation of cyanohydrins from aldehydes and ketones, where hydrogen cyanide (

• provides a cyanide ion for the reaction.
• results in a nucleophilic addition process.

Electrophilicity

Electrophilicity refers to the tendency of an electron-deficient atom or group to attract electrons, making it prone to chemical reactions with nucleophiles. In organic chemistry, carbonyl compounds exhibit high electrophilicity due to the presence of the polar carbon-oxygen double bond.
The carbon atom holds a partial positive charge, creating a site where nucleophiles can attack.
In 3-butenal and 3-buten-2-one, the carbonyl carbon is the main electrophilic center. This is because it can readily accept electrons from nucleophiles like cyanide ions.
While both the carbonyl carbon and the alkene in these compounds can potentially serve as electrophilic sites, the carbonyl group generally remains more electrophilic.
This persistent characteristic is why in 3-buten-2-one, the nucleophile often prefers the carbonyl group over the double bond, leading to specific addition reactions.

Cyanohydrin Formation

Cyanohydrin formation is an important chemical reaction wherein a nitrile group (\(-CN\)) from hydrogen cyanide (

• adds to the electrophilic carbon of a carbonyl compound.
• converts the compounds into stable cyanohydrin products.

Upon reaction with hydrogen cyanide, the cyanide ion acts as a nucleophile. It attacks the electrophilic carbon in the carbonyl group, creating a tetrahedral intermediate.
Subsequently, the addition of hydrogen atom results in the formation of a cyanohydrin.
This conversion is essential in synthetic organic chemistry for constructing more complex chemical structures. In molecules like 3-buten-2-one, cyanohydrins result from the preference of cyanide ions to the highly electrophilic carbonyl carbon, demonstrating the efficiency and selectivity of these reactions in creating targeted products.

Thermodynamic Stability

Thermodynamic stability refers to the overall energy balance of a chemical reaction and the resulting compound's relative stability. A product is more thermodynamically stable if it possesses lower potential energy compared to its reactants.
In organic chemistry reactions, thermodynamic stability often influences which reaction pathway is favored.
When assessing the addition of hydrogen cyanide to 3-butenal or 3-buten-2-one, the formation of cyanohydrins illustrates this concept well.
The reaction leads to a more stable product when the cyanide ion adds to the carbonyl group rather than the double bond in 3-buten-2-one.
The stability of the resulting cyanohydrin is due to:

• the presence of strong covalent bonds in the new molecule.
• the tetrahedral geometry of the carbon atom post-reaction, which adds to its stability.
• resonance stabilization and other electronic effects that decrease the energy of the system.

Understanding such stability considerations helps in predicting the most likely outcome of chemical reactions.