Question

Generally aldehydes are more reactive than ketones because of : (A) Less steric crowding (B) Hydrogen bonding (C) More electrophilic carbon (D) Both (A) and (C)

Short answer

The short answer: Aldehydes are generally more reactive than ketones because of both (A) Less steric crowding and (C) More electrophilic carbon. This makes it easier for nucleophiles to attack the carbonyl carbon and results in higher reactivity for aldehydes.

Step-by-step solution

Understanding the structure of aldehydes and ketones

Aldehydes and ketones are both carbonyl compounds, having a carbon-oxygen double bond (C=O). In an aldehyde, the carbonyl group is bonded to a hydrogen atom and an alkyl group (or aryl group), whereas in a ketone, the carbonyl group is bonded to two alkyl groups (or aryl groups). Aldehyde: R-CHO Ketone: R-CO-R' Now let's analyze each provided option and compare the properties of aldehydes and ketones. Step 2:

Analysing option (A): Less steric crowding

Steric crowding refers to the hindrance caused by the surrounding groups in a molecule. In aldehydes, one side of the carbonyl group is bonded to a hydrogen atom, while in ketones, two alkyl groups are attached to the carbonyl carbon. Thus, aldehydes have less steric crowding around the carbonyl carbon, which makes it easier for the nucleophiles to attack the carbonyl carbon, resulting in higher reactivity. Step 3:

Analysing option (B): Hydrogen bonding

Hydrogen bonding can affect the reactivity of a molecule, but in the context of comparing aldehydes and ketones, it is not a significant factor. Both aldehydes and ketones do not participate in hydrogen bonding with each other; therefore, this option is not applicable in our comparison. Step 4:

Analysing option (C): More electrophilic carbon

Electrophilicity refers to the ability of an atom to attract electrons and participate in bonding. In both aldehydes and ketones, the electrophilic center is the carbonyl carbon. However, due to the inductive effect of the attached alkyl groups in ketones, the positive charge on the carbonyl carbon is reduced, making it less electrophilic than the carbonyl carbon in aldehydes (which has no such inductive effect). As a result, aldehydes have more electrophilic carbonyl carbon, which leads to a higher reactivity compared to ketones. Step 5:

Selecting the correct option(s)

Based on our analysis, we can conclude that both (A) Less steric crowding and (C) More electrophilic carbon are contributing factors for the higher reactivity of aldehydes as compared to ketones. Therefore, the correct answer is (D) Both (A) and (C).

Key concepts

Carbonyl Compounds

Carbonyl compounds are fundamental in chemistry due to the presence of a carbon-oxygen double bond, known as the carbonyl group. This functional group is characterized by a carbon atom covalently bonded to an oxygen atom. It’s often written as C=O, and is known to occur in several main classes of organic molecules, such as aldehydes, ketones, carboxylic acids, and esters.
In the context of aldehydes and ketones, these carbonyl compounds differ mainly by what is bonded to the carbonyl carbon. Aldehydes have at least one hydrogen atom attached, making them R-CHO, while ketones have two carbon-based groups, represented as R-CO-R'.
The carbonyl compound's distinct reactivity arises from the polar nature of the carbon-oxygen bond as oxygen attracts more electrons due to its higher electronegativity. This polarization sets the stage for reactions involving nucleophilic attack, as the carbonyl carbon becomes an attractive site for electron-rich nucleophiles.

Steric Hindrance

Steric hindrance refers to the prevention of chemical reactions due to the spatial arrangement of atoms within a molecule. In terms of carbonyl compounds like aldehydes and ketones, this concept explains how molecular structure affects reactivity.
In aldehydes, the carbonyl carbon is bonded to a hydrogen atom and, usually, only one larger substituent. This means there is minimal steric hindrance, making the carbonyl carbon an accessible target for nucleophiles. By contrast, ketones exhibit greater steric hindrance as the carbonyl carbon is bonded to two alkyl groups, creating a crowded environment that hinders reactions.
Reduced steric hindrance in aldehydes not only enhances reactivity but also facilitates nucleophilic attack, as less bulky groups are present to block the approach of nucleophiles. This difference in steric environment is a critical factor in understanding why aldehydes are generally more reactive than ketones.

Electrophilicity

Electrophilicity is a key concept in understanding the reactivity of carbonyl compounds. It describes the tendency of an atom or molecule to attract electrons and form bonds with electron-rich species, called nucleophiles.
Aldehydes and ketones both exhibit electrophilic character at the carbonyl carbon, however, the level of electrophilicity differs between them. In aldehydes, the presence of hydrogen allows the carbonyl carbon to remain highly electrophilic because there is less electron-donating influence compared to ketones. The inductive effect from the alkyl groups in ketones tends to share electrons with the carbonyl carbon, diminishing its electrophilicity.
This high electrophilicity in aldehydes enhances their susceptibility to attacks from nucleophiles, making them more reactive compared to ketones. Understanding electrophilicity helps explain why aldehydes are excellent candidates for reactions involving nucleophilic attack.

Nucleophilic Attack

A nucleophilic attack is a fundamental mechanism in organic chemistry where a nucleophile, which is a species rich in electrons, attacks an electron-deficient site to form a bond. This process is integral for understanding reactions involving carbonyl compounds like aldehydes and ketones.
The carbonyl carbon in these compounds is partially positive due to the electronegative oxygen, making it an ideal target for nucleophiles. In aldehydes, with less steric hindrance and higher electrophilicity, nucleophilic attacks occur more readily. The nucleophile approaches the carbonyl carbon, sharing its electrons to form a new bond.
Once this attack is initiated, the carbonyl carbon transitions to a tetrahedral intermediate. This is a crucial step in many organic reactions, leading to the formation of alcohols, carboxylic acids, or other derivatives. Understanding nucleophilic attack aids in predicting and manipulating the chemical behavior of carbonyl compounds in diverse synthetic applications.