Question

Among the given compounds, the most susceptible to nucleophilic attack at the carbonyl group is (a) \(\mathrm{MeCOCl}\) (b) \(\mathrm{MeCHO}\) (c) MeCOOMe (d) \(\mathrm{Me} \mathrm{CO}-\mathrm{O}-\mathrm{CO} \mathrm{Me}\)

Short answer

(a) Methanoyl chloride (MeCOCl) is the most susceptible to nucleophilic attack.

Step-by-step solution

Identify the Functional Groups

The compounds given are: (a) C u00a0Methanoyl chloride (C MeCOCl), (b) C C u00a0Methanal (C MeCHO), (c) C Methyl acetate (MeCOOMe), and (d) C C Dimethyl carbonate (C MeCO-O-COMe). Identify that (a) is an acyl chloride, (b) is an aldehyde, (c) is an ester, and (d) is a carbonate ester.

Understand Reactivity to Nucleophilic Attack

Acyl chlorides are generally very reactive towards nucleophilic attack at the carbonyl carbon due to the presence of the highly electronegative chlorine leaving group. Aldehydes are less reactive than acyl chlorides, but more reactive than esters, due to less steric hindrance and stronger electrophilic character of the carbonyl carbon. Esters and carbonate esters are generally less reactive because the leaving group (alkoxy group) is less electronegative than chlorine.

Evaluate the Leaving Group Ability

The leaving group ability is crucial for the reactivity towards nucleophilic attack. Chloride ions are excellent leaving groups due to their stable nature when dissociated. In contrast, methoxide ions (found in ester and carbonate esters) are not as good leaving groups in comparison to chloride.

Determine Susceptibility to Nucleophilic Attack

Given the reactivity trends, acyl chloride (MeCOCl) will be the most susceptible to nucleophilic attack due to its highly electronegative chlorine atom, making the carbonyl carbon more electrophilic and the leaving chloride ion more stable.

Key concepts

Reactivity of Acyl Chlorides

Acyl chlorides, also known as acid chlorides, are highly reactive compounds, especially when it comes to nucleophilic attacks on the carbonyl group. This high reactivity can be attributed to the presence of the chlorine atom, which is significantly electronegative. This electronegativity increases the partial positive charge on the carbon of the carbonyl group, making it an excellent target for nucleophiles. Additionally, the carbonyl group is planar, creating minimal steric hindrance, allowing nucleophiles easy access to attack the carbon atom.
This reactivity of acyl chlorides is often utilized in organic synthesis. They readily undergo nucleophilic substitution reactions, where nucleophiles replace the chlorine atom. This can lead to the formation of other functional groups, such as esters, amides, and carboxylic acids. Such transformations are invaluable in the production of more complex organic molecules.

Carbonyl Group Electrophilicity

In organic chemistry, the carbonyl group is a common site of reaction due to its electrophilic nature. Electrophilicity in this context refers to the tendency of the carbonyl carbon to act as an electron acceptor during chemical reactions. The carbon-oxygen double bond features a significant dipole moment as oxygen is more electronegative than carbon, attracting electron density.
This dipole moment increases the positive character of the carbon atom, making it more susceptible to attacks by nucleophiles seeking to donate electron pairs. The degree of electrophilicity of the carbonyl carbon can vary, depending on the attached groups. For instance, acyl chlorides, due to the presence of electronegative chloro groups, exhibit high electrophilicity. This allows them to react more rapidly with nucleophiles compared to compounds with less electrophilic carbonyls, like esters or amides.
Understanding the reactivity of the carbonyl group is critical in predicting and manipulating chemical pathways in synthesis.

Leaving Group Ability in Organic Compounds

The ability of a group to depart as a leaving group is a fundamental concept in understanding the reactivity of many organic reactions. A good leaving group is typically a weak base and stabilizes easily upon departure from the parent molecule. In nucleophilic substitution reactions, chloride ions (Cl⁻) serve as excellent leaving groups due to their stable nature and low nucleophilicity.
This property is particularly evident in acyl chlorides. When a nucleophile attacks the carbonyl carbon, the chlorine atom is displaced easily because it can stabilize itself as a chloride ion in solution. Comparatively, alkoxy groups, such as those found in esters, are poorer leaving groups since their corresponding anion – the methoxide ion (CH₃O⁻) – is a stronger base and less stable on its own.
The efficiency of a leaving group dictates the rate and likelihood of nucleophilic substitution reactions occurring, playing a crucial role in the outcome of organic reactions.