Question

Which of the following compounds does not undergo aldol-type condensation? (A) Phenylacetaldehyde (B) Nitromethane (C) Isobutyraldehyde (D) Chloral

Short answer

The compound that does not undergo aldol-type condensation among the given options is (D) Chloral, as it contains a carbonyl group but lacks α-Hydrogen atoms.

Step-by-step solution

Understanding Aldol Condensation requirements

Aldol condensation is an organic reaction that requires the presence of a compound with an α-Hydrogen atom, which is acidic and can be removed to generate an enolate ion. The enolate ion then acts as a nucleophile and reacts with a carbonyl compound to form a β-hydroxyaldehyde or β-hydroxyketone.

Analyzing Phenylacetaldehyde

Phenylacetaldehyde has the structure: \[ Ph-CH_{2}-C(=O)-H \] It contains an α-Hydrogen atom and a carbonyl group. The α-Hydrogen atom can be removed to generate an enolate ion, and therefore, phenylacetaldehyde can undergo aldol condensation.

Analyzing Nitromethane

Nitromethane has the structure: \[ H_{3}C-NO_{2} \] It has no carbonyl group and thus, cannot undergo aldol condensation.

Analyzing Isobutyraldehyde

Isobutyraldehyde has the structure: \[ (CH_{3})_{2}CH-C(=O)-H \] It contains an α-Hydrogen atom and a carbonyl group. The α-Hydrogen atom can be removed to generate an enolate ion, and therefore, isobutyraldehyde can undergo aldol condensation.

Analyzing Chloral

Chloral has the structure: \[ CCl_{3}-C(=O)-H \] It has a carbonyl group, but lacks α-Hydrogen atoms. Therefore, chloral cannot undergo aldol condensation.

Conclusion

Based on the analysis of the four compounds, option (B) Nitromethane and option (D) Chloral both do not undergo aldol-type condensation. However, Chloral is more relevant to the aldol reaction since it contains a carbonyl group but lacks α-Hydrogen atoms. So, the answer is option (D) Chloral.

Key concepts

Enolate Ion Formation

Enolate ion formation is a fundamental step in many organic reactions, especially in aldol condensation. Imagine a molecule that has two specific features: an alpha hydrogen and a carbonyl group. The alpha hydrogen is the hydrogen atom connected to the carbon atom next to the carbonyl group. This proximity to the carbonyl makes it significantly acidic.

When a base comes into play, it can remove this acidic alpha hydrogen from the molecule. This removal creates a negatively charged carbon atom, forming what we call an "enolate ion."

• The enolate ion has a resonance structure: one form exhibits the charge on the carbon, the other on the oxygen.
• This double form interaction makes the enolate a very reactive nucleophile.

In the context of aldol condensation, the enolate ion's ability to act as a nucleophile allows it to react with a carbonyl group in another molecule. This gives rise to a larger, more complex structure: a β-hydroxyaldehyde or a β-hydroxyketone.

Alpha Hydrogen

The concept of the alpha hydrogen is crucial when discussing reactions such as aldol condensation. To understand the role of the alpha hydrogen, let's start by defining where it is located. The alpha hydrogen atom is the hydrogen attached to the carbon that is directly beside the carbonyl carbon. This unique positioning makes it more acidic than you might expect for a typical hydrogen in a molecule.

The presence of this alpha hydrogen is what determines whether a compound can undergo enolate ion formation. This is a key requirement for processes like aldol condensation, because one of the steps involves a base removing the alpha hydrogen to create an enolate ion. Without an alpha hydrogen, this step is impossible.

• In turn, without enolate ions, the nucleophilic attack on the carbonyl group cannot happen.
• Examples where alpha hydrogens are present include compounds like phenylacetaldehyde and isobutyraldehyde.

However, compounds like chloral lack alpha hydrogens. Despite having a carbonyl group, chloral cannot form enolate ions, and thus, cannot proceed with aldol condensation.

Carbonyl Compounds

Carbonyl compounds are a group of organic molecules that contain a carbon-oxygen double bond. This functional group is central to many reactions in organic chemistry, including aldol condensation. Think of carbonyls as the backbone that supports and connects other important features in a molecule.

Being highly polar, the oxygen atom in the carbonyl group enjoys extra electron density, while the carbon atom is relatively electron-deficient. This makes the carbon atom a prime target for nucleophiles. When an enolate ion—a strong nucleophile—attacks this carbon, a new bond forms, leading to the construction of more complex molecules.

• Carbonyl compounds that are common in aldol condensation include aldehydes and ketones.
• In the reaction, these molecules serve as both substrates and targets, forming β-hydroxyaldehydes or β-hydroxyketones after the reaction progresses.

It's worth noting that just the presence of a carbonyl group isn't enough for aldol condensation. The compound must also have alpha hydrogens to facilitate enolate ion formation.