Question

Assume that an equimolar mixture of formaldehyde and trimethylacetaldehyde (each undergoes the Cannizzaro reaction by itself) is heated with sodium hydroxide solution. Write equations for the various possible combinations of Cannizzaro reactions which might occur. Would you expect formaldehyde used in excess to primarily reduce or oxidize trimethylacetaldehyde? Why?

Short answer

The various possible combinations of Cannizzaro reactions for an equimolar mixture of formaldehyde (HCHO) and trimethylacetaldehyde (Me3CCHO) in the presence of sodium hydroxide (NaOH) are: 1. Cannizzaro reaction for formaldehyde: \(2HCHO + NaOH \rightarrow HCOONa + CH_3OH\) 2. Cannizzaro reaction for trimethylacetaldehyde: \(2Me_3CCHO + NaOH \rightarrow Me_3CCOONa + Me_3CH_2OH\) 3. Cross Cannizzaro reaction (HCHO gets oxidized, and Me3CCHO gets reduced): \(HCHO + Me_3CCHO + NaOH \rightarrow HCOONa + Me_3CH_2OH\) 4. Cross Cannizzaro reaction (HCHO gets reduced, and Me3CCHO gets oxidized): \(HCHO + Me_3CCHO + NaOH \rightarrow Me_3CCOONa + CH_3OH\) If formaldehyde is used in excess, it would primarily reduce trimethylacetaldehyde due to being more reactive towards the Cannizzaro reaction and more likely to get oxidized, while trimethylacetaldehyde is more likely to get reduced.

Step-by-step solution

Understanding the Cannizzaro Reaction

Cannizzaro reaction involves the disproportionation of two aldehyde molecules, where one molecule gets oxidized to the carboxylic acid, and the other gets reduced to the primary alcohol. This reaction requires a non-enolizable aldehyde in the presence of a strong base.

Cannizzaro Reaction for Formaldehyde

Let's first write the Cannizzaro reaction for formaldehyde (HCHO) in the presence of sodium hydroxide (NaOH). Oxidation Reaction (Formic Acid Formation): \(HCHO + OH^− → HCOO^− + H_2O\) Reduction Reaction (Methanol Formation): \(HCHO + HCOO^− → CH_3OH + CO_2\) So, the Cannizzaro reaction for formaldehyde \(2HCHO + NaOH → HCOONa + CH_3OH\)

Cannizzaro Reaction for Trimethylacetaldehyde

Now, let's do the same for trimethylacetaldehyde (Me3CCHO). Oxidation Reaction (Carboxylic Acid Formation): \(Me_3CCHO + OH^− → Me_3CCOO^− + H_2O\) Reduction Reaction (Alcohol Formation): \(Me_3CCHO + Me_3CCOO^− → Me_3CH_2OH + CO_2\) So, the Cannizzaro reaction for trimethylacetaldehyde \(2Me_3CCHO + NaOH → Me_3CCOONa + Me_3CH_2OH\)

Cross Cannizzaro Reaction

Now, consider all cross Cannizzaro reactions that could occur between formaldehyde (HCHO) and trimethylacetaldehyde (Me3CCHO). 1. HCHO gets oxidized, and Me3CCHO gets reduced: \(HCHO + Me_3CCHO + NaOH → HCOONa + Me_3CH_2OH\) 2. HCHO gets reduced, and Me3CCHO gets oxidized: \(HCHO + Me_3CCHO + NaOH → Me_3CCOONa + CH_3OH\)

Evaluating the Reactivity

Formaldehyde (HCHO) is more reactive than trimethylacetaldehyde (Me3CCHO) towards the Cannizzaro reaction. The electron-donating methyl groups on trimethylacetaldehyde act as electron-releasing groups, making the aldehyde carbon less electrophilic compared to the aldehyde carbon in formaldehyde. Consequently, the nucleophilic attack by the hydroxide ion is more favorable for formaldehyde. In conclusion, if formaldehyde is used in excess, it would primarily reduce trimethylacetaldehyde, as formaldehyde is more likely to get oxidized, and trimethylacetaldehyde is more likely to get reduced in this scenario.

Key concepts

Formaldehyde

Formaldehyde, with the simple chemical formula HCHO, is the simplest aldehyde and is known for its high reactivity. This small, yet significant molecule serves as a starting material for many chemical syntheses, particularly in polymer and resin production. Given its reactivity, it readily undergoes a variety of chemical transformations, including the Cannizzaro reaction.

In this reaction, formaldehyde acts interestingly—it can either be oxidized to form formic acid (HCOOH) or reduced to yield methanol (CH3OH), depending on reaction conditions. Since formaldehyde lacks any alkyl groups that might donate electrons, it is more susceptible to nucleophilic attack, meaning it is more readily oxidized compared to bulkier aldehydes.

Trimethylacetaldehyde

Trimethylacetaldehyde, also known as 2,2,2-trimethylacetaldehyde, represents a more complex structure than formaldehyde, with three methyl groups attached to the carbonyl carbon. This steric hindrance, generated by the bulky methyl groups, makes trimethylacetaldehyde less reactive in comparison to formaldehyde in reactions such as the Cannizzaro.

When it comes to its reduction in the Cannizzaro reaction, trimethylacetaldehyde is transformed into a corresponding alcohol, trimethylcarbinol. These characteristics of trimethylacetaldehyde play a vital role in predicting the outcomes of reactions with formaldehyde, especially when considering reaction kinetics in processes like chemical disproportionation.

Chemical Disproportionation

The concept of chemical disproportionation is showcased by a reaction where a single compound undergoes oxidation and reduction simultaneously, transforming into two different products. The Cannizzaro reaction is a classic example of disproportionation, involving aldehydes without alpha-hydrogen atoms in the presence of a concentrated base.

In our discussion, formaldehyde and trimethylacetaldehyde react with themselves or each other, leading to products like alcohols and carboxylates. A crucial aspect of disproportionation in this context is that reactivity contrasts have significant influences on which compound gets reduced or oxidized, guiding the outcome of the Cannizzaro reaction.

Aldehyde Reactions

Aldehyde reactions are rich and varied in organic chemistry. These reactions are often governed by the electron-rich carbonyl group, which makes aldehydes prime candidates for nucleophilic addition reactions, such as the Cannizzaro reaction.

This particular reaction is unique because it does not result in carbon-carbon bond formation, rather it highlights a balance between the reducing and oxidizing tendencies within the same type of functional group. Understanding aldehyde reactivity, particularly in the Cannizzaro reaction, requires insight into the structural features of the molecules involved, such as the presence of electron-donating groups, which influence reactivity.