Question

(a) Reaction of 1,2-dimethylpyrrolidine (1) with ethyl iodide leads to two isomeric ammonium ions, \(\mathrm{C}_{8} \mathrm{H}_{18} \mathrm{IN}\). Explain. (b) However, when 2-methylpyrrolidine (2) undergoes a similar reaction with ethyl iodide, followed by treatment with weak base, analysis of the product by \({ }^{1} \mathrm{H}\) NMR spectroscopy reveals only one set of signals for \(\mathrm{C}_{7} \mathrm{H}_{15} \mathrm{~N}\). Explain.

Short answer

(a) Two isomeric ammonium ions form because ethyl iodide can attach to either methyl. (b) Only one product forms as there is only one substitution position.

Step-by-step solution

- Understanding 1,2-Dimethylpyrrolidine Isomerism

1,2-Dimethylpyrrolidine has two methyl groups attached to the nitrogen atom. When ethyl iodide reacts with 1,2-dimethylpyrrolidine, the ethyl group can attach to either of the nitrogen-located methyls, leading to two different isomers of the resulting ammonium ion.

- Formation of Ammonium Ions

The reaction of 1,2-dimethylpyrrolidine with ethyl iodide produces two different ammonium ions, \(\text{C}_{8}\text{H}_{18}\text{IN}\). This happens because the attachment of the ethyl group can occur at two different positions, generating isomers.

- Analysis of 2-Methylpyrrolidine Reaction

2-Methylpyrrolidine has a single methyl group attached to the nitrogen atom. When ethyl iodide reacts with this compound and is followed by treatment with a weak base, only one product is formed, as there is only one position on the nitrogen for substitution.

- \(^{1}H\) NMR Spectroscopy Analysis

\(^{1}H\) NMR spectroscopy of the product from 2-methylpyrrolidine reaction shows only one set of signals, which indicates the formation of a single product. This is because there is no positional isomerism in the substitution reaction of 2-methylpyrrolidine.

Key concepts

Understanding 1,2-Dimethylpyrrolidine

1,2-Dimethylpyrrolidine is an organic compound composed of a pyrrolidine ring with two methyl groups attached to the nitrogen atom. Understanding the positions helps in predicting reaction outcomes.
This compound can participate in reactions where substituents are added to the nitrogen. Since the nitrogen in the pyrrolidine ring can bind to different groups, it requires careful analysis of the resulting products.
When 1,2-dimethylpyrrolidine reacts with other chemicals, the position of the methyl groups can cause the creation of different isomers, leading to varied outcomes in chemical reactions.

Formation and Characteristics of Isomeric Ammonium Ions

Isomeric ammonium ions are significant in the study of organic chemistry because their different structures can affect how they behave in reactions.
When 1,2-dimethylpyrrolidine reacts with ethyl iodide, it can form two different ammonium ions, each having the molecular formula \(\text{C}_{8}\text{H}_{18}\text{IN}\).
The difference in these isomers comes from the ethyl group attaching to either one of the two nitrogen-attached methyl groups.
This flexibility in attachment sites leads to two distinct isomers of ammonium ions, which can be identified through various analytical methods.

NMR Spectroscopy Analysis of Reaction Products

NMR spectroscopy, particularly \(^1H\) NMR, is a powerful tool in determining the structure of organic compounds. In the context of 2-methylpyrrolidine reaction, NMR helps in identifying the unique characteristics of the products formed.
By analyzing the NMR spectra, one can determine the number and environment of hydrogen atoms in the molecule.
For 2-methylpyrrolidine reacting with ethyl iodide, the NMR spectroscopy reveals only one set of signals. This indicates that only one product is formed, as opposed to multiple products from isomeric reactions.
This single set of signals signifies no positional isomerism, providing a clear understanding of the product's molecular structure.

The Reaction Between Ethyl Iodide and Pyrrolidines

Ethyl iodide is an alkyl halide that readily participates in substitution reactions with nitrogen-containing compounds like pyrrolidines.
When ethyl iodide reacts with 1,2-dimethylpyrrolidine, it adds the ethyl group to the nitrogen, leading to isomeric ammonium ions.
In the case of 2-methylpyrrolidine, the reaction followed by treatment with a weak base results in a single substitution product due to the lack of positional isomers on the nitrogen.
These reactions show the importance of molecular structure in determining the outcomes of organic reactions and how variation at the molecular level influences the final product.