Question

The alkaloid hygrine is found in the coca plant. Suggest a structure for it on the basis of the following evidence: Hygrine \(\left(\mathrm{C}_{8} \mathrm{H}_{1}{ }_{5} \mathrm{ON}\right)\) is insoluble in aqueous \(\mathrm{NaOH}\) but soluble in aqueous HC1. It does not react with benzenesulfonyl chloride. It reacts with phenylhydrazine to yield a phenylhydrazone. It reacts with NaOI to yield a yellow precipitate and a carboxylic acid \(\left(\mathrm{C}_{7} \mathrm{H}_{13} \mathrm{O}_{2} \mathrm{~N}\right)\). Vigorous oxidation by \(\mathrm{CrO}_{3}\) converts hygrine into hygrimic acid \(\left(\mathrm{C}_{6} \mathrm{H}_{11} \mathrm{O}_{2} \mathrm{~N}\right)\)

Short answer

In summary, the most likely structure for hygrine (\(\mathrm{C}_{8} \mathrm{H}_{15} \mathrm{ON}\)) contains a carbonyl group (either an aldehyde or a ketone) and an alkyl halide group attached to a tertiary carbon atom, considering its reactions with various reagents and its solubility in aqueous HCl. The structure can be represented as: \[ \begin{array}{c} \mathrm{C=O} \\ \mathrm{R_{1}} \mathrm{C}^{\ast}\mathrm{R}_{2}\mathrm{R}_{3} \mathbf{-} \mathrm{X} \\ \end{array} \] where R1, R2, and R3 are alkyl groups, and X is a halogen atom.

Step-by-step solution

Solubility in aqueous HCl and insolubility in aqueous NaOH

The first evidence suggests that hygrine contains acidic functional groups, because it is soluble in aqueous HCl (acidic solution) but not in a basic solution (aqueous NaOH). Based on this, the most common acidic functional groups in organic chemistry are the carboxylic acid (-COOH) and phenol (-OH) groups. However, phenols typically react with NaOH to form soluble salts, which is not the case for hygrine. Thus, we can rule out the phenol group. In addition, since hygrine reacts with phenylhydrazine to form a phenylhydrazone (see evidence #3), it must contain a carbonyl group (C=O).

Reaction with benzenesulfonyl chloride

The fact that hygrine does not react with benzenesulfonyl chloride indicates that it does not have a primary or secondary amine group (NH2 or NHR, where R is an alkyl group) since these groups react with benzenesulfonyl chloride.

Reaction with phenylhydrazine

As mentioned in step 1, the reaction of hygrine with phenylhydrazine to form a phenylhydrazone suggests the presence of a carbonyl group (C=O). This group can be found in aldehydes and ketones.

Reaction with NaOI

Reaction with NaOI yields a carboxylic acid with the empirical formula \(\mathrm{C}_{7} \mathrm{H}_{13} \mathrm{O}_{2} \mathrm{~N}\). We can deduce that hygrine loses one carbon atom during the reaction, suggesting the presence of an alkyl halide group (R-X, where R is an alkyl group, and X is a halogen atom) that can be substituted in a nucleophilic substitution reaction (SN2) with NaOI. Since there is no indication that a reduced or decarboxylated product is formed, this suggests that the IO- ion does not attack a carbonyl group.

Oxidation by CrO3

Vigorous oxidation by \(\mathrm{CrO}_{3}\) converts hygrine into hygrimic acid \(\left(\mathrm{C}_{6} \mathrm{H}_{11} \mathrm{O}_{2} \mathrm{~N}\right)\). This result implies that hygrine loses two carbon atoms during the oxidation process, suggesting the presence of an adjacent tertiary carbon atom and alkyl halide group (alkyl halides can be easily oxidized).

Suggesting a structure for hygrine

Considering all the given information, the structure of hygrine must contain a carbonyl group (either an aldehyde or a ketone) along with an alkyl halide attached to a tertiary carbon atom. The most likely structure for hygrine is: \[ \begin{array}{c} \mathrm{C=O} \\ \mathrm{R_{1}} \mathrm{C}^{\ast}\mathrm{R}_{2}\mathrm{R}_{3} \mathbf{-} \mathrm{X} \\ \end{array} \] where R1, R2, and R3 are alkyl groups, and X is a halogen atom.

Key concepts

Alkaloids

Alkaloids are naturally occurring chemical compounds, predominantly found in plants, that usually contain at least one nitrogen atom. These compounds often have significant pharmacological effects and are commonly used in medications. Morphine and quinine are classic examples of alkaloids.

The study of alkaloids covers understanding their chemical structure, production process, and the role they play in different plant species. Hygrine is an alkaloid found in coca plants. Its structure determination involves understanding how various chemical reactions correspond to its functional groups. Alkaloids can have diverse structures, but the presence of nitrogen is a common feature. When identifying an alkaloid like hygrine, observing its interactions with acids and bases provides critical insights into its solubility properties and potential functional groups.

Acidic Functional Groups

Acidic functional groups are a common and essential part of organic chemistry. These groups can donate a proton (H+) in reactions, and include the carboxylic acid group \(-COOH\) and the phenol group \(-OH\). When discussing hygrine's solubility, it's suggested it contains acidic functional groups because it dissolves in aqueous HCl.

However, hygrine's lack of solubility in \(NaOH\) indicates a phenol group is unlikely, as phenols typically react with bases. Thus, we often consider the presence of a carboxylic acid group in such scenarios. Understanding solubility behavior is crucial in deducing which functional group an alkaloid might have and why it might react or not react with certain chemicals.

Carbonyl Group

A carbonyl group (\(\text{C=O}\)) is a fundamental feature in organic chemistry found in aldehydes and ketones. It encompasses a carbon atom double-bonded to an oxygen atom. Hygrine's reaction to form a phenylhydrazone with phenylhydrazine indicates the presence of this group.

This reaction is characteristic because phenylhydrazine easily forms hydrazones with carbonyl compounds. This feature helps in identifying aldehydes and ketones during structure determination. Knowing how hygrine interacts with phenylhydrazine offers insight into its structural elements, guiding chemists toward identifying it as an aldehyde or ketone despite its complex alkaloid nature.

Reactions with Oxidizing Agents

Hygrine undergoes reactions with oxidizing agents, like \(\text{CrO}_3\), which transform its molecular structure into different compounds such as hygrimic acid. The process of oxidation involves the loss of electrons and an increase in oxidation state, often resulting in breaking down molecules or rearranging structural components.

The oxidation of hygrine indicates the presence of tertiary carbon atoms and possibly alkyl halides, given the loss of carbon atoms during this reaction. In organic chemistry, such reactions are invaluable as they provide information on the presence and location of certain functional groups within the compound. By examining how hygrine changes upon oxidation, chemists can hypothesize about the types of bonds and carbon structures present, contributing to deriving a more accurate structural representation of hygrine.