Question

Citral, \(\mathrm{C}_{10} \mathrm{H}_{16} \mathrm{O}\), is a terpene that is the major constituent of lemongrass oil. It reacts with hydroxylamine to yield a compound of formula \(\mathrm{C}_{10} \mathrm{H}_{17} \mathrm{ON}\), and with Tollens' reagent to give a silver mirror and a compound of formula \(\mathrm{C}_{10} \mathrm{H}_{16} \mathrm{O}_{2}\). Upon vigorous oxidation citral yields acetone, oxalic acid \((\mathrm{HOOC}-\mathrm{COOH})\), and levulinic acid \(\left(\mathrm{CH}_{3} \mathrm{COCH}_{2} \mathrm{CH}_{2} \mathrm{COOH}\right)\). (a) Propose a structure for citral that is consistent with these facts and with the isoprene rule. (b) Citral has two isomers, geranial and neral, which yield the same oxidation products. What is the most likely structural difference between these two isomers? (c) Geranial is obtained by mild oxidation of geraniol; neral is obtained in a similar way from nerol. On this basis assign structures to geranial and neral.

Short answer

The most likely structural difference between the isomers geranial and neral is the configuration of one of the double bonds, with geranial having an E configuration and neral having a Z configuration. The structures of geranial and neral are: Geranial (E configuration): \(\mathrm{CH_3}-\mathrm{C}(CH_3)=\mathrm{CH}-\mathrm{CH_2}-\mathrm{CH}=\mathrm{C}-\mathrm{CH_2}-\mathrm{C}(O)H\) Neral (Z configuration): \(\mathrm{CH_3}-\mathrm{C}(CH_3)=\mathrm{CH}-\mathrm{CH_2}-\mathrm{CH}(-\mathrm{CH_2}-\mathrm{C}(O)H)=\mathrm{C}\)

Step-by-step solution

Analyze the reactions and products

First, we will analyze the given reactions and products to find any relevant information that will help to deduce the structure of Citral. 1. Citral reacts with hydroxylamine to yield a compound of formula \(\mathrm{C}_{10} \mathrm{H}_{17} \mathrm{ON}\). 2. Citral reacts with Tollens' reagent to give a silver mirror and a compound of formula \(\mathrm{C}_{10} \mathrm{H}_{16} \mathrm{O}_{2}\). 3. Upon vigorous oxidation, Citral yields acetone, oxalic acid, and levulinic acid The reactions show the presence of an aldehyde group in Citral, evident from the silver mirror test and the formation of oxalic acid and levulinic acid.

Apply the isoprene rule

The isoprene rule states that terpenes can be synthesized by the repetitive joining of isoprene units. We can fit the four isoprene units together in various ways to fit this rule. Considering the facts given and the isoprene rule, a proposed structure for Citral is: \(\mathrm{CH_3}-\mathrm{C}(CH_3)=\mathrm{CH}-\mathrm{CH_2}-\mathrm{CH}= \mathrm{C}-\mathrm{CH_2}-\mathrm{C}(O)H\) This structure fits the reactions and products mentioned, along with the isoprene rule. #b) Structural difference between geranial and neral#

Analyze the reactions

Citral has two isomers, geranial and neral, which yield the same oxidation products. Thus, the structural difference must not have any significant impact on the oxidation process. The most likely possible structural difference would be the configuration of one of the double bonds since it does not affect the oxidation reactions.

Determine the structural difference

The most likely structural difference between geranial and neral is the configuration of one of the double bonds, making one isomer have an E configuration and the other one have a Z configuration. #c) Assigning structures to geranial and neral#

Analyze the mild oxidation

Geranial is obtained by mild oxidation of geraniol, and neral is obtained in a similar way from nerol. Therefore, we will analyze the structural difference between geraniol and nerol. Geraniol (E configuration): \(\mathrm{CH_3}-\mathrm{C}(CH_3)=\mathrm{CH}-\mathrm{CH_2}-\mathrm{CH}= \mathrm{C}-\mathrm{CH_2}-\mathrm{C}(O)H\) Nerol (Z configuration): \(\mathrm{CH_3}-\mathrm{C}(CH_3)=\mathrm{CH}-\mathrm{CH_2}-\mathrm{CH}(-\mathrm{CH_2}-\mathrm{C}(O)H)=\mathrm{C}\)

Assign structures

Based on the mild oxidation and the structural difference between geraniol and nerol, we can assign the following structures: Geranial (E configuration): \(\mathrm{CH_3}-\mathrm{C}(CH_3)=\mathrm{CH}-\mathrm{CH_2}-\mathrm{CH}= \mathrm{C}-\mathrm{CH_2}-\mathrm{C}(O)H\) Neral (Z configuration): \(\mathrm{CH_3}-\mathrm{C}(CH_3)=\mathrm{CH}-\mathrm{CH_2}-\mathrm{CH}(-\mathrm{CH_2}-\mathrm{C}(O)H)=\mathrm{C}\)

Key concepts

Isoprene Rule

The isoprene rule is a guiding principle in understanding the structure of terpenes. It states that terpenes are built from isoprene units, which are five-carbon building blocks with the formula C₅H₈.

When constructing terpene molecules, the isoprene rule helps us to visualize how multiple isoprene units link together to form the complete structure. In the case of citral, the isoprene rule helps to identify that its structure is consistent with the joining of two isoprene units. This also introduces students to the concept of terpene biosynthesis in nature.

Terpenes

Terpenes are a large and diverse class of organic compounds, produced primarily by plants. They are known for their strong odors, which is why they are commonly found in essential oils, such as lemongrass oil that contains citral. Terpenes play diverse roles, including as biological building blocks and in plant defense mechanisms.

In our exercise, citral is identified as a terpene that conforms to the structure suggested by the isoprene rule. This is a critical link between structure and function in organic chemistry, as it connects the compound back to its natural origins and its biosynthetic pathways.

Oxidation Reactions in Organic Chemistry

Oxidation reactions are fundamental in organic chemistry and involve the gain of oxygen, the loss of hydrogen, or the loss of electrons. Upon oxidation, many organic compounds undergo structural changes leading to the formation of new functional groups.

Citral's reaction products upon oxidation, such as acetone and oxalic acid, provide valuable clues to its structure. Recognizing the result of oxidation reactions is crucial for students in understanding the intricate changes occurring at the molecular level during these reactions.

Geometric Isomerism

Geometric isomerism, or cis-trans isomerism, is a form of stereoisomerism. It occurs due to the inflexibility of double bonds that restricts the rotation of the attached groups. As a result, you can have two or more forms of the same molecular formula with different spatial orientations.

In citral, this concept helps explain the existence of its two isomers, geranial and neral. The difference lies in the spatial positioning around the double bond, not affecting the oxidation products, providing students a clear example of how isomers can have different physical and chemical properties.

Tollens' Reagent Test

Tollens' reagent is a chemical reagent used to determine the presence of aldehydes. When an aldehyde is present, the reagent oxidizes it and is itself reduced, causing the deposition of metallic silver, known as the 'silver mirror' effect.

This test is significant in the analysis of citral's structure in our exercise. The positive Tollens' reagent test confirms the presence of an aldehyde group within the citral molecule, thus guiding students through the identification process of functional groups in organic compounds.

Hydroxylamine Reaction

The reaction of organic compounds with hydroxylamine (NH₂OH) is typically used to identify carbonyl groups, such as aldehydes and ketones. The product of this reaction is an oxime, which has the general formula R₂C=NOH.

In our exercise, citral's reaction with hydroxylamine resulting in a C₁₀H₁₇ON product indicates the conversion of an aldehyde group to an oxime. This provides students with practical insight into the behavior and reactivity of carbonyl compounds, which are a significant topic in organic chemistry.

Aldehyde Functional Group

The aldehyde functional group consists of a carbonyl center (a carbon double bonded to oxygen) with at least one hydrogen attached to that carbon. Aldehydes are highly reactive and are characterized by their distinctive odors. They are susceptible to a variety of chemical reactions, including oxidation and nucleophilic addition reactions.

In citral, the presence of an aldehyde group is confirmed through reactions with tollens' reagent and hydroxylamine, as well as the oxidation products. Teaching students to recognize and predict the behavior of aldehydes is crucial, as these functional groups are widely present in naturally occurring and synthetic compounds.