Question

Limonene is one of the most common inexpensive fragrances. Two isomers of limonene can be isolated from natural sources. They are shown below. The one on the left has the odor of lemons, and the one on the right has the odor of oranges. C=C(C)C1CC=C(C)CC1 Isomers of Limonene (a) What kind of isomers are they? (b) Are \(E, Z\) isomers possible in limonene? (c) Why do these two isomers smell different?

Short answer

Answer: The given limonene isomers are enantiomers, which are a type of stereoisomers. They smell differently due to their different molecular structures, which cause them to interact differently with the olfactory receptors in our nose. The shape and size of the molecules they interact with are detected by the olfactory system, leading to the perception of different odors.

Step-by-step solution

(Step 1: Identify the type of isomers)

To identify the type of isomers, we need to examine the structural differences between the two. In this case, we see that both isomers have the same molecular formula but different spatial orientation of the substituents around the chiral center. This means they are enantiomers, which are a type of stereoisomers.

(Step 2: Determine if \(E, Z\) isomers are possible in limonene)

\(E, Z\) isomers, also known as geometrical isomers, are a type of isomer that occurs due to the different orientation of substituent groups around a double bond. In both limonene isomers, we can see that there is a double bond present. However, the double bond is between two carbon atoms that are both connected to two identical methyl groups. Therefore, there is no possibility for different orientation of the substituent groups, and consequently, \(E,Z\) isomers are not possible in limonene.

(Step 3: Explain why the two isomers smell different)

The difference in smell between the two isomers can be explained by their different molecular structures and how they interact with the olfactory receptors in our nose. Enantiomers, like the two isomers of limonene, have different spatial arrangements of their atoms, causing them to interact differently with the various receptors in our olfactory system. The olfactory receptors in our nose are highly sensitive to the shape and size of the molecules they interact with. Since the two isomers have distinct molecular shapes, they can trigger different responses from the olfactory receptors, leading to the perception of different odors.

Key concepts

Enantiomers

Enantiomers are a type of stereoisomer where molecules are mirror images of each other but cannot be superimposed onto one another. Imagine your hands; they are similar in structure but mirror images and not identical. Similarly, each enantiomer in a pair has the same molecular formula but a different arrangement in three-dimensional space, leading to different interactions with other chiral molecules.

For instance, in the case of limonene, each enantiomer interacts with the olfactory receptors differently, giving rise to distinctive scents. Being non-superimposable mirror images, enantiomers often display notable differences in their biological and chemical behavior despite having comparable physical properties such as melting points and densities.

Stereoisomers

Stereoisomers are molecules that have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in space. Apart from enantiomers, another common type of stereoisomers are diastereomers, which are not mirror images of each other.

Diastereomers can have different physical and chemical properties and, unlike enantiomers, they can be separated fairly easily due to these differences. Stereoisomerism plays a crucial role in organic chemistry and biochemistry, affecting properties such as solubility, reactivity, and boiling points.

Molecular Structure and Olfaction

Molecular structure has a profound impact on the sense of smell, or olfaction, because olfactory receptors are exquisitely sensitive to a molecule's shape and functional groups. It is the lock-and-key principle that applies here; only certain shapes of molecules fit well with specific olfactory receptors, triggering a nerve impulse that the brain interprets as a distinct smell.

Different enantiomers of limonene interact with receptors in varied ways, hence one smells like lemons and the other like oranges. The enantiomers' three-dimensional orientation determines how effectively they bind to receptors and which scent signal is transmitted to the brain.

(E, Z) Isomers

The (E, Z) isomerism, also known as geometric isomerism, occurs due to the differing spatial positions of substituents around a double bond. The 'E' (from the German 'entgegen') denotes the opposite sides of a double bond, and 'Z' ('zusammen') refers to the same side. These are particularly important in olefins, compounds that contain carbon-carbon double bonds.

In limonene, the double bond's terminals are connected to identical groups, precluding the existence of (E, Z) isomerism. This is because for (E, Z) isomers to exist, each carbon of the double bond must be attached to two different groups, allowing the geometric distinction.