Chapter 11: Problem 88
The number of optical isomers formed by hydrogenation of the compound, \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{CHCH}_{3}\) are (a) 0 (b) 1 (c) 2 (d) 3
Short Answer
Expert verified
(a) 0 optical isomers.
Step by step solution
01
Identify the Compound Structure
First, we need to understand the structure of the given compound before hydrogenation. The compound \((\mathrm{CH}_{3})_{2} \mathrm{C}=\mathrm{CHCH}_{3}\) is a molecule with two methyl groups attached to a double-bonded carbon. Its structure can be expressed as follows: a central carbon atom connected by a double bond to another carbon atom, which is then connected to a methyl group. This double bond is crucial for understanding the stereochemistry after hydrogenation.
02
Understand Hydrogenation Effect
Hydrogenation involves adding hydrogen across the double bond, converting it into a single bond. Thus, \((\mathrm{CH}_{3})_{2} \mathrm{C}=\mathrm{CHCH}_{3}\) becomes \((\mathrm{CH}_{3})_{2} \mathrm{CH}-\mathrm{CH}_{2}\mathrm{CH}_{3}\). This creates a new chiral center at the central carbon, which was part of the double bond before hydrogenation.
03
Determine Chiral Centers
After hydrogenation, check the molecule for chiral centers. A chiral center is typically a carbon atom bonded to four different groups. In the resulting compound \((\mathrm{CH}_{3})_{2} \mathrm{CH}-\mathrm{CH}_{2}\mathrm{CH}_{3}\), the carbon atom with two methyl groups is bonded to a hydrogen and the rest of the molecule, forming a stereocenter. However, this is not actually chiral since it is symmetrical with two identical methyl groups.
04
Calculate the Number of Optical Isomers
Optical isomers occur due to the presence of chiral centers that result in non-superimposable mirror images (enantiomers). Since the molecule does not have a true chiral carbon (due to identical methyl groups), it does not form optical isomers. Therefore, the number of optical isomers is 0.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Chiral Centers
A chiral center, also known as a stereocenter, is a carbon atom that is connected to four different atoms or groups. This unique configuration gives rise to the property of chirality, which means the molecule can have non-superimposable mirror images, much like our left and right hands.
In organic chemistry, identifying chiral centers is key to understanding the potential for optical isomerism in a molecule.
In the molecule \((\mathrm{CH}_{3})_{2} \mathrm{CH}-\mathrm{CH}_{2}\mathrm{CH}_{3}\), the central carbon atom initially appears to be a candidate for a chiral center. However, because it is bonded to two identical methyl (\(\mathrm{CH}_{3}\)) groups, it does not meet the requirement of being attached to four different groups.
Therefore, no true chiral center exists in this case. Chiral centers are crucial as their presence or absence determines whether a molecule can exhibit optical activity.
In organic chemistry, identifying chiral centers is key to understanding the potential for optical isomerism in a molecule.
In the molecule \((\mathrm{CH}_{3})_{2} \mathrm{CH}-\mathrm{CH}_{2}\mathrm{CH}_{3}\), the central carbon atom initially appears to be a candidate for a chiral center. However, because it is bonded to two identical methyl (\(\mathrm{CH}_{3}\)) groups, it does not meet the requirement of being attached to four different groups.
Therefore, no true chiral center exists in this case. Chiral centers are crucial as their presence or absence determines whether a molecule can exhibit optical activity.
Hydrogenation Reaction
A hydrogenation reaction involves the addition of hydrogen (\(\mathrm{H}_{2}\)) across a double bond \(\mathrm{C}=\mathrm{C}\) in a molecule, converting it into a single bond \(\mathrm{C}-\mathrm{C}\). This process is significant in transforming unsaturated compounds (like alkenes) into saturated ones (like alkanes).
During hydrogenation, the chemical structure changes substantially because the double bond is broken, and hydrogen atoms add to the previously double-bonded carbons.
This has substantial effects on the molecule's properties, including its stereochemistry.
In our exercise, hydrogenation of \((\mathrm{CH}_{3})_{2} \mathrm{C}=\mathrm{CHCH}_{3}\) leads to the formation of \((\mathrm{CH}_{3})_{2} \mathrm{CH}-\mathrm{CH}_{2} \mathrm{CH}_{3}\), altering the molecule's structural and potentially chiral properties.
During hydrogenation, the chemical structure changes substantially because the double bond is broken, and hydrogen atoms add to the previously double-bonded carbons.
This has substantial effects on the molecule's properties, including its stereochemistry.
In our exercise, hydrogenation of \((\mathrm{CH}_{3})_{2} \mathrm{C}=\mathrm{CHCH}_{3}\) leads to the formation of \((\mathrm{CH}_{3})_{2} \mathrm{CH}-\mathrm{CH}_{2} \mathrm{CH}_{3}\), altering the molecule's structural and potentially chiral properties.
Stereochemistry
Stereochemistry refers to the 3D spatial arrangement of atoms in molecules. It's crucial because it affects how molecules interact with each other, especially in biological systems.
In the context of our exercise, understanding stereochemistry helps us evaluate changes in molecular shape and symmetry when performing reactions like hydrogenation.
The original compound, \((\mathrm{CH}_{3})_{2} \mathrm{C}=\mathrm{CHCH}_{3}\), has a double bond that allows for planar configuration, but once hydrogenated, the new carbon framework forms a single-bonded structure, which can rotate freely around the single \(\mathrm{C}-\mathrm{C}\) bond.
This potential for different spatial arrangements is a central topic within stereochemistry.
In the context of our exercise, understanding stereochemistry helps us evaluate changes in molecular shape and symmetry when performing reactions like hydrogenation.
The original compound, \((\mathrm{CH}_{3})_{2} \mathrm{C}=\mathrm{CHCH}_{3}\), has a double bond that allows for planar configuration, but once hydrogenated, the new carbon framework forms a single-bonded structure, which can rotate freely around the single \(\mathrm{C}-\mathrm{C}\) bond.
This potential for different spatial arrangements is a central topic within stereochemistry.
Enantiomers
Enantiomers are a type of optical isomer, pairs of molecules that are mirror images of each other but cannot be superimposed. This unique property is a direct result of chirality at one or more chiral centers.
In molecules with a single chiral center, such as some amino acids, enantiomers can have vastly different effects in biological systems.
However, for the molecule \((\mathrm{CH}_{3})_{2} \mathrm{CH}-\mathrm{CH}_{2}\mathrm{CH}_{3}\), hydrogenation results in no chiral centers because the central carbon is attached to two identical methyl groups.
Without chiral centers, true enantiomers cannot form, and thus the compound has no optical activity stemming from enantiomerism.
Understanding when a molecule can have enantiomers is important for predicting its chemical behavior and interactions.
In molecules with a single chiral center, such as some amino acids, enantiomers can have vastly different effects in biological systems.
However, for the molecule \((\mathrm{CH}_{3})_{2} \mathrm{CH}-\mathrm{CH}_{2}\mathrm{CH}_{3}\), hydrogenation results in no chiral centers because the central carbon is attached to two identical methyl groups.
Without chiral centers, true enantiomers cannot form, and thus the compound has no optical activity stemming from enantiomerism.
Understanding when a molecule can have enantiomers is important for predicting its chemical behavior and interactions.
