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Chapter 23: Problem 130

For each category of compounds the minimum number of carbons required for optical isomerism to be possible is given. Find the correct match (es). (a) Alkane \(-7\) (b) Alkene \(-6\) (c) Alkyl halide \(-4\) (d) Alkadiene - 7

Short Answer

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(a) 7 carbons for alkane, (b) 6 carbons for alkene, (c) 4 carbons for alkyl halide, (d) 7 carbons for alkadiene.

Step by step solution

01

Understanding Optical Isomerism

Optical isomerism occurs due to the presence of a chiral center in a molecule. A carbon atom can be a chiral center if it is bonded to four different substituents.
02

Minimum Carbon Requirement for Alkanes

For alkanes, to have a chiral center, a branch must involve a carbon atom with four different groups attached. Let's try with 7 carbon atoms: a chiral center can be created in a branched structure, such as in 3-methylhexane.
03

Minimum Carbon Requirement for Alkenes

In alkenes, optical isomerism can arise from restricted rotation around the double bond. The simplest chiral alkene might be an unsymmetrical disubstituted double bond, like 3-methylpent-2-ene, but typically requires 6 carbons for such a configuration.
04

Minimum Carbon Requirement for Alkyl Halides

For alkyl halides, a carbon atom can become a chiral center if bonded to a halogen and three different other groups. An example with 4 carbon atoms is 2-bromobutane, which has a chiral center at the second carbon.
05

Minimum Carbon Requirement for Alkadienes

Alkadienes typically need more complexity for optical isomerism. For certain structures, such as hepta-1,3-diene with specific substitutions, 7 carbon atoms are required to achieve a scenario for chiral centers or cis-trans isomerism.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Chiral Center
A chiral center is key to understanding optical isomerism. It is usually a carbon atom bonded to four different substituents or groups, which creates asymmetry in the molecule. This asymmetry is crucial because it allows the molecule to exist in two non-superimposable mirror image forms, known as enantiomers. These enantiomers can rotate plane-polarized light in different directions; one clockwise and the other counterclockwise. This property is essential for the concept of chiral centers.
  • Chiral centers cause molecules to exhibit optical activity
  • Presence of four different groups is necessary for a chiral center
  • Leads to enantiomers, causing different rotations of light
Understanding chiral centers can help us determine if a molecule has optical isomerism potential.
Alkanes
Alkanes are the simplest organic compounds, consisting entirely of single-bonded carbon and hydrogen atoms. While typically not chiral because of their simple structure, they can achieve optical isomerism under certain conditions. To create a chiral alkane, at least one carbon atom must have four different groups attached. In straight-chain alkanes, this requires introducing branches.
  • Typically not chiral due to symmetry
  • Branched alkanes with a chiral center can show optical isomerism
  • Minimum of 7 carbons is often required for complexity
Examples like 3-methylhexane show how alkanes can indeed exhibit optical isomerism.
Alkenes
Alkenes contain carbon-carbon double bonds, which offer a unique structural feature that can result in optical isomerism through cis-trans isomerism and chiral centers from branching. The double bond restricts rotation, and when combined with asymmetrical substitutions, can result in chiral geometries.
  • Double bond leads to restricted rotation, influencing optical isomerism
  • Minimum of 6 carbons can typically allow formation of a chiral alkene
  • Disubstituted alkenes like 3-methylpent-2-ene can be chiral
Understanding these principles helps recognize how alkenes can illustrate optical isomerism.
Alkyl Halides
Alkyl halides are a category of organic compounds where a halogen atom is bonded to an alkane framework. Chiral centers in alkyl halides can be achieved when the halogen attaches to a carbon that has three other different groups. This makes alkyl halides a suitable candidate for studying optical isomerism.
  • Presence of a halogen atom introduces the necessary asymmetry for chiral centers
  • Typically requires 4 carbons for potential chiral center formation
  • Example: 2-bromobutane, where the bromine attachment leads to chirality
Through this, alkyl halides cleverly illustrate how even small structural changes impact optical properties.
Alkadienes
Alkadienes possess two carbon-carbon double bonds. Their complexity can allow optical isomerism to arise not only from chiral centers but also through cis-trans isomerism if the double bonds are appropriately substituted. This makes alkadienes intriguing for studying stereochemistry.
  • Contains two double bonds adding structural complexity
  • 7 carbons are often needed for sufficient complexity for optical isomerism
  • Chirality can result from specific substitution patterns
Certain substitutions in hepta-1,3-diene can result in chiral centers, showcasing the intricate ways in which alkadienes can exhibit optical isomerism.

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Most popular questions from this chapter

The hybridization of carbon atoms in \(\mathrm{C}-\mathrm{C}\) single bond of \(\mathrm{HC} \equiv \mathrm{C}-\mathrm{CH}=\mathrm{CH}_{2}\) is (a) sp \(^{3}-\mathrm{sp}^{3}\) (b) \(\mathrm{sp}^{2}-\mathrm{sp}^{3}\) (c) \(\mathrm{sp}^{2}-\mathrm{sp}\) (d) \(\mathrm{sp}^{3}-\mathrm{sp}\)

How many optically active stereoisomers are possible for butan- 2,3 -diol? (a) 1 (b) 2 (c) 3 (d) 4

On monochlorination of n-pentane, the number of isomers formed is are (a) 4 (b) 3 (c) 2 (d) 1

The IUPAC name of the compound shown below is [2006] ClC1CCCC(Br)C1 (a) 2-bromo-6-chlorocyclohex-1-ene (b) 6 -bromo-2-chlorocyclohexene (c) 3-bromo-1-chlorocyclohexene (d) 1-bromo-3-chlorocyclohexene

How many chiral carbon atoms are present in 2,3 , 4-trichloropentane? (a) three (b) two (c) one (d) four
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