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Chapter 4: Problem 17

Draw all the stereoisomers of the following molecules. Indicate which of the molecules you've drawn are chiral and which are achiral. (a) 2,3 -dibromobutane (b) 2,2 -dibromo-3,3-dichlorobutane (c) 2,3 -dibromo-2,3-dichlorobutane (d) 2,3 -dibromo-2-chloro-3-fluorobutane

Short Answer

Expert verified
2,3-Dibromobutane has 4 stereoisomers (2 are chiral, 2 are meso). 2,2-Dibromo-3,3-Dichlorobutane is achiral with no stereoisomers. 2,3-Dibromo-2,3-Dichlorobutane has 4 stereoisomers (2 are chiral, 2 are meso). 2,3-Dibromo-2-Chloro-3-Fluorobutane has 4 stereoisomers (all chiral).

Step by step solution

01

Identify Chiral Centers in Each Molecule

Look at each carbon atom in the given molecules. Identify if they have four different groups attached. A chiral center (stereocenter) must have four different atoms or groups of atoms attached to it.
02

Draw Stereoisomers for 2,3-Dibromobutane

2,3-Dibromobutane has two chiral centers at carbon 2 and carbon 3. Draw all combinations of the bromine atoms being in the same or different orientations (R/R, S/S, R/S, S/R).
03

Determine Chirality for 2,3-Dibromobutane

Check each of the drawn structures. Diastereomers are not mirror images and have different physical properties. Enantiomers are non-superimposable mirror images and will be chiral. Meso compounds are superimposable on their mirror image and are achiral.
04

Draw Stereoisomers for 2,2-Dibromo-3,3-Dichlorobutane

This compound doesn't have any chiral centers because each carbon with the bromine or chlorine atoms attached is attached to identical groups. Therefore, no stereoisomers are present.
05

Define Chirality for 2,2-Dibromo-3,3-Dichlorobutane

Since there are no chiral centers, this molecule is achiral.
06

Draw Stereoisomers for 2,3-Dibromo-2,3-Dichlorobutane

There are two chiral centers, one at carbon 2 and one at carbon 3. Draw all the possible combinations of bromine and chlorine being in different orientations (R/R, S/S, R/S, S/R).
07

Evaluate Chirality for 2,3-Dibromo-2,3-Dichlorobutane

Check the drawn structures. Determine if they are non-superimposable mirror images (chiral) or not. Meso compounds will typically have plane symmetry and will be achiral.
08

Draw and Evaluate 2,3-Dibromo-2-Chloro-3-Fluorobutane

This molecule has chiral centers at carbon 2 and carbon 3. Draw all possible combinations of the bromine, chlorine, and fluorine atoms. Evaluate each structure for chirality as well. Check if any meso forms exist.

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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, or stereocenter, is a carbon atom with four different atoms or groups attached to it. This unique configuration creates a non-superimposable mirror image, known as chirality. Chiral centers are essential in creating stereoisomers.
To identify a chiral center, inspect each carbon atom in the molecule. If a carbon atom connects to four different substituents, it's a chiral center.
For instance, in 2,3-dibromobutane, the carbon atoms at positions 2 and 3 each have four distinct groups attached, making them chiral centers.
In short, chiral centers lead to the diversity of stereoisomers, including enantiomers and diastereomers.
Diastereomers
Diastereomers are stereoisomers that are not mirror images of each other. They arise from having multiple chiral centers in a molecule.
Key characteristics of diastereomers include:
  • Different physical properties: They have different melting points, boiling points, and solubilities.
  • Non-superimposable: You cannot overlay one diastereomer on top of the other perfectly.

For example, in 2,3-dibromobutane, when one chiral center has an R configuration and the other an S, you get diastereomers (R/S and S/R).
The importance of diastereomers lies in their distinct chemical and physical properties, which are crucial in synthesis and drug design.
Enantiomers
Enantiomers are pairs of stereoisomers that are non-superimposable mirror images of each other. They occur when molecules have one or more chiral centers.
Key attributes of enantiomers include:
  • Identical physical properties in a symmetrical environment: They have the same melting points, boiling points, and densities.
  • Optical activity: They rotate plane-polarized light in opposite directions - one clockwise (dextrorotatory) and the other counterclockwise (levorotatory).

An example is 2,3-dibromobutane, where the R,R and S,S configurations form a pair of enantiomers.
Understanding enantiomers is vital for their roles in biological systems and pharmaceuticals, as one enantiomer can have vastly different effects from its mirror image.
Meso Compounds
Meso compounds are unique achiral molecules that contain multiple chiral centers. Despite having chiral centers, they are superimposable on their mirror images due to an internal plane of symmetry.
Characteristics of meso compounds include:
  • Achirality: They do not exhibit optical activity.
  • Internal symmetry: An internal plane of symmetry divides the molecule into two mirror-image halves.

For example, 2,3-dibromo-2,3-dichlorobutane can form meso compounds because the molecule is symmetrical, thus achieving superimposability.
Meso compounds are significant in stereochemistry as they illustrate how symmetry can negate chirality, leading to achiral molecules despite the presence of chiral centers.

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Most popular questions from this chapter

There are six isomers of dichlorocyclobutane. Draw them all and name them correctly.

Draw 1-pentene. Consider the constitutional isomers obtained by replacing any hydrogen of 1-pentene with a methyl group. Draw the only isomer that is chiral. Draw both enantiomers and provide the IUPAC name for each. Don't forget the \((R)\) or \((S)\) description in the name.

Observe the reactions "Unimolecular nucleophilic substitution" and "Unimolecular elimination." The first intermediate in each reaction has a carbon that is planar. What is the hybridization of that carbon in each case? What would be the outcome if a stereogenic carbon were involved in the reaction? Would chirality be retained? Why or why not? PROBLEM 4.61 Observe the "Bimolecular elimination" reaction. When the base reacts with the hydrogen in the first step of the reaction, what is the spatial relationship between that hydrogen and the bromide leaving group? Do you suppose this relationship is important? Why or why not?

Draw three-dimensional pictures of all the stereoisomers of 3,4 -dimethylheptane and 3,5 -dimethylheptane. It is probably easiest to draw them in the eclipsed arrangement, even though this is not a low-energy conformation. Determine the absolute configuration \((R)\) or \((S)\) of each stereogenic carbon. Designate the stereogenic carbons with an asterisk.

Draw \((2 R, 3 S)-2,3\)-dichloropentane. Draw the enantiomer of \((2 R\), \(3 S)-2,3\)-dichloropentane. Draw a diastereomer of \((2 R, 3 S)-2,3\)-dichloropentane.
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