Problem 13 - Basic Principles of Organic Chemistry

Question

Designate which of the following structures are chiral, prochiral, and/or achiral. Specify which carbon atoms are prochiral centers. a. ethenylbenzene (styrene) b. cis-2-butene c. 2 -propanone (acetone) d. 2 -butanone e. glycine, \(\mathrm{H}_{2} \mathrm{NCH}_{2} \mathrm{CO}_{2} \mathrm{H}\) f. butanedioic acid, \(\left(\mathrm{CH}_{2} \mathrm{CO}_{2} \mathrm{H}\right)_{2}\) g. 2-methylbutanedioic acid, \(\mathrm{CH}_{3} \mathrm{CH}\left(\mathrm{CH}_{2} \mathrm{CO}_{2}\right.\) \(\mathrm{H}) \mathrm{CO}_{2} \mathrm{H}\) h. 1 -chloro-2-phenylethane

Step-by-Step Solution

Answer

Cis-2-butene, ethenylbenzene, and glycine are achiral; 2-methylbutanedioic acid and 1-chloro-2-phenylethane are chiral. 2-methylbutanedioic acid has a prochiral center.

Step 1: Understand Key Definitions

Before we begin, let's review key terms. A chiral molecule is one that is not superimposable on its mirror image. An achiral molecule is superimposable on its mirror image and lacks chirality. A prochiral center is an atom in a molecule that can become a chiral center by changing one of two identical groups.

Step 2: Analyze Ethenylbenzene (Styrene)

Ethenylbenzene (styrene) contains a vinyl group attached to a benzene ring. Since the structure is symmetrical through the plane of the benzene ring and there is no chiral center, it is achiral. There is no prochiral center since no chiral center can be created by substitutions.

Step 3: Analyze Cis-2-Butene

Cis-2-butene is symmetrical due to the presence of the methyl groups on the same side of the double bond, making this structure achiral. There are no tetrahedral carbon atoms with identical substituents that could be made chiral, so it lacks prochiral centers.

Step 4: Analyze 2-Propanone (Acetone)

2-Propanone (acetone) is achiral due to its symmetry (plane of symmetry present). Additionally, both methyl groups are identical, meaning no prochiral centers are possible.

Step 5: Analyze 2-Butanone

2-Butanone has a plane of symmetry and thus is achiral. The carbonyl carbon is not tetrahedral, and the methyl and ethyl groups make no potential prochiral centers in nearby carbons.

Step 6: Analyze Glycine

Glycine is the only amino acid that is achiral due to the hydrogen atoms being identical and symmetric across the molecule. It lacks any chiral or prochiral centers.

Step 7: Analyze Butanedioic Acid (Succinic Acid)

Butanedioic acid is symmetrical through the central carbon chain and is therefore achiral. There are no chiral centers nor any potential prochiral centers.

Step 8: Analyze 2-Methylbutanedioic Acid

2-Methylbutanedioic acid has a single chiral center at the carbon next to the methyl branch, making it chiral. Additionally, the same carbon is considered prochiral due to the potential creation of a second chiral center by substitution.

Step 9: Analyze 1-Chloro-2-Phenylethane

1-Chloro-2-phenylethane has two different substituents on the chiral carbon (chlorine and phenyl group), so it is chiral. This structure lacks prochiral centers.

Key Concepts

Chiral Molecules

Chiral molecules have a unique property that makes them fascinating in the world of chemistry. These molecules are not superimposable on their mirror images, much like how your left hand is not superimposable on your right hand. This property is called "chirality," and it plays a crucial role in the behavior of many organic compounds.
Imagine a molecule as a three-dimensional object. If you took this object and its mirror image, you would not be able to align all parts of the two objects in a way that makes them the same. This is an indication of chirality.
Chiral molecules often contain an "asymmetric carbon" or "chiral center." This is a carbon atom that is bonded to four different groups or atoms. The unique three-dimensional arrangement of these groups gives the molecule its chiral nature. For example, in the case of 1-chloro-2-phenylethane, the presence of a chiral carbon results in the molecule being chiral because it has distinct substituents like chlorine and a phenyl group that cannot be superimposed on its mirror image.

Achiral Molecules

Achiral molecules, on the other hand, can be superimposed on their mirror images. This means that the molecule and its mirror image are identical in all respects. Achiral molecules are characterized by having a plane of symmetry, where one half of the molecule is a mirror image of the other half.
Many simple organic compounds are achiral. For example, acetone and glycine are both achiral. In the case of acetone (2-propanone), the molecule has a mirror plane cutting through the double-bonded oxygen and directly across the molecule, making it identical to its mirror image.
It's important to identify achiral molecules, especially when studying chemical reactions and properties, because they do not exhibit the same optical activity as chiral molecules. This means that they do not rotate plane-polarized light, which is a key characteristic of chirality.

Prochiral Centers

A prochiral center is a concept used to describe certain atoms in molecules that aren't chiral yet, but could become chiral through a chemical transformation. Specifically, a prochiral center is an atom that can transform into a chiral center when one of its substituents is replaced by a different group.
This typically involves tetrahedral carbon atoms with two identical groups. For instance, in 2-methylbutanedioic acid, the carbon adjacent to the methyl group is both a chiral center and considered prochiral. A reaction could replace one of the hydrogen atoms on this carbon with a different atom or group, thereby creating a new chiral center.
Understanding prochiral centers is crucial when designing syntheses in organic chemistry, as it allows chemists to predict and control the formation of chiral centers in molecules.

Molecular Symmetry

Symmetry in molecules is about balance and proportion. A molecule is symmetrical if it has at least one symmetry element. These symmetry elements can include planes, axes, or centers of symmetry within the molecule.
The presence of molecular symmetry often leads to a molecule being achiral. For example, cis-2-butene has a symmetrical structure because the methyl groups are positioned on the same side of the carbon-carbon double bond. This symmetry means that the molecule can be superimposed on its mirror image, rendering it achiral.
Symmetry is a key concept in determining many properties of molecules, from their chemical reactivity to their optical activity. Often, finding a plane of symmetry can quickly help identify if a molecule is achiral, as seen in various simple organic molecules like butanedioic acid.