Question

The following equation shows the reaction of trans-2,3-diphenyloxirane with hydrogen chloride in benzene to form 2-chloro-1,2-diphenylethanol. (a) How many stereoisomers are possible for 2 -chloro- 1,2 -diphenylethanol? (b) Given that opening of the epoxide ring in this reaction is stereoselective, predict which of the possible stereoisomers of 2-chloro-1,2-diphenylethanol is/are formed in the reaction.

Short answer

Question: Determine (a) the number of possible stereoisomers for 2-chloro-1,2-diphenylethanol, and (b) predict which stereoisomer(s) will be formed in the reaction considering the stereoselective opening of the epoxide ring. Answer: (a) There are 4 possible stereoisomers for 2-chloro-1,2-diphenylethanol. (b) The major product formed in the reaction should be the stereoisomer where both chiral centers are inverted compared to the starting trans-2,3-diphenyloxirane.

Step-by-step solution

Identify chiral centers of 2-chloro-1,2-diphenylethanol

To find the number of stereoisomers, we should first identify the chiral centers in the molecule. Chiral centers are usually carbon atoms that have four different groups attached. In 2-chloro-1,2-diphenylethanol, the chiral centers are the carbons at positions 1 and 2. Carbon-1 has an OH group, a chlorine, a phenyl group, and is connected to carbon-2. Carbon-2 has two phenyl groups, a chlorine, and is connected to carbon-1.

Calculate the number of stereoisomers

Now that we have identified the chiral centers, we can use the formula 2^n to calculate the number of possible stereoisomers (where n is the number of chiral centers). In this case, n = 2, so there are 2^2 = 4 possible stereoisomers.

Predict the stereoisomer(s) formed in the reaction

The given reaction is the opening of an epoxide ring, which is stereoselective, meaning that it favors the formation of one stereoisomer over the others. In this case, the nucleophile (HCl) attacks from the back side of the molecule, which results in the opening of the epoxide ring and formation of a new configuration around the two chiral centers. In this reaction, the HCl attack will lead to inversion of stereochemistry at both chiral centers, as the bulky phenyl groups are on the opposite side of the epoxide ring opening. Therefore, the major product will have both chiral centers inverted compared to the starting material (trans-2,3-diphenyloxirane).

Conclusion:

In summary, there are 4 possible stereoisomers for 2-chloro-1,2-diphenylethanol. The major product formed in the reaction should be the stereoisomer where both chiral centers are inverted compared to the starting trans-2,3-diphenyloxirane.

Key concepts

Chiral Centers

In organic chemistry, chiral centers play a fundamental role in determining the three-dimensional arrangement of molecules. These centers, often carbon atoms, are unique in that they are bonded to four different groups or atoms, leading to non-superimposable mirror images, called enantiomers.

When analyzing compounds such as 2-chloro-1,2-diphenylethanol, identifying chiral centers is crucial. This molecule contains two chiral centers: one is bonded to an OH group, a chlorine atom, a hydrogen atom, and a carbon attached to two phenyl groups; the other, while also bonded to the first carbon, differs by being attached to a chlorine atom and the aforementioned two phenyl groups. Each chiral center can exhibit two configurations ('R' or 'S'), contributing to the molecule's chirality.

Understanding chiral centers helps predict the number of stereoisomers a compound can have. It is calculated using the formula \( 2^n \), where \( n \) is the number of chiral centers. For 2-chloro-1,2-diphenylethanol with its two chiral centers, there are \( 2^2 = 4 \) possible stereoisomers.

Stereoselective Reactions

The concept of stereoselective reactions is paramount in the field of stereoisomer preparation and synthesis. These reactions are characterized by the preference for the formation of a particular stereoisomer when multiple stereoisomers are possible.

During the reaction of trans-2,3-diphenyloxirane with hydrogen chloride, the opening of the epoxide ring is not arbitrary; it occurs in a way that is stereoselective. This leads to the preferential formation of one stereoisomer over others. The bulky phenyl groups and the incoming nucleophile's direction - in the case of this problem, the chloride ion's attack angle - influence which isomer results. As a chloride ion approaches, it tends to attack from the most accessible direction, often the least hindered side of the molecule. Subsequently, the nucleophile adds to the carbon atom opposite to the larger substituents, causing a specific stereoisomer to be favored.

Stereoselectivity is a key concept in many pharmaceutical syntheses, as the biological activity of drugs can be highly stereospecific. Hence, controlling the stereochemistry of reactions is essential.

Epoxide Ring Opening

Another fascinating area in organic chemistry is the epoxide ring opening. Epoxides are three-membered cyclic ethers and are known for their high strain and reactivity.

Their ring strain makes epoxides prone to nucleophilic attack, where a nucleophile, such as the chloride ion from hydrogen chloride (HCl), targets an electron-deficient carbon atom. In the ring-opening process, the nucleophile attacks the less hindered side, leading to the cleavage of the strained ring and formation of a more stable, open-chained product.

The epoxide ring opening is particularly interesting because it's a textbook example of a stereoselective reaction. It often involves the creation of new chiral centers, as seen in the formation of 2-chloro-1,2-diphenylethanol from trans-2,3-diphenyloxirane — where the nucleophile attacks from the back side, causing inversion of stereochemistry at the carbon atoms that were part of the epoxide ring.

This type of reaction is instrumental in the synthesis of complex molecules where specific stereochemistry is desired. Understanding its mechanism provides deeper insight into how the structure of substrates influences the outcome of the reaction and why certain stereoisomers are favored over others in the resulting products.