Q50E - Organic Chemistry (Mcmurry)

Question

Imagine that you have created (2R, 3R)-2,3-epoxy-3-methylpentane with aqueous acid to carry out a ring opening reaction.

1. Draw the epoxide showing stereochemistry.
2. Draw and name the product showing stereochemistry.
3. Is the product chiral? Explain.
4. Is the product optically active? Explain.

Step-by-Step Solution

Answer

a) 

2,3-epoxy-3-methylpentane

b) 

c) Yes, the product is chiral.

d) Yes, the product is optically active.

Step 1: Ring opening reaction

2,3-epoxy-3-methylpentane undergoes ring opening reaction by acid hydrolysis. When nucleophilic water attacks the epoxide oxygen in aqueous condition it gets protonated. The final product will be 1,2- diol with OH group trans to each other.

Step 2: Stereochemistry of epoxide (a)

After including stereochemistry,  (2R, 3R)-2,3-epoxy-3-methylpentane looks as follows:

Stereochemistry of 2,3-epoxy-3-methylpentane

Step 3: Stereochemistry of product

Following product is formed after the reaction:

Stereochemistry of 3-methylpentan- 2,3- diol

The ring opening reaction of epoxide result the product 1,2-diol. The given epoxide is asymmetric so, the water nucleophile is attacking more substituted epoxide carbon. Thus, the final product is 3-methylpentan- 2,3- diol.

Step 4: Chirality of 3-methylpentan- 2,3- diol

The obtained product is chiral, because there is no plane of mirror in the structure. The left- and right-hand side is not mirror images of each other.

Step 5: Optical activity of 3-methylpentan- 2,3- diol

The obtained product is optically active. All chiral compounds are optically active. 3-methylpentan- 2,3- diol is capable of optical rotation.