39 E - Organic Chemistry (Mcmurry)

Question

Question: Assume that you have carried out a radical chlorination reaction on

and have isolated (in low yield) . How many stereoisomers of the product are formed, and in what ratio? Are any of the isomers optically active? (See Problem 10-38.)

Step-by-Step Solution

Answer

Two stereoisomers of the product are formed, which are unequal in ratio, so they are not in a 1:1 ratio. One product is optically active. 

Step-by-Step Solution Step 1: R and S- configuration

A counterclockwise direction is an S configuration, and a clockwise direction is an R configuration.

Step 2: Sequence Rules for Specifying Configuration

Rule 1

Number all the four atoms directly attached to the chirality center according to their atomic number. The atom with the highest atomic number has the highest ranking (first), and the atom with the lowest atomic number has the lowest ranking (fourth).

· Rule 2

If a decision can’t be reached by ranking the first atoms in the substituent, then look at the second, third, or fourth atoms away from the chirality center until finding the first difference.

· Rule 3

If two atoms are multiple-bonded, it is counted as equivalent to the same number of single-bonded atoms.

· After ranking the four groups attached to a chiral carbon, we orient the lowest priority group to the rear side (away from the observer) and then describe the stereochemical configuration around the carbon by using three remaining substituents that appear to radiate toward us.

· If a curved arrow is drawn from the highest to second-highest to third-highest ranked substituent, it is clockwise. The chirality center has the R configuration, and if the arrow is counterclockwise, then the S configuration.

Step 3: Optically inactive

A compound unable to show optical rotation is called optically inactive. Most achiral compounds are optically inactive. 

Step 4: Chiral carbon

Chiral carbons have a non-superimposable mirror image. All the groups present on 3D carbon should be different

Step 4: Chiral radical intermediate formation

For the formation of 2-chloropentane 2,4-2-dichloropentane   a chiral radical formation occur when hydrogen is abstracted 2-chloropentane . The reaction of this intermediate with chlorine occurs with unequal probability on both sides due to steric hindrance and repulsion. So the two diastereomers formed are not in a 1:1 ratio. 

Diastereomers possible

Step 5: Products

Here the first product is optically active while the second is a meso compound having symmetry in its molecule hence optically inactive.