`(S)`-sec Buty`1` chloride is subjected to free-radical chlorination followed by fractional distillation to isolate the various isomeric products. How many frecations are isolated?

Options

A. Four
B. Five
C. Six
D. Three

Answer

B

Explanation

In analyzing the sterochemistry of such reactions follow the guidelines:
(1) Unless a bond to an original chiral center is broken, the configuration is retained center and (2) if a new, chiral center is generated, both possible configuration about the new center results.
`(S)-CH_(3)overset(CI)overset(|)CHCH_(2)CH_(3)overset(CI_(2))underset(hv)rarrunderset(1,2-"Dichlorobutane")underset((A))(CICH_(2)overset(**)CHCICH_(2)CH_(3))+underset(2,2-"Dichlorobutane")underset((B))underset(CI)underset(|)overset(CI)overset(|)(CH_(3)C CH_(2)CH_(3))+underset(2,3-"Dichlorobutane")underset((C ))(CH_(3)underset(**)overset(CI)overset(|)CHoverset(CI)overset(|)underset(**)CHCH_(2)) +underset(1,3-"Dichlo robutan e")underset((D))(CH_(3)overset(CI)overset(|)underset(**)CHCH_(2)CH_(2)CI`
Monochlorination of sec-buty`1` chloride can occur at any one of four carbons. The staring meterical is the optically active compound `(S)`-sec buty`1` chloride.

`underset("see-Buty1 chloride")(CI gt C gt C gt C-H gt H)`
`(A)` is still chiral but the `C-2` configuration is unchanged because no bonds to the chiral carbon `(C-2)` have beem broken. However because of the change in priorities `(CH_(2)CI gt CH_(2)CH_(3))`, it is `R`.

`underset(("optically active"))underset(1,2-"Dichlorobutane")(CI gt C-CI gt C-C gt H)`
`(B)` is achiral because during its formation the chiral center `(C-2)` is destroyed as `C-2` with two identical ligands (two `CI's`) has a plane of symmetry.

Two diastereomers corresponding to `(C )` are formed. `C-2` is steill `(S)` as none of its bond is broken but `C-3` can be `R` or `S` as the flat radical has enantiotopic faces. These diastereomers are `(2S,3S)` which is optically active and meso `(2S,3R)`.

Attack at `C-4` does not effect the configuration at `C-2` because no bond to the chiral center is broken. `(D)` is also chiral and `(S)` because the priority order does not changes (ot is just accidental that `-CH_(2)CH_(2)Ci` happens to be of the same relative priority as the `-CH_(2)CH_(3))`.

Note that both `(A)` and `(B)` and `(D)` are formed with retention of configuration but `(A)` has the opposite sepecification while `(D)` has the same specification.
Thus a total of five diastereomeric fractions are obtained of which two are optically inactive and three optically active. Monomchlorination of `(R)`-sec-buty`1`chloride will yield similar result i.e. five diastereomeric fractions (two inactive and three active). Monochlorination of racemic sec-buty`1`chloride also gives five diastereomeric fractions but all optically inactive. They will be the same as discussed above except that each chiral structure is now accompanied by an equal amount of its enantiomer.