Question

Alkenes can be converted to alcohols by hydroboration-oxidation. Draw the structure of the alcohol(s) formed in the following reaction sequence. If applicable, draw hydrogen at a chirality center and use wedge-and-dash bonds to designate the stereochemistry.

 

Select all that apply: The alcohol product(s) of the reaction is characterized as being


_____ R,R

_____ R,S (and/or S,R)

_____ S,S

_____ achiral

_____ racemic

_____ diastereomers

_____ R

_____ S

Answer 1

Concepts and reason

The concept used to solve this problem is hydroboration of alkene followed by the oxidation. Diborane is used as an electrophile. The addition of diborane to the alkene occurs in a cis manner. The hydrogen and boron add to the double bond from the same side of the double bond. The addition is antimarkonikov in nature.

The more electronegative part adds to the double bonded carbon having more number of hydrogen across the double bond.

Asymmetric carbon (chiral center) can be identified, when the carbon atom is bonded with four different groups or atoms. Assign the R and S configurations to the asymmetric carbon atoms.

Fundamentals

Chirality center: An atom (usually carbon) which is bonded to four different atoms or groups is known as chiral center. In general, it is represented with an asterisk (*).

R and S configuration can be assigned to the compound by using the following CIP (Cahn-Ingold-Prelog) sequence rules:

Rule 1: Select the chiral center in the compound and assign priorities to the atoms based on its atomic number. The atom which is having highest atomic number gets the first priority and atom which is having least atomic number (usually hydrogen) gets fourth priority.

Rule 2: If the isotopes of same atom are attached to the chiral center, then then atom with the higher atomic mass receives higher priority.

Rule 3: If the first atom of each substituent is same then give priority to the second atom in the each substituent.

Rule 4: If the substituents have multiple bonds, then the multiple bonded atoms are considered as same number of single boned atoms.

If the rotation of the numbering is in clockwise direction, then the configuration of the isomer is R. Similarly, if the rotation of the numbering is in anticlockwise direction, then the configuration of the isomer is S.

The reaction is as follows:

The configurations for the formed alcohols are as follows:

Ans:

The structures of the formed alcohols are as follows:

The alcohol products of the reaction are characterized as follows: