Question

The compounds whose molecules are shown below would have:

• A. different melting points
• B. equal but opposite optical rotation
• C. the same melting poin
• D. more than one of the above

Answer 1

The correct option is D more than one of the above

The $R$ and $S$ configuration is used to name enantiomers of a chiral compound.

(Refer to Image 1)

Rules for assigning priority:-

1) Prioritize the $4$ atoms or groups of atoms attached to chiral center based on the atomic number of atom that is bonded directly to chiral carbon. Higher the atomic number higher the priority.

$H$ will have least priority as it has least atomic number.

2) If $2$ or more of the atoms that are bonded directly to chiral center are same, then prioritize the atoms adjacent to directly bonded atoms. Priority is assigned at first point of difference.

• If $2$ atoms have substituents of same priority, higher priority is assigned to atom with more of there substituents.

$H$ $H$

$-\underset{|}{\stackrel{|}{C}}-Cl$ $>$ $-\underset{|}{\stackrel{|}{C}}-H$

$Cl$ $Cl$

$-CHC{l}_2$ $-C{H}_{2}Cl$

higher priority

• Isotopes like $H,D,T$ have same atomic number, so mass number is used to prioritize them $T>D>H$

If a multiple bond is encountered treat it as if the atoms are attached by same number of single bonds.

Example:- $-C=C\Rightarrow -\underset{|}{\stackrel{|}{C}}-\underset{|}{\stackrel{|}{C}}-$

$C$ $C$

$C$ $C$

$-C\equiv C\Rightarrow -\underset{|}{\stackrel{|}{C}}-\underset{|}{\stackrel{|}{C}}-$

$C$ $C$

3) Determine orientation of least $\left(4^{th}\right)$ priority group.

• If it is oriented into plane of paper (away from viewer's side) then clockwise rotation corresponds to $R$ configuration and anti clockwise rotation corresponds to $S$ configuration.

If it is oriented towards us then clockwise rotation corresponds to $S$ configuration and anticlockwise rotation corrsponds to $R$ configuration.

By using these rules we will assign configuration to following compounds:-

(Refer to Image 2)

Least priority group towards viewer's side so configuration reverts. So, configuration is $S$.

(Refer to Image 3)

Least priority group towards viewer's side. So, configuration reverts$\Rightarrow R$

So, both the compounds are enantiomers as their absolute configuration is opposite.

Properties of enantiomers:

1) They have same chemical and physical properties (boiling points, melting points, etc.)

2) They rotate direction of plane polarised light to equal bu opposite angles.

So, option B and C both are correct. So answer is Option D.