Question

Draw (1S,3S)-3-methylcyclopentanol and explain your reasoning.

Answer 1

Answer) (1S,3S)-3-methylcyclopentanol

Cycloentanol is tha parent chain

Cyclo means it is a cyclic ring of carbon atoms

pent means this ring consist of 5 carbon

Give all the carbons a number by counting from 1 to 5

-ol at the end without any number means the -OH(hydroxyl) group is at the first carbon in counting.

3-methyl means there is a methyl (CH₃) substituent at the third carbon in counting.

(1S,3S) means both the carbon 1 and 3 have S configuration.

To assign dash and wedge , follow the rules below:

#1 If the lowest priority group is below the plane(shown by dash) then

clockwise rotation is R configuration and

anticlockwise rotation is S configuration.

#2 If lowest priority group is above the plane(shown by wedge) then

clockwise rotation is S configuration and

anticlockwise rotation is R configuration.

To decide configuration we have see the priority order:

First priority is given to the group with highest mass and so on last priority is given to the group with lowest mass .

C1

The first carbon (C1), there is a hydroxyl group below the plane, it is given the first priority because oxygen has greater mass compared to carbon and

the right side of this the carbon (C1) has the side of the ring with methyl group near it , this side is given second priority because there is a methyl group on this side and

third priority is given to the left side carbon as there is no other additional substituent attached the

lowest priority (fourth priority) is given to the hydrogen attached to this carbon .

(Note : hydrogen is not shown in line diagrams, but as -OH group is below plane and ring is on the plane , so hydrogen is above the plane (dash))

when you make a loop from 1-2-3 . The loop is moving clockwise and as we know that the lowest priority group is above the plane so this carbon (C1) has S configuration.

C3

Similarly for third carbon (C3)

We have to see the four groups connected to this third carbon

First priority is given to the carbon that has hydroxyl group the nearest

second priority is given to the carbon on the other side of the Ring (which is away from -OH group )

third priority is given to the methyl group and

fourth priority is given to the hydrogen

(Note: As the methyl group is drawn above the plane and the other two carbons are on the ring i.e. on the plane , so the hydrogen is below the plane)

Make the loop from 1-2-3 . The loop is moving anticlockwise and as we know that the lowest priority group is below the plane, so this carbon (C3) has S configuration.