Topic 17.1 Chirality

Question 1

A chiral molecule

Answer 1

One that cannot be superimposed on its mirror image
(has 4 different groups attached)

Question 2

Achiral

Answer 2

The opposite of chiral

Question 3

Enantiomers

Answer 3

–> The mirror image of chiral molecule
Optical isomers or enantiomers are chiral molecules (when they can’t be superimposed)

Question 4

Optical isomers examples

Answer 4

Lactic acid is chiral.
The two forms have identical chemical reactions and physical properties except for the effect on plane-polarised light.
–> Optical isomers polarise light in opposite directions (rotate 90° oppositely)

Question 5

Racemate

Answer 5

A mixture containing equal quantities (1:1) of the enantiomers is called a racemic mixture or racemate.
-They have no overall rotation (as they are rotated to same so there is no change)
-Optically inactive
—> Made in nucleophilic addition

Question 6

SN2 reactions

Answer 6

S- substitution
N- nucleophile
2- bimolecular (2 molecules involved is RDS)
Results in optically active product due to inversion.
(The dashed lines in the transition state show partial bonds- those being woken and formed)

Question 7

SN1 reactions

Answer 7

S- substitution
N- nucleophile
1- 1 molecule involved in RDS
Products, in theory, will be a racemic mixture due to planar intermediate.

Question 8

Polarised light

Answer 8

Chiral molecule interacts with linear plane of polarised light
-Molecule will rotate the plane
Dextroration- clockwise
Levorotation- anticlockwise

Question 9

polarimetry

Answer 9

use of a polarimeter to measure optical activity
light passes through a polariser which converts unpolarised to polarised. then through sample tube containing substance. if optically activated plane will be rotated. analyser is rotated