Optical Isomers

Question 1

Stereoisomers

Answer 1

molecules that have the same structural formula but a different arrangement of atoms in space.

Question 2

Types of Stereoisomers

Answer 2

E-Z isomers And Cis trans isomers (these are geometric isomers)
Optical isomers

Question 3

Optical isomers

Answer 3

non-superimposable mirror images which rotate the plane of plane-polarised light in an equal amount but opposite directions

Question 4

What is plane polarised light

Answer 4

Light that oscillates in one plane

Question 5

How to measure rotation of plane polarised light?

Answer 5

Using a polarimeter

Question 6

Chrial carbon

Answer 6

Carbon that has four different groups bonded to it

Question 7

Enantiomers

Answer 7

How you refer to optical isomers of the same molecule

Question 8

Racemic mixture

Answer 8

A 50-50 mixture of the two enantiomers of an optical isomer
Does not rotate the plane of plane polarised light

Question 9

Drawing enantiomers

Answer 9

In a tetrahedral shape
Draw a chairal carbon centre which has 4 different groups attached to it
Draw a line to be the mirror and a mirror image thus this shows molecules with same structural formula but a different arrangement of atoms in space

Question 10

What must be present for a molecule to be an enantiomer of each other

Answer 10

A molecule with a chiral carbon

Question 11

How to find a chiral carbon

Answer 11

Ignore any CH3/CH2 as these aren’t bonded to 4 different things
Ignore involved in double bond
If it’s a ring molecule, consider its symmetry

Question 12

Sn1 mechanism RDS

Answer 12

Nucleophilic substitution where only 1 molecule is present in the first step (RDS)
Which is the heterolytic fission of the C-X bond to form a carbocation
NO INTERMEDIATE with square brackets

Question 13

Sn1 mechanism: where can the nucleophile attack from

Answer 13

Because the first step forms a carbocation trigonal planar molecule,
Nucleophile can attack above and below the plane to form 2 enantiomers of an alcohol

Question 14

Sn1 mechanism will form a….

Answer 14

Racemic mixture because a trigoal planar carbiocation was formed in step 1 which has equal chance of the nucleophile attacking above or below plane
So forms equal concentrations of enantiomers = reflection of plane polarised light is cancelled out

Question 15

Sn2 mechanism RDS

Answer 15

In the first step both the nucleophile attacks as the C-X bond undergoes heterolytic fission at the same time (2 molecules involved in RDS) from opposite directions
To form a trigonal bipyramidal intermediate

Question 16

The intermediate in an Sn2 mechanism

Answer 16

In square brackets
Dashed lines to show leaving group and nucleophile added on OPPOSITE SIDES
Trigonal bipyramidal

Question 17

Does sn2 form a racemic mixture?

Answer 17

No because nucleophile can only be added from direction other group is leaving so only 1 enantiomer formed which is not a racemic mixture

Question 18

Does sn2 reflect the plane of plane polarised light?

Answer 18

Yes because the reflection isn’t cancelled out by 2 enantiomers which reflect in diff directions

Question 19

When HCN reacts with carbonyls…. Will the product always contain a chiral carbon?

Answer 19

No, if it’s a symmetrical ketone then will contain equal length carbon chains bonded to central carbon

Question 20

Why is HCN + carbonyl product optically inactive?

Answer 20

Because the carbonyl compound is Trigonal planar shape so nucleophile can attack above and below the molecules plane to form equal concentrations of enantiomers (hydroxynitrile) = racemic mixture