Wrapping Up: Subdivision of Isomers

Isomers(different compounds with same molecular formula)

Constitutional Isomers

(isomers whose atoms have a different

connectivity)

Stereoisomers(isomers that have the same

connectivity but differ in spatial arrangement of their

atoms)

Enantiomers(stereoisomers that are

nonsuperimposable mirror

images of each other)

Diastereomers(stereoisomers that areNOT mirror images of

each other)

Determining the relationship between

molecules• If the molecular formula is different they are not

isomers

Example: benzene vs. cyclohexane

© 2014 by John Wiley & Sons, Inc. All rights reserved.

Determining the relationship between

molecules• If the molecular formula is identical, but they are

connected differently they are structural isomers

Example: cis-1,2 vs. cis-1,3-dimethylcyclohexane

Example: diethyl ether vs. 1-butanol

Determining the relationship between

molecules• If the connectivity is identical, but they differ in

the 3-D arrangement of atoms in space they are steroisomers.

Determining the relationship between

molecules• If the molecules are not superimposable and

mirror images, they are enantiomers:

Determining the relationship between

molecules• If the molecules are not superimposable nor are

they mirror images, they are diastereomers:

Determining the relationship between

molecules• In molecules with multiple stereocenters, this

may be difficult to visualize.

• Simply, if every R, S center flips between the molecules you are comparing, they are enantiomers:

Determining the relationship between

molecules• If at least one center remains the same and at

least one other flips, they are diastereomers!

Physical Properties of Stereoisomers• Enantiomers have identical physical properties

(e.g. melting point, boiling point, refractive index, solubility etc.)

Compound bp (oC) mp (oC)

(R)-2-Butanol 99.5

(S)-2-Butanol 99.5

(+)-(R,R)-Tartaric Acid

168 – 170

(–)-(S,S)-Tartaric Acid 168 – 170

(+/–)-Tartaric Acid 210 – 212

© 2014 by John Wiley & Sons, Inc. All rights reserved.

Physical Properties of Stereoisomers• Enantiomers also:

o Have the same chemical properties (except reaction/interactions with chiral substances)

o Show different behavior only when they interact with other chiral substances (enzymes)

o Rotate plane-polarized light in equally in opposite directions - this property of enantiomers is called optical activity

© 2014 by John Wiley & Sons, Inc. All rights reserved.

Optical Activity• The property possessed by chiral substances of

rotating the plane of polarization of plane-polarized light

• The electric field (like the magnetic field) of light is oscillating in all possible planes

• When this light passes through a polarizer (Polaroid lens), we get plane-polarized light (oscillating in only one plane)

Polaroidlens

Optical Activity – Measuring on

Polarimeter• Polarimeter – instrument to measure optical

activitya = observed optical

rotation

Optical Activity – Measuring on

Polarimeter

Optical Activity – Measuring on

Polarimeter

Optical Activity – Measuring on

Polarimeter

Optical Activity – Calculating Specific

Rotation

D[] =

25 c x

temperature

wavelength of light(e.g. D-line of Na lamp, l = 589.6 nm)

concentration of sample solutionin g/mL

length of cellin dm (1 dm = 10 cm)

ℓ

observed rotation

Optical Activity – Calculating Specific

Rotation• The value of a depends on the particular

experiment (since there are different concentrations with each run)

• But specific rotation [aD] should be the same regardless of the concentration

Optical Activity – Calculating Specific

Rotation• Two enantiomers should have the same value of

specific rotation, but the signs (+/-) are opposite

mirror

HO

*

CH2CH3

CH3

H

[] = + 13.5o25

D

OH

*

H3CH2C

CH3

H

[] = 13.5o25

D

Optical Activity – Calculating Specific

Rotation• An equimolar mixture of two enantiomers is called

a racemic mixture (or racemate or racemic form)

• A racemic mixture causes no net rotation of plane-polarized light

HC2H5

CH3

OH

(R)-2-Butanol

HC2H5

H3C

HO

(S)-2-Butanol(if present)

rotation

equal & opposite rotation by the enantiomer

Racemic Forms and Enantiomeric Excess• A sample of an optically active substance that

consists of a single enantiomer is said to be enantiomerically pure or to have an enantiomeric excess of 100%

• An enantiomerically pure sample of (S)-(+)-2-butanol shows a specific rotation of +13.52

• A sample of (S)-(+)-2-butanol that contains less than an equimolar amount of (R)-(–)-2-butanol will show a specific rotation that is less than 13.52 but greater than zero

Calculating Enantiomeric Excess or Optical

Purity• Also known as optical purity

• It can also be caluclated from optical rotation data

% enantiomericexcess

moles of oneenantiomer

moles of otherenantiomer

total moles of both enantiomers

= x 100

% enantiomericexcess *

observed specific rotation

specific rotation of thepure enantiomers

= x 100

Calculating Enantiomeric Excess or Optical

Purity• A mixture of the 2-butanol enantiomers showed

a specific rotation of +6.76o

• The specific rotation of pure (S)-2-butanol is +13.5o

• The enantiomeric excess of the (S)-(+)-2-butanol is 50%

% enantiomericexcess *

+6.76+13.52

= x 100 = 50%

Calculating Enantiomeric Excess or Optical

Purity• It is important to know what this means. Since

any R impurity will ‘cancel’ the rotation of an equal amount of S:

• A sample with an ee of 50% S is actually 50% pure S and 50% racemic R/S.

• The total S enantiomer in the sample is actually 75%!

Physical Properties of Diastereomers• Stereoisomers that are not enantiomers

• Unlike enantiomers, diastereomers usually have substantially different chemical and physical properties

Other Chirality In Organic Chemistry

R1

SiR2

R4 R3

R1

GeR2

H R3

R1

NR2

R4 R3

X SR2 R1

O

Other Chirality In Organic Chemistry

P(Ph)2

P(Ph)2

(Ph)2P

(Ph)2P

(S)-BINAP (R)-BINAP

enantiomers

Other Chirality In Organic Chemistry

enantiomers

C C

Cl

H

CCl

HCC

Cl

H

CCl

H

mirror