Topic 20.3 - Isomerism

Question 1

Isomerism

Answer 1

Isomers have the same molecular formula but are different based on either structure or

1 - Structural isomerism (same molecular formula)
2- Stereoisomerism (same molecular formula, same structure, different atom arrangement in space)

Question 2

Structural isomerism

Answer 2

1 - Chain isomers (butane / methylpropane)
2 - Position isomers (but-2-ene / but-1-ene)
3 - Functional group isomers (propanol / propanone)

Question 3

Stereoisomerism

Answer 3

1 - Conformational (can be rotated around a bond)

2 - Configurational (breaking and reforming a bond)

Question 4

Conformational isomerism

Answer 4

Rotation around a single bond occurs frequently. Rotation around a double bond does not easily occur without breaking the bond.

Question 5

Configurational isomerism

Answer 5

1 - Cis-trans / E-Z (E/trans-but-2-ene / Z/cis-but-2-ene)

2 - Optical (around a chiral centre)

Question 6

Optical isomerism

Answer 6

1 - enantiomers (non-superimposable: distinguishable from its mirror image, have no plane of symmetry)
2 - diastereomers (non-superimpossible, forms no mirror images, two or more stereocentres (at least one differs configurationally from one other centre))

Question 7

Chiral centres

Answer 7

A chiral centre is determined by the carbon having four unique bond groups (ie one H, one CH₃, one OH, and one C₂H₆)

Question 8

Polarimeter

Answer 8

Compounds rotated around a plane of polarised light. They rotate in opposite directions: + enantiomers move clockwise and - enantiomers are rotated anticlockwise

Question 9

Racemic mixtures

Answer 9

Equal mixture of + and - enantiomers, does not rotate plane-polarised light.

Question 10

Use of polarimeters

Answer 10

Purity can be established by comparing the enantiomers with the enantiomers of the desired products.

Question 11

Physical and chemical properties of optical isomers

Answer 11

Enantiomers -> Identical physical properties, share most chemical properties.
Diastereomers -> different physical properties, different chemical properties