isomerism

Question 1

what are the 2 types of isomerism?

Answer 1

• stereoisomerism
• constitutional isomerism

Question 2

what are the 2 forms of stereoisomerism

Answer 2

• cis-trans isomerism
• enantiomerism

Question 3

what is the definition of stereoisomerism?

Answer 3

• stereoisomers have the same structure & functional groups, but differ in the way their atoms are arranged in space

Question 4

what are the criteria required for cis-trans isomerism to occur?

Answer 4

1. restricted rotation about a carbon-carbon bond
2. 2 different substituents bonded to each C in the restricted bond

Question 5

what is a cis-isomer?

Answer 5

• the isomer with 2 identical groups on the same side of the double bond

Question 6

what is a trans-isomer?

Answer 6

• the isomer with 2 identical groups on different sides of the double bond

Question 7

does the trans-isomer have any dipole movement?

Answer 7

• NO, so it is non-polar

Question 8

does the cis-isomer have any dipole movement?

Answer 8

• YES, so it is polar

Question 9

does the cis & trans isomers have the same physical properties?

Answer 9

• NO, THEY DO NOT

Question 10

generally, what are the properties of the cis-isomer?

Answer 10

• it has a higher boiling point due to its higher polarity.
• but has a lower melting point due to its poorer packing of molecules in the solid state

Question 11

do cis & trans isomers generally have the same chemically properties?

Answer 11

• YES, THEY DO

Question 12

why do certain cis & trans isomers have different chemical properties?

Answer 12

• due to the difference in the proximity of the functional groups

Question 13

what compounds can exhibit cis-trans isomerism?

Answer 13

• cycloalkanes
• cyclodiols
• alkenes

Question 14

what are the criteria required for enantiomerism to occur?

Answer 14

• have a mirror image that is non-superimposable on itself
• have a sp3 C bonded to 4 different substituents (or if in a ring, all 4 substituents bonded to it must have different bonding patterns)
• have no plane of symmetry

Question 15

what is formed when there are equal percentages of + & - enantiomers present, and what property does it have?

Answer 15

• a racemic mixture!
• it is optically inactive (unable to rotate plane-polarised light)

Question 16

what are the chemical properties of enantiomers?

Answer 16

• they have identical chemical properties, except in their interaction with another chiral molecule

Question 17

what are the physical properties of enantiomers?

Answer 17

• they have identical physical properties, except for the direction in which they rotate plane-polarised light

Question 18

explain why a molecule with chiral centers may be achiral, and explain its optical activity.

Answer 18

• even though a molecule has chiral centers, the mirror images are superimposable onto each other, 7 may also have a plane of symmetry
• hence, the molecule is achiral & a sample of it is optically inactive

Question 19

what is the formula used to calculate the maximum number of stereoisomers a compound can have?

Answer 19

• 2^(n+m),
• where n is the number of chiral centers
• & m is the number of C=C bonds which can exhibit cis-trans isomerism

Question 20

what are meso-isomers?

Answer 20

• they are stereoisomers that contain chiral centers but is optically inactive