Isomers

Question 1

What is a constitutional isomer?

Answer 1

Compounds that have the same molecular formula, different atom to atom bonding sequence

Question 2

What is a double bond equivalent (DBE)?

Answer 2

Rings or pi bonds

Question 3

What is the DBE formula?

Answer 3

DBEs = 1/2 (2n4 + n3 - n1 + 2)

where n4 = C
n3 = N
n1 = H, halogens

Question 4

Can constitutional isomers have different functional groups?

Answer 4

Yes

Question 5

What is a stereoisomer?

Answer 5

Molecules with different arrangement of atoms and groups of atoms in space

Question 6

What are the two types of stereoisomers?

Answer 6

Conformational and configurational

Question 7

What are conformational isomers?

Answer 7

Same molecular formula, same atom to atom bonding but the arrangement in space differs by rotation about a single bond

Question 8

What are configurational isomers?

Answer 8

Molecules that have a different arrangement of atoms or groups of atoms in space that requires bond breaking for interconversion

Question 9

Are conformational isomers the same compound?

Answer 9

Yes, they interconvert easily via rotation about a single bond and hence can be considered the same molecule

Question 10

What is a conformer?

Answer 10

A specific shape adopted by the molecule as a result of the rotation about a single bond

Question 11

What are the two types of conformers?

Answer 11

Staggered and eclipsed

Question 12

What is an eclipsed conformer?

Answer 12

One in which the groups of adjacent carbon-carbon are lined up - hence least stable

Question 13

What is a staggered conformer?

Answer 13

One in which the groups of adjacent carbon-carbon are separated furthest apart - hence most stable

Question 14

How does a conformer go from staggered to eclipsed?

Answer 14

Rotation of 60 degrees about the C-C bond

Question 15

What are the two types of staggered conformers?

Answer 15

Anti and Gauche

Question 16

What is an anti conformer?

Answer 16

A type of staggered conformer in which the largest groups of the C-C bond are separated furthest apart

Question 17

What is a gauche conformer?

Answer 17

A type of staggered conformer in which the largest groups of the C-C bond are 60 degrees apart (close together)

Question 18

What are the types of eclipsed conformer?

Answer 18

syn eclipsed conformer

Question 19

What is a syn conformer?

Answer 19

A type of eclipsed conformer in which the largest groups of the C-C bond are closest together and hence least stable

Question 20

What is the most stable conformer of alkanes?

Answer 20

Anti Staggered

Question 21

What is the least stable conformer of alkanes?

Answer 21

Syn eclipsed

Question 22

What are the conformers of cyclohexane molecules?

Answer 22

Boat and chair

Question 23

What is the boat conformer?

Answer 23

A boat shaped conformer of cyclohexane. When looked at from the ‘tip’ of the boat front-on all bonds will be eclipsed hence least stable conformer

Question 24

What is the chair conformer?

Answer 24

A chair shaped conformer of cyclohexane. When viewed from the side of the chair there are no eclipsed bonds. Hence, lowest energy conformer.

Question 25

What is the least stable cyclohexane conformer?

Answer 25

Boat conformer

Question 26

What is the most stable cyclohexane conformer?

Answer 26

Chair conformer

Question 27

What is a ring flip?

Answer 27

Rotation of bonds in which all axial groups become equatorial and all equatorial groups become axial

Question 28

What is axial methylcyclohexane?

Answer 28

A chair conformer of methylcyclohexane in which the substituted methyl group is in the axial plane (vertical)

Question 29

How does gauche interactions occur with methylcyclohexane?

Answer 29

The CH3 group in the axial plane interacts with nearby hydrogen making the molecule less stable

Question 30

For substituted cyclohexanes, what conformer is the most stable?

Answer 30

Equatorial chair conformer - no gauche interactions when largest atoms/groups are in the equatorial position

Question 31

What are the types of configurational isomers?

Answer 31

Enantiomers and diastereomers

Question 32

What are enantiomers?

Answer 32

Non-superimposable mirror images

Question 33

What are diastereomers?

Answer 33

Not mirror images, same molecular formula, cannot be interconverted without breaking of bonds Many types - all configurational isomers excluding mirror images (enantiomers)

Question 34

What are cis/trans isomers?

Answer 34

Diastereomers that form on cycloalkane rings where the position of substituents on sp3 carbon of the ring can either be above or below the plane of the ring Cis and trans are relative terms

Question 35

What are E/Z isomers?

Answer 35

Isomerism across a double bond where the pi bond fixes atoms in space as rotation cannot occur. Requires different atoms or groups of atoms on each carbon of the double bond

Question 36

What is the CIP rule?

Answer 36

The relative priority of groups attached to carbon can be determined in order of their relative molecular mass (high to low) If same groups are attached on one carbon, extend out to the atom or group of atom that is bonded to the first group and compare relative molecular mass If a double bond or triple bond exists, treat them as 2, 3 bonds respectively

Question 37

What is a Z isomer?

Answer 37

An isomer in which there are different groups of atoms on each carbon of a double bond and the higher priority groups according to the CIP rules are on ze zame zide

Question 38

What is an E isomer

Answer 38

A diastereomer in which there are different groups of atoms on each C atom adjacent to a double bond and the higher priority groups and opposite from each other

Question 39

What is a stereogenic carbon?

Answer 39

A carbon with 4 different atoms or groups of atoms bonded to it. A molecule with a stereogenic carbon is called chiral

Question 40

What is a chiral molecule?

Answer 40

A molecule containing a stereogenic carbon

Question 41

What are non-chiral molecules called?

Answer 41

Symmetrical molecules called achrial

Question 42

What is a racemic mixture?

Answer 42

A mixture of equal amounts of the two enantiomers - + and -

Question 43

What happens to a pair of enantiomers when exposed to plane polarised light?

Answer 43

They interact differently with one of the enantiomers rotating clockwise + and the other rotating anticlockwise -

Question 44

What are R / S enantiomers?

Answer 44

Naming of the configuration of enantiomers based on the priority of the 4 groups attached Place the 4th priority group away from you and determine whether the priority increases clockwise or anticlockwise Clockwise - to the right hence R enantiomer