Isomers Flashcards
What is a constitutional isomer?
Compounds that have the same molecular formula, different atom to atom bonding sequence
What is a double bond equivalent (DBE)?
Rings or pi bonds
What is the DBE formula?
DBEs = 1/2 (2n4 + n3 - n1 + 2)
where n4 = C
n3 = N
n1 = H, halogens
Can constitutional isomers have different functional groups?
Yes
What is a stereoisomer?
Molecules with different arrangement of atoms and groups of atoms in space
What are the two types of stereoisomers?
Conformational and configurational
What are conformational isomers?
Same molecular formula, same atom to atom bonding but the arrangement in space differs by rotation about a single bond
What are configurational isomers?
Molecules that have a different arrangement of atoms or groups of atoms in space that requires bond breaking for interconversion
Are conformational isomers the same compound?
Yes, they interconvert easily via rotation about a single bond and hence can be considered the same molecule
What is a conformer?
A specific shape adopted by the molecule as a result of the rotation about a single bond
What are the two types of conformers?
Staggered and eclipsed
What is an eclipsed conformer?
One in which the groups of adjacent carbon-carbon are lined up - hence least stable
What is a staggered conformer?
One in which the groups of adjacent carbon-carbon are separated furthest apart - hence most stable
How does a conformer go from staggered to eclipsed?
Rotation of 60 degrees about the C-C bond
What are the two types of staggered conformers?
Anti and Gauche
What is an anti conformer?
A type of staggered conformer in which the largest groups of the C-C bond are separated furthest apart
What is a gauche conformer?
A type of staggered conformer in which the largest groups of the C-C bond are 60 degrees apart (close together)
What are the types of eclipsed conformer?
syn eclipsed conformer
What is a syn conformer?
A type of eclipsed conformer in which the largest groups of the C-C bond are closest together and hence least stable
What is the most stable conformer of alkanes?
Anti Staggered
What is the least stable conformer of alkanes?
Syn eclipsed
What are the conformers of cyclohexane molecules?
Boat and chair
What is the boat conformer?
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
What is the chair conformer?
A chair shaped conformer of cyclohexane. When viewed from the side of the chair there are no eclipsed bonds. Hence, lowest energy conformer.
What is the least stable cyclohexane conformer?
Boat conformer
What is the most stable cyclohexane conformer?
Chair conformer
What is a ring flip?
Rotation of bonds in which all axial groups become equatorial and all equatorial groups become axial
What is axial methylcyclohexane?
A chair conformer of methylcyclohexane in which the substituted methyl group is in the axial plane (vertical)
How does gauche interactions occur with methylcyclohexane?
The CH3 group in the axial plane interacts with nearby hydrogen making the molecule less stable
For substituted cyclohexanes, what conformer is the most stable?
Equatorial chair conformer - no gauche interactions when largest atoms/groups are in the equatorial position
What are the types of configurational isomers?
Enantiomers and diastereomers
What are enantiomers?
Non-superimposable mirror images
What are diastereomers?
Not mirror images, same molecular formula, cannot be interconverted without breaking of bonds
Many types - all configurational isomers excluding mirror images (enantiomers)
What are cis/trans isomers?
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
What are E/Z isomers?
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
What is the CIP rule?
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
What is a Z isomer?
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
What is an E isomer
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
What is a stereogenic carbon?
A carbon with 4 different atoms or groups of atoms bonded to it. A molecule with a stereogenic carbon is called chiral
What is a chiral molecule?
A molecule containing a stereogenic carbon
What are non-chiral molecules called?
Symmetrical molecules called achrial
What is a racemic mixture?
A mixture of equal amounts of the two enantiomers - + and -
What happens to a pair of enantiomers when exposed to plane polarised light?
They interact differently with one of the enantiomers rotating clockwise + and the other rotating anticlockwise -
What are R / S enantiomers?
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
