L13: Isomerism Flashcards
Define isomerism
Molecules which have the same molecular formula, but different arrangement of their atoms
What is the difference between structural isomers and stereoisomers?
Structural isomers differ in their bonding sequence.
Stereoisomers differ only in the arrangement of their atoms in space.
What are the 4 types of structural isomer?
1) Chain or skeletal isomerism
2) Position isomerism (position of the FG)
3) Functional isomerism (identity of the FG)
4) Tautomerism (movement of bonds and a proton (H))
What are the two types of stereoisomerism?
1) Geometric (cis/trans)
2) Optical
True or false: Trans isomer has two alkyl groups on opposite sides of the double bond
True
Fill in the gap:
In a cis isomer, two alkyl groups are on the ______ side of the double bond
same
What type of isomer rotates the plane of polarised light to the right (clockwise)?
Dextrorotary (d-isomer or +)
What type of isomer rotates the plane of polarised light to the left (anticlockwise)?
Laevorotary (l-isomer or -)
If an isomer can rotate the plane of polarised light, what type of isomerism is displayed?
Optical
True or false: Compounds with more than one chiral centre will show optical activity
False. They may or may not depending on whether they are chiral (non-superimposable) or achiral (superimposable)
True or false: Compounds without chiral centres do not normally show optical activity
True
True or false: Compounds with one chiral centre may or may not show optical activity
False. Compounds with one chiral centre will always show optical activity
What is a meso compound?
A molecule with two or more stereogenic centres, but the molecule itself can be superimposed on its mirror image
What are two types of tautomer?
1) Keto-enol
2) Ring-chain
How do you form an enol?
Take a ketone C=O, bond a hydrogen to the O and move the double bond to either the immediate left or right of the O-H to make a C=C bond
When drawing out a keto-enol isomer, where do you position the double bond?
It doesn’t matter unless there is another double bond to which it can be conjugated
How do you form a ring-chain isomer?
Take a C-OH from one end of an open chain, and take a bond from a C=O on the other end to cyclise the molecule with a C-O-C bond. The H moves to the O that is missing a bond
True or false: Keto-enol and ring-chain reactions are reversible
True
True or false: For geometrical isomerism, the molecule must have a double bond and it must have two different groups either side of the C=C
True
How can polarised light tell you if a molecule is an optical isomer?
If the molecules rotate the plane of polarised light, the molecules are optical isomers
True or false: Compounds can be identified by the amount they rotate the plane of polarised light
True. Samples of the same molecule will rotate the plane of polarised light the same amount, assuming they are pure.
What are enantiomers?
Optical isomers with 2+ chiral centres, both in opposing configuration
True or false: Enantiomers have different physical properties
False. Enantiomers have identical physical properties in all respects, except in their interaction with plane of polarised light
What are diastereomers?
Stereoisomers that are non-superimposable and non-mirror images. They have opposite configurations at some chiral centres (but not all of them)
True or false: Diastereomers have different physical properties
True
Where are entantiomers found in biology?
Bacterial cell wall - protect bacteria from proteases
Fill in the gap:
Enantiomers have _______ physical and chemical properties
identical
True or false: Diastereomers may rotate the plane of polarised light but by different amounts
True
True or false: Diastereomers rotate the plane of polarised light by equal amounts in opposite directions
False. Enantiomers rotate the plane of polarised light by equal amounts in opposite directions
True or false: Enantiomers are impossible to separate by normal physical methods
True
Fill in the gap:
Diastereomers have ________ physical and chemical properties
different
True or false: Both enantiomers and diastereomers differ in taste and smell
True
What is a 1:1 mixture of enantiomers called?
A racemate
Can enantiomers be distinguished by NMR spectroscopy?
No
Can diastereomers be distinguished by NMR spectroscopy?
Yes
How many isomers can you expect to find for diastereomers?
2^n
n = number of chiral centres
