Lecture 13-Organic chemistry III: Isomerism Flashcards
What is isomerism?
Molecules which have the same molecular formula, but differ in the arrangement of their atoms
How do structural / constitutional isomers differ?
In their bonding sequence / molecular skeleton
How do stereoisomers differ?
Only in the arrangement of the atoms in space
What are structural isomers?
Different compounds with different physical + chemical properties
Name the types of structural isomers?
Chain / skeletal isomerism
Position isomerism
Functional isomerism
Tautomerism
What is tautomerism?
(The movement of bonds + a proton)
pairs of molecules readily converted by reaction that includes the transfer of a H atom from one part of the molecule to another involving the switch of formation of single to double bond
What is keto-enol?
2 isomers are in equilibrium based on stability (99% vs 1) allowing carbonyls to react at the C next to the carbonyl group
What is pyranose?
Hemiacetal
What is steroisomerism?
Same structural formula ,but different 3D arrangement of atoms in space
What are the 2 types of stereoisomerism?
Geometric -cis/trans E/Z
Optical isomerism
How does geometric isomerism occur?
Results from restricted rotation around a bond ,because of single sigma bonds
Why can’t double pi bonds have geometric isomerism?
The rotation breaks the bond = not allowed without energy input
What does geometric isomerism require?
Different groups at each end of this bond
Different physical + chemical properties
What is a cis isomer?
2 alkyl groups on same side of double bond
What is a trans isomer?
2 alkyl groups on the opposite side of the double bond
What ability do optical isomers have?
To rotate plane polarised light in opposite direction
What isomer rotates the plane of polarised light to the RIGHT?
dextrorotatory
d-isomer / +-isomer
What isomer rotates the plane of polarised light to the LEFT?
levorotatory
l-isomer
(-)-isomer
What molecule has no plane symmetry?
chiral
What is a chiral centre / asymmetric carbon atom?
A C atom bonded to 4 different substituents lacks a plane of symmetry
Do compounds without chiral centres show optical activity?
Not usually
Optical isomers of compounds with 1 chiral centre will show ……. and are ……?
optical activity ……. chemically identical
Will compounds with more than 1 chiral centre show optical activity?
Depends on whether they are non-superimposable on their mirror/superimposable images
What are enantiomers?
the non-superimposable mirror image forms of a chiral molecule
What are meso compounds?
They contain a chiral C ,but not chiral overall
What are the systematic nomenclature for chiral centres?
R and S
R= dashed line
S= solid line
Enantiomers represent 2 optical isomers how?
(+) and (-)
Why do enantiomers have opposite rotatory powers?
Due to the opposite arrangements of groups around each asymmetric C atom
Describe the physical properties of enantiomers?
identical ,except in their interaction with plane of polarised light
Do enantiomers interact similarly or differently with other chiral molecules?
Differently
Generally are enantiomers not interconverted under ordinary conditions
Yes
What is a racemic mixture?
A 1:1 mixture of the 2 enantiomers of a compound
Describe what’s in a racemic mixture?
- Equal quantities = d- and l- enantiomers
- Notation = d,l or +/=
- No optical activity
- The mixture may have different mp / bp from the enantiomers
A structure with n chiral centres has?
2n possible isomers
What are diastereomers?
Stereoisomers that are NOT mirror images of each other
Diastereomers are most common in….
Molecules with 2/+ chiral compounds where AT LEAST 1 ,but not all differ
2 diastereomers have different ….
physical properties e.g. mp , bp + stability
Diastereomers have different chemical reactivity with ,,,,,
both achiral + chiral reagents
What is the importance of enantiomers?
Proteins/receptors/enzymes made up of L-AA -chiral environment (differentiate between isomers)
D-alanine
Component of bacterial cell wall not found in mammalian proteins
-Protects bacteria from protease
S-Tirofiban
R-Tirofiban
Anti-platelet drug
1000 fold less active
S-thalidomide
R-thalidomide
Anti-angiogenic,teratogen
Sedative
Compare the polarised light for enantiomers + diastereomers?
E = equal amounts D = different amounts
Compare the physical,chemical and NMR spectroscopy for enantiomers + diasteremoers?
E = identical D = different
Are enantiomers or diastereomers able to separate by physical methods (achiral)?
Diastereomers
Do enantiomers react differently with chiral molecules?
Yes -allows separation via chiral techniques
Do diastereomers react differently with chiral molecules or achiral molecules?
BOTH
Enantiomers form what type of mixture?
1:1 racemate
