Isomers Flashcards
Constitutional/structural isomers
Chain isomerism
Positional isomerism
Functional group isomerism
same molecular formula different structural formula
Chain isomerism
different arrangement of carbon skeleton
Positional isomerism
different location of substitutional/functional groups on C chain
Functional group isomerism
different functional groups
Stereoisomerism
Criteria
Presence of chiral center
cis trans isomerism
same molecular and structural formula but different spatial 3d arrangement of atoms
chain and positional isomers
have different physical properties due to difference in SA of contact between molecule but have the same chemical properties due to the fact that they have the same functional group
Functional group isomers
types 1. alcohols (OH), ethers(OCH3) CnH2n+2O 2. alkenes and cycloalkenes CnH2n 3.carboxylic acids(OH-C=O) and esters(O-C=O) CnH2nO2 4. aldehydes(H-C=O )and ketones( CH3-C=O) CnH2nO
have different physical and chemical properties
Cis trans isomerism
two conditions for it to exist
restricted rotation about a bond can be due to
1. C=C in a str8 chain
2. Ring structure
two different groups on each of the carbon atoms with restricted atoms
3. cyclic alkenes do not exhibit cis trans despite fulfilling both conditions
cis isomers
have identical atoms groups of atoms on the same side of the double but never on the same carbon
trans isomers
have identical atoms/groups of atoms on diagonal sides of the double bond
Properties of cis trans isomers
same chemical but different physical
1.due to differences in polarity
Molecules containing two or more cis trans
a molecule with ‘m’ cis trans C=C has a max of 2^m stereoisomers
Optical isomers/enantiomers
exists when molecules contain at least one chiral centre causing them to lack a point and plane symmetries
Chirality
if C atom is attached to 4 diff atoms/groups of atoms
labelled with a *
Enantiomers
pair of compounds with same connectivity but opposite 3d shapes. non superimposable images mirror images of each others
Properties of optical isomers
- enantiomers
rotate the plane of plane polarised light by equal angles in the opposite directions when equal conc of enantiomers are being analysed
similar chemical properties(exc when interacting w a chiral mol.) and physical properties(exc in the direction in which they rotate plan polarised light)
different
different biological properties
2.racemic mixture
mixture containing equal conc of enantiomers the equal but opp rotation of plane polarised light by each enantiomers results in net zero optical activity. mixture is optically inactive despite having chiral molecules
Molecules containing two or more chiral centers
a molecule with n chiral centers has a max of 2^n stereoisomers
there are some molecules with 2 chiral centers with less than 4 stereoisomers
- might have isomers that are identical/superimposable
Meso compound
contains 2 chiral centers but is superimposable on its mirror image as it has a plane of symmetry
optically inactive as the effect on the plane polarised light by each half of the mol. is equal in magnitude but opposite in directions. Hence they cancel each other out to give a net zero effect on plane polarised light
in general a molecule with m C=C bonds that exhibit cis-trans isomerism and n chiral centers has a max of 2^m+n stereoisomers
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Biological properties of optical isomers
the arrangement of atoms between a pair of enantiomers are recognised
- enantiomers can interact differently with the chiral receptor cells in taste buds to detect taste
- enantiomers can interact differently with chiral receptor cells in the nose to detect smell
- drugs work by interacting with the appropriate receptor site via IMF which are very specific
