isomerism Flashcards
what are isomers?
isomers are molecules with the same molecular formula, but different arrangement of the atoms
what are constitutional isomers?
constitutional isomers are molecules with the same molecular formula but different structural formula.
structural formula may differ due to different carbon chains (chain isomers), different positions of the same functional group (positional isomers), or different functional groups (functional group isomers)
how to deduce number of double bonds/triple bonds/ring(s)?
useful info! js read thru
if a compound with n number of carbon atoms has fewer H atoms than 2n+2, it must contain double/triple bonds and/or rings
- every decrease of 2 H atoms from the alkanegeneral formula contributes to 1° unsaturation -> indicates that either a double bond or a ring is present
- 2° saturation indicates that the compound has 2 double bonds/2 ring structures/1 double bond & 1 ring structure/1 triple bond
systematic way to draw all constitutional isomers with a given molecular formula
useful info! js read thru
- from molecular formula, identify possible functional group(s) & no. of double/triple bonds
- using no. of carbon atoms, draw all possible carbon chains, starting with straight chain first then gradually increasing to draw all possible branched chains
- for each carbon chain from step 2, draw structures with functional group(s) placed at all possible different positions on the carbon chain
- repeat step 3 with other likely functional groups untill all possibilities are exhausted
what are stereoisomers?
stereoisomers are molecules with the same molecular formula and structural formula but different arrangement in space. they differ only in the way the atoms are arranged in 3D space
cis-trans isomers and enantiomers are types of stereoisomers
what are the conditions for a molecule to exhibit cis-trans isomerism?
- a covalent bond with restricted rotation (eg a π bond or ring structure)
- non-identical groups attached to each end of the bond with restricted rotation
what is the difference between cis and trans isomers?
the cis isomer has identical groups on the same side of the double bond
the trans isomer has identical groups on the opposite sides of the double bond
if there are n C=C double bonds in a molecule, the maximum number of cis-trans isomers is 2^n
what molecules canNOT exhibit cis-trans isomerism?
molecules that have identical groups attached to one or both ends of the double bond
can cycloalkenes exhibit cis-trans isomerism?
cycloalkenes have ring structure with 1 double bond
cycloalkenes with small rings like cyclohexene and cyclopentene cannot exhibit cis-trans isomerism as they only exist in the cis isomer form at rtp. the trans isomer of cycloalkenes with smaller rings are unstable due to the large ring strain.
only LARGER rings with 8 or more atoms can accomodate the trans form
can ring structure (not cycloalkenes) exhibit cis-trans isomerism?
yes! because it is not possible for single bonds that form a ring to rotate freely without first breaking the ring, hence there is restricted rotation about single bonds that form part of a ring -> can exhibit cis-trans isomerism
so ring structure with non-identical groups attached to 2 or more atoms in the ring can exhibit cis-trans isomerism
what are the conditions for a molecule to exhibit enantiomerism?
- contain one or more chiral centres
- do not possess any internal plane of symmetry
- its mirror image must be non-superimposable onto itself
what is a chiral centre?
a chiral centre is any atom with a tetrahedral geometry bonded to 4 different groups
if there are m chiral centres in a molecule, the maximum number of stereoisomers will be 2^m
what are meso compounds?
meso compounds are stereoisomers with 2 or more chiral centres but no optical activity due to an internal plane of symmetry
what is a racemic mixture and why doesn’t it rotate plane-polarised light?
a racemic mixture is a mixture containing equal proportions of each enantiomer. each enantiomer rotates plane-polarised light by the same angle but in opposite directions. a 50-50 mixture of the 2 enantiomers doesn’t rotate plane-polarised light as the clockwise rotation caused by 1 enantiomer is exactly cancelled by the anticlockwise rotation of the other enantiomer
how to deduce the total number of stereoisomers (enantiomers + cis-trans isomers)
if a molecule has m chiral centres and n double bonds that can give rise to cis-trans isomers, the maximum number of stereoisomers it can have is 2^(m+n)
the actual number of stereoisomers is determined by considering any internal plane of symmetry in the possible structures
