Stereochemistry Flashcards
what does non-superimposable mean?
two molecules that cannot be perfectly overlapped by rotation and translation.
what does chiral mean?
a molecule that is non-superimposable on its mirror image.
what does achiral mean?
if an object can be superimposed upon its mirror image, then it is achiral.
what are enantiomers?
chiral molecules that are mirror images of each other, they cannot be superimposed.
often labelled as ‘R’ or ‘S’.
can be extended to any molecule containing a tetrahedral carbon with 4 different substituents attached.
how do enantiomers react when in the presence of plane polarised light?
they rotate plane polarised light in opposite directions.
what is optical activity?
a property of chiral molecules that enables them to rotate plane polarised light. It is rotated by an angle proportional to the concentration of that molecule and path length.
rotation also indicates the purity of the sample; if only one enantiomer is present, the sample is optically pure and will rotate plane polarised light in one direction.
If there are a mixture of enantiomers present, the polarisation will be less pronounced.
what is a racemic mixture?
an equimolar mixture of two enantiomers is called a racemic mixture.
they do not rotate plane-polarised light.
how to you assign configurations to each enantiomer?
4 substituents at the centre carbon are ranked based on atomic number.
arrange so lowest priority substituent is pointing away.
if highest to second highest is clockwise, assign R.
if highest to second highest is anti-clockwise, assign S.
if two of the atoms are the same, we look to the atom that they are directly attached to.
what does (+) mean?
(-)
what does (dl) mean?
racemic mixture/racemate.
d means the enantiomer is positive and rotates plan polarised light to the right.
l means the enantiomer is negative and rotates plane polarised light to the left.
stick to R/S
if molecule has more than one stereogenic centre, the number of stereoisomers usually increases according to the following formula:
no. of stereoisomers = 2^N
N = no. of stereogenic centres.
what do you call stereoisomers that are not enantiomers?
diastereoisomers.
they are non-superimposable, non-mirror images.
what is the difference between enantiomers, diastereoisomers and conformers?
enantiomers - invert all stereocentres.
diastereoisomers - invert at least one, but not all.
conformers - no change, same.
what is a meso compound?
a compound that contains two or more stereocentres but is superimposable with its mirror image.
what are achiral diastereoisomers?
diastereoisomers are non-superimposable, non-mirror images.
alkenes are usually achiral, they possess a mirror plane.
spatial arrangements are different in cis and trans isomers.
what are configurational/geometric isomers?
a type of stereoisomerism that arises from the restricted rotation around a double bond or ring.
referred to as cis/trans or E/Z isomers.
