stereochemistry Flashcards
(37 cards)
what is stereochemistry?
study of isomers
what are isomers?
compounds that have the same molecular formula but are chemically distinct from each other
-the same way in which the isomers differ depends on the type of isomerism
what are two types of isomerism?
-structural
-stereoisomerism
what is structural isomerism?
-where compounds have the same molecular formula but differ in the position that the atoms are connected to each other
-they can have very different physical and chemical properties
what is stereoisomerism?
where compounds have the sane molecular formula and have their atoms bonded in the same order but can differ in their three dimensional shape
what are the three types of stereoisomerism?
-geometric
-optical
-conformational
when does geometric isomerism occur?
-when there is rigidity in a molecule so it can lead to restricted rotation
state the two common cases where there is rigidity in a molecule that leads to restricted rotation…
-when there is Pi bonds in alkenes
–in cyclic structures
what are the two types of geometric isomers?
-Cis and Trans geometric isomers
-E/Z geometric isomers
what happens in cis/trans isomerism?
-the double bonds in the alkenes restricts the free rotation of the bond at room temp and atmospheric pressure.This is because the Pi bond would have to be broken to allow rotation
why can cis-trans isomerism not occur if the atoms bonded to one edge of the double bond are the same?
-because the structures are flat and so by turning the molecule over (not requiring the breakage of any bonds) you have the same molecule
when does E/Z isomerism occur?
when there is more than two different groups attached to the alkene double bond so you use the E/Z nomenclature system rather than cis/trans
how do you do E/Z isomerism?
-Prioritise the groups on either side of the double bond and label them 1 (top priority) or 2 (lower priority).
-if the two top priority groups are on the same side this is known as the Z isomer (similar to Cis). If they are on opposite sides then this is known as the E isomer (similar to trans)
how do you determine priority for E/Z isomerism?
-Look at the atom directly attached to the double bond. The higher its molecular mass the greater its priority e.g. I > Br >Cl > O >N > C
-With isotopes the higher mass isotope has priority e.g. 37Cl > 35Cl or 3H > 1H
If the two atoms bound to the double bond are the same, look one bond away and find out which one has the highest mass atom (e.g. Br). When this is the same then count the number of these higher mass atoms. If you can still not distinguish priority then one more bond away and repeat the process.
Multiply bonded groups i.e. C=O, C=C, or C≡C are counted as almost twice (double bond) or three times (triple bond). Except –it is important to note that two single bonded atoms have greater priority than a double bonded atom.
how do you work out priority in E/Z isomerism if the two atoms bound to the double are the same?
-If the two atoms bound to the double bond are the same, you look one bond away and find out which one has the highest mass atom (e.g. Br).
-When this is the same then count the number of these higher mass atoms. If you can still not distinguish priority then one more bond away and repeat the process.
how do you work out priority in E/Z isomerism if you have multiple bonded groups?
-if you have multiple bonded groups i.e. C=O, C=C, or C≡C they are counted as almost twice (double bond) or three times (triple bond).
- Except it is important to note that two single bonded atoms have greater priority than a double bonded atom.
what is a chiral centre?
-a carbon with 4 different groups attached to it
what are enantiomers?
-any compound that cannot be superimposed onto its mirror image,the two mirror image forms are optical isomers of each other
what 2 properties are different for a single pair of enantiomers?
-interactions with other chiral substances
-interactions with plane polarised light
what does it mean when a compound is optically active?
-it can rotate plane-polarised light
what does it mean if an enantiomer is dextrorotatory?
-the enantiomer can rotate plane polarised light to the right
-it is sometimes given the symbol D
what does it mean if an enantiomer is levorotatory?
-its mirror image of the other enantiomer will rotate plane polarised light to the left
-it is sometimes given the symbol L
what is a racemic mixture?
-a mixture of equal proportions of any pair of enantiomers
what are the rules for assigning R and S nomenclature to an enantiomer?
-rank the 4 groups surrounding the chiral centre according to priority (RMM), 1for the highest,4 for the lowest
-rotate the compound so that the lowest priority group faces away from you
-if the priority numbers increase in a clockwise direction then this enantiomer is the R enantiomer
-if the priority numbers increase in an anticlockwise direction then this enantiomer is the S enantiomer
