Question

MCAT Tutorial: Isomers

Answer 1

\({\bf{Definitions:}}\)
- isomer: compound with the same chemical formula but different arrangement of atoms and different chemical properties
> two subtypes, constitutional isomers and stereoisomers

- structural/constitutional isomers: isomer with distinct chemical bonding

- stereoisomers: same connectivity but different spatial arrangement of bonds
> subtypes: enantiomers and diastereomers
- enantiomers: non-superimposable mirror images
- diastereomers: stereoisomers that are not enantiomers
> subtypes of diastereomers: conformational isomers and cis/trans isomers

- conformational: differ about a single bond rotation
- cis/trans isomer: differ in the arrangement of their bonds wrt a double bond

Answer 2

\({\bf{Chirality:}}\) a property given to an object that is non-superimposable upon its mirror image, like a pair of gloves (one right handed and one left handed. they are mirror images but they cannot be perfectly overlapped across each other)
- chirality center: an atom (usu. carbon) bonded to four different atoms/groups. note that this chiral center, when mirrored, will not superimpose itself.



I will do a quick example of how to identify chirality centers on a molecule:

Answer 3

finally this IS a chirality center because it has four different groups associated with it

making these two atoms the only two chirality centers on the molecule

Answer 4

\({\bf{R~S~Designations:}}\) for chirality centers; label each attached atom of the chirality center in order from highest atomic # to loweset atomic # (if two are the same, go to the next atom attached to the bond until you reach the first point of difference)

you should have 4 atoms labeled (1,2,3,4). next, draw/visualize the molecule being rotated such that the lowest priority atom is going into the page. then, see whether 1,2,3 are arranged in clockwise (R) or counterclockwise (S) order



\({\bf{Diastereomers:}}\) not mirror images; must contain 2= chirality centers, designated RR,SS,SR, RS, etc. Put the R or S in parentheses in front of the chirality center being labelled, still following the typical IUPAC naming convention. note the relationship between these isomers, noting which ones are enantiomers and which ones are diastereomers



\({\bf{Meso~Compounds:}}\) subtype of diastereomer that has a plane of symmetry and is not optically active

Answer 5

trans/cis designations: for alkenes with at least two groups/atoms in common across the double bond, if the similar atoms are on the same side of the double bond, it is designated cis, if they're on opposite sides of the double bond, they're trans

note: in this case the sides are determined by splitting the bond along its axis ("horizontally")



E/Z designation for alkenes: for alkenes with no like groups. split the double bond "vertically" (not along the axis of the bond, but the perpendicular direction) and assign priorities (1 or 2) to each of the groups on the left side, then the right. If the 1's are on equal sides (horizontally) then give Z designation, otherwise give E designation.

This is probably easier to see with an example so:

Answer 6

Anyway, that's the end of my tutorial, I hope it was a helpful resource. Source material is the 2nd Edition Barron's Prep book for the new MCAT