MC- molecular shape : conformation

- eclipse, staggered graphs of energy

What is a Conformational isomer?

• they only require a sigma bond to rotate in the molecule

NOTE- stereoisomers - are configurational isomers ( a bond has to be broken for the molecules to interconvert

Are the bond easy to rotate in conformational isomers?

• energy is required to rotate a bond

• this energy depends on the rest of the molecule eg. is there any conjugation

• it is harder to rotate when sigma bonds when there are π bonds present 

Why are rotations important in drugs ?

• they are important for binding right the first time 

How are conformational isomers structured?

• they can be staggered or eclipsed 

• staggered - not in line as far apart as possible ( less repulsion lower energy ) 

• eclipsed- in line ( higher energy more repulsion )

How do we draw the different conformers of a molecule ?

• use newman projections

• looking at it from a straight down the bond

• there is an infinite number of conformers as it rotates by one degree = new conformer ..

How do we rank the different conformers in energy?

eclipse- always the worst , Highest energy arrangement because the electrons clouds repel each other - not stable

staggered - best , lowest energy the electron clouds are the furthest apart so don’t repel - more stable

NOTE - highest energy when the biggest molecules are closest together

lowest energy when the biggest molecules ar ethe furthest apart

How do graphs represent energy of conformers ?

• each point is a conformation

• Can have some conformers that are identical in energy - because they have 2 of the same substituents

• syn periplanar - global maximum- highest energy ( eclipse)

• antiperiplanar global minimum- lowest energy `( staggered) \also have local maximum, local minimum

What affects the energy of a conformation?

when substituents are hydrogens - general rules:

-Staggered conformations always have lower energies

 -Repulsion occurs between the electrons in the bonds

-The repulsion is greater when the bonds are aligned (eclipsed conformations) than when there is a larger dihedral angle between the bonds (staggered conformation)

-Each arrangement or conformation has a slightly different energy and stability.

-If there are no other factors the molecule will preferentially sit in the lowest energy staggered conformation.

when the substituents are larger:

-Steric (size) interactions begin to matter

-Some staggered conformations will be of relatively higher energy (lower stability)

-Some eclipsed conformations will be of relatively lower energy (higher stability)