Stereochemistry

Primary Carbon

carbon atoms bonded to exactly one other carbon atom

secondary carbon

carbon atoms bonded to exactly two other carbon atoms

tertiary carbon

carbon atoms bonded to exactly 3 other carbon atoms

Quaternary carbon

carbon atoms bonded to exactly 4 other carbon atoms

methyl carbons

exactly 3 attached hydrogen atoms

methylene carbons

exactly 2 attached hydrogen atoms

methine carbons

exactly 1 attached hydrogen atom

normal line on 3D depiction

the bond is in the same plane as the page

hashed line on 3D depiction

the bond is projected into the page away from the viewer

bold wedged line on 3D depiction

bond is projected out of the page towards the viewer

Unambiguous depiction

the wedge and hash must be adjacent and the in plane bonds must be adjacent

stereoisomer

compounds with the same molecular formula and the same connectivity between atoms but a different 3D arrangement of those atoms in space

conformational isomers or conformers

stereoisomers arising from rotation around a single bond

Newman Projections

depicts the 3D arrangement of bonds on 2 adjacent atoms as viewed along the bond joining 2 atoms

Staggered conformers

are a type of conformational isomer where substituents on adjacent carbons are oriented to minimize steric strain, resulting in a lower energy state.

Eclipsed conformers

are conformational isomers where the substituents on adjacent carbons are aligned to maximize steric strain, resulting in a higher energy state.

torsional strain

energetic penalty for eclipsing interactions of hydrogen-hydrogen interactions on adjacent atoms

steric effects

in eclipsed conformers, the hydrogen atoms are closer in space and repel each other

result from repulsive interactions between atoms that are close in space

anti-conformer

the lowest energy, staggered conformer that has 180 degree angle between the two largest groups - the global minimum

gauche conformer

gauche conformation - local minimum with a 60 degree angle between the two largest groups, resulting in some steric strain

syn conformer

the staggered conformer with a 0 degree angle between the two largest groups, leading to increased steric strain. - global max

cycloalkanes

organic compounds containing only C-C and C-H single bonds and at least one ring

general formula for cycloalkanes with exactly 1 ring

CnH2n

naming cycloalkanes

same as IUPAC just use prefix cyclo- in name of parent hydrocarbon

cycloalkanes cannot rotate around single bonds bc…

there is not enough room for rotation to occur without rupturing the ring → results in cis/trans isomers

cis isomers

the groups are on the same side of the ring

trans isomers

the groups are on opposite sides of the ring

when are the cis/trans labels only used

disubstituted rings - two groups - although the ring can be of any size

angle strain

since an ideal sp3 hybridized carbon atom has bond angles of 109.5 degrees - significant bond angle distortions would be expected for very small rings and very large rings

overall ring strain

combination of angle strain, torsional strain, and steric strain

strain trends

-3 membered and 4 membered rings are highly strained

-6 membered rings and rings with 14 or more atoms are essentially strain free

-7-11 membered rings (medium rings) are moderately strained

most commonly encountered rings in Orgo

5-6 atoms - these are often found in naturally occurring compounds such as steroids' and sugars

cyclohexane

the most important ring in orgo

it is virtually strain free bc it can adopt 2 chair conformations with bond angles very close to the ideal 109.5

axial bond positions in a chair conformer

up and down alternate as you move around the ring

equatorial bonds in a chair conformer

up and down alternate as you move around the ring

ring flip or chair flip

the process of switching between the 2 chair conformers

“pulling down” one corner of the chair while simultaneously “pulling up” the other corner

there is significant energy available at room temp to make the interconversion rapid

Tracking substituent groups during chair flip

1. a chair flip converts all axial bonds into equatorial bonds and vice versa

2. a chair flip maintains the “up” or “down” designation of each bond within the ring

substituted cyclohexanes

one or more of the hydrogens have been replaced by a different group

axial vs equatorial positions relative energies

axial conformers have higher energy than equatorial conformers - bc there are more analogous gauche interactions in axial conformers

A-value

difference in energy between the axial and equatorial isomers for a particular monosubstituted cyclohexane

Trends in A-values

• chloro-, bromo-, and iodo- substituents have almost the same a-value - due to atoms getting bigger but bonds getting longer which cancels

• methyl, ehtyl, and isopropyl groups have almost the same A-value - bond rotation minimizes unfavorable steric interactions

• tert-butyl group has a very lg A-value - no way to minimize steric interactions via bond rotation

Wedge and hash in chair conformers

wedge = up position in chair conformer

hash = down position in chair conformer

hidden gauche interactions

there are “hidden” gauche interactions for any chair conformer in which at least one of the substituent groups is equatorial

in general only occur when 2 of the same group are right next to each other

bicyclic compound

organic compound that contains 2 rings

fused bicycles

2 rings share a common bond and the ring fusion can be either trans or cis

spiro compounds

2 rings share a common atom

bridged bicycles

contain a chain of one or more atoms that bridge a larger ring

atoms at the intersection of the larger ring and bridge - bridgehead atoms

polycyclic compounds

compounds that contain more than 2 rings

superimposable

when 2 objects are superimposable - they can be overlapped such that all corresponding parts match up exactly

can completely align all atoms and bonds

chiral

not superimposable with its mirror image

mirror images but cannot rotate in space and make identical

best way for an atom to be chiral - sp3 hybridized with 3 different groups attached

4 different groups (chiral center) and no symmetry

achiral

is superimposable with its mirror image

• any object that has an internal plane of symmetry MUST be achiral

has symmetry and can have a chiral center

chiral center/stereogenic center/stereocenter

most common way for an organic molecule to be chiral is for it to contain an atom with tetrahedral geometry bonded to 4 different substituent groups (chiral center)

such carbon atom is an asymmetric carbon or chiral carbon

Meso compound

specific molecule with 2 chiral centers but has symmetry

• cannot have enantiomers bc have symmetry

enantiomers

have a mirror-image relationship

• needs to be chiral (4 different groups and no symmetry)

• flip all wedges to hashes and vice versa or draw mirror image

• S and R flip

  • S + S → R + R

  • R + S → S + R

enantiomers always come in pairs

a compound and its non-superimposable mirror image

enantiomer configuration

the configuration at a chiral carbon CANNOT be changed via molecular rotations or translations → would require breaking or reforming bonds

a molecule or atom’s configuration

the specific molecular geometry arising from the spatial arrangement of atoms and bonds

enantiomer physical properties

in achiral environment

• enantiomers have identical chemical and physical properties

  in a chiral environment

• enantiomers can have different physical properties

Diastereomer

2 compounds to be stereoisomers but are not mirror images

• typically arise when we have more than 1 chiral center in a particular molecule

Meso or achiral compound

• cis/trans association switching

How to draw a diastereomer

Keep 1 dash or hash and switch others or switch cis/trans depending on molecule

• S/R - keep 1

• RRS → RSS or RSR

• R S → RR or SS

If you see a double bond you usually have a diastereomer - can distinguish between cis/trans - rotation about the pi bond

Diastereomer physical properties

Have completely different chemical and physical properties

racemic mixture

50:50 mixture of two enantiomers

optically inactive - does not rotate plane-polarized light

half of the molecules rotate light clockwise and other half rotate light counter-clockwise - canceling

What compounds are optically inactive?

racemic mixtures and achiral compounds

Absolute configuration of an individual stereocenter

R or S configuration

• lowest priority must be pointing back in space (hash)

• if not pointing back and is pointing forward (wedge) switch R and S

R or S included in front of name if there are stereogenic centers

R configuration

moving around the circle in clockwise direction based on priority groups

S configuration

moving around the circle in counter-clockwise direction based on priority groups

How to determine total possible number of stereoisomers

determine number of stereogenic centers = n

2^n = total possibilities

What is the stereochemical relationship between axial and equatorial chair conformers?

They are either enantiomers or diastereomers due to the differences in the axial and equatorial positions. If the original structure is achiral it cannot be an enantiomer so it is a diastereomer.

When are cis/trans applicable to naming?

In achiral molecules - have symmetry

When is R and S applicable to naming?

In chiral molecules - have stereocenters

Can chirality occur at any atom?

Yes, not just carbon