OCHEM Ch. 3 Conformations and Stereochemistry

Newman Projection

• used to relate stability of different conformations of molecules

Drawing Newman Projections

1. look down C-C bond

2. see how the substituents are arranged

3. front carbon looks like Y with closed circle

   1. wedge goes to top right

   2. dash goes to top left

4. back carbon looks like upside down Y with open circle

   1. wedge goes to bottom right

   2. dash goes to bottom left

anti-periplanar (anti)

• most stable

• dihedral angle = 180°

• lowerst energy confirmation and torsional strains

eclipsed

• dihedral angle = 120°

gauche

• dihedral angle = 60°

• steric strain

steric strain

repulsion experienced between two bulky groups forced close together in space

• anti-periplanar 180°

• gauche 60°

• fully eclipsed 0°

torsional strain

eclipsed (0°) vs staggered (60°)

fully eclipsed

• least stable

• dihedral angle = 0°; groups are overlapping

• highest energy conformation and torsional strain

staggered conformations are ______ stable than eclipsed

more

ideal C-C-C bond angle to minimize angle strain

109.5°

• cyclopropane = 60°

• cyclobutane = 90°

• cyclopentane = 108°

• cyclohexane = 120°

• cycloheptane = 129°

most stable (lowest energy) conformation of cyclohexane is

chair conformation

cyclohexane chair conformation

• low-energy

• tetrahedral geometry and C-H bonds are staggered

• each carbon has 1 axial and 1 equtorial substituent

axial substituents

bonds that go straight up or down the page

equatorial substituents

bonds that are at an angle upwards or downwards

• most stable because they avoid 1,3-diaxial interactions (steric strain)

when a chair is flipped

• all axial substituents become equatorial and all equatorial substituents become axial

• all of the substituents that were pointed up, remain pointed up, and the substituents that were pointed down, remain pointed down

most stable chair conformation

bulky groups in EQUATORIAL positions due to less steric strain

• equatorial most stable because they avoid 1,3-diaxial interactions (steric strain)

• bulkier substituents in equatorial position are heavily favored

cis disubstituted rings

substituents pointing in the same directions

trans disubstituted rings

substituents point in opposite directions

stereocenter

any point within a molecule that can give rise to stereoisomers

chiral center

sp³ hybridized carbon atom with 4 unique substituents

• configuration of a chiral center is indicated by R/S designation

To assign R/S designation:

1. Locate chiral centers: sp³ hybridized carbon with 4 different groups

2. Assign priority (Cahn-Ingold-Prelog)

   1. Higher atomic # = higher priority

   2. Compare further atoms if direct ones are the same

3. Trace circle from 1 to 2 to 3 and Orient lowest priority (4):

   1. On dash: assign R (clockwise) or S (CCW)

   2. On wedge: assign then reverse designation

   3. On line: swap pairs to move 4 to dash and assign as usual

4. Naming: (R)-, (S)-, or (2R,3S)- as prefix in IUPAC names

R/S designation with lowest priority group on a dashed bond

R: CW

S: CCW

R/S designation with lowest priority group on a wedged bond

reverse the designation

R/S designation with lowest priority group on a line

swap pairs to move 4 to dash and assign as usual

If there is more than one chiral center in the compound,

include locants before each R/S designation.

Stereoisomers

molecules with the same molecular formula and connectivity but different spatial arrangement

Enantiomers

stereoisomers that are non-superimposable mirror images of each other and possess the same physical properties

• opposite configurations at all chiral centers

• differ only in optical activity

Diastereomers

stereoisomers that are non-superimposable, non-mirror images of each other and possess different physical properties

• opposite configurations at some, but not all, stereocenters

  • need more than one stereocenter

2ⁿ rule

max # of stereoisomers

n = # chiral centers

ex: molecule with 3 chiral centers, 2³=8 possible stereoisomers

constitutional / structural isomer

share the same molecular formula, but differ in the connectivity of their atoms

two ways to draw enantiomer

1. switch all wedges to dashes and all dashes to wedges

2. draw a reflection over an imaginary line of reflection

optical activity

• chiral molecules rotate plane-polarized light

• enantiomers: rotate light equally but in opposite directions

• racemic mixture (50:50) = optically inactive

T/F: A molecule with one chiral center is always chiral.

True

meso compound

• have at least 2 chiral centers + internal plane of symmetry

• achiral (optically inactive) despite having chiral centers

• # of stereoisomers for meso compound: 2ⁿ - 1

Fischer Projections

• depicts stereochemistry of chiral molecules

• horizontal lines = wedges (out of plane)

• vertical lines = dashes (into plane)

• R/S designation determined by lowest priority group

R/S configuration of a stereocenter in a Fischer projection

1. assign priorities to each group based on atomic number

2. if lowest priority on vertical, assign R/S normally

3. if lowest priority on horizontal, assign → reverse

E configuration of alkene

higher priority groups on both carbons are on OPPOSITE sides of the double bond

Z configuration of alkene

higher priority groups on both carbons are on the SAME side of the double bond

E/Z configuration for tied alkenes

If both atoms attached to one side of the alkene are identical, we assign priority by comparing the elements directly bonded to the atoms that are tied. We keep doing this until the first point of difference.

E/Z (Cis/Trans) Alkene Naming

include locants and stereochemistry at the beginning of IUPAC name