Chapter 5

• Chirality is the property of molecules that allows a compound to exist in 2 non-superimposable mirror image forms

Nonsuperimposable mirror images, different molecules with different properties

• Carbons with four different groups attached are chiral • It’s mirror image will be a different compound (enantiomer)

When the images can be superposed the compound is achiral

• Notice the plane can bisect atoms.

A molecule that has a plane of symmetry is achiral.

Cis-1,2-dichlorocyclohexane is achiral because the molecule has an internal plane of symmetry. Both structures above can be superimposed.

Trans-1,2-dichlorocyclohexane does not have a plane of symmetry so the images are nonsuperimposable and the molecule will have two enantiomers.

(R) or (S) assignment

assigned with respect to a reference sample or another stereocenter in the same molecule • relative configuration is easier to deduce from experimental data than absolute configuration

1) assign priorities to the groups on the asymmetric (chiral) center 2) orient the molecule with the lowest priority group pointing away from you 3) trace a path from group 1 to group 2 to group 3

clockwise path is the R configuration

counterclockwise is the S configuration

1) If the atoms attached to the asymmetric center are all different, priority depends on atomic number 2) When relative priority cannot be decided by Rule 1, compare the next set of atoms in the groups working outward from the stereogenic center (ignore H when atoms within a rank are compared) 3) A double-bonded atom is treated as if there were two such single bonds (triple-bonded atom treated as three such single bonds)

• For multiply bonded atoms, expand the bonding by putting the appropriate number of atoms in parentheses • Real atoms take priority over parenthetical atoms

• R or S configuration goes in front of the name in parentheses, followed by sign of rotation (if known) • For molecules with more than one stereocenter, the position number of the stereocenter is added to the R or S

Enantiomers have identical physical properties except: –The way they interact with plane polarized light (equal magnitude/opposite direction) –The way they interact with other chiral molecules

compounds rotate the plane of polarized light

compounds do not rotate polarized light

• Optical rotation measured by a polarimeter – 1) monochromatic light passed through a polarizer 2) the plane-polarized light is passed through a sample 3) the analyzer (another polarizing filter) is adjusted to allow the maximum light through, and the angle is noted Clockwise rotation is (+) dextrorotatory-d Counter-clockwise rotation is (-) levorotatory-l

• Equal quantities of d- and l- enantiomers. • Notation: (d,l) or (±) • No optical activity. • The mixture may have different boiling point (b. p.) and melting point (m. p.) from the enantiomers!

• optical purity (enantiomeric excess or ee) measures how much more of one enantiomer is present than the other • optical purity = observed rotation / known rotation (x 100%) • optical purity = % major enant – % minor enant

• If equilibrium exists between two chiral conformers, the molecule is not chiral. • Judge chirality by looking at the most symmetrical conformer. • Cyclohexane can be considered to be planar, on average.

Have more than one stereocenter • Diastereomers are non-mirror image stereoisomers • Diastereomers differ in configuration in at least one, but not all, stereogenic centers • Diastereomers have: different physical properties and can be separated by ordinary physical methods • Diastereomers have same formula, connectivity, but different 3D arrangement and are not enantiomers