Lecture 6, 'Chirality', introduces the concept that atoms with identical functional groups can have different three dimensional structures. This phenomenon is call chirality and can have significant affects on the biological and biochemical properties of seemingly identical organic molecules. We will focus on how to recognize and describe these 3D differences, and on why molecules with different chirality can have different effects on living organisms.
This material corresponds to Chapter 11 of Daley & Daley.
Lecture 6 slide sets: Chirality
(Click on the links below to download each slide set for the lecture.)
6.1 Symmetry & asymmetry
6.2 Nomenclature of stereocenters
6.3 Properties of asymmetric (chiral) molecules
6.4 Optical isomerism
6.5 Fisher projections
6.6 Molecules with two stereocenters
6.7 Resolution of enantiomers
6.8 Stereocenters other than carbon
- Key concepts
1. Chiral molecules are asymmetrical.
2. Chiral carbons are bonded to four different substituents.
a. Symmetrical objects are superimposable.
b. Enantiomers are chiral molecules: mirror image pairs of molecules (stereoisomers or
configurational isomers) that contain chiral carbons and are non-superimposable.
c. Cahn-Ingold-Prelog allows us to label enantiomers R or S. The substituents of chiral
carbons are prioritized by atomic mass: highest mass is highest priority. With the lowest
priority group facing away from you, determine whether high to low priority requires
clockwise (R ) or counterclockwise (S) rotation.
d. Fisher projections show 3D structure on a 2D page. Groups on the horizontal axis come
forward (out of the page) while groups shown on the vertical axis recede backwards (into
e. Meso isomers have two chiral carbons (of opposite rotation) with an internal plane of
symmetry and are thus achiral; they don't rotate light.
f. Non-carbon atoms can also be chiral centers: Si, N, P, S.
i. Molecules with an internal plane of symmetry are achiral even if they have chiral carbons.
ii. A center (or point) of symmetry connects two identical groups in a symmetrical molecule.
iii. When prioritizing substituents for CIP go atom by atom. So, a COOH group has a higher
priority than a CH2OH group.
iv. Enantiomeric pairs have identical physical & chemical properties, with two exceptions: 1)
they rotate plane-polarized light in opposite directions, though to the same degree; 2) their
biological properties will differ since most biological molecules that entantiomers interact
with are also chiral.
v. Flipping Fisher projections by 180 degrees doesn't change them, but flipping them 90
degrees does change them.
vi. Racemic mixtures of enantiomers appear not to rotate plane-polarized light, since each
enantiomer rotates light in opposite directions.
vii. Dextrorotary (D) enantiomers rotate light to the right, while levorotary (L) enantiomers
rotate light to the left. D & L don’t correspond to R & S in any systematic way.
viii. Biological synthesis produces only one an enantiomer while chemical synthesis produces
ix. Enantiomers can be ‘resolved’ or separated by attaching each to the same molecule,
separating the joined molecules by their (now) different physical and chemical properties,
and then reversing the join.
Links & items of interest:
> VIDEO: Optical isomerism (University of Surrey, 2011)
> VIDEO: What is chirality and how did it get in my molecules? (Michael Evans, TedEd, 2012)
> VIDEO: Chirality (Brad Pentelute, MIT OpenCourseWare, 2014)
> VIDEO: Fisher projection introduction (Khan Academy)
> VIDEO: Fisher projection practice (Khan Academy)
> Why is our world chiral? (Emily Singer, QuantaMagazine, 26 November 2014)
> Atryn, on old McDonald's Pharm (Charles Choi, Scientific American, 10 January 2009)
> Chiral drugs (Khan Academy)
Resources (not mandatory):