Name ______________________________________
Read section 7.4 on optical rotation.
1 The structures of two popular analgesics, aspirin and ibuprofen are on p 479 of the text.
(a) Draw ibuprofen in skeletal notation.
(b) How many other words in the English language begin with "ibu..."? (hint: approximately 0)
(c) Why do you think that name starts with "ibu"? (that's a serious question — look at the
structure and make an educated guess)
(d) Ibuprofen is chiral. Mark the stereocenter in your drawing above with a *.
(e) Only the S enantiomer of ibuprofen has medicinal value. Draw the S enantiomer below.
(f) The S enantiomer rotates polarized light in the clockwise direction. How is this possible?
Doesn't "S" mean counterclockwise? Shouldn't there be some relationship between the
configuration label and the direction of optical rotation? Explain briefly.
Lecture outline
Enantiomers have identical physical properties — e.g., mp, bp, density, solubility, etc; however...
... they rotate the plane of polarized light in opposite directions (by equal amounts, of course) —
thus, chiral compounds are said to be optically active.
light —
"end view" of oscillating
E-vector
Electric
field
— plane polarized light can be made by passing ordinary (unpolarized) light
through a polarizer
!
unpolarized
light
polarizer
planepolarized
light
soln of one enant
of a chiral cmpd
plane-polarized
light — rotated
by angle !
If rotation is cw,
If rotation is ccw,
Important: There is no simple relationship between +/– rotation and R/S
configuration labels!
Not surprisingly, the angle of rotation is proportional to the concentration of
the solution and the length of the sample cell — if we divide these
out, we get the specific rotation of the compound, [α].
[!] =
!
observed rotation
cl
path length (in decimeters (dm; 1 dm = 10 cm))
concentration (in g/ml)
Specific rotation depends on temperature and wavelength of light used, thus it's often
written as, e.g.,
T = 20 °C
[!]20
D
Na vapor lamp "D-line" (589 nm = yellow)
\
Achiral compounds do not rotate plane-polarized light they are "optically inactive".
Mixtures of exactly equal amounts of enantiomers — called racemic mixtures (or
"racemates") — are optically inactive.
Racemic mixtures are often indicated by "(±)")
e.g., (±)-2-bromobutane =
Optical rotation is commonly used to determine the "enantiomeric excess" in a sample
containing unequal amounts of two enantiomers. For example, consider the following problem:
A chemist did a reaction she hoped would produce the good enantiomer, E, of a chiral drug, and
none of the evil enantiomer, E'. Pure enantiomer E is known to have [α]D = –150°. The specific
rotation of her rxn product is –120°. What are the %s of the good and evil enantiomers in her
mixture?
Two enantiomers of a chiral compound must behave the same way in a achiral
environment. For example, their solubilities in an achiral solvent, their reactions with an achiral
reagent, or their interactions with an achiral catalyst must all be identical by symmetry.
But the way two enantiomers behave in a chiral environment must be different. The way
enantiomeric forms of drugs interact with biological systems, nearly all of which are chiral, can
be very different.