Lecture 3, 'Molecular conformations', discusses the varied three-dimensional shapes that molecules can assume because of rotational freedom around single bonds, or because of the lack of rotation around single bonds in ring structures. Lecture 3 references Chapter 3 of Daley and Daley.
Lecture 3 slide sets: Molecular conformations
(Click on the links below to download each slide set for the lecture.)Key concepts
1. Most molecules are flexible and can adopt a variety of conformations.
2. Molecules tend to seek and remain in their lowest energy conformations.
a. Conformation = the 3D shape of a molecule
b. Alkanes rotate through conformations known as conformers or rotamers like staggered and eclipsed.
c. If molecular conformers are plotted vs. energy, staggered conformations are seen at energy minima
while eclipsed conformations are seen at energy maxima.
d. Steric hindrance is 3D crowding or interference by adjacent atoms or functional groups. Large
substitutents or functional groups like butyl groups are more likely to sterically hinder their neighbors
than hydrogen atoms. Tert-butyl groups (1,1-dimethylethyl) are famously bulky and exert significant
e. With the exception of cyclopropane, cycloalkane rings are not planar but puckered in order to maintain
bond angles as close as possible to 109.5° bond angles for carbons with four substitutents.
f. Rings may have cis and trans stereoisomers. Generally, the isomer that places large substituents in the
equatorial position is the most stable.
i. Molecular conformation can effect the physical, chemical & biological properties of molecules.
ii. Alkanes rotate spontaneously through staggered & eclipsed conformations.
iii. Little energy is required for rotation around single bonds.
iv. The staggered conformation is more stable (lower energy) than the eclipsed conformation; staggered is favored more than 90:10.
v. Conformers (or rotamers) can be represented by dash-wedge, sawhorse or Newman projection
v. Torsion angles (aka dihedral angles) are angles of rotation between adjacent substituents in molecular
vi. Gauche refers to groups separated by 60° torsion angles.
vii. Anti refers to groups separated by 180 degree torsion angles.
viii. If a molecule includes two large substituents then the staggered conformation in which the two are
anti is more stable than the conformation in which they are gauche. Likewise, the least stable eclipsed
conformation occurs when the two large groups are directly eclipsed in front of and behind each other.
ix. Cyclohexane puckers into either chair or boat forms. The chair form is more stable because chair
hydrogens are staggered while boat hydrogens are eclipsed and because the chair gives each
substituent (particularly flagpole substituents) maximal space.
x. In the chair form of cyclohexane, each carbon has both axial and equatorial substituents. Axial
positions are directly above and below the average plane of the ring. Equatorial positions surround the
ring and give substituents more space and thus less steric hindrance.
xi. Flipping cyclohexane chairs causes all axials to become equatorial and vv.
xii. Ring stability is maximized when large substituents are placed in equatorial positions.
Links and items of interest:
Resources for students: