Review Sheet for Chem 32 First Exam
by Elena Franklin
The following represents an outline of material covered so far in Chem
32, Fall 1999.
- Symmetry operation: A movement about a symmetry
element which leaves the molecule indistinguishable from its
- Symmetry element: The line, plane, or point
about which a symmetry operation is done.
- Point group: A name for a collection of symmetry
elements which are commonly found together. A compound which
has all of the symmetry elements of a point group is said to
belong to that point group.
- Polarity and Chirality: can both be determined
by looking at the point group of the molecule. Polar
molecules are Cn, Cnv, or Cs
only. Chiral molecules have no improper rotations, including
i (S2)or plane of symmetry (S1).
- Organic Chemistry
- Structure notations: Know how to read and write
line bond, condensed, and "chickenwire" notations.
- Nomenclature: Read appendix B. Be able to draw
a structure from a name you are given. See handout for one
example. If this is hard for you and you need more
examples/help, let me know or send me email. Also
check out the nomenclature problems on previous exams.
- Functional Groups: A portion of a molecule which
reacts in certain characteristic wyas which are
different from the reactions of alkanes. Some
functional groups are alkenes, ketones, ethers, esters,
carboxylic acids, amides, and alcohols
just to name a few. Be familiar with these and be able to
identify and name them from a complicated molecule.
- Hybridization: Carbon usually bonds to 2, 3, or
4 other atoms. In order for all of these bonds to be
equivalent (like in methane, CH4), we combine the s
and 1, 2, or 3 p orbitals to form hybrid orbitals. There are
written as sp, sp2, and sp3. Know what
these notations mean and know how to predict what the
hybridization will be on a carbon atom (Nitrogen and oxygen
follow the same rules. Be able to do them too!)
- Formal Charge: The formal charge on an atom is
the number of electrons in the free atom minus the number of
electrons the atom owns in the molecule of interest. What
does it own? Half of the electrons in all the bonds it forms
plus all of the electrons in its lone pairs.
- Unsaturation: A molecule is unsaturated
if it contains pi bonds (double or triple bonds). Each pi
bond reduces the number of hydrogens in the molecular formula
by 2, so we can predict the number of sites of
unsaturation by looking at the molecular formula. But
wait, rings also reduce the number of hydrogens by 2, so we
can't distinguish between rings or pi bonds by counting. So
informally, we count a ring as a site of
unsaturation as well. Note that this is just to help us
determine structures; rings are strictly speaking not
- Reactive intermediates: Carbon tends to react
through one of four reactive intermediates: carbonium ions,
radicals, carbanions, and carbenes. These are important
because we use our knowledge of them in helping us to predict
- Resonance: This is a big one! Resonance
structures occur when two or more structures can be drawn for
a molecule by changing only the positions of electrons.
Nuclei do not move in resonance structures. Usually,
resonance only occurs when you have a double bond next to a
heteroatom, another double (or triple) bond, or a charged
atom. Get a lot of practice at drawing resonance structures.
If you are having trouble with this, ask any TA to show you
the arrow pushing formalism which should help.
- Aromaticity: A molecule is aromatic if it
possesses an unusually stable conformation such as benzene.
In order to be aromatic a molecule must follow Huckel's rule:
It must have a continuous ring of p orbitals and have
4n+2 pi electrons. Note: if an atom can become part of an
aromatic system by changing its hybridization (from
sp3 to sp2) then it usually
- Y and Y*: Y is
the wave function which describes an atomic orbital. These are
calculated using the Schrodinger equation. Y2 is a probability distribution
which tells us where in space the electron(s) are likely to
- Quantum numbers: "n" describes the radial (how
far from the origin) part of the atomic orbital. "l" and
"m1" describe the angular part of the atomic
orbital and "ms" describes the spin part. For
hydrogen, "n" determines the energy of a particular orbital.
For all other atoms, you need to know "n" and "l" to know the
energy. Pauli principle: all 4 quantum numbers must be
different for each electron. The periodic table gets its
shape from the pauli principle: can you explain why?
- LCAO-MO Method: We are going to build molecular
orbitals (MO's) by using an approximation: make MO's by
linearly combining atomic orbitals (LCAO). Basically, add and
subtract AO's to get MO's. This usually gives us a pretty
- MO's: Bonding MO's have most of the
electron density on the line between the nuclei (the
"internuclear axis"), while Antibonding MO's have most
of the electron density on the outside of the molecule.
Nonbonding MO's have most of the electron density
situated on only one of the atoms in the molecule. s MO's have spherical symmetry about the
internuclear axis. p MO's change
sign if you rotate them 180o about the internuclear
- Bond Order: The bond order can be calculated by
the number of electrons in bonding MO's minus the number of
electrons in antibonding MO's and dividing the whole thing by
2. Bond strength goes up with increasing bond order while
bond length goes down.
- Chirality and Stereoisomerism
- Stereoisomers: Molecules which have the same
connectivity (order of bonding) but differ in the way atoms are
arranged in space. Note: different conformations of a
rotationally free molecule are not stereoisomers. They
only count as stereoisomers if we can separate and store them at
room temperature. So ethane's arrangements are not stereoisomers
but cis and trans arrangements about a double bond
- Chiral: A molecule is chiral if it has a
nonsuperimposable mirror image, or to use group theoretical
terms, if it does not possess an improper rotation axis. The
molecule is different from its mirror image. If the
molecule is the same as its mirror image in any
accessible conformation then it is not chiral. A "chiral
center" is a carbon with four different groups attached, but
note that molecules can be chiral even if they have no chiral
centers (e.g. bromochloroacetonitrile) and they can be achiral
even if they do have chiral centers (e.g. 2,3-dibromo
2,3-dichloroethane, a meso compound).
- Enantiomers: Stereoisomers which are
non-superimposable mirror images of each other. Ex: two
different trans-dibromocyclohexane enantiomers. One pure
enantiomer will rotate the plane of polarized light. The
other one (if pure) will rotate polarized light the same
amount but in the opposite direction. Remember that
the direction in which light is rotated (clockwise or
counterclockwise) has nothing to do with whether the center is
R or S. A racemic mixture (a 50/50 mixture) of the two will
not rotate the plane of polarized light at all.
- Diastereomers: Stereoisomers which are not
enantiomers. Cis and trans diastereomers are sometimes called
geometric isomers. Get used to the old and new notations,
they are everywhere in chemistry! Ex cis and trans
- Meso structure: A molecule which is not chiral
but contains chiral centers. Why don't the chiral centers
make it chiral? The molecule has an internal plane of
symmetry, which is the same as an S1 axis (remember
- 2nrule: A molecule with n chiral
centers will have AT MOST 2n stereoisomers. If
some of them are meso structures though, that reduces the
number. Ex Tartaric acid has 2 chiral centers but only 3
stereoisomers since one is a meso structure (R,S and S,R are
the same molecule).
- R S naming system: Get a lot of practice at this
one, it can be tricky to see. To name a chiral center giving
its absolute configuration:
- Assign a priority sequence to the
substituents--highest atomic number first (if you have
isotopes, higher atomic mass is higher), go down chains
until the first point of different, double bonds
count as two bonds to that type of atom.
- Turn the molecule so that the lowest priority atom is
pointed away from you.
- If going around from the highest priority to the next
and then the third you traverse a clockwise circle the
carbon is R. If it's counterclockwise it's
- Biological Systems: Chirality is everywhere in
biological systems. It's what makes things smell different,
taste different, and do different things in your body.
Proteins, DNA, cholesterol, enzymes, etc. are all chiral.