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.

  1. Symmetry

    • Symmetry operation: A movement about a symmetry element which leaves the molecule indistinguishable from its original conformation.
    • 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).

  2. 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 unsaturation.
    • 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 reaction mechanisms.
    • 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 will.

  3. Chemical Structure

    • 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 be.
    • 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 good approximation.
    • 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 axis.
    • 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.

  4. 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 are.
    • 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 dibromocyclohexane.
    • 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 your symmetry!).
    • 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:
      1. 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.
      2. Turn the molecule so that the lowest priority atom is pointed away from you.
      3. 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 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.