Supplemental Instruction

January 22nd, 2014:

 

Question: Explain more about the Molecular Orbital Theory, and the differences in the potential arrangments of the electrons.

 

Answer: Molecular Orbital Theory describes the possible arrangement and energy of electrons within a molecule. When electrons are in a “bonding” molecular orbital, which is formed by “additive” interaction of atom orbitals, they contribute to the formation of a stable bond because electrons in a “bonding” orbital have lower energy. The reverse is also true. Electrons in an “antibonding” orbital, which is formed by “subtractive” interaction of atomic orbitals, have higher energy and these electrons make molecules less stable. When electrons have the choice, they would like to stay in the “bonding” orbitals because going to “antibonding” orbitals means higher energy. However, when molecules with electrons in the bonding orbitals (we call the molecules at “ground” state) somehow absorb energy (for example, UV irradiation), the electrons can move into antibonding orbitals. Since the molecules are now at an unnatural higher energy state, we call the molecules at “excited” state.

 

 

January 26th, 2014

 

Question: What are some guidelines for creating a structure from a molecular formula in regards to formal charges?

 

Answer: The goal when drawing any structure from a molecular formula is to have the lowest formal charge for all of the atoms in the given molecule, while maintaining the octet rule. If there is no way to avoid placing having a formal charge, a negative charge should be placed on the more electronegative atoms. If two structures (A and B) are possible for a given molecular formula, and A has formal charges on all of the atoms while B has a formal charge on only one of the atoms, then structure B will be the favored structure. 

 

 

Question: How can I know when to use a resonance form and where the three atom unit is?

 

Answer: Resonance forms are used when there is a three atom unit that contains at least two of three potential markers: a positive charge, a negative charge (lone pair of electrons) or a pi bond (double bond). When the three atom unit it identified, the three atom unit can experience electron pushing, represented by curly arrows in a mechanism format. All of the resonance forms drawn for a molecule, when compiled, form the actual structure of the molecule, a hybrid for all of the resonance structures. 

 

March 26th, 2014

 

Question: In problem six, why aren't two products drawn for the reaction with NBS and HBr?

 

Answer:

 

" For the reaction of the tetrasubstituted alkene and NBS in the practice exam, only the major product is shown in the answer. The other product (a tertiary bromide) might form, but likely in very small amount because the radical center is very crowded compared with the other radical center. In the exam, if I need the students to list all possible products, I will ask them specifically.Same for the other question, only the major product was shown (somewhat similar to Markovnikov selectivity). "

 

- Dr. Yang

 

March 30th, 2014

 

Question: Would S2- be a better nucleophile than HS-? And would the same apply to O2- and OH-?

 

Answer: Hypothetically, yes. On paper, both S2- and O2- would be much better nucleophiles. However, it would be nearly impossible to maintain both of these nucleophiles in a solution, because they are so basic, they would end up pulling at least one proton from solution.