Answers for Problem Set 1

1.2. The ability to maintain aromaticity (4n + 2 electrons in a continuous loop; n=0,1,2,...) is the important observation.
a. Place the electrons from the central double bond in the 5-member ring. To confirm the validity of this approach, look at the electrostatic potential map (red=negative, blue=positive; calculation at the PM3 semiempirical level):

b. Conjugate the electron pair of the amine with the nitro group.
c. Draw the resonance structures for each in which a heteroatom lone pair participates in pi bonding.

1.3. All can be rationalized on the basis of resonance. When resonance in the reactant does not help you, draw resonance structures for all possible protonated products; this is usually more informative than examination of reactants.
a. On the nitrogen.
b. On oxygen.
c. C-2! (Count resonance structures)
d. The nitrogen in the ring (compare aniline vs. pyridine).

1.4. All of these deal with resonance delocalization of the amide lone pair into the π bond of the carbonyl.
a. Both resonance and MO pictures can be modeled on an allyl anion.
b. The N=C-O form is more polar (+)N=C-O(-) than the N-C=O form; solvent interactions will thus make it more important (by stabilizing it).
c. The bicyclic molecule cannot delocalize the lone pair (the four substituents on the putative N=C bond are not coplanar). Thus, the carbonyl is more electron-deficient, and more reactive to nucleophiles.

2.1. a. Diastereomers
b. Enantiomers
c. Enantiomers
d. Diastereomers (both are meso!)
e. Enantiomers
f. Enantiomers

2.2. a. S
b. R
c. R
d. R
e. S
f. 3 stereocenters!C-1: R. C-2: R. C-4: S.
g.The sulfur has a lone pair; atomic number = 0, so the given structure is R.

2.26.a. Chiral
b. Chiral
c. Chiral
d. Achiral (3 planes)
e. Achiral
f. Chiral
g. Chiral (Hexahelicene; there is a spiral twist.)

h. Chiral
i. Chiral
j. Chiral
k. Chiral
l. Chiral
m. Chiral; see the two forms:

Supplemental problems.

B.1.

B2.

B3.

a. cis-1,2-aminomethylcyclohexane (NOT bis-(aminomethyl)cyclohexane!)
b. A gauche pentane Others are possible
c. 1-Methylnorbornene (1-methylbicyclo[2.2.1]hept-2-ene)
d. Z-bicyclo-[4.3.1]-dec-1-ene (Z indicates the geometry of the double bond; 1-ene makes it a bridgehead alkene with the double bond in the largest ring.)
e. trans-decalin (trans-bicyclo[4.4.0]decane)
f. cis-decalin
g. trans-4-tert-butylcyclohexanol Note that the diequatorial form limits steric clashes; the diaxial form has significant H-H interactions with the other axial hydrogens (1,7 interactions).
Isobutane (show all hydrogens)