Class Test #1

ANSWERS

October 30, 1996

1. When nitromethane (pK_a 10.2) and 4-methoxypyridine (pK_a 7.0) are mixed together in water, the following acid-base equilibrium is established:

(a) Draw complete Lewis line-dot structures for ...

(b) Draw resonance structures for the two ions on the right-hand side of the equilibrium, and for each, indicate which are major contributors.

(c) For the -CH₂NO₂ anion, identify the hybridization of the ... carbon:    sp² ... nitrogen:    sp² ... oxygens:    sp² and    sp².

(d) What is the orientation of the lone pair of electrons on the nitrogen atom of 4-methoxypyridine with respect to the plane of the ring?    in the plane (parallel)  

(e) Does the equilibrium lie on the right-hand side (RHS) or the left-hand side (LHS)?    LHS  

(f) The standard free energy for this reaction is G^o =    +18.2   kJ/mol at 25 ^oC.

Calculation:

G^o = -RTln K_eq = -2.303RTlog K_eq

K_eq = K_a^NM / K_a^MP      (NM = nitromethane; MP = 4-methoxypyridine)

log K_eq = log K_a^NM - log K_a^MP

             = pK_a^MP - pK_a^NM

             = -3.2

G^o = -2.303(0.0083)(298)(-3.2) = 18.2 kJ/mol.

(i) complete the Lewis structures for the pertinent parts of the molecules,

(ii) label the nucleophiles ("N") and electrophiles ("E"), and

(iii) draw curly arrows showing the movement of electrons that take place to convert reactants to products.

See: Ege, Problem 4.21(b)

(a) 1-chloro-3-ethyl-4-methylpentane

(b) 4-(1-methylethyl-5-methyl-2-phenyl-3-hexanol

(a) Draw the two chair conformations of fluorocyclohexane and indicate by the length of arrows between the two, which conformer predominates at equilibrium.

(b) Draw the two chair conformations of trans-1-fluoro-3-methylcyclohexane and indicate by the length of arrows between the two, which conformer you would expect to predominate at equilibrium.

6. (a) A compound with the molecular formula C₇H₁₂ has    2    sites of unsaturation. Thus, the molecule contains either    1 triple bond   ,    2 double bonds   ,    1 ring and 1 double bond   , or    2 rings    (there are four possibilities). For each of the four categories of compounds given as possibilities for the molecular formula C₇H₁₂, draw one structural formula.

(b) Adding 2 moles of hydrogen to the compound gives 2,3-dimethylpentane. In light of this evidence, what structures are possible for the C₇H₁₂ compound?

12nov96; wjl