CHE 171
Fall, 2005
Quiz 2
Name_______________________________
Section:
101 102 103
M
W
F
(circle one)
1. Tetrahydrocannabinol (THC) is the most psychotropic of the more than 400
chemicals found in the cannabis sativa plant (marijuana). A Lewis structure of
THC is shown below. Answer questions a-c in the spaces provided.
CH3
H
C
H
C
H
H
C
H
C
most acidic proton
H
O
H
H
H3C
C
C
H
C
C
C
C
C
C
O
H
C
H 3C
H
H
H
C
C
H
C
H
H
C
H
H
C
H
H
H
(a) Draw the skeletal structure (bond-line form) of THC below. (5 pts)
OH
O
(b) Which is the most acidic proton in the THC molecule? Circle it in either
the Lewis structure provided or in your skeletal structure. (5 pts)
(c) Is THC a polar or non-polar molecule? Explain. (5 pts)
OH
O
THC is non-polar. The individual
dipole moments within the molecule
cancel to give NO net dipole moment.
2. Draw the products of each of the following acid-base reactions and predict
whether the equilibrium favors the starting materials or products (draw an arrow
above the equilibrium arrows either towards products or reactants to show this; a
pKa table is on the next page). Use curved-arrow notation to show electron flow.
(5 pts each)
O
(a)
O
H
O
+
O
H
+
H2
(b)
NH2
+
H
O
NH3
+
H2O
+
O
(c)
HO
+
H C C H
C C H
3. Acetonitrile (CH3CN) has a pKa of 25, making it more acidic than many other
compounds having only C-H bonds. Draw Lewis structures for acetonitrile and
its conjugate base. Use resonance structures to account for the acidity of
acetonitrile. (5 pts)
See Smith Problem 2.16
H
H C C N
H
+
H
H
B
C C NH
C C N
H
H
acetonitrile
4. In the reaction shown below, label the nucleophile and electrophile. Used curved
arrows to show the movement of electron pairs. (6 pts)
H3C
CH3
+
H3C
H Br
CH3
nucleophile
(Lewis base)
H3C
H3C
electrophile
(Lewis acid)
H
CH3
CH3
+
Br
5. The pKa of three different C-H bonds is given below. (9 pts)
(a) For each compound, draw the conjugate base, including all possible
resonance structures.
(b) Explain the observed trend in pKa.
CH3CH2CH2-H
CH3
H 2C C
CH2-H
pKa = 50
CH3CH2CH2
H 3C
O
C
CH2-H
pKa = 19.2
pKa = 43
CH3
H 2C C
CH2
H 3C
O
C
CH3
H 2C C
CH2
H3C
CH2
O
C
CH2
The most acidic compound has a conjugate base that is resonance stabilized, where
the electron density is shared by the electronegative oxygen atom. See Smith Problem
2.39.