Name:
1
Chemistry 250A -- Exam #1 Answer Key -- September 23, 2008
(There are 5 pages.)
1. (15 pts) Show the major product(s) from the following acid-base reactions. If there is
no reaction then say “No Reaction”.
a)
H2N
H
O
C
C
O
CH3
+ HCl
H3N
OH
b) H
H
C
C
F
OH
c)
O
O
CH3 + Cl
O
H
OH
C
H
+ CH3CH2OK
K
CH3
O
C
C
C
F
O
O
C
d)
C
O
C
H
H
H + NaNH2
H
C
O
C
H
CH3
O
HO
O
OH
C
H
H
O
O
Na
O
C
C
H
O
H
OH
H
O
H
CH3
+ CH3CH2OCH2CH3
Br
No Reaction
H
H
e) NaHCO3 + H2SO4
HSO4 Na
+ CO2 + H2O
2. (3 pts) Circle the compound that is most soluble in water.
CO2CH3
CO2Na
+ NH3
H
H
CO2H
+ CH3CH2OH
Name:
2
3. (20 pts) Rank the following compounds according to the indicated property.
c) acidity
H
H
H
N
NH2
O
A
CH3CO2H
C
B
B
most acidic
C
>
D
A
>
>
D
least acidic
b) basicity
O
O
F
O
S
O
NaOH
OH
O
A
B
B
most basic
D
C
C
>
D
>
H2C
N
C
>
A
least basic
c) relative importance of resonance structures
H2C
N
A
N
C
most important
H2C
N
B
>
D
N
B
>
>
N
H2C
N
D
A
least important
d) boiling point
O
CH3CH2CH2CH2OH
CH3CH2CH2CH2CH3
CH3CH2CCH3
C
D
B
A
A
>
highest boiling point
CH3CH2CH2CH3
D
>
B
>
C
lowest boiling point
N
Name:
3
4. (16 pts) Amoxicillin (shown below) is a widely-prescribed antibiotic that is active against a
variety of bacteria.
Amine
Amide
NH2
109.5°
Bond angle
*
*
HO
*
*
*
*
*
H
N
Thioether
H
S
O
*
CH3
N
CH3
Amide O
Phenol
C
*
O
Amoxicillin
OH
Carboxylic Acid
a) Identify the functional groups in amoxicillin. (I've done one for you.)
b) Put an asterisk by all sp2 hybridized carbon atoms.
c) Provide the approximate value for the indicated bond angle.
d) Although this is the structure that is usually drawn for amoxicillin (e.g., see
Wikipedia), it is not accurate. Considering what you know about the acid-base properties
of the functional groups in Amoxicillin, draw a more accurate representation.
NH3
H
N
O
H
S
CH3
N
HO
CH3
O
C
O
O
The carboxylic acid and the amine will react with each other!
5. (10 pts) Allene has the structure H2C=C=CH2.
a) Draw an accurate 3D structure of allene. (Hint: It is not planar.) On this structure show
the p-orbitals and how they overlap to form the pi bonds in allene. Clearly indicate the
3D orientation of the p-orbitals relative to the atoms in the molecule and to each other.
pi bond coming in and out of paper
H
sp2 sp
C
H
C
sp2
C
H
H
pi bond in plane of paper
p-orbitals coming
in and out of paper
b) Label the hybridization on each carbon atom.
c) What are the approximate CCC and HCH bond angles? 180° and 120° respectively.
Name:
4
6. (24 pts) The reaction of butyllithium with diisopropylamine produces LDA, a
commonly used base in organic reactions.
CH3CH2CH2CH2Li +
N
LDA + CH3CH2CH2CH3
(Butane)
H
Diisopropylamine
a) Redraw butyllithium and diisopropylamine showing all atoms, bonds, lone pairs, and
non-zero formal charges explicitly.
H
H
H
H C
H
H
H
H C H
H CH
C
N
C
H
H
H
H
H
H
C
C
C
C
H
H
H
H
H
H C
H
H
C H
H
Li
H
H
H
H C H
H CH
C
H
H
H
H
H
C
C
C
C
H
H
H
H
N
C
Li
LDA
H
H
C H
H
H
b) Use curved arrows to show the reaction of butyllithium with disopropylamine to
form LDA and butane. (Draw a complete Lewis structure for LDA.)
c) Provide a brief rationale for why this reaction is favorable.
The nitrogen is more electronegative than the carbon, therefore the negative charge
"prefers" to reside on the nitrogen, thus making the reaction favorable.
d) LDA undergoes an acid-base reaction with acetone to produce an enolate. Use curved
arrows to show this reaction. Draw a full Lewis structure for the product and use curved
arrows to find other important resonance forms. Circle the most important resonance
form.
H
H
H
H C
H
Li
H CH
C
H
H C H
N
C
H
H
H
H
H C
H
H
C H
H
H
O
H
C
C
C
H
H
H
H
H C H
H CH
H
C
N
C
H
H
H
C
H
O
H
C
C
H
H
C H
H
H
H
C
O
H
C
C
H
H
H
Acetone
e) Provide a brief rationale for why the reaction of acetone with LDA is favorable.
The negative charge is delocalized by resonance in the product enolate, but not in
the LDA. The most important resonance form of the enolate has the negative
charge on oxygen which is more electronegative than nitrogen. Both of these effects
result in the negative charge being more stable in the product enolate than in the
starting LDA, thus making the reaction favorable.
Name:
5
f) Draw an accurate 3D structure of the enolate of acetone and, using dashed lines to
represent partial bonds and deltas to represent partial charges, depict the structure of the
"resonance hybrid".
H
!"
C C
H
"
!
O
C
H
H
H
g) Is trichloroacetone more or less acidic than acetone? Briefly explain.
Cl
Cl
O
H
C
C
C
H
Cl
H
Trichloroacetone
Trichloroacetone is more acidic than acetone. The electronegative chlorine atoms
inductively withdraw electron density, thus stabilizing the adjacent negative charge
in the conjugate base. This makes deprotonation more favorable.
7. (12 pts) Draw complete Lewis structures for the molecules indicated below. Show all
atoms, bonds, lone pair electrons, and non-zero formal charges.
a) Nitric acid (HNO3). (Hint: There will be formal charges.)
O
O
N
H
O
b) Draw 3D-accurate Lewis structures for the three stable isomers with the formula
C2H2Cl2. Show all lone pairs. Circle the isomer that has no dipole moment. For the
other two, show the direction of the net dipole moment.
Cl
Cl
C
H
Cl
C
H
C
H
Cl
Cl
C
H
C
H
H
C
Cl