Chemistry 2050 Introduction to Organic Chemistry
Fall Semester 2005, Dr. Rainer Glaser
Examination #1
“Bonding, Alkanes, Alcohols & Alkyl Halides”
Wednesday, 09/14/05, 11–11:50 am.
Name:
Answer Key
Question 1. Structural Formula of Organic Molecules
20
Question 2. Atomic Structure, Lewis Structures & Bonding
20
Question 3. Isomers and Combustion
20
Question 4. Conformation and Stereoisomers
20
Question 5. Alcohols and Alkylhalides
20
Total
—1—
100
Question 1. Structural Formula of Organic Molecules. (20 points)
(a) Draw a complete structural formula of
pentane. (4 points)
H
(b) Draw a condensed structural formula
of pentane. (1 points)
H
H
H
H
H
C
C
C
C
C
H
H
H
H
H
H
H3 C−CH 2 −CH 2 −CH 2 −CH 3
(c) Draw a simplified structural formula
(aka bond-line formula) of pentane. (1
points)
(d) Provide the complete structural formula and the name of one structural isomer of pentane. (6 p.)
Structure:
Name:
H
H
H
H
C
H
H
H
C
C
C
C
H
H
H
H
2-methylbutane
H
(e) Size and shape of acyclic alkanes. (8 p.)
Typical length of a C−C single bond: _154_ pm
Approximate angle ∠(C−C−C)
in acyclic alkanes: > 109.5°
Typical length of a C−H bond: __105_ pm
Approximate angle ∠(H−C−C)
in acyclic alkanes: > 109.5°
Hybridization of C in acyclic alkanes: sp3
Tetrahedral angle: 109.5°
—2—
Question 2. Atomic Structure, Lewis Structures & Bonding. (20 points)
(a) A neutral oxygen atom contains __8_ protons, __8_ neutrons, _2_ core electrons, and __6_ valence
electrons. In the left box, complete the electron configuration of a neutral oxygen atom by provision of
the exponents (e.g. the number of electrons in that AO). On the right, draw all valence electrons as
“arrows”. (8 points)
1s2 2s2 2px2 2py1 2pz1
↑↓ ↑↓ ↑ ↑
(b) Draw the complete Lewis structures (all atoms, all bonds, all lone pairs, draw all pairs as dashes, use
dots only for unpaired electrons, indicate formal charges if any). (6 points)
Carbon dioxide, CO2
O
C
Nitrous Acid, HONO
H
O
N
O
O
-
(c) Draw the three important resonance forms of nitrate anion, NO3 . Draw “resonance arrows” between
the structures. Draw complete Lewis structures. (6 points)
O
O
O
N
N
O
O
O
N
O
—3—
O
O
Question 3. Isomers and Combustion. (20 points)
(a) Give the molecular formula and the name for
the following compound. (4 points)
C10H22
2,2-dimethyl-4-ethyl-hexane
(b) Rewrite the condensed structural formula as a
bond-line drawing and give its name. (4 points)
(H3C)3CCH2CH2CH3
2,2-dimethyl-pentane
(c) Circle the isomers. (4 points)
butane ⋅ cyclobutane ⋅ isobutane ⋅ 2-methylbutane
(CH3 )3 CCH2 CH2 CH3 ⋅ (CH3 )3 CCH2 CH(CH3 )3 ⋅ H3 C(CH2 )5 CH3 ⋅ H3 CCH2 CH=CHCH2 CH2 CH3
(d) Provide the reaction formula for the complete combustion of octane (e.g. what happens in your
car’s engine). (3 points)
H3 C(CH2 )6 CH3 + _12.5_ O 2 → _8_ CO2
+ _9_ H2 O
(e) Provide the reaction formula for the incomplete combustion of octane. Note that 10 O2 are
available. [Hint: One more product is formed.] (5 points)
5 fewer O-atoms, 4 CO are formed instead of 5 CO2
H3 C(CH2 )6 CH3 + 10 O2 → 5 CO + 3 CO2 + 9 H 2 O
—4—
Question 4. Conformation and Stereoisomers. (20 points)
(a) Draw the perspective drawing of staggered
ethane. (4 points)
H
H
H
(b) Draw the Newman projection of the trans
conformation of butane. (4 points)
H
H
H
CH3
H
H
H
H
CH3
(c) Draw the two chair conformations of ethylcyclohexane in which the substituent is either axial or
equatorial. Circle the structure that is preferred. (6 points)
Axial ethyl.
Equatorial ethyl.
H
C2H5
H
C2H5
(d) Draw the best conformation of cis-1-methyl4-isobutylcyclohexane. (6 points)
H
C(CH3)3
H
CH3
—5—
Question 5. Alcohols and Alkyl Halides. (20 points)
(a) Nomenclature. Draw the complete structure
of 2-chlorobutane; draw every atom, every bond,
every lone pair. (3 points)
H
Give the IUPAC name of the structure shown. (3
points)
H
Cl
H
H
C
C
C
C
H
H
H
H
Cl
Cl
H
Name:
2,2-dichloropentane
(b) When tert.-butanol, (CH3 )3 C−OH, is treated with HCl, the first step of the reaction consists in the
protonation. Draw the product of this protonation. The next step is the elimination of water and the
formation of the tert.-butyl cation. Draw this cation and indicate the hybridization of the central Catom. For both structures, show all atoms, show all lone pairs, show formal charges. (6 points)
Protonated tert.-butanol
Tert.-butyl cation
H3C
H
C
CH3
O
H3C
H3C
C
H3C
H
CH3
sp2
(c) Complete the structure of water by adding lone pairs and indicating partial charges on all the atoms.
Then draw an arrow that shows the direction of the dipole moment of water (from plus to minus). On
the right, again complete the structure of the water molecule shown and then draw a second water that is
hydrogen-bonded to water molecule drawn via the hydrogen atom on your right. Try to get the relative
distances as right as possible. (8 points)
Water
Water Dimer
H
δ
δ
H
O
H
H
O
H
O
δ
—6—
H