12/9/2009 1
Chem 105 Wed 12-9-09
Today: Chapt 10 Carbon Compounds (cont’d)
1) Final Exam Information
2) Optical Isomers – mirror image molecules
3) Alkenes (CnH2n)
4) Saturated and unsaturated fats
5) Aromatic hydrocarbons
6) Petroleum chemistry
7) IAS questionnaire
12/9/2009 2
Final Exam Information
1. Friday, Dec. 18, 1-3 pm (this room)
2. Covers Chap 1-10 Kotz, lecture and lab
3. American Chemical Society “General Chemistry
First Semester”
4. 70 Multiple choice questions (a-d)
5. Extra sheet with several substitute questions
(ideal gas OUT, organic chemistry IN.)
6. Use the ACS book Preparing for Your ACS
Examination in General Chemistry
12/9/2009 3
Sections Not covered in lecture, OWL, Final Exam
10.5 Polymers
NO molecular (I.U.P.A.C) names of:
Alcohols, aldehydes, ketones, carboxylic acids, or
fatty acids
Do need to name, recognize the functional groups
and draw formulas for specific examples of
alkane, alkene, alkyne, alcohol, ether, aldehyde,
ketone, carboxylic acid, carboxylic amide.
Alkanes and Alkenes, YES
12/9/2009 4
Optical Isomers
Molecules containing a tetrahedral atom with four different atoms
or groups can exist in “right-handed” and “left-handed” forms.
These have identical physical properties, except how they interact
with polarized light. One isomer twists polarized light beam
clockwise, the other isomer twists it counterclockwise.
Hence, they are “optical isomers” . This type of molecule is “chiral”
(ky’ rəl).
C
X
Y
Z
W
CHClFH3CH3C
H
Cl
F
12/9/2009 6
Optical Isomers
“L-phenylalanine” (l) tastes bitter. “D-phenylalanine” (r) tastes sweet.
The “L” isomers of amino acids occur naturally in proteins. “D” isomers are rare.
H
NH2HO2C
H
H2NCO2H
12/9/2009 7
Alkenes contain C=C bond (CnH2n)
Some alkenes can exist in cis- or trans- geometric isomers
H3C
CH3H
H
H3CCH3
H H cis-
trans-
Same atoms or groups
– compare their
positions on C=C.
12/9/2009 8
Geometric isomers (cis and trans on C=C)
C=C bonds contain π-bond that depends on parallel orientation of the 2 p-
orbitals on bonded carbons. This prevents twisting around the bond. Therefore
C=C bonds containing different atoms can exist as cis- or trans- isomers
C l C l
HH
C l C l
H C H3
cis isomers
No
C l
C l
H
H
C l
C lH
C H3
trans isomers
12/9/2009 9
Naming alkenes
(1) Find longest chain containing BOTH CARBONS of C=C.
(2) Number the carbons starting at the end nearer C=C.
(3) Locate the C=C along the chain using # of FIRST C=.
cis-2-pentene
Location of C=C
Main chain 5 C’sMain chain
contains C=C
H3C CH3
H H
1
23
4 5
pentene
2-pentene
12/9/2009 11
Alkanes are referred to as “saturated hydrocarbons ”. They
have the maximum number of H’s for a given # of C’s.
Alkenes are “unsaturated” - they have fewer H’s.
Unsaturated fats contain C=C bonds along the fatty acid
chain. These are usually cis- because the C=C plays a
structural role in lipid bilayers.
12/9/2009 12
Saturated fatty
acyl group
glycerol
phosphate
“Lard” is derived from cell
membranes of animals. It is
rich in saturated fatty acid
components.
The straight chains pack
tightly into a lipid bilayer with
proper biological function at
37°C.
Section of
lipid
bilayer
12/9/2009 13
Plant and bacterial cell
membranes contain fatty acid
components rich in cis-double
bonds.
These chains are kinked by
the cis bond, resulting in
disordered lipid bilayer that is
more fluid. It remains flexible
at low temperatures, which
maintains the proper biological
function of the cell membrane.
C=C
12/9/2009 14
trans-double bonds in fat
are man-made.
They are created during
partial hydrogenation
reactions that add H2 to
C=C bonds using a metal
catalyst.
The trans-acid packs into
membranes in a manner similar
to saturated fats. This may be the
basis of the unhealthy effects of
fats containing lots of trans fatty
acids.
http://en.wikipedia.org/wiki/Trans_fat
12/9/2009 15
Alkynes contain a C≡C triple bond.
HH
Each sp-hybridized carbon contains
two unhybridized p-orbitals
C C HH
The four p-orbitals combine
to form two π-bonds.
12/9/2009 16
C CCH3CH2 CH2
CH2 CH
CH3
CH3
1 2 3 4
5
6 7
7-methyl-3-octyne
8
H-C≡C-H common name is “acetylene”
(“ethyne” is the systematic name, but is not used.)
Uses: acetylene is welding gas
larger alkynes used as synthesis intermediates because
the C≡C bond can be modified in many ways.
Some natural molecules such as dynemicin anti-cancer
drug contain C≡C bonds.
12/9/2009 18
Aromatic hydrocarbons contain one or more benzene rings.
Benzene itself is C6H6. It is a resonance hybrid of two
equivalent forms. This leads to an unusually stable molecule.
This is called an “aromatic ring” which refers to the special
stability of 6 π-electrons in a ring of sp2-atoms.
C
CC
C
CC
H
H
H
H
H
HC
CC
C
CC
H
H
H
H
H
H
Sometimes written this way to
emphasize bonding symmetry:or
12/9/2009 19
H
H H
H
HH
Enhanced stability of the aromatic ring is due to formation of a
low-energy π-molecular orbital extending around the ring.
12/9/2009 20
Toluene (methylbenzene) is 28 kcal/mol more
stable than the next-most stable C7H8 isomer
-1610
-1600
-1590
-1580
-1570
-1560
-1550
-1540
-1530
Bin
din
g e
nerg
y (
kcal/
mo
l)*
* calculated with PM3 semi-empirical method
CH3
CH2
CH3 H2C
H3C CH2
CH2
There are >100 isomers of C7H8!
Toluene
Not
aromatic
Aromatic
12/9/2009 21
H
H
H
H
H
CH3
H
H
H
H
CH3
CH3
H
H
H
CH3
H
CH3
H
HH
H
CH3
CH3
Toluene
(methylbenzene)
ortho-xylenemeta-xylene
para-xylene
12/9/2009 29
300-600 oC
"catalysts"
Refinery chemistry:
Reforming gives lots more gasoline
Crude oil Lots of gasoline
12/9/2009 30(from North Pole refinery)
The benzene content of
Alaska’s distilled-only gasoline
is much greater than regular
gas manufactured in Lower 48
refineries.
Courtesy of R. Stolzberg