Chem 106
Tues 1-25-2011
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Systematic naming of alkanes
Alkenes
Alkynes
Aromatic compounds (benzene etc)
Hydrocarbon physical properties
Petroleum chemistry
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pencil : paper :: paint brush :
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Hydrocarbons
Alkanes – all sp3 carbons
CnH2n+2
Cycloalkanes – all sp3 carbons in a ring
CnH2n (one ring)
Alkenes – one (or more) C=C bond (sp2 C)
CnH2n (one C=C)
Alkynes – one (or more) C≡C bond (sp C)
CnH2n-2 (one C≡C)
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Alkane names are based on length of longest chain
Need to know C1 – C10 names.
not obvious –
need to learn
if you don’t
know them
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Systematic names of complex alkanes (“I.U.P.A.C.”)
Substituents
Parent chain
M
“location-numbersubstituentsparent”
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Substituents are sometimes called “radicals”.
They are molecules but with a H atom removed to create an attachment point.
methane
ethane
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methyl substituent (CH3)
ethyl substituent (CH3CH2)
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Then substituents can be attached somewhere along a longer chain.
2-methyloctane
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Alkane systematic names
CH3
(1) Find longest chain in molecule
(2) Number the chain starting at end
closest to any substituents.
(3) Identify the number (di, tri, tetra etc)
and type (methyl, ethyl, etc) of
substituents.
H3C
1
H
H
C
C
C
C
CH3
H
H
H
2
3
H
4
5
CH3
6
2,2-dimethylhexane
all sp3
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flip
CH3
H
6 5 4
2
1
3
H3C C C C C CH3
H
H
H
H
H
CH3
This is also 2,2-dimethylhexane. It has the same
name, therefore it’s the same molecule
This is an
isomer: It has a
different NAME
and structure.
H3C
CH3
H
H
H
C
C
C
C
CH3 H
H
H
CH3
2,3-dimethylhexane
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Structure  name
Name  structure
3,3,4,4,5-pentamethyldecane
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Naming an alkane: What is the parent chain of the
following compound?
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5
e
no
na
n
an
e
oc
t
he
pt
an
e
0
ne
hexane
heptane
octane
nonane
he
xa
1.
2.
3.
4.
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What substituent is not present in the systematic name
of the following compound?
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m
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py
l
et
hy
l
et
hy
l
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pr
o
1. methyl
2. ethyl
3. propyl
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Show 2,2,3-trimethylpentane in ACD ChemSketch and HyperChem.
1/25/2011
Free download at www.acdlabs.com
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Cycloalkalkanes have formula CnH2n (one ring)
H
cyclopropane
H
CH2
H2C
CH2
H
H
H
H
cyclohexane
cycloheptane
Largest known cycloalkane:
Cyclodotetracontane (C42H84)
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Alkenes contain C=C bond (CnH2n)
CH3
H
H 3C
H
H
H
H 3C
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CH3
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Naming alkenes
(1) Find longest chain containing BOTH CARBONS of C=C.
(2) Number the carbons starting at the end nearest C=C.
(3) Locate the C=C along the chain using # of FIRST C=.
Location of C=C
H
H3C
1
2
H
3
2-pentene
4
CH3
5
Main chain 5 C’s
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Main chain
contains C=C
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Some alkenes exist in cis- or trans- geometric isomers
CH3
H
H3C
H
H
H
H3C
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trans-
Same atoms or groups – compare
their positions on C=C.
trans-2-pentene
cisCH3
cis-2-pentene
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No
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
H
Cl
H
Cl
H
CH
Cl
Cl
cis isomers
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H
Cl
Cl
H
H
Cl
Cl
CH3
trans isomers
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trans- 4,5- diethyl- 4-nonene
7
6
4
5
H3C
2
9
8
CH3
CH3
3
1 CH3
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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. As found in nature, these are usually cis- because the
C=C plays a structural role in lipid bilayers.
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polar
Saturated fatty acid
“Lard” is derived from cell
membranes of animals. It is rich
in saturated fatty acid
components.
The straight chains pack tightly
into the cell’s membrane
bilayer.
Crosssection
through
cell
membrane
Has proper structure and
function at 37°C.
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Plant cell membranes contain
unsaturated fatty acids 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 functions
of the cell membrane.
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trans-double bonds in fats
are man-made.
They are created during
partial hydrogenation
reactions that adds 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
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Alkynes contain a C≡C triple bond.
Each sp-hybridized carbon contains
two unhybridized p-orbitals
H
H
The four p-orbitals combine
to form two π-bonds.
H
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C C
H
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H-C≡C-H common name is “acetylene”
(“ethyne” is the systematic name, but is not used.)
8
6
1
2
CH3CH2
7
CH3
CH2 CH
3
4
C
C CH2
CH3
5
7-methyl-3-octyne
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 an anticancer drug contain C≡C bonds.
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Alkynes are used in organic synthesis reactions to
create specific organic molecules. “2-hexyne” cannot
be obtained from crude oil or other natural sources.
H3C
C
C
H
+ Base
H3C
C
C
+ HBase
acetylide ion
Cl
H3C
C
C
CH2CH2CH3
H3C
C
C
CH2CH2CH3
+ Cl
2-hexyne
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Synthesis products are larger than “feedstock”
hydrocarbons, they have specific structures,
and they often have useful chemical or
biological properties.
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Aromatic hydrocarbons
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Aromatic hydrocarbons contain one or more benzene rings.
H
H
H
H
C
C
C
C
C
C
H
H
H
H
H
C
C
H
C
C
C
C
H
H
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.
Sometimes written this way to
emphasize bonding symmetry:
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or
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Enhanced stability of the aromatic ring is due to formation of a
low-energy π-molecular orbital extending around the ring.
H
H
H
H
H
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H
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CH3
CH3
CH3
H
H
H
H
H
H
H
H
Toluene
ortho-xylene
(methylbenzene)
H
H
H
H
H
H
H
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CH3
CH3
H
CH3
CH3
H
meta-xylene
para-xylene
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Toluene (methylbenzene) is 28 kcal/mol more
stable than the next-most stable C7H8 isomer
H3C
-1530
Binding energy (kcal/mol)*
CH3
CH2
H2C
-1540
-1550
CH2
Not
aromatic
CH2
-1560
-1570
-1580
-1590
-1600
Aromatic
CH3
-1610
Toluene
* calculated with PM3 semi-empirical method
There are >100 isomers of C7H8!
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Hydrocarbon properties
Boiling points vs molar mass
GC separation
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High molecular weight hydrocarbons - strong induced dipoles (van
der waals forces) due to their large exposed surface area.
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decane
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Gas chromatogram of C1-C6 hydrocarbons
Courtesy of R. Stolzberg
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Courtesy of R. Stolzberg
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Courtesy of R. Stolzberg
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Distillation
Courtesy of R. Stolzberg
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Courtesy of R. Stolzberg
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Refinery chemistry:
Reforming gives lots more gasoline
300-600 oC
"catalysts"
Crude oil
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Lots of gasoline
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(from North Pole refinery)
Courtesy of R. Stolzberg41
Courtesy of R. Stolzberg
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