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Organic Compounds:
Alkanes 1828: Friedrich Wohler first synthesized an
organic compound from an inorganic source
Seager & Slabaugh, 2004
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Organic chemistry is the study of carbon containing
compounds except elemental carbon (diamond, graphite,
coal), CO2, CO, carbonates (CO32-group) and cyanides
(CN-group) Inorganic chemistry studies the elements and everything
else.
Seager & Slabaugh, 2004
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Bonding Characteristics
In carbon, the 2s and three 2p orbitals can mix to produce
four new sp3hybrid orbitals
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The four hybrid sp3orbitals allow carbon to form four
bonds. When carbon is joined to four substituents (i.e.
CH4), the resultant configuration is tetrahedral in shape.
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Isomerism
Isomers: Compounds that have identical molecular
formulas, but different arrangement of atoms.
Structural isomers: A type of isomerism in which the
atoms bond in different patterns.
Ball-and-stick models of the isomers of C2H6O. Ethyl
alcohol is a liquid at room temperature and completely
soluble in water, whereas dimethyl ether is a gas at room
temperature and only partially soluble in water.
Seager & Slabaugh, 2004
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Representing Organic Compounds
Expanded structural formulas show all atoms with bonds.
Condensed structural formulas list all the atoms in orderimplying how they are bound together:
CH3CH2CH2CH3 or CH3(CH2) 2CH3
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Classification of Hydrocarbons
Hydrocarbons contain only carbon and hydrogen. A
hydrocarbon that contains only single bonds is a saturated
hydrocarbon or alkane.
Seager & Slabaugh, 2004
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The simplest alkane is methane (CH4)the primary
compound in natural gas.
Ethane (C2H6)a minor component of natural gas.
Propane (C3H8)a fuel for heating homes and cooking.
Seager & Slabaugh, 2004
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More complex alkanes can be straight chained (normal) or
branched.
C C C C C
normal alkane
C
|
C C C
|C
branched alkane
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Conformations of Alkanes
There is free rotation around C-C bonds.
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Which of the following pairs represent structural isomers,
and which are simply the same compound?
Which are normal alkanes and which are branched
alkanes?
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Naming Alkanes
The IUPAC method consists of:
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Root NameMeth-eth-
prop-but-pent-hex-hept-oct-
non-dec-
# of Carbons12
345678
910
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Naming Alkanes, cont.
The endingane signifies the alkane family.
Step 1:Identify and name the longest carbon chain. Thisgives the root and ending.
Step 2:Number the longest carbon chain to give the
lowest number to any carbon to which a group is attached.
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Step 3: Locate and name the attached alkyl group.
Example:
CH35 CH3
CH2CH2CH CH3 (pentane)
| |
4 3 2 1
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Naming Alkanes, cont.
Step 4: Combine the longest chain and the branches into
the name.
Example:
CH35 CH3
CH2CH2CH CH3| |
4 3 2 1
2-methylpentane
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Naming Alkanes, cont.
Step 5:
For multiple branches, show the location of each branch
with numbers.
For multiple branches of the same type, modify the name
with di-, tri-,
List multiple branches alphabetically
the di-, tri-
, dont
count.
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|
1CH3 2CH 3CH 4CH 5CH2
6CH2 7CH3
| |
CH3 CH CH
CH3
CH3
|
4-isopropyl-2, 3-dimethylheptane
Example:
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Naming Cycloaklanes
Cycloalkanes are alkanes containing rings of carbon
atoms.
The prefix cyclo- is used before the alkane name.
When two or more substituents are attached to the
cycloalkanes, the ring numbering begins with the first
group alphabetically and proceeds to give lowest numbers
possible.
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Example:CH2CH3
1
23
CH3
1-ethyl-3-methylcyclopentane
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The Shape of Cycloalkanes
Stereoisomerscompounds with the same structural
formula but different spatial arrangements of atoms.
Geometric isomersa type of stereoisomerism that gives
cis- and trans- orientations.
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Rotation about C-C single bonds occurs in open-chain
compounds but not within rings.
Geometric isomerism can result in two geometric isomers
of 1,3-diemthylcyclopentane.
Cis-substituents on the same side.
Trans-substituents on the opposite side.
Seager & Slabaugh, 2004
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Physical Properties of Alkanes
Non-polar moleculescannot hydrogen bond.
Not soluble in water (hydrophobic).
Low density (less dense than water).
Melting points increase with molecular size.
Boiling points increase with molecular size.
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Alkane Reactions
Alkanes are the least reactive of all organic compounds.
The most significant reaction of alkanes is combustion
(rapid oxidation).
Many alkanes are used this wayas fuels.
Methanenatural gasPropaneused in gas grills
Butanelighters
Gasolinea mixture of hydrocarbons
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Complete Combustion (in the presence of adequate Oxygen)
Incomplete Combustion (where there is not enough Oxygen
available)
Seager & Slabaugh, 2004
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