Chapter 11:
Organic Compounds: Alkanes
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ORGANIC COMPOUNDS:
• In 1828, Friedrich Wöhler first synthesized an organic
compound from an inorganic source.
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• Organic chemistry is the study of carbon containing
compounds except elemental carbon (diamond, graphite,
coal), CO2, CO, and carbonates (CO32- group).
• The main “ingredients” of of food, fuels, wood construction,
and clothing are all organic compounds.
I need a new coat to keep
me warm and dry in Seattle
& I wonder what atoms and
molecules these are made
of?
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BONDING: for sp3 hybridized carbon
• In carbon, the 2s and three 2p orbitals can mix to produce
four new sp3 hybrid orbitals.
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• An sp3 orbital has a two-lobed shape, similar to the shape
of a p orbital but with different-sized lobes.
• Each carbon-hydrogen bond in methane arises from an
overlap of a C (sp3) and an H (1s) orbital.
• The sharing of two electrons in this overlap region creates
a sigma (σ) bond.
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• The four hybrid sp3 orbitals 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.
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• Functional Groups: Unique reactive combination of atoms
that differentiate organic compounds into classes.
• Examples:
• Except for alkanes, each functional group contains a
multiple bond or at least one oxygen or nitrogen atom.
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REPRESENTING ORGANIC COMPOUNDS
• Expanded structural formulas show all atoms with bonds.
• Condensed structural formulas list all the atoms in order
implying 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.
• Unsaturated hydrocarbons are called alkenes, alkynes,
and aromatics and contain double bonds, triple bonds, or
ring systems with alternating double bonds.
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• Alkanes can be represented by the general formula
CnH2n+2, where the n is the number of carbon atoms in the
molecule.
• The simplest alkane is methane (CH4), which is the
primary compound in natural gas.
• Ethane (C2H6) is a minor component of natural gas.
• Propane (C3H8) is used as a fuel for heating homes and
cooking.
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• More complex alkanes can be straight chained (normal) or
branched.
C
|
C—C—C—C—C
C—C—C
|
normal alkane
C
branched alkane
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CONFORMATIONS OF ALKANES
• There is free rotation around C-C bonds.
• The different arrangements of atoms in space achieved by
rotation about single bonds are called conformations.
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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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NAMING ALKANES, CONT.
Step 1: Identify and name the longest carbon chain. This
gives the root and ending. (The ending –ane signifies the
alkane family.)
Step 2: Number the longest carbon chain to give the
lowest number to any carbon to which a group is attached.
5 CH
CH3
|
|
Example:
CH2 — CH2 — CH — CH3 (pentane)
3
4
3
2
1
Step 3: Locate and name the attached alkyl groups.
Step 4: Combine the longest chain and the branches into
the name.
2-methylpentane
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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-, tetra-, penta-, etc. and separate the position
numbers by commas.
• List multiple branches alphabetically. Ignore the di-, tri-,
sec-, and t- prefixes.
Example:
1CH —2CH
3
—3CH —4CH —5CH2 —6CH2 —7CH3
|
|
|
CH3 CH3
CH–CH3
|
CH3
4-isopropyl-2,3-dimethylheptane
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NAMING CYCLOALKANES
• 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.
Example:
CH2CH3
1
CH3
3
2
1-ethyl-3-methylcyclopentane
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THE SHAPE OF CYCLOALKANES
• Stereoisomers are compounds with the same structural
formula but different spatial arrangements of atoms.
• Geometric isomers are molecules with restricted rotation
around C-C bonds that differ in the three-dimensional
arrangements of their atoms in space and not in the order
of linkage of atoms.
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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 1,2-dimethylcyclopentane.
• Cis-substituents on the same side.
• Trans-substituents on the opposite side.
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PHYSICAL PROPERTIES OF ALKANES
•
•
•
•
•
Non-polar molecules with weak intermolecular forces
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 as fuels.
Methane – natural gas
Propane – used in gas grills
Butane – lighters
Gasoline – a mixture of hydrocarbons
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Complete Combustion (in the presence of adequate oxygen)
Incomplete Combustion (not enough oxygen available)
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