Spencer L. Seager
Michael R. Slabaugh
www.cengage.com/chemistry/seager
Chapter 11
Organic Compounds: Alkanes
Jennifer P. Harris
ORGANIC COMPOUNDS
• In 1828, Friedrich Wöhler first synthesized an organic
compound from an inorganic source.
• This discredited the “vital force” theory.
ORGANIC CHEMISTRY
• Organic chemistry is the
study of carbon containing
compounds. Other elements
often combined with nitrogen
include nitrogen, oxygen,
sulfur, phosphorus, and
hydrogen.
• Inorganic chemistry studies
the elements and everything
else.
• The principle components of
food, fuels, wood
construction, and clothing are
organic compounds.
ORGANIC CHEMISTRY (continued)
• An estimated 250,000 inorganic compounds
• More than 6 million organic compounds
• Thousands of new ones are synthesized or isolated each
year.
BONDING CHARACTERISTICS
• In carbon, the 2s and three 2p orbitals can mix to produce
four new sp3 hybrid orbitals.
BONDING CHARACTERISTICS (continued)
• 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.
BONDING CHARACTERISTICS (continued)
• 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.
BONDING CHARACTERISTICS (continued)
• Carbon can also bond to other carbon atoms.
• In principle, there is no limit to the number of carbon atoms
that can bond covalently.
• Organic molecules range from the simple molecules like CH4
to very complicated molecules containing over a million
carbon atoms.
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.
FUNCTIONAL GROUPS
• 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.
FUNCTIONAL GROUPS (continued)
FUNCTIONAL GROUPS (continued)
FUNCTIONAL GROUPS (continued)
FUNCTIONAL GROUPS (continued)
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
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, or
aromatics and contain double bonds, triple bonds, or ring
systems with alternating double bonds.
CLASSIFICATION OF HYDROCARBONS
(continued)
ALKANES
• 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.
ALKANES (continued)
• More complex alkanes can be straight chained (normal) or
branched. (Hydrogens are not shown)
C—C—C—C—C
normal alkane
C
|
C—C—C
|
C
branched alkane
CONFORMATIONS OF ALKANES
• The different arrangements of atoms in space achieved by
rotation about single bonds are called conformations.
Free rotation
around
C-C bond
CONFORMATIONS OF ALKANES (continued)
• Which of the following pairs represent structural isomers,
and which are simply the same compound?
• Which are normal alkanes and which are branched
alkanes?
CONFORMATIONS OF ALKANES (continued)
• The top two molecules are both normal alkanes containing
five carbon atoms. They are two conformations of the same
molecule.
• Hint: When trying to determine if a molecule is normal or
branched, look at the carbon bonding. A branched
molecule will have at least one carbon atom bonded to three
or more other carbon atoms. A normal molecule will not
have any carbon atoms bonded to more than two other
carbon atoms.
CONFORMATIONS OF ALKANES (continued)
• The bottom two molecules are both branched alkanes
containing five carbon atoms with four carbon atoms in the
longest chain and one CH3- group attached to the second
carbon atom in the longest chain. These are two
conformations of the same molecule.
• The top molecules and the bottom molecules are structural
isomers of each other. The formula for both is C5H12, but the
top molecules are normal, while the bottom molecules are
branched.
ALKYL GROUPS
• An alkyl group is a group differing by one hydrogen from an
alkane.
COMMON NONALKYL GROUPS
NAMING ALKANES
• The IUPAC method consists of:
NAMING ALKANES (continued)
• 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
NAMING ALKANES (continued)
• 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
THE SHAPE OF CYCLOALKANES
THE SHAPE OF CYCLOALKANES (continued)
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
ISOMERISM & 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.
• Rotation about C-C single
bonds occurs in
open-chain compounds
but not within rings.
ISOMERISM & CYCLOALKANES (continued)
• Geometric isomerism can result in two geometric isomers
of 1,2-dimethylcyclopentane.
• Cis-substituents on the same side.
• Trans-substituents on the opposite side.
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
PHYSICAL PROPERTIES OF ALKANES
(continued)
• A homologous series is a group of compounds with the
same functional class that differ by a –CH2– group.
PHYSICAL PROPERTIES OF ALKANES
(continued)
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
ALKANE REACTIONS (continued)
• Complete Combustion (in the presence of adequate oxygen)
CH4 + 2O2 → CO2 + 2H2O + 212.8 kcal/mol
• Incomplete Combustion (not enough oxygen available)
2CH4 + 3O2 → 2CO + 4H2O
CH4 + O2 → C + 2H2O