Chapter 11

Organic Compounds: Alkanes

Spencer L. Seager

Michael R. Slabaugh

www.cengage.com/chemistry/seager

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 except elemental

carbon (diamond, graphite,

coal), CO2, CO, carbonates

(CO32- group) and cyanides

(CN- group).

• 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 have been

identified, but more than 6 million organic compounds are

known, and 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.

C — C — C — C — C

normal alkane

C

|

C — C — C

|

C

branched alkane

ALKANES (continued)

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.

CONFORMATIONS OF ALKANES (continued)

• This figure shows perspective models and carbon skeletons

of two conformations of n-butane.

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.

ALKYL GROUPS

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.

• Step 3: Locate and name the attached alkyl groups.

• Step 4: Combine the longest chain and the branches into the name.

Example:

CH3 5 CH3

CH2 — CH2 — CH — CH3 (pentane)

| |

4 3 2 1

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.

|

1CH3 —2CH —3CH —4CH —5CH2 —

6CH2 —7CH3

| | CH3

CH3 CH–CH3

CH3

|

4-isopropyl-2,3-dimethylheptane

Example:

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

2 3

CH3

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)

• Incomplete Combustion (not enough oxygen available)

CH4 + 2O2 → CO2 + 2H2O + 212.8 kcal/mol

2CH4 + 3O2 → 2CO + 4H2O

CH4 + O2 → C + 2H2O