Chapter 11Organic Compounds: Alkanes

Spencer L. SeagerMichael 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 (CO3

2- 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:

CH35 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: