1 Chapter 11: Organic Compounds: Alkanes. 2 ORGANIC COMPOUNDS: In 1828, Friedrich Wöhler first...

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Chapter 11:Organic Compounds: Alkanes

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ORGANIC COMPOUNDS: • In 1828, Friedrich Wöhler first synthesized an organic

compound from an inorganic source.

• This discredited the “vital force” theory.

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• 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.

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• 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.

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BONDING CHARACTERISTICS• 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

normal alkane

C|

C — C — C|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 ALKANESThe 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.

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

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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.

|1CH3 —2CH —3CH —4CH —5CH2 —6CH2 —7CH3

| |CH3 CH3

CH–CH3

CH3

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4-isopropyl-2,3-dimethylheptane

Example:

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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

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CH3

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)