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Organic Compounds:

Alkanes 1828: Friedrich Wohler first synthesized an

organic compound from an inorganic source

Seager & Slabaugh, 2004

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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.


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Bonding Characteristics

In carbon, the 2s and three 2p orbitals can mix to produce

four new sp3hybrid orbitals

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The four hybrid sp3orbitals 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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Representing Organic Compounds

Expanded structural formulas show all atoms with bonds.

Condensed structural formulas list all the atoms in orderimplying 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.


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The simplest alkane is methane (CH4)the primary

compound in natural gas.

Ethane (C2H6)a minor component of natural gas.

Propane (C3H8)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.

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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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Root NameMeth-eth-

prop-but-pent-hex-hept-oct-

non-dec-

# of Carbons12

345678

910

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Naming Alkanes, cont.

The endingane signifies the alkane family.

Step 1:Identify and name the longest carbon chain. Thisgives the root and ending.

Step 2:Number the longest carbon chain to give the

lowest number to any carbon to which a group is attached.

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Step 3: Locate and name the attached alkyl group.

Example:

CH35 CH3

CH2CH2CH CH3 (pentane)

| |

4 3 2 1

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Step 4: Combine the longest chain and the branches into

the name.

Example:

CH35 CH3

CH2CH2CH CH3| |

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-,

List multiple branches alphabetically

the di-, tri-

, dont

count.

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|

1CH3 2CH 3CH 4CH 5CH2

6CH2 7CH3

| |

CH3 CH CH

CH3

CH3

|

4-isopropyl-2, 3-dimethylheptane

Example:

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Naming Cycloaklanes

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.

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Example:CH2CH3

1

23

CH3

1-ethyl-3-methylcyclopentane

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The Shape of Cycloalkanes

Stereoisomerscompounds with the same structural

formula but different spatial arrangements of atoms.

Geometric isomersa type of stereoisomerism that gives

cis- and trans- orientations.

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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 1,3-diemthylcyclopentane.

Cis-substituents on the same side.

Trans-substituents on the opposite side.


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Physical Properties of Alkanes

Non-polar moleculescannot hydrogen bond.

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 this wayas fuels.

Methanenatural gasPropaneused in gas grills

Butanelighters

Gasolinea mixture of hydrocarbons

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Complete Combustion (in the presence of adequate Oxygen)

Incomplete Combustion (where there is not enough Oxygen

available)


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