UNIT 5

ORGANIC CHEMISTRY

What makes a compound organic?

• Organic compounds– Contain both carbon and hydrogen,

Ex. C6H12O6

• Inorganic compounds– Do not contain both carbon and hydrogen

Ex. H2O, CO2

Hydrocarbons

• Compounds that contain only hydrogen and carbon atoms

• Most come from living sources: fossil fuels (decomposition of once-living organisms)

Classification:

1. aliphatic: open-chain or cyclic

- includes alkanes, alkenes, alkynes and alicyclics (ring structures)

2. aromatic: cyclic hydrocarbons where some electrons are shared, ex. benzene

1. ALIPHATIC Classified by the kinds of carbon-carbon

bonds in their molecules

– Saturated: containing only single C-C bonds

– Unsaturated: containing at least one double or triple bond between carbon atoms

a. Alkanes

• Saturated Hydrocarbons– nonpolar– general formula CnH2n+2

www.rjclarkson.demon.co.uk/found/found6a.htm

                  

                                      

Cows frequently produce methane after consumption of grass...

Penguins usually don't...

Alkane nomenclature

• Methane : 1C– Product of anaerobic decomposition

– Formula: CH4

– Structural formula:

• Ethane: 2C– Formula: C2H6

– Condensed formula: CH3-CH3

– Structural formula:

• Propane: 3C– Formula: C3H8

– Condensed formula: CH3-CH2-CH3

– Structural formula:

A special case: cyclohexane

• cyclical alkanes are named by adding cyclo- in front of name,

• ex. cyclohexane

Properties of Alkanes:

1. boiling point: increases with the number of C atoms

- the longer the chain the greater the intermolecular forces

- branched-chain alkanes have a lower b.p. than straight-chain alkanes with the same number of C atoms

2. melting point:

- increases with the mass and chain length

3. density

- increases with chain length

4. solubility

- non-polar, therefore not water soluble, dissolve in other non-polar substances

b. Alkenes: Unsaturated Hydrocarbons

• contain one or more double bonds

• general formula CnH2n

                               

C

H

H

H

C

H

H

H

C

H

H2

H

+C

H

H

C2H6 C2H4

C

H

H

H

C

H

H

H

H ...... CH

H.. .. .. HC

H

H.. .. CH

H.. .. .. HC

H.. H H..+

. .

..CH

H.. .. H H..+..

..

CH

H..

C

H

H

+C

H

H

H2

UNSATURATED HYDROCARBON

SATURATED HYDROCARBON

Properties of Unsaturated Hydrocarbons

• nonpolar• more reactive than

alkanes• lower boiling point

than alkanes ex. ethane: -89oC,

ethene: -104oC,

library.thinkquest.org/C005377/content/images/ethene.gif

Alkene Nomenclature- "ane" is replaced by "ene"

2 carbon atoms

a double bond

ETHENE

C2H4

C C

H

HH

H

Alkenes are hydrocarbons that contain at least one double bond

3 carbon atoms

a double bond

PROPENE

C3H6

C C C

HH

HH

H

H

But… How do you tell these two apart?

1-butene

2-butene

• SO…1)the “butene” tells you how many C we have

C C C C

2) the “ene” tells you there is a double bond

3) the numerical prefix (ex. 2- ) tells you which carbon the double bond is on

4) Fill in the bonds and the H’s!

H

H

H

HH

H

H

H

What about funny-looking things like this…

-Find the longest C-chain:prop

-Find the ending:ene

-Locate the additional group, changing its ending to “yl”:

meth-yl

-Locate the double bond

-Put it all together:

methylpropene

• When two or more double bonds are present the ending –diene (or –triene etc.) is used and two numbers are required to specify the position of the double bonds

• Ex. 1,3 - butadiene

c. Alkynes: unsaturated hydrocarbons

• Contain one or more triple bonds• General formula CnH2n-2

Alkyne nomenclature

• "ane" is replaced by "yne"

Ex. ethyne

Ex. propyne

2. Aromatics:ex. benzene

• Aromatic cyclical alkene• electron distribution can vary

ISOMERS

- compounds that have the same molecular formula but a different molecular structure

a) Structural Isomers (“Positional Isomers”)Ex. 1-butene vs. 2-butene - differ in the position of the multiple bond

b) geometric isomers (stereoisomers or cis-trans isomers)

- have atoms that are connected in the same order but with different three-dimensional arrangements

ex. cis-2-butene (both methyl groups are on the same side of the double bond)

trans-2-butene (methyl groups on opposite sides)

REACTIONS OF HYDROCARBONS

- alkenes and alkynes are more chemically reactive than alkanes

-double and triple bonds are easily converted to single bonds

1. Combustion Reactions

• All hydrocarbons are combustible (they burn)

• Combustion can be – i. complete: excess oxygen is present

+ energy

ii. Incomplete: there is not enough oxygen present:

CH4 (g) + O2 (g) C (s) + 2 H2O(l)

2 CH4 (g) + 3 O2 (g) 2 CO (g) + 4 H2O(l)

• The combustion of hydrocarbons is always exothermic: it releases energy

• Law of conservation of energy:– Energy can be neither created nor destroyed

During a chemical reaction (ex. combustion) two things have to happen:

1. bonds have to be broken: requires energy

2. new bonds have to form: releases energy

Energy change: the difference between energy required and energy released

Exothermic reactions

- energy released by product formation is greater than energy needed to break bonds

Endothermic reactions

- energy released by product formation is lesser than energy required to break bonds

Thermochemical equation

- a balanced chemical equation in which the heat of reaction is included:

+ energy

2. Addition Reactions

- a reaction of an alkene or alkyne in which a molecule is added across a multiple bond

C

H

H

H

C

H

H

H

C2H6

C

H

H2

H

+C

H

H

C2H4

Addition reactions with other compounds

• Works through the same mechanism as addition of H2

ex. ethene and chlorine gas --> chloroethane

C HH

H

C

HCl

Cl

C2H5Cl2

C

H

Cl2

H

+C

H

H

C2H4

3. Substitution Reactions

• Involve alkanes: react chemically by replacing hydrogen atoms with other atoms, require energy

• Ex. CH4 (g) + Cl2 (g) CH3Cl (g) + HCl (g)

UV light

4. Polymerization Reactions

• joining of identical units to form a large organic molecule:

• Check out:

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