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 cycloin 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
H
C2H6
H
H
C
C
H
H
H
H
H
C
H
C2H4
+ H2
C
H
H
H H
H
+H H
..
..
..
..
..
..
H
. .
C C
.. ..
..
H
.. H
..
H C C H
.. ..
H H
H
..
C C
SATURATED
UNSATURATED H
HYDROCARBON
HYDROCARBON
H
H
H
C
C
H
H
..
H
H
H
H
C
H
+H H
+ H2
C
H
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"
ETHENE
H
2 carbon atoms
a double bond
C
H
C
H
H
C2H4
Alkenes are hydrocarbons that
contain at least one double bond
PROPENE
3 carbon atoms
H
H
a double bond
C
H
C
C
H
H
C3H6
H
But… How do you tell these two
apart?
1-butene
2-butene
• SO…
1)the “butene” tells you how many C we have
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
C
C
H
H
H
H
C
C
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
H
H
C
C
H
+
H
C2H4
H2
H
H
H
C
C
H
H
C2H6
H
Addition reactions with other
compounds
• Works through the same mechanism as
addition of H2
ex. ethene and chlorine gas --> chloroethane
H
Cl
H
C
C
H
Cl
H
C
C
H
+
H
C2H4
Cl2
H
C2H5Cl2
H
3. Substitution Reactions
• Involve alkanes: react chemically by
replacing hydrogen atoms with other atoms,
require energy
UV light
• Ex. CH4 (g) + Cl2 (g)  CH3Cl (g) + HCl (g)
4. Polymerization Reactions
• joining of identical units to form a large
organic molecule:
• Check out:
http://www.tvo.org/iqm/plastic/hydrocarbons.
html