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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
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
• 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
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)
- 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