Carbon Chemistry
Classification of Hydrocarbons
and Organic Functional Groups
The Families of Hydrocarbons:
alkanes, alkenes, alkynes, and aromatics
Alkanes, revisited
• Alkanes are saturated hydrocarbons with linear, branched,
and even cyclic structures.
• Linear or branched molecules with the same molecular
formula are called “isomers” of each other, e.g.
CH3CH(CH3)CH3 is an isomer of CH3CH2CH2CH3.
• Names of alkanes are derived from the number of carbons in
the molecule (but naming is complicated with branching).
• Physical properties of linear alkanes vary systematically, e.g.
Boiling point increases steadily with increasing Mr (branching
tends to alter the trend, however).
• Chemically, alkanes are not very reactive, but what is
important is that all hydrocarbons can undergo combustion!
Alkenes
• Like alkanes, alkenes can be linear or branched.
• Alkenes contain a carbon-carbon double bond
and so have fewer hydrogens per molecule than
alkanes.
• The simplest alkene is called “ethene” and has
the structural formula, CH2CH2.
• The position of the double bond along the carbon
chain determines a different alkene. For example,
CH2CHCH2CH3 (but-1-ene) is a different alkene
from CH3CHCHCH3 (but-2-ene).
• Alkenes are much more chemically reactive than
alkanes due to the carbon-carbon double bond.
Examples of Alkenes
H
H
C
C
C
H
H
H
H
C
H
H
H
CH2CHCH2CH3 , but-1-ene
H
H
C
C
C
C
H
H
H
H
H
CH3CHCHCH3 , but-2-ene
Other Hydrocarbons**
• Hydrocarbons that contain a carbon-carbon
triple bond are called “alkynes.” An example is
ethyne (acetylene), a welding torch fuel.
• Hydrocarbons that form rings of alternating
double bonds between carbons are called
“aromatics.” An example is benzene, a powerful
solvent.
** We will not study these other families of hydrocarbons.
Concept of “Saturation”
• Alkanes are “saturated” with hydrogen; they
contain the maximum number of hydrogen
atoms per carbon atom possible.
• Alkenes, for example, are “unsaturated.”
They have fewer hydrogen atoms per carbon
atom than alkanes.
• Unsaturated compounds have at least one
double or triple bond between carbon atoms.
• E.g., “unsaturated fats” in foods contain one
or more C=C bonds in their carbon chain
Structure of a Natural Fat
Effect of Double Bonds on Fat Structure
Beef Fat
Flax Seed Oil
C=C bonds cause stiff kinks along the chains, weaken the
attraction between chains, and lower the melting point of
fats. At room temperature, saturated fats are solids, but
unsaturated fats are liquids.
Exercise 1
1. Name the molecule with structural formula, CH3CHCH2.
2. Draw the complete structural diagrams for all the unique
isomers of the alkene, C5H10. Use the molecular models
to help sort out the differences.
3. Complete this table: Fill in “liquid” or “solid”
Type of Fat
Saturated
Mono-unsaturated
Polyunsaturated
Room Temperature
In refrigerator
Organic Functional Groups
• “Functional groups” can introduce elements other
than carbon and hydrogen.
• We will study only one important functional group
which is formed by incorporating the element
oxygen in the molecule as –O-H (“hydroxyl group”).
• Functional groups are so-named because they
result in an organic compound with a whole new
function, i.e., they significantly alter the physical
and chemical properties of the parent hydrocarbon.
• Functional groups (along with isomers) are mainly
responsible for the extensive variety of known
natural and man-made molecules.
Exercise 2
1. Alkanes (linear or branched) have the general molecular
formula CnH2n+2. What is the general formula for alkenes?
2. Simple alcohols are like alkanes in which one of the –H
atoms is replaced by an –O-H “hydroxyl group”. Write the
molecular formula and draw the structural diagram of the
SIMPLEST possible alcohol. Alcohols are named like
alkanes, but have “ol” at the end instead of “ane.” Name
this simplest alcohol.
3. Draw a structural diagram for each of the possible linear
and branched isomers of the alcohols derived from the
alkane, butane (hint: an alcohol will have only one
hydroxyl group per molecule).
Alcohols
The functional group of alcohols
(called the “hydroxyl” group)
Example alcohol: ethanol
Molecular
formula
C2H6O
H3C OH
(see below *)
Structural
formula
CH3CH2OH
Structural
diagram
H
H
H
C
C
H
H
O
H
*Derived from the hydrocarbon, ethane. Compare the molecular formula,
structural formula, and structural diagram of ethane.
Comparing Ethanol with Ethane
Ethanol Ethane
Physical state at 20oC
Boiling Point
Color
Odor
Combustible (a fuel)
Energy of combustion
Solubility in water
Primary source
Biochemical
liquid
78oC
colorless
“alcohol”
yes
30 kJ/g
miscible
synthetic
yes
gas
-89oC
colorless
none
yes
48 kJ/g
insoluble
natural
no
Some Uses for Different
Types of Organic Compounds
• Alkanes – fuels, solvents, lubricants
• Alkenes – solvents, building blocks*
• Alcohols – fuels, solvents, beverages,
antiseptics, building blocks*
* for other organic molecules
More complex molecules
ibuprofen
(Tylenol)
glutamic acid
glucose (a sugar)
[O in red and H in white]
(a protein building block)