|Chapter 11 - Organic Chemistry
The element carbon is the world champion when it comes to
forming compounds. Carbon is unique because its atomic
structure allows it to form many different molecules. A carbon
atom has four valence electrons. It can form four single covalent
bonds with other atoms to make a full, stable outer shell. It can
also form double and triple bonds. In double bonds, two pairs of
electrons are shared. Similarly, in triple bonds, three pairs of
electrons are shared. A carbon atom can form single, double, and
triple bonds with other carbon atoms as well as with other kinds of
atoms. As a result, there are over five million different molecules that
can be formed with carbon atoms. You might think that other
members of the carbon family would also form millions of
different compounds. These other elements also have four valence
electrons. However, atoms of other elements in the carbon family all
have more shells of electrons than carbon atoms. These extra shells
make it more difficult for the atoms to form bonds. Carbon,
therefore, is truly unique.
Compounds that contain carbon are called
organic compounds. The study of organic
compounds is called organic chemistry. The
word organic was first used many years ago.
At that time it was believed that all matter
could be divided into three categories;
animal, vegetable, and mineral. The animal
and vegetable, or living part of the world,
was called organic. Because organic material
came from living things, it was thought to be
made of substances different from those in
nonliving, mineral material. The nonliving
material was called inorganic.
Scientists no longer use the word organic to
describe only living matter. It is now known
that living things are made of the same
substances as nonliving things
Both living and nonliving things are made of elements. Today, the word
organic is used to describe any carbon compound whether or not it is
found in living matter. Therefore, things such as toothbrushes, which
contain carbon, are said to be made of organic compounds, even
though toothbrushes are not alive, never have been alive, and never will
Hydrocarbons are compounds made of only carbon and hydrogen.
Hydrocarbons are the basic parts of all organic compounds. The
simplest hydrocarbon is methane. Methane, CH4, is made of one
carbon atom and four hydrogen atoms. The hydrogen atoms are joined
to the carbon atom by single covalent bonds.
Covalent molecules can be illustrated by using electron dot diagrams.
Two dots represent a single covalent bond. When dashes are used in
place of the pairs of dots, the result is called a structural formula. The
image above shows the electron dot diagram and the structural formula
for a methane molecule. Structural formulas are a little neater way to
represent molecules with many atoms. For this reason they are used to
Structural formulas give an idea of the structure of the molecule. However, you should always
remember that they do not give the exact structure. They are drawn on a flat sheet of paper,
and the molecules are three dimensional. For example, the actual shape of a methane molecule
is a pyramid with four faces. The carbon atom is in the center, and the hydrogen atoms are on
the points of the pyramid.
Other hydrocarbon molecules similar to methane are made by linking together one or more
carbon atoms in a chain. For example, in the ethane molecule, C2H6, two carbon atoms are
bonded together and surrounded by hydrogen atoms. In propane, C3H8, three carbon atoms
are bonded together. Each time another carbon atom is added to the chain, a new molecule is
formed. These compounds are called straight chain hydrocarbons. The carbon atoms are
bonded together in a single line.
Sometimes the hydrogen and carbon atoms do not form single chains.
Instead they form branched or looped chains. Molecules formed with
branched or looped chains have properties different from straight chain
hydrocarbons. Two hydrocarbon molecules may have the same numbers of
carbon and hydrogen atoms bonded together in different ways. That is, the
molecular formulas are the same, but the structural formulas are different. For
example, butane, C41 H10, is a straight chain hydrocarbon. Isobutane,
C4H10 is a hydrocarbon with a branched chain. Butane and isobutane have
the same molecular formulas but different structural formulas and different
Two compounds with the same molecular formulas but different structural
formulas are called isomers. Sometimes the carbon atoms of a hydrocarbon
are joined by double and triple bonds. Hydrocarbon compounds having
double or triple bonds are grouped together. One of the more familiar
members of this group is acetylene, C2H2. The two carbon atoms in the
acetylene molecule are joined by a triple bond. The triple bond causes
acetylene to burn with a very hot flame. For this reason, acetylene is used in
Hydrocarbon chains of different lengths have different properties and uses. In general, the shorter the chain is, the
lighter the molecule is, and the lower the boiling point is. Because short chain hydrocarbons evaporate and burn
easily, they are used as fuels. Cooking gas and gasoline are short chain hydrocarbons. The heavier long chain
hydrocarbons are used as lubricating oils, as waxes, and as asphalt. They do not evaporate easily and are liquids or
solids at room temperature.
Because they have different boiling points, hydrocarbons in crude oil from wells can be separated in a process called
distillation. In distillation a liquid is changed to the gas state, and then the gas is collected and cooled, returning it to
the liquid state. You start with a liquid and end up with a liquid. How does this process separate the hydrocarbons in
crude oil? The different hydrocarbons boil and change to a gas at different temperatures. As the temperature rises,
each hydrocarbon evaporates to the gas state and separates from the liquid crude oil. The hydrocarbon gas is then
cooled and returned to the liquid state. After the hydrocarbons are separated, they are put through other chemical
processes. Many useful products such as gasoline, kerosene, diesel fuel, asphalt, lubricating oil, wax, and petroleum
jelly are produced in this way. Because the distillation of crude oil separates the oil into many parts, or fractions, it
is often called fractional distillation.
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