ORGANIC CHEMISTRY
Organic chemistry is the study of compounds that contain carbon. It is one of
the major branches of chemistry.
Organic chemistry was first defined as a branch of modern science in the early 1800's
by Jon Jacob Berzelius. He classified chemical compounds into two main groups:
organic if they originated in living or once-living matter and inorganic if they came
from "mineral" or non-living matter. Like most chemists of his era, Berzelius believed in
Vitalism - the idea that organic compounds could only originate from living organisms
through the action of some vital force.
It was a student of Berzelius' who made the discovery that would result in the
abandonment of Vitalism as a scientific theory. In 1828, Frederich Wöhler discovered
that urea - an organic compound - could be made by heating ammonium cyanate (an
inorganic compound).
Wöhler's discovery was a turning point in science history for two reasons. First, it
undermined the idea of Vitalism because an organic compound was produced from an
inorganic one. However, it also represented the discovery of isomerism - the
possibility of two or more different structures (ammonium cyanate crystals and urea
crystals) based on the same chemical formula (N2H4CO).
Chemists started searching for reasons to explain isomerism. That in turn led to
theories about the structure of chemical compounds. By the 1860's, chemists like
Kékulé were proposing theories on the relationship between a compound's chemical
formula and the physical distribution of its atoms. By the 1900's chemists were trying
to determine the nature of chemical bonding by developing models for electron
distribution. During all of this time the number of known organic compounds was
increasing rapidly year by year.
During the 20th century, organic chemistry branched into sub-disciplines such as
polymer chemistry, pharmacology, bioengineering, petro-chemistry, and numerous
others. During that century, millions of new substances were discovered or synthesized.
Today over 98% of all known compounds are organic.
There are three generally accepted sources of organic compounds:
carbonized organic matter – the fossil fuels of coal, oil, and natural gas
living organisms
invention/human ingenuity (plastics for example)
Why Carbon?
Stability of Carbon to Carbon Bonds
Carbon atoms form stable covalent bonds with other carbon atoms. Carbon to carbon
bonds are very strong - a lot of energy is required to break them compared to other
covalent bonds.
The tremendous diversity of organic compounds is due mainly to the ability of carbon
atoms to form stable chains, branched chains, rings, branched rings, multiple rings, and
multiple bonds (double and triple bonds). Add to this the ability to bond to many other
nonmetal atoms, and you can certainly see why organic compounds outnumber all
other classes of compounds combined by a huge margin!
Do you remember the days when you played with building blocks? It was common to
build one structure with the blocks, take it apart, and build a completely different
structure using the exact same blocks.
In chemistry, atoms are the building blocks. Because of carbon's unique bonding
properties, it is possible to make a number of different molecules using the same group
of atoms. Consider these two structures:
butane
methylpropane
Count the number of carbon and hydrogen atoms in each (remember an open bond
symbol is assumed to have a hydrogen atom at the open end). What do you notice? Do
they have the same molecular formula?
Now compare the configuration of carbon atoms in each molecule. How do the
structures differ?
Structures that have the same molecular formula but different structural formulas are
called structural isomers.
The possibility of more than one structure for a single molecular formula is called
isomerism. It is a key reason for the tremendous diversity of organic compounds. For
example, there are 75 possible isomers of C10H22; 366, 319 possible isomers of C20H42;
and 4, 111, 846, 763 possible isomers of C30H62. Note the use of the word "possible".
It is used because relatively few of the isomers for these molecular formulas have
actually be isolated.
Example 1
Using structural formulas, draw all the possible isomers of C5H12.
Answer
Begin by drawing the simplest isomer - a chain of five carbons.
Shorten the chain by one carbon to make a branched chain isomer.
Then shorten the chain to three carbons with two branches.
This example shows you a nice simple systematic way to construct models for the
isomers of a molecular formula. You will develop this skill as you work through future
lessons. The main point to note right now is that one chemical formula may have two or
more legitimate structural formulas, and this contributes to the diversity of organic
compounds.
Draw all the isomers of C6H14 (hint: there are five possible isomers)
CLASSIFYING ORGANIC COMPOUNDS
It is helpful to divide organic compounds into two major groups based on composition
only. The hydrocarbons are compounds that consist of carbon and hydrogen atoms
only (e.g. methane, CH4). The hydrocarbon derivatives are compounds in which one
or more hydrogen atoms is replaced by another nonmetallic atom (e.g. bromomethane,
CH3Br).
Hydrocarbons
There are two main classes of hydrocarbons: aliphatic and aromatic hydrocarbons.
Aliphatic hydrocarbons consist of carbon atoms bonded together in straight chains,
branched chains, rings, branched rings, multiple rings or branched multiple
rings. Aromatic hydrocarbons are distinguished by the presence of a special group of six
carbons known as the benzene ring.
Three classes of aliphatic hydrocarbons can be defined based upon carbon to carbon
bond type. They are the alkanes, alkenes and alkynes. Note that each term differs
from the other two by a single letter. The endings, -ane, -ene, and -yne, indicate the
presence of single, double and triple bonds respectively. Non-benzene rings of three or
more carbon atoms are known as alicyclic hydrocarbons and these are further
classified based on the types of bonding within the rings.
Each class of aliphatic hydrocarbons may be represented by a general formula showing
the ratio of carbon atoms to hydrogen atoms. Applying these general formulas will
assist you in the classification of organic compounds in this course.
Table 1: Some general formulas for common classes of aliphatic hydrocarbons.
General Formula
Class of Hydrocarbon
CnH2n+2
alkanes
CnH2n
alkenes (one double
bond)
cycloalkanes
CnH2n-2
alkynes (one triple
bond)
cycloalkenes (one
double bond)
A general formula can be used to determine the molecular formula of a compound.
They can also be used to classify compounds when you are given chemical formulas.
Write the molecular formula for a 10 carbon alkane.
Alkanes have the general formula CnH2n+2 ; therefore, substituting the number of
carbon atoms into the general formula, the molecular formula for the compound
becomes C10H22.
Classify C2H4.
C2H4 fits the general formula CnH2n. Since a minimum of three carbons are needed to
produce a ring structure, C2H4 may be classified as an alkene.
Most hydrocarbon derivatives are classified on the basis of a functional group - an
atom or group of atoms that give the compound its unique chemical and physical
properties.
Which substance is classified as a hydrocarbon derivative? NaCl, H2O, C3H8, or C2H5OH?
Sodium chloride and water are inorganic compounds because they lack carbon. Propane
is a hydrocarbon because it contains carbon and hydrogen and carbon atoms only.
Ethanol is a hydrocarbon derivative because in addition to carbon, it contains oxygen.