Organic Chemistry: Organic chemistry is a branch of chemistry dealing with
compounds involving one or more carbon atoms; any compound that doesn't have a
carbon atom in it, such as HF (hydrofluoric acid), is an inorganic compound. For a long
time, it was believed that you couldn't get any organic matter out of inorganic
compounds. However, compounds that were previously thought inorganic were shown to
yield obviously organic compounds. It was thus shown that chemical principles don't
need to be divided into organic and inorganic principles.
Carbon is special because a) it has 4 valence electrons it's willing to share and b) it's so
small. The 4 shareable valence electrons allows it to assume very complex forms and the
small size allows the bonds it form to be very strong. For instance, silicon also has four
valence electrons, but because of its large size, those bonds aren't as strong. Thus,
complex structures involving silicon aren't nearly as stable as organic structures.
Hydrocarbons: The simplest type of organic molecule is the hydrocarbon. These are
molecules that, as befits their name, contain nothing by carbon and hydrogen. The name
of the particular molecule depends upon the number of carbons within it, the structure
they assume when they are connected to each other, and the bond that exists between
adjacent carbon atoms (be it all single bonds or maybe some double or triple bonds). For
instance, one carbon atom surrounded by four hydrogen atoms is methane, CH4. Two
hydrogen atoms connected via a single bond and then surrounded by hydrogen atoms is
ethane, C2H6. Three carbon atoms connected via a single bond and surrounded by
hydrogen atoms is pentane, C3H8. Four such carbon atoms yields butane, five give
pentane, six gives hexane, seven gives heptane, eight gives octane, nine gives nonane and
ten gives decane (more exist, but you should get the idea by now).
The different structures yield what are called "isomers". For instance, I can have four
singly-bonded carbon atoms in a row and have butane. However, suppose I have three
singly-bonded carbon atoms in a row and a fourth carbon atom attached to the middle
carbon atom instead of an end carbon atom. This has four singly-bonded carbon atoms
and is therefore a type of butane. To distinguish it from having four in a row, this type of
butane is called iso-butane whereas the one with four in a row is called n-butane.
Other possible isomers can occur when the carbon atoms form a circle. Such structures
lead to the prefix “cyclo-“ in front of the name of the molecule, e.g., cyclopropane would
have the three carbon atoms connected to each other with the remaining bonds filled with
hydrogen atoms.
Unsaturated Hydrocarbons: If any double or triple bonds exist among the carbon
atoms, we have what is called an unsaturated hydrocarbon, "unsaturated" because the
number of hydrogen atoms is less than what it could conceivably be otherwise. For
instance, if we have two carbon atoms connected with a double bond and then the
remaining valence vacancies are filled with hydrogens, we have ethene (replace the "a" in
"-ane" with "e" to get "-ene" and it means you have a hydrocarbon with a double bond).
If the two carbon atoms are connected via a triple bond, you have ethyne (also called
"acetylene", but in general, to represent the fact that you have a triple bond you need to
replace the "a" in "-ane" with "y" to get "-yne"). For longer unsaturated hydrocarbons,
there's also a numerical prefix to indicate where the connected occurs. For instance, you
can have 1-propene to indicate that the first carbon in the three-carbon long molecule is
connected to the second one through a double bond: C3H6. If you have the molecule 1,2propene, then there is a double bond connecting the first to the second and a double bond
connecting the second to the third, C3H4. Propyne also exists, indicating that there's a
triple bond. Examination of this molecule, though, shows that propyne can exist in only
one way.
A more sophisticated unsaturated hydrocarbon is the benzene ring (Figure 14.9). This
molecule is a little unusual in that while it alternates single bonds with double bonds, it’s
really as if there are 1½ bonds connecting every single carbon atom to the next one in the
circle. This molecule and longer molecules that include it tend to have strong odors and
are therefore called aromatic compounds. In essence, aromatic compounds have a single
bond leading from one of the carbon molecules and connecting to a carbon molecule in
the benzene ring, as shown in Figure 14.9.
Petroleum Products: Much of our everyday lives depend upon organic compounds
found in raw petroleum. These compounds tend to be dominated by hydrocarbons. For
instance, methane and ethane are found in crude oil; these are extracted and sold as
natural gas. Propane and butane are also extracted, liquefied and sold. Higher-order
hydrocarbons, from isomers of pentane to isomers of dodecane (which has 12 carbon
atoms) are used in gasoline. Compounds with 12 to 15 carbons wind up in kerosene.
Those with 14 to 18 carbons go into diesel fuel. Lubricating oils—including motor oil—
are composed of hydrocarbons with 16 to 18 carbon atoms. Petroleum jelly has 16 to 32
carbon atoms. Wax typically has compounds with 20 or more carbons per molecule and
asphalt will have 36 or more carbon atoms per molecule.
Another important product of crude oil is ethene, also called ethylene, C2H4. This has a
double bond between the carbon atoms. When it is heated, the double bond breaks up,
leaving only a single bond between the two carbon atoms and permitting the remaining
bonds to hook up with other broken-up C2H4 molecules. Long strands form; these are
called “polymers.”
Noncarbon Atoms in Organic Molecules: Pure hydrocarbons aren't the only type of
organic molecule, though they form the basis for most of organic chemistry. However, it
is very easy to replace one or more carbons with a different atom and/or one or more
hydrogen atoms with a different atom (or atoms). Certain types of replacements are very
common and lead to certain groups of molecules. If a hydrogen connected to a carbon at
the end of a chain is replaced with -O-H, then we have a molecule of the alcohol group.
If that hydrogen is replaced with -N-H2, then it's an amine. If an interior carbon atom
with two hydrogen atoms is replaced with a single oxygen atom, then the molecule in the
ether group. Other groups are listed in Table 14.5.
Despite the complexity that can quickly grow, the nomenclature remains as consistent as
possible. For instance, there is a substance known as 1,5-pentanediamine (which is also
called cadaverine). If we examine the name, we see the word "pentane" within it, to
indicate there are 5 carbon atoms; since it isn't iso-pentane or neo-pentane or anything
else, that indicates these 5 carbons atoms are lined up in a row. We also see the word
"diamine", which indicates that there are 2 (from the "di" prefix) amines. The "1,5" that
exists at the beginning tells us that one of the amines is connected to the first carbon atom
and the other amine is connected to the fifth carbon atom. In another example, we can
look at the name "diethyl ether". The "ether" tells us that two carbon atoms are
connected by way of an oxygen atom in the middle. The "diethyl" tells us that there are
two (from the "di") ethane-type molecules ("ethyl" is the adjectival form of "ethane").
We thus see that the oxygen is actually connecting two ethane-type molecules, as is
shown in Figure 21.18. We could also have ethylmethyl ether, which would indicate an
oxygen atom connecting a methyl molecule to an ethyl molecule, or dimethyl ether,
indicating an oxygen connecting two methyl molecules.
Alcohols are an important functional group. The name of the alcohol is typically the
name of the hydrocarbon but with an ending of “-ol.” For instance, CH3OH is a methane
molecule where one of the hydrogens has been replaced with –OH. Therefore, this is
“methanol.” We could also have “ethanol” or “propanol.” However, if we have propanol
we have to distinguish between n-propanol, where the –OH is off one of the end carbons,
or iso-propanol, where the –OH is off the middle carbon. This second type of propanol is
also known as “isopropyl alcohol”, or rubbing alcohol.
If the alcohol had 2 –OH attachments, it belongs in the sub-category of “glycols.”
Perhaps the most well-known glycol is ethylene glycol, used in anti-freeze. If there are 3
–OH attachments, then the molecule belongs in the sub-category of “glycerols” or
“glycerins.” Nitroglycerin is perhaps the best known of the glycerols as it provides the
explosive power in dynamite.