Chapter 9
We earlier defined a class of compounds called hydrocarbons
(containing C and H and nothing else). Hydrocarbons form the
backbone of an important area of chemistry called Organic
Chemistry. More than 95% of all known substances are organic
substances.
•Original Definition – Any substance formed from living
organisms or once living organisms. It was believed that there
were organic and non-organic (inorganic) substances. Each were
completely separate. No crossover.
•Then in 1828, a chemist, named Friedrich Wohler showed that an
organic substance could be made from inorganic substances and
since then we know that this is common. A different definition
was needed.
•Modern Definition – Any substance containing C ( except CO,
CO2, substances containing CO3-2 or CN-1). Often called Carbon
Chemistry.
The uniqueness of C chemistry, is that C has 4
valence electrons and always forms exactly 4
covalent bonds, (not necessarily to 4 atoms;
there can be double and/or triple bonds) but
unlike N, O and F, C cannot form C2 molecule
(a quadruple bond would be required and this
does not happen.
•The result is that when C bonds to itself in compounds
there is always at least one electron left over to bond to
other atoms, most commonly H but also frequently O, N
and S. This results in C chains, sometimes extremely long
containing thousands of C atoms.
•Basic Structure and Nomenclature – There are so many
organic compounds, that it is convenient to break down the
many compounds into smaller groups with similar properties.
The largest breakdown is into 2 major classes:
•hydrocarbons and substituted hydrocarbons.
Hydrocarbons are further broken down into several types:
1. Alkanes – Every C atom has 4 single covalent bonds
connected to it, either to other C atoms or to H atoms.
Every bond is a single bond. Alkanes are said to be
saturated, because there is no room for any more atoms
to bond to the C atoms. We will explain this in more
detail later.
2. Alkenes – Contains one or more C=C double bonds
3. Alkynes – Contains one or more CC triple bonds
4. Aromatics – We will come back to these later
It is also important to know more than the molecular formula for
organic compounds. We also need to know the structure, or what
atom is bonded to what atom and where in space. Let’s illustrate this
with a couple of examples:
H
H
C
H
methane
H
H
H
H
C
C
H
H
ethane
H
H C
H
H
H
H
H
C
C
C
H
H
H
2-methylpropane
H
•Before continuing our discussion of organic compounds, we need
to understand another concept that we haven’t discussed so far;
isomerism.
•Isomers are 2 or more compounds with the same molecular
formula but different structures or shapes. The most common are
what are called structural isomers. There are 2 structural isomers
of C4H10:
CH 3
CH 2
CH 2
CH 3
CH 3
CH 3
CH
CH 3
•These actually have different names: butane and 2-methyl
propane.
•Also note, that the structures above look different than the
first structures we saw. This is a condensed structural
formula, just showing where the carbons are, since any H
atoms must be bonded to the C atom using single covalent
bonds.
•These can be further condensed by eliminating all bond
lines, as seen in Table 9.2 on page 232.
•Also listed there are the names of the first 10 (containing 1
through 10 C atoms) alkane names, assuming all C atoms in
one long continuous chain.
•Also the number of possible isomers is listed for each. Note
how that number increases dramatically as the number of C
atoms increases.
Alkanes are the simplest hydrocarbons and the simplest of these
have all the C atoms bonded to each other in a straight chain. We
need to learn how to name hydrocarbons and the first step is to
learn the names of these straight chained hydrocarbons.
•You will need to know the names of these 10 basic alkanes.
REVIEW NOTE: There are several ways to draw structures. We
have already seen 2 ways. A third example does not show the
individual C-H bonds. Sometimes we will write structures with no
H's written in, since it is assumed everyone can fill in the H's so that
every C atom has 4 bonds. This would be the easiest and fastest.
Let’s look at a couple of examples.
•Alkenes are very similar to alkanes except that they are
unsaturated while alkanes are saturated.
•C always has 4 covalent bonds attached to it(remember the octet
rule). If there are 4 atoms bonded also, then each bond must be
single and it is impossible to fit any more atoms bonding to it. Thus
if all the C atoms have 4 atoms bonded to them, no more atoms can
be added to the molecule and it is said to be saturated.
•If there are any multiple bonds, such as in alkenes, then the C’s with
the multiple bond will have less than 4 atoms bonded to them, and it
is possible to break the second and/or third bond of these multiple
bonds, and form all single bonds.
•This would require adding other atoms to the molecule. Thus
alkenes can fit more atoms in and are said to be unsaturated.
This is the cause for the other difference between alkenes
and alkanes. Alkenes are more chemically reactive than
alkanes.
Sometimes, instead of chains, C atoms will bond to each
other to form a geometric shape (triangle, square, etc).
These are called cyclic compounds. There are
cycloalkanes, cycloalkenes and even cycloalkynes
(although rare).
Aromatic Hydrocarbons: - Cyclic hydrocarbon that has
the equivalent of several C=C double bonds. However,
instead of the normal arrangement for the second bond
between 2 atoms, in aromatic compounds all the C atoms
forming the ring share all of the electrons in the second
bonds of all the double bonds. This is called
delocalization, which gives significant extra stabilization
energy for this structure, called delocalization energy.
The important net effect is on the chemical behavior of
aromatic hydrocarbons.
1. The ring structure almost always stays intact.
2. It behaves like a saturated compound rather than the very
unsaturated compound, that it is.
3. The bond lengths are all equal and are in between single
and double bond lengths
To indicate that all of the extra electrons are shared equally
by all the C atoms in the ring, the multiple bonds are
represented most commonly by a circle inside the ring,
such as benzene, which is the simplest aromatic
hydrocarbon, C6H6:
Each corner represents a C atom and 1 H atom is bonded to
each C atom:
CH
HC
CH
HC
CH
CH
•The second large class of organic compounds, the Nonhydrocarbons are also called substituted hydrocarbons.
• They are called this because in these compounds an
element or group of elements replaces one or more of the
hydrogens to form different classes of compounds.
•These new classes are identified and their properties are
controlled by these replacements, which are called
Functional Groups.
A list of many of these functional groups and their class of
compounds is found in Table 9.4 on page 242.
Functional Group
Name
Hydroxyl (OH)
}
carbonyl
( )
O
C
carboxyl (COOH)
(COOR)