Chapter 9 and 10
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 80% 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 Werner 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 bomds) 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 classified into smaller
groups. We will look at only one of these.
Alkanes – Every C atom has 4 single covalent bonds connected to it, either to other
C a toms 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.
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
H
methane
H
H
H
C
C
H
H
ethane
H
H
H
H
H C H
H
C
C
C
H
H
H
H
2-methylpropane
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:
CH3 CH2 CH2 CH3
CH3
CH3 CH CH3
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 242.
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. You will need to know the names of these 10
basic alkanes.
Sometimes the number of C atoms in a molecule reaches very large numbers,
perhaps hundreds or thousands, a giant molecule is created, which we call a
polymer.
Polymer means many parts, because usually these molecules are formed by linking
many smaller molecules (called monomers) together, like an elephant train.
There are 2 sources of polymers. There are many natural polymers found in
Nature, some of which we will study in later chapters. Then beginning in 1909, the
first synthetic polymer was produced in a laboratory and this has spawned a
multibillion dollar industry. All plastics are synthetic polymers. We will now look
briefly at some of these.
2 Types
1) Addition or vinyl
2) Condensation
Addition polymers – Large molecule composed of one or sometimes 2 smaller
molecules (called monomers), linked together end to end, basically (sort of like a
train of elephants).
Requirments of monomers
For additon, at least one double bond – Linking takes place by breaking the
double bonds and adding each monomer at that point.
A few common addition polymers are described in your book on pages 281 – 283.
You should read these. Note that in each case, the monomer has at least one
double bond. Table 10.2 on page 285 has a more detailed list of common addition
polymer plastics and their uses.
Generally, plastics are insulators (don’t conduct heat or electricity. But on page 286,
a plastic called polyacetylene is described which does conduct electricity and is
sometimes used as a metal substitute. The polymer, unlike most other addition
polymers, still has alternating double and single bonds. The electrons in the second
bond of a double bond are more loosely held than those in a single bond, thus
allowing electrons to flow through the plastic.
By adjusting the reaction conditions, chemists can adjust the exact structure of these
compounds. The same general polymer, such as polyethylene can be made elastic
or rigid, soft or hard, clear or opaque. We won’t go into these specifics, except in the
following situation:
Diene Polymerization - C=C-C=C. Usually only one C=C is utilized, leaving
...-C-C=C-C... , leaving one C=C in the middle.
Natural rubber is a polymer of a diene: The repeating unit is found to be:
CH2 C
C
CH2
CH3
This can be made from the monomer isoprene:
C C C C
C
When this was done for the first time in a laboratory, the product had some rubberlike properties, but not all & it was basically not usable (sticky & tacky)
Further study showed that natural rubber was almost all cis around C=C, while
the synthetic was a mixture of cis & trans. Using special catalysts the synthetic with
almost all cis was made & it was just as good as natural rubber.
Various variations on monomers have yielded various different synthetic
rubbers.
Even natural rubber was not a very useful product until Charles Goodyear in
1844, discovered that if you heat natural rubber in the presence of S, cross links are
made between different strands of the polymer utilizing S atoms as the bridge. This
process is called vulcanization. It produced a much tougher & less gummy
product.
The second type of polymer :
Condensation Polymers – Polymer formed when 2 or more monomer molecules
linked together via a condensation reaction, each reaction eliminating a small
molecule, usually H2O or HCl.
The monomer requirement for condensation is that it must contain 2 functional
groups – One functional group on the first monomer reacts with the opposite
functional group on the second monomer producing a larger molecule, that still has 2
functional groups. This process can continue on and on.
Let’s digress into what a functional group is.
With organic compounds, the most common type is a hydrocarbon (containing C and
H and nothing else. Substituted hydrocarbons are compounds that have special
groups bonded to C, instead of H. These special groups are called functional
groups, because they direct the special behavior of one group from another. They
direct the function of these groups.
A list of many of these functional groups is found in Table 9.4 on page 251.
Condensation Polymers:
1. polyamide - Nylon (different types) & protein.
Fibers of nylon are formed by the following:
Melt spinning - Heat to just above M.P., force at high pressure through tiny
holes into stream of N2 gas to cool. Solidifies quickly. Then drawn into 4
times original length, which orients polymer chains into regular side by side
position --> increases MP - harder, denser, tougher. (extensive H bonding)
2. Polyesters – polymer formed when a molecule containing 2 COOH groups
(a di-acid) and a molecule containing 2 OH groups, a di-alcohol or glycol
3. Phenol-Formaldehyde and related materials – Bakelite (the first plastic,
named after its inventor, Leo Baekeland) is a condensation polymer between a
compound named phenol and a compound named formaldehyde. This forms a
spider-web like network of bonds, leading to a very hard plastic. This was originally
used as a substitute for ivory in billiard balls.
Another related plastic uses urea instead of phenol and a third uses melamine (see
page 291) instead of phenol. This last case leads to a plastic that has been widely
used in plastic dinnerware and formica countertops.
Another way we classify plastics is whether they can be remolded or not. A
thermoplastic polymer can be softened by heat and pressure and then reshaped.
This can be done repeated times. Theoretically, these could be recycled with some
effort. A thermosetting plastic cannot be softened by heat and remolded. Heating
causes discoloration and decomposition.