Transcript Petrochemicals: Builder Molecules

Petrochemicals:
Builder Molecules
Chemists as Molecular Architects
• Just as an architect uses knowledge of
available construction materials in designing
a building, a chemist—a “molecular architect”—
uses knowledge of available molecules in
designing new molecules
• Architects must know about the structures and
properties of common building materials.
Likewise, chemists must understand the
structures and properties of their raw
materials, common hydrocarbon ‘builder’
molecules.
Today, many common objects and materials created by
the chemical industry are unlike anything seen or
used by citizens of the 1800s or even the mid-1900s.
Early 1900s
What similarities
and differences do
you see?
Early 2000s
Petrochemicals
• The multitude of useful compounds and materials
made directly or indirectly from hydrocarbons (oil
or natural gas) are called petrochemicals.
• Until the early 1800s, all objects and materials used by
humans were either created directly from wood or
stone or crafted from metals, glass, and clays.
• Available fibers included cotton, wool, linen, and
silk.
• All medicines and food additives came from natural
sources.
• Celluloid (from wood) and shellac (from animal
materials) were the only sources for commercially
produced polymers, which are large molecule
‘chains’ typically composed of 500 to 20,000 or more
repeating units of simpler molecules known as
monomers.
Direct and Indirect Petrochemical Use
• Many of these differences are due to the use of
petrochemicals.
• Some petrochemicals, such as detergents,
pesticides, pharmaceuticals, and cosmetics, are
used directly.
• Most petrochemicals, however, serve as raw
materials in producing other synthetic substances,
particularly a wide range of polymers.
• Synthetic polymers include paint components,
fabrics, rubber, insulating materials, foams,
adhesives, molding, and structural materials.
• Worldwide production of petroleum-based polymers
is more than four times that of aluminum products.
The astonishing fact is that it takes relatively
few builder molecules (small-molecule
compounds) to make thousands of new
substances, including many polymers.
• One builder molecule is ethene, C2H4, a
hydrocarbon compound commonly called
ethylene.
The two carbon atoms in an ethene molecule share two
pairs of electrons in a double covalent bond.
Double Bonds are Highly Reactive
• Ethene or ethylene is an alkene, a hydrocarbon with
a double covalent bond between the carbon atoms.
• Because of the high reactivity of its double bond,
ethene is readily transformed into many useful
products.
• A simple example—the formation of ethanol (ethyl
alcohol) from water and ethene—illustrates how
ethene reacts:
Acid catalyst
Ethene
Water
Ethanol
Addition Reaction
• In this reaction, the water molecule (H2O)
“adds” to the double-bonded carbon atoms
by adding an H to one carbon atom and an
OH group to the other carbon atom. This
type of chemical change is called an
addition reaction.
Notice that the double
bond between the
carbon atoms has
been broken.
Addition Reactions
• In these reactions, the second bond
between the carbons is broken (requiring
energy), and new bonds are formed
between the carbon atoms and added
atoms (releasing energy).
• Addition reactions can ONLY take place
at a multiple bond. This enables
formation of new bonds without cleaving
(chopping up) the original molecule.
Long Chain Polymers
• Ethene can also undergo an addition reaction
with itself. Because the added ethene molecule
also contains a double bond, another ethene
molecule can be added, and so on.
• This creates a long-chain polymer called
polyethene, commonly known as polyethylene, a
polymer consisting of 500 to 20,000 or more
repeating units of ethene (ethylene) monomer.
This is the ‘shorthand’
structure of polyethylene.
•Polymers formed by reactions such as this are
called—sensibly enough—addition polymers.
•Polyethylene is commonly used in grocery
bags and packaging. The United States
produces millions of kilograms of polyethylene
annually.
More Addition Polymers
• We can make a great variety of addition
polymers from monomers that closely
resemble ethene.
• The most common variation is to replace
one or more hydrogen atoms in ethene
with an atom or atoms of another
element.
• In the following examples, note the
replacements for hydrogen atoms in each
monomer and polymer.
Orlon fabric,
telephone sets,
shoe soles, auto
parts
Acrylonitrile monomer
The ring shown
here is benzene,
which we will learn
about shortly.
POLYPROPYLENE
Bottle caps, buckets, carpets, long johns,
ropes, chairs, crates, straws, tents, etc.
POLYVINYL CHLORIDE
Raincoats, PVC pipe;
bottles for oil, shampoo,
baby products; plastic
wrap; garden hoses
POLYSTYRENE
Styrofoam food containers, egg
cartons, building insulation, coat
hangers, CD cases, disposable pens
How Are Polymers’ Molecules
Arranged?
• During your computer assignment, you
saw the molecular arrangement of
polyethylene. What was it?
• What would a collection of these
molecules look like?
• Sketch this arrangement on your paper,
using a pencil or pen line to represent
each linear polymer molecule.
• Ductility refers to the ability of a
material to be drawn out, as into
thin strands. For most polymers like
the ones you just drew, flexibility
and ductility depend on
temperature. When the material is
warm, the polymer chains can slide
past one another easily. The
polymer becomes more rigid
when it cools.
• Such polymers, classified as
thermoplastics, are used in
many everyday products,
such as soft drink bottles,
milk bottles, and plastic bags.
These include PVC,
polyethylene, polystyrene and
polyacrylonitrile
• The flexibility of a polymer
can also be enhanced by
adding molecules that act
as internal lubricants
among the polymer
chains.
• For example, untreated
polyvinyl chloride (PVC) is
used in rigid pipes and
house siding.
• With added lubricant
molecules, PVC becomes
flexible enough for use in
such consumer goods as
raincoats and inflatable pool
toys.
Branched Polymers
• The reactions that form polymer chains can also
take place perpendicular to the main chain,
forming side chains. These polymers are called
branched polymers.
• Branching changes the properties of a polymer
by affecting the ability of chains to slide past
one another and by altering intermolecular
forces.
Draw at least two different
models of branched-chain
polymers. Try to vary your
representations—the forms of
branched polymers can differ
greatly.
Cross Linking
• Another way to alter the properties of polymers
is through crosslinking.
• Polymer rigidity can be increased if the
polymer chains are cross-linked so that they
can no longer move or slide readily. (Think
about the cap on a soda bottle.)
• Cross-linking is having links between polymer
chains.
Draw several linear
polymer chains that
have been cross-linked.
• Chemists can make even more different
kinds of polymers by using BOTH branching
and cross-linking.
Draw several cross-linked, branched polymers.
Then describe how cross-linked branched
polymers compare to cross-linked linear
polymers.
Aligned Linear Chains
• Chemists can control polymer strength and toughness. To
do this, the polymer chains are arranged so that they lie
in the same general direction. The aligned chains are
stretched until they uncoil.
• Polymers that remain uncoiled after this treatment make
strong, tough films and fibers. Such materials include
polyethene, which is used in everything from garbage
bags to artificial ice rinks, and polypropylene, which is
used in bottles and some carpeting.
Draw several aligned linear polymer chains.
Draw several aligned and cross-linked linear polymer chains.
Saturated and Unsaturated
Hydrocarbons
• Each carbon atom in an alkane molecule is
bonded to four other atoms. Compounds such
as alkanes are called saturated hydrocarbons
because each carbon atom forms as many
single covalent bonds as it can.
• In some hydrocarbon molecules, though, carbon
atoms bond to three other atoms, not four.
Members of this series of hydrocarbons are
called alkenes. You already learned about
ethene, C2H4.
• The carbon–carbon bonding that characterizes
alkenes is a double covalent bond. In a double
bond, four electrons (two electron pairs) are
shared between the bonding partners.
Unsaturated Hydrocarbons
• Alkenes, which contain carbon–carbon
double bonds, are described as
unsaturated hydrocarbons which means
that not all carbon atoms are bonded to
their full capacity with four other
atoms.
• Because of their double bonds, alkenes
are more chemically reactive—and
therefore better builder molecules—than
are alkanes.
Substituted Alkenes
• The substituted alkenes make up
another class of builder molecules. In
addition to carbon and hydrogen, these
molecules contain one or more other
atoms, such as oxygen, nitrogen, chlorine,
or sulfur. Remember PVC?
• Adding atoms of other elements to
hydrocarbon structures significantly
changes their chemical reactivity.
More Builder Molecules
Cycloalkanes and
Aromatic Compounds
Another alkane structure
Consider hexane C6H14, a straight chain
molecule
CH3 ̶ CH2 ̶ CH2 ̶ CH2 ̶ CH2 ̶ CH3
What happens if we remove one hydrogen
from each end and bond the end carbons to
each other?
Cyclohexane!
(used for making nylon)
Cycloalkanes
• Cycloalkanes are saturated
hydrocarbons made up of carbon atoms
joined in rings
• Cyclo- is from the Greek for “circle.”
cyclohexane, cyclobutane, cyclopentane
What’s this molecule?
Aromatic Compounds
• Another class of hydrocarbon builder molecules
with distinctly different properties are the
AROMATIC COMPOUNDS.
• The simplest aromatic compound is benzene,
C6H6. When it is shown without any letters, each
vertex of the six-carbon hexagon represents a
carbon atom with its hydrogen atom.
How is the structure of
benzene different from
cyclohexane?
Aromatic Compounds
Chemists have discovered, however, that
benzene doesn’t behave like it has ANY
double bonds (its chemical reactivity isn’t
strong enough), so this structural
representation isn’t correct.
Experiments have determined that all
carbon – carbon bonds in benzene are
identical, with the bonding electrons shared
among all 6 atoms. Benzene’s symbol is
composed of two parts: the inner ring
represents the equal sharing of bonding
electrons among the carbon atoms, and
the outer hexagon represents the bonding
of the 6 carbon atoms to each other.
Aromatic Compounds
• Although only small amounts of aromatic
compounds are found in petroleum,
fractionation and cracking produce large
quantities which are primarily used as
building molecules.
• Used in the pigment and dye and
pharmaceutical manufacturing industries.
Also used in the production of soaps,
tanning agents, insecticides, fungicides,
plastics, and processing of certain foods.
Polycyclic Aromatic Hydrocarbons
• Polycyclic aromatic
hydrocarbons (PAH) are
composed of two or more
benzene rings which are
fused together when a pair
of carbon atoms is shared
between them. The resulting
structure is a molecule
where all carbon and
hydrogen atoms lie in one
plane (i.e., they’re flat)
• These substances are toxic
to aquatic organisms or
carcinogenic.
Builder Molecules Containing Oxygen
• Recall ethanol (ethyl alcohol), CH3-CH2-OH,
and methanol (methyl alcohol), CH3-OH.
• These molecules have an –OH group
attached to a carbon atom. This general
structure is characteristic of a class of
compounds called alcohols.
• The –OH group is one type of a functional
group, an atom or group of atoms that give
characteristic properties to organic
compounds.
• NOTE: The –OH group is NOT the same as a
OH- ion found in ionic compounds.
Alcohols
• The general formula for an alcohol is
R ̶ OH
where R is the rest of the molecule (everything
except the functional group) and the line
represents the covalent bond linking the oxygen
of the –OH group with the adjacent carbon atom in
the molecule. In ethanol R is CH3CH2– .
• In these alcohols, what would R represent?
CH3CH2CH2OH
1-Propanol
Cyclohexanol
More Builder Molecules with
Oxygen
• Two more classes of oxygen-containing
organic compounds are carboxylic acids and
esters.
• The functional group for each of these classes
of compounds contains TWO oxygen atoms,
one doubly bonded to the carbon atom and
the other singly bonded to the same carbon
atom:
Notice that esters
have TWO ‘rest of
molecule’ groups
Carboxylic acids
Esters
Alcohols Carboxylic acids Esters
Isopropyl alcohol
Ethanoic acid or
acetic acid
Methyl ethanoate
(methyl acetate)
Formic acid
Carboxylic acids
Esters
Alcohols
Condensation Polymers
• So far we have seen how ADDITION
POLYMERS are formed from alkene
monomers (substitution alkenes like PVC,
polyethylene and polystyrene).
• Are natural polymers—like proteins,
starches, and cellulose—addition
polymers?
• No, they’re not! And neither are some
familiar synthetic polymers like nylon and
polyester.
Condensation Polymers
• These polymers are formed with the loss
(removal) of simple molecules, such as water,
when monomer units join, instead of the
breaking of a double bond.
• The term condensation reaction applies to this
second type of polymer-making process, and the
resulting product is called a condensation
polymer.
• Here is a simple representation of this process:
-R1–O–H + H–O–R2- → -R1–O–R2- + H–OH
Monomer 1
Monomer 2
Condensation
polymer
water
Condensation Process
• One common condensation polymer is
polyethylene terephthalate (PET).
• This polymer is often used in large soft-drink
containers. It has many other applications—as
thin film for videotape and as Dacron for
clothing, surgical tubing, and fiberfill.
• More than two million kilograms of PET are
produced each year in the United States.
Can all PET be recycled?
• While some PET may be recycled, material
containing high quantities of additives, such as
dyes or other polymers, is either disposed of in a
landfill or is incinerated.
• Another option is a DuPont process that breaks
down PET into its monomers, which are then
repolymerized into high-quality PET products.
This technology decreases the need for new
petroleum-based builder molecules as well as
decreases the quantity of PET discarded in
landfills.
More uses for condensation reactions
• Condensation reactions can be used to
make small molecules as well as
polymers. In tomorrow’s investigation, you
will use condensation reactions to produce
several esters. These reactions illustrate
how you can chemically combine organic
compounds to create new, useful
substances.
Other Uses for Condensation Rx
• Condensation reactions can be used to
make small molecules as well as
polymers.
• In tomorrow’s investigation, you will use
condensation reactions to produce several
esters.
• These reactions illustrate how you can
chemically combine organic compounds to
create new, useful substances.