NOTES: Carbon Chemistry
Name______________________________
Date__________ Block____
Objective 1. Explain how the structure of the carbon atom affects the type of bonds it
forms in organic and inorganic molecules.
A carbon atom has four valence electrons (2s1 2p3) and therefore can
make four single covalent bonds, one double covalent and two single
covalent bonds, or one single covalent bond and one triple covalent
bond. This ability to form different kinds of covalent bonds allows
carbon to form thousands of possible molecules.
Only carbon is able to form stable, extended chains of atoms bonded
through single, double, or triple bonds. No other element has this
ability.
I will use structural diagrams of hydrocarbons (molecules composed of carbon and hydrogen) to
illustrate some of the different possible arrangements of carbon atoms in molecules.
Methane, CH4, is an example of one carbon atom bonded to four other atoms.
Each C—H bond is a single covalent bond (one pair of valence electrons is shared by two
atoms).
H
|
H–C–H
|
H
Two carbon atoms can form a double covalent bond between them and also form single covalent
bonds with other atoms. Ethylene, C2H4, illustrates this bonding. Each C—H bond is a single
covalent bond. The two carbon atoms are joined by a double covalent bond (two pairs of valence
electrons are shared by two atoms).
H
H
C=C
H
H
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Two carbon atoms can also form a triple covalent between them and also form one single
covalent bond with other atoms. Acetylene, C2H2, illustrates this bonding arrangement. The two
carbon atoms are joined by a triple covalent bond (three pairs of valence electrons are shared by
two atoms). Each C—H bond is a single covalent bond.
H–C
C–H
Carbon atoms can bond to other carbon atoms to form long chain molecules such as hexane,
C6H14.
H
|
H–C–
|
H
H H
|
|
C – C–
|
|
H H
H
|
C –
|
H
H
|
C -|
H
H
|
C–H
|
H
Carbon atoms may form branched chains. In each of the molecules below, an ethyl group, C2H5,
has been added to the 6-carbon chain of hexane. The position of the branch on the longest carbon
chain determines the chemical properties of the molecule. In the upper molecule, the branch is
attached at the 4th carbon in the longest chain. In the lower molecule, the branch is at the 3rd
carbon atom in the longest chain. Both molecules have the same chemical formula, C6H18, but
are different compounds because of their different structures. Compounds that have the same
chemical formula but different structures are called isomers.
H
H
H
|
|
|
H–C – C – C
|
|
|
H
H
H
H
H
|
|
H – C -- C
|
|
H
H
H
|
H–C–H
|
H–C–H
|
– C –
|
H
H
|
H–C–H
|
H–C–H
|
– C –
|
H
H
H
|
|
C -- C – H
|
|
H
H
H
H
H
|
|
|
C – C -- C – H
|
|
|
H
H
H
2
3-ethylhexane
3-ethylhexane
Carbon atoms may also form rings such as benzene, C6 H6.
H
H
C
H
C
||
H–C
C
|
C–H
C
H
Structures containing rings and chains may also be formed.
H
H
H
C
H
H
H
H
|
|
|
C – O -- C -- C -- C -- H
|
|
|
|
C–H
H H
H
H–C
|
H–C
H
C
H
H
H
Other elements can be bonded to the carbon atoms that are in chains or rings. Functional groups
such as methyl (--CH3), ethyl (CH3CH2), alcohol (-- OH), carboxyl (-- COOH), carbonyl (a
double bonded oxygen atom (=O), and amine (-NH2) may also bond to various carbon atoms in a
chain or ring.
H
|
H–C –
|
H
H
|
H
H
H
H H--C--H
|
|
|
|
|
C – C – C – C -- C – H
|
|
|
|
|
H
H
H
H
H
3
methyl group (-CH3) is in bold
n-heptane
H
|
H–C–
|
H
H H
|
|
C – C–
|
|
H H
H
|
C –
|
H
H
|
C -|
H
H
|
C–O
|
H
H
H
|
H–C–
|
H
H H
|
|
C – C–
|
|
H H
H
|
C –
|
H
H
|
C -|
H
H O
|
||
C–C
|
O -- H
H
alcohol group (-OH) is in bold
carboxyl group (-COOH) is in bold
Objective 2. Describe combustion reactions of hydrocarbons and their resulting byproducts.
A combustion reaction is a reaction of a substance with oxygen,
usually with the rapid release of heat to produce a flame. Remember a
flame is the light energy released by excited electrons of a gaseous substance as the electrons
return to their ground state.
Sometimes a combustion reaction occurs very slowly as when iron metal reacts with oxygen in
the air to form rust. This reaction can take several weeks or months.
Other times the reaction occurs very quickly as when magnesium metal is heated in air: if the
magnesium is hot enough, the magnesium atoms will react with oxygen in the air to form
magnesium oxide (a white powder) and releases a lot of heat and light energy. Once this
reaction starts, the magnesium does not have to be heated by a Bunsen burner: enough heat is
released by the reaction for the reaction to continue on its own.
When heated with enough oxygen in the atmosphere, most organic
compounds (molecules containing carbon) react completely with
oxygen to form only carbon dioxide gas and water vapor. If there is
not enough oxygen gas available, the products are carbon monoxide,
unreacted carbon, and water vapor.
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Examples:
Methane gas, CH4, reacts with sufficient oxygen gas, O2, to form carbon dioxide gas, CO2, and
water vapor, H2O. There is a lot of heat released but very little visible light; the visible light
emitted is pale blue. The small amount of visible light emitted is indicative of complete
combustion – all of the carbon compound has reacted and produces only carbon dioxide gas and
water vapor. This is the pale blue flame we used in the flame test lab: a nonluminous flame.
When there is insufficient oxygen, the methane reacts with oxygen to form carbon monoxide gas,
CO, unreacted carbon, and water vapor. There is heat released but not as much as the amount of
heat released by the complete combustion reaction. There is a lot of visible light emitted: mostly
yellow light emitted by excited electrons of the unreacted carbon atoms. This is the luminous
flame we observed when learning how to use the Bunsen burner properly.
Objective 3. Explain the general formation and structure of carbon-based polymers,
including synthetics polymers such as polyethylene, and biopolymers such as
carbohydrates.
A polymer is a chemical molecule having very high molecular mass
and that is made from many units of low molecular mass covalently
linked together.
The term polymer means “many mers.” A mer is a basic unit such as a small
molecule. The small compound used to make a polymer is called a monomer.
A simple example is polyethylene which consists of many ethylene units,
--CH2CH2--, covalently bonded to one another to form long-chain molecules
consisting of thousands of ethylene units. One ethylene molecule is:
H
H
C=C
H
H
A short piece of a polyethylene molecule is shown; one ethylene unit is in bold.
H
|
H–C–
|
H
H H
|
|
C – C–
|
|
H H
H H H H
|
|
|
|
C – C -- C -- C -|
|
|
|
H H H H
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single carbon electron available to bond
with the next ethylene unit
To form the polyethylene, a methyl group, -- CH3, is bonded to an ethylene
group, CH2CH2. The first reaction involves methane and ethylene. In this
process, several chemical changes occur:
a) the methane loses a hydrogen atom and forms a methyl group, -- CH3;
the methyl group can now form a single covalent bond with another atom
b) the double bond between the two carbon atoms in the ethylene is changed
to a single covalent bond and two valence electrons of the carbon become
available to form bonds with other atoms; the ethylene unit is able to form
two single covalent bonds with one bond at each end of the ethylene unit
H
|
H–C– H
|
H
H
+
H
C = C
H
methane
H
|
H – C -|
H
→
H
ethylene
+
methyl group
H
H
|
|
-- C -- C -|
|
H
H
ethylene unit
Then the methyl group and the ethylene unit now form a single covalent bond to begin the
polymer:
H
|
H–C–
|
H
methyl
group
H H
|
|
C – C–
|
|
H H
ethylene
unit
single valence electron of carbon ready to form a
single covalent bond with the next ethylene unit
BIOPOLYMERS include starches (made of smaller sugar units), proteins
(made of small amino acid units), fatty acids (made of one methyl group
(--CH4), many ethyl units (–CH2), and ending with a carboxyl group
(--COOH), and the nucleic acids, DNA and RNA, each composed of
nucleotides (phosphate/deoxyribose sugar/ A,T,C,G in DNA,
and phosphate/ribose sugar/A,U,C,G in RNA.)
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Objective 4. Explain how simple chemical monomers can be combined to create linear,
branched and/or cross-linked polymers.
LINEAR POLYMERS: see above for the formation of polyethylene
BRANCHED POLYMERS: see above for the structures of 3- and 4ethylmethylhexane.
CROSS-LINKED POLYMERS: cross-linked polymers have linkages
between the individual strands of polymers.
Example: two hydrocarbon chains that are cross-linked by a sulfur atom:
H
|
H–C–
|
H
H H
H
|
|
|
C – C– C –
|
|
H
H
H
|
C -|
H
H
|
C–
|
H
H
|
C -|
H
H
|
C–
|
H
S
H
|
H–C–
|
H
H
|
C – C–
|
|
H H
H
|
C –
|
H
Cross-linked polymers can form flexible, stretchy materials if there are few
cross-links along the length of the two chains.
The greater the number of cross-links between polymer chains, the less
flexible and stretchable the polymer becomes.
Objective 5. Explain how the chemical structure of polymers affects their physical
properties.
FIBERS are long, thin, threadlike bits of material that are characterized by
great tensile (pulling) strength in the direct of the fiber. Natural fibers
include cotton, silk, and wool. Common synthetic fibers include nylon,
spandex, Dacron (a polyester), and acrylic fibers such as Orlon.
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The key requirements of a fiber are a linear molecular shape that allows sideby-side alignment, and strong intermolecular attractions (NOT actual
chemical bonds!) to maintain the alignment. Fibers remain aligned when
stretched and when the stretching force is removed. Because the
intermolecular attractions are so strong in maintaining the alignment, these
fibers have limited ability to stretch.
ELASTOMERS are polymers that have a lot of elasticity characteristic of
rubber: it can be stretched to eight times its original length and yet return to
its original shape and size. Natural elastomers include rubber, latex, and
chicle (the base for chewing gum!) An important synthetic elastomer is SBR,
used instead of natural rubber in the manufacture of automobile tires.
Here, as in fibers, the molecules are long and thin and have random
formations. The fibers line up only when they are stretched. HOWEVER,
when the stretching force is removed, the fibers return to their random
formations. These molecules do not stay aligned because the intermolecular
attractive forces between the molecules are too weak to hold them aligned.
The long chains of elastomers are held together by occasional cross-links:
enough cross-links to keep the molecules from slipping past each other, but
not so many cross-links that the chains lose the flexibility needed for
stretching and returning to random formation.
PLASTICS are made of polymers that are long molecules that either linear or
branched, and space-network molecules.
Most linear and branched plastics have many cross-links between the
molecules. These polymers soften when heated and for this reason are called
thermoplastics. Thermoplastics are used to make items such as plastic food
wrap, film, molded toys, knobs, dishes, and toothbrushes.
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Certain thermoplastics do not have cross-links between the molecules but
rather are held together by strong intermolecular attractive forces. These
materials are called glasses: they reflect little light and are therefore
transparent and are brittle. Plexiglas and Lucite are two examples of this
type of thermoplastic.
Space-network polymers (also called resins) are highly cross-linked and form
rigid three-dimensional structures. A sample of such material is one giant
molecule; heating does not soften it but in fact causes additional cross-links to
form which makes the material harder. These plastics are called
thermosetting. Thermosetting plastics are used to manufacture valves, gears,
automobile bodies (such as for Formula-1 and Indy cars), fences, suitcases,
and siding for houses
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