Chapter 22 Hydrocarbon Compounds

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Chapter 22Hydrocarbon Compounds
kerosene
Diesel fuel
gasoline
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Organic Chemistry Hydrocarbons
Scientists used to think that only living
organisms could synthesize the carbon compounds
found in their cells. They called the study of
these compounds organic chemistry. However,
Friedrich Wohler (1800s) was able to use
inorganic substances to synthesize urea. ( a
carbon compound found in urine) Today - Organic
chemistry includes the chemistry of almost all
carbon compounds regardless of their origin.
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Hydrocarbons
There are many more organic compounds than
inorganic compounds The simplest organic
compounds contain only carbon and hydrogen and
are called hydrocarbons. The two simplest
hydrocarbons are methane (CH4) and ethane C2H6.
Methane is a major component of natural gas and
is formed by the action of bacterial on decaying
plants in swamps and other marshy
areas. Livestock and termites also emit
substantial quantities of methane as a product of
digestion.
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Methane
Carbon has 4 valence electrons and hydrogen has
1, so the carbon atom can form a single covalent
bond with four hydrogen atoms. Because carbon
has four valence electrons, a carbon atom always
forms four covalent bonds.
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Ethane
Methane is not typical of the vast majority of
organic compounds, because there isnt a bond
between carbon atoms in a methane molecule.
Ethane has a carbon-carbon bond. Two carbons
atoms share a pair of electrons. The remaining
six valence electrons form bonding pairs with the
electrons from six hydrogen atoms.
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Alkanes
The ability of carbon to form stable
carbon-carbon bonds is one reason that carbon can
form so many different compounds. Alkanes are
hydrocarbons in which there are only single
covalent bonds. In alkanes, all the
carbon-carbon bonds are single covalent bonds and
all other bonds are carbon hydrogen bonds. The
carbon atoms in an alkane can be arranged in a
straight change or in a chain that has
branches Alkenes have the general formula CnH2n
2 (C2H6)
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Straight-Chain Alkanes
Ethane is the simplest of the straight-chain
alkanes, which contain any number of carbon
atoms, one after the other in a chain. Propane
(C3H8) has three carbon atoms bonded in a chain
with eight electrons shared with eight hydrogen
atoms. Butane (C4H10) has four carbon atoms
bonded in a chain with 10 hydrogen
atoms. Homologous series a constant increment
of change in molecular structure from one
compound in the series to the next.
(straight-chain alkanes are an example)
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Straight-Chain Alkanes
First Ten Straight-Chain Alkanes First Ten Straight-Chain Alkanes First Ten Straight-Chain Alkanes First Ten Straight-Chain Alkanes
Name Molecular Formula Structural Formula BP (ºC)
Methane CH4 CH4 -161.0
Ethane C2H6 CH3CH3 -88.5
Propane C3H8 CH3CH2CH3 -42.0
Butane C4H10 CH3CH2CH2CH3 0.5
Pentane C5H12 CH3CH2CH2CH2CH3 36.0
Hexane C6H14 CH3CH2CH2CH2CH2CH3 68.7
Heptane C7H16 CH3CH2CH2CH2CH2CH2CH3 98.5
Octane C8H18 CH3CH2CH2CH2CH2CH2CH2CH3 125.6
Nonane C9H20 CH3CH2CH2CH2CH2CH2CH2CH2CH3 150.7
Decane C10H22 CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3 174.1
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Naming Alkanes
Every alkane has a name that ends with the suffix
ane. For the straight-chain alkanes with 1
to 4 carbon atoms, the official names and the
common names are the same. (Methane, ethane,
propane, and butane) A mixture of Latin and Greek
prefixes are used to name the hydrocarbons having
straight chains longer than four carbon atoms.
The prefixes are pent- 5, hex- 6, hept- 7) oct-
8, etc. A complete structural formula shows all
the atoms and bonds in a molecule. Sometimes,
shorthand structural formulas work just as well.
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Condensed Structural Formulas
In a condensed structural formula, some bonds
and/or atoms are left out of the structural
formula. (even thought the bonds and atoms do not
appear, you must understand that they are
there) C4H10 CH3 CH2 CH2 CH3 CH3(CH2)2CH3 C
C C - C
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Branched-Chain Alkanes
Because a carbon atoms form four covalent bonds,
it can bond not only to one or two other carbon
atoms, but also to three or even four other
carbons, resulting in branched chains. Branches
on a hydrocarbon chain are discusses as if they
were substituted for a hydrogen atom on the
chain. A atom or group of atoms that can take
the place of a hydrogen atom on a parent
hydrocarbon molecule is called a substituent.
The longest continuous carbon chain of a
branched-chain hydrocarbon is called the parent
alkane. All other carbon atoms or groups of
carbon atoms are regarded as substituents.
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Branched-Chain Alkanes
C C -- C C
Substituent
Parent alkane (propane)
Substituents
C C C
C --- C - C - C - C --- C
Parent alkane (hexane)
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Alkyl Groups

• A hydrocarbon substituent is called an alkyl
  group.
• An alkyl group can be one carbon or several
  carbons long.
• Alkyl groups are name by removing the ane ending
  from the parent hydrocarbon name and adding yl.
• The three smallest alkyl groups are
• methyl group (CH3 --)
• ethyl group (CH3CH2 --)
• propyl group (CH3CH2CH2 --)
• An alkyl group consists of an alkane with one
  hydrogen removed.

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Naming Branched-Chain Alkanes
When a substituent alkyl group is attached to a
straight-chain hydrocarbon, branches are formed.
An alkane with one or more alkyl groups is
called a branched-chain alkane. The name of a
branched-chain alkane is based on the name of the
longest continuous carbon chain. Each alkyl
substituent is named according to the length of
its chain and numbered according to its position
on the main parent chain.
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Naming Branched-Chain Alkanes
7 6 5
4 3 2 1 CH3 ---
CH2 - CH2 - CH - CH --- CH ---CH3

CH2 CH3
CH3
CH3
4-ethyl-2,3-dimethylheptane
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Naming Branched-Chain Alkanes

1. Find the longest chain of carbon atoms (parent)
2. Number the carbons in the main chain in sequence.
   (Start at the end that will give the groups
   attached to the chain the smallest numbers.)
3. Add numbers to the names of the substituent
   groups to identify their positions on the chain.
   (these numbers become prefixes to the name of the
   alkyl group)
4. Use prefixed to indicate the appearance of the
   same group more than once in the formula. (di-,
   tri-, tetra-, penta-)
5. List the names of alkyl substituents in
   alphabetical order. (ignore the prefixes di-,
   tri-, etc.)
6. Use proper punctuation. Commas are used to
   separate numbers. Hyphens are used to separate
   numbers and words. Entire name written without
   any spaces.

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Properties of Alkanes
The electron pair in a carbon-hydrogen or a
carbon-carbon bond is shared almost equally by
the nuclei of the atoms form the bond.
Molecules of hydrocarbons, such as alkanes, are
nonpolar molecules. The attractions between
nonpolar molecules are weak van der Waals forces,
so alkanes of low molar mass tend to be gases or
liquids that boil at a low temperature. The
nonpolar hydrocarbon compounds will not form
solutions with polar compounds.
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End of Section 22.1
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Alkenes
Organic compounds that contain the maximum number
of hydrogen atoms per carbon atom are called
saturated compounds. (alkanes) Compounds that
contain double or triple carbon-carbon bonds are
called unsaturated compounds. Alkenes are
hydrocarbons that contain one or more
carbon-carbon double covalent bonds. At least
one carbon-carbon bond in an alkene is a double
covalent bond. Other bonds may be single
carbon-carbon and carbon-hydrogen bonds. Alkenes
have the general formula CnH2n (C2H4)
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Naming Alkenes

• Ethene (common name ethylene) is the simplest
  alkene.
• To name an alkene by the IUPAC system
• Find the longest chain in the molecule that
  contains the double bond. (this will be the
  parent chain). It has the root name of the alkane
  with the same number of carbons plus the ending
  -ene.
• The chain is numbered so that the carbon atoms of
  the double bond have the lowest possible numbers.
  
• Substituents on the chain are name and numbered
  in the same way they are for the alkanes.

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Alkenes
Ethene (ethylene)
1- butene
Propene
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Alkynes
Hydrocarbons that contain one or more
carbon-carbon triple covalent bonds are called
alkynes. Alkynes have the general formula
CnH2n-2 (C2H2) The simplest alkyne is the gas
ethyne, which has the common name
acetylene. Straight chain and branched chain
alkanes, alkenes, and alkynes are aliphatic
hydrocarbons. The major attractions between
aliphatic molecules are weak van der Waals
forces. (the introduction of a double or triple
bond into a hydrocarbon does not have a dramatic
effect on physical properties such as boiling
point. )
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End of Section 22.2
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Structural Isomers
Structures of some hydrocarbons differ only in
the positions of substituent groups or of
multiple bonds in their molecules. Compounds
that have the same molecular formula but
different molecular structures are called
isomers. Isomers have different properties
from each other.
butane
2-methylpropane
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Structural Isomers
Structural isomers are compounds that have the
same molecular formula, but the atoms are joined
together n a different order. Structural isomers
differ in physical properties such as boiling
point and melting point. They also have different
chemical reactivities. In general, the more
highly branched the hydrocarbon structure, the
lower the boiling point of the isomer compared
with less branched isomers.
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Stereoisomers Isomers

• Stereoisomers are molecules in which the atoms
  are joined in the same order, but the positions
  of the atoms in space are different.
• There are two types of steroisomers
• Geometric isomers
• optical isomers.

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Geometric Isomers
A double bond between two carbon atoms prevents
them from rotating with respect to each other.
Because of this lack of rotation, groups on
either side of the double bond can have different
orientations in space. Geometric isomers have
atoms joined in the same order, but differ in the
orientation of groups around a double bond.
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Geometric Isomers
In the trans configuration, the methyl groups are
on opposite sides of the double bond. In the
cis configuration, the methyl groups are on the
same side of the double bond. Trans-2-butene and
cis-2-butene have different physical and chemical
properties.
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Geometric Isomers
The groups attached to the carbons of the double
bond do not need to be the same. Geometric
isomerism is possible whenever each carbon of the
double bond has at least one substituent.
cis-2-pentene
2-methyl-1-butene
Trans-2-pentene
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Optical Isomers
Whenever a carbon atom has four different atoms
or groups attached, optical isomers
occur. Asymmetric carbon a carbon with four
different atoms or groups attached. The
relationship between the molecules attached to
the carbon is similar to the relationship
between right and left hands.
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Optical Isomers
Pairs of molecules that differ only in the way
that four different groups are arranged around a
central carbon atom are called optical
isomers. The molecules cannot be superimposed
because they are mirror images of each other.
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End of Section 22.3
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Hydrocarbon Rings
In some hydrocarbon compounds, the carbon chain
is in the form of a ring. These are called cyclic
hydrocarbons.
Cyclopropane
Cyclopentane
Cyclohexane
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Aromatic Hydrocarbons
There is a class of organic compounds that are
responsible for the aroma of spices such as
vanilla, cinnamon, cloves and ginger. These
compounds were originally called aromatic
compounds because they have distinct pleasant
odors. However, not all compounds currently
classified as aromatic have an odor. Molecules
of aromatic compounds contain a single ring or a
group of rings. Benzene (C6H6) is the simplest
example of an aromatic compound. Aromatic
compound is an organic compound that contains a
benzene ring or other ring in which the bonding
is like that of benzene.
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Aromatic Hydrocarbons
Another name for an aromatic compound is an
arene. Because of the structure of benzene, the
properties of aromatic compounds are quite
different from those of aliphatic compounds.
Benzene is a six-membered carbon ring with a
hydrogen atom attached to each carbon. One
electron from each carbon is free to participate
in a double bond.
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Benzene Structure
In a benzene molecule, the bonding electrons
between carbon atoms are shared evenly around the
ring. Recall that when two or more equally valid
structures can be drawn for a molecule, resonance
occurs. Benzene and other molecules that
exhibit resonance are more stable than similar
molecules that do not exhibit resonance. Thus,
benzene is not as reactive as six-carbon alkenes.
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Substituted Aromatic Compounds
Compounds containing substituents attached to a
benzene ring are named as derivatives of benzene.
When the benzene ring is a substituent on an
alkane, the C6H5 group is called a phenyl group.

methylbenzene
ethylbenzene
3-phenylhexane
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Disubstituted Benzenes
Some derivatives of benzene have two
substituents. These derivatives are called
disubstituted benzenes. There are three
structural isomers for the liquid aromatic
compound dimethylbenzene. Common names for
disubstituted benzenes use the terms ortho, meta,
and para in place of numbers.
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End of Section 22.4
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Natural Gas

• Much of the worlds energy is supplied by burning
  fossil fuels. Fossil fuels are carbon-based
  because they are derived from the decay of
  organisms.
• Typically, natural gas is composed of
• about 80 methane
• 10 ethane
• 4 propane
• 2 butane.
• The remaining 4 consists of nitrogen and
  hydrocarbons of higher molar mass.
• also contains a small amount of He)
• Methane, the major constituent of natural gas is
  especially prized for combustion because it burns
  with a hot, clean flame.

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Combustion of Hydrocarbons
Propane and butane are separated from the other
gases in natural gas by liquefaction and sold in
pressurized tank as liquid petroleum gas
(LPG) Oxygen is necessary for the efficient
combustion of a hydrocarbon. If there is not
enough O2 available, the combustion is
incomplete. Complete combustion gives a blue
flame. Incomplete combustion gives a yellow
flame. Carbon monoxide, a toxic gas, is also
formed along with carbon dioxide and water during
incomplete combustion.
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Petroleum
The organic compounds found in petroleum, or
crude oil, are more complex than those in natural
gas. Most of the hydrocarbons in petroleum are
straight-chain and branched-chain alkanes.
Petroleum also contain small amounts of aromatic
compounds and sulfur, oxygen, and nitrogen
containing organic compounds. Petroleum is a
mixture of hydrocarbons having from one to 40
carbon atoms. Without further treatment,
petroleum is not very useful. The mixture must be
separated, or refined, into parts called
fractions.
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Petroleum Fractionation
The refining process starts with the
distillation of petroleum (crude oil) into
fractions according to boiling point.
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Hydrocarbon Cracking
The amounts of products obtained by fractional
distillation are not in proportion to the demand
of the market. Gasoline is the most commonly used
product, so other processes are used to make the
supply meet the demand. Cracking is a controlled
process by which hydrocarbons are broken down or
rearranged into smaller, more useful molecules.
(with the aid of a catalyst and with
heat) Example fractions containing compounds of
higher molar mass are cracked to produce the
more useful short-chain components of gasoline
and kerosene.
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Coal Formation - Peat
The first stage in coal formation is an
intermediate material known as peat. Peat is a
soft, brown, spongy, fibrous material with a very
high water content. When first dug out peat has a
very high water content. After it is allowed
to dry, it produces a low cost but smoky fuel.
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Coal Formation - Lignite
If peat is left in the ground, it continues to
change. After a long period of time, peat loses
most of its fibrous texture and becomes lignite,
or brown coal. Lignite is much harder than peat
and has a higher carbon content (about 50). The
water content is still high.
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Coal Formation - Bitumious
Continued pressure and heat slowly change lignite
into bitumious, or soft coal. Bitumious coal
has a lower water content and higher carbon
content (70 80) than lignite.
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Coal Formation - Anthracite
In some regions of Earths crust, even greater
pressures have been exerted and in those places
(eastern PA), soft coal has been change into
anthracite, or hard coal. Anthracite has a
carbon content that exceeds 80, making it an
excellent fuel source. Coal is classified by its
hardness and carbon content.
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Coal Mining
Coal, which is usually found in seams from 1 3
meters thick, is obtained from both underground
and surface mines. In North America, coal mines
are usually less than 100 meters
underground. Much of the coal is so close to the
surface that it is strip-mined. Many coal mines
in Europe and other parts of the world extend
1000 to 1500 meter below Earths surface.
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Composition of Coal
Coal consists largely of condensed aromatic
compounds of extremely high molar mass. These
compounds have a high proportion of carbon
compared with hydrogen. Due to the high
proportion of aromatic compounds, coal leaves
more soot upon burning than do the more aliphatic
fuels obtained from petroleum. The majority of
the coal that was once burned in North America
contained about 7 sulfur, which burns to form
the major air pollutants SO2 and SO3
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Composition of Coal
Coal may be distilled to obtain a variety of
products coke, coal tar, coal gas and ammonia.
Coke is the solid material left after coal
distillation. It is used as a fuel in many
industrial processes and is the crucial reducing
agent in the smelting of iron ore. Because coke
is almost pure carbon, it produces intense heat
And little or no smoke when it burns.
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Composition of Coal
Coal gas consists mainly of hydrogen, methane,
and carbon monoxide, all of which are
flammable. Coal tar can be distilled further into
benzene, toluene, napthalene, phenol and pitch.
The ammonia from distilled coal is converted to
ammonium sulfate for use as a fertilizer.
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Quick Summary
Petroleum and natural gas are derived from marine
organisms. Buried under ocean sediments. These
compounds have a high proportion of carbon
compared with hydrogen. Due to the high
proportion of aromatic compounds, coal leaves
more soot upon burning than do the more aliphatic
fuels obtained from petroleum. The majority of
the coal that was once burned in North America
contained about 7 sulfur, which burns to form
the major air pollutants SO2 and SO3
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End of Chapter 22