Organic Chemistry: Basic Concepts
Millions and Millions of Organic
Compounds
• Carbon is unique among
elements in that it can bond to
other carbon atoms to form
chains containing as many as
several thousand atoms.
Millions and Millions of
Organic Compounds
• Because a carbon atom can bond to as
many as four other atoms at once, these
chains can have branches and form closedring structures that make possible an
almost endless variety of compounds.
Millions and Millions of Organic
Compounds
• In addition, carbon can bond strongly
to elements such as oxygen and
nitrogen, and it can form double and
triple bonds.
Saturated Hydrocarbons
A hydrocarbon in which all the carbon atoms
are connected to each other by single bonds is
called a saturated hydrocarbon. Another
name for a saturated hydrocarbon is an
alkane
Alkanes
• Alkanes are the simplest hydrocarbons. The
carbons in an alkane can be arranged in a
chain or a ring, and both chains and rings can
have branches of other carbon chains
attached to them. Alkanes that have no
branches are called straight-chain alkanes.
Methane, CH4; ethane, C2H6; propane,
• C3H8; and butane, C4H10 are all common
fuels.
Alkanes
• The carbons in an alkane can be arranged in
a chain or a ring,
and both chains
and rings can have
branches of other
carbon chains
attached to them.
Alkanes
• Alkanes that have no branches are called
straight-chain alkanes.
Alkanes
• Some alkanes have a
branched structure.
• In these compounds, a chain
of one or more carbons is
attached to a carbon in the longest
continuous chain, which is called the parent
chain.
Alkanes
• The carbon atoms in alkanes can also link up to
form closed rings. The most common rings
contain five or six carbons.
• The structures of these compounds can be
drawn showing all carbon and hydrogen
atoms.
Alkanes
• Structural diagrams can be
simplified by using straight
lines to represent the bonds
between atoms in the rings.
• In these ring diagrams, each
corner represents a carbon
atom.
Alkanes
• Structural diagrams of straight- and branched-chain
hydrocarbons also can be written in a simplified way
by leaving out some of the bonds.
• For example, the formula for
propane can be written as
CH3—CH2—CH3.
Alkanes
• In an even more simplified type of shorthand, the
condensed structural formula for propane can be
written as CH3CH2CH3.
• Here, the bonds both between C and C and
between C and H are understood.
The First Ten Alkanes
Naming Alkanes
• To name a branched alkane, you must be able to
answer three questions about its structure.
1.
How many carbons are in the longest
continuous chain of the molecule?
How many branches are on the longest
chain and what is their size?
3. To which carbons in the longest chain
are the branches attached?
2.
Naming Alkanes
• For convenience, the
carbon atoms in organic
compounds are given
position numbers.
• In straight-chain hydrocarbons, the numbering can
begin at either end. It makes no difference.
• In branched hydrocarbons, the numbering begins at
the end closest to the branch.
.
• Four carbons are in the longest continuous chain,
so butane is the parent chain and will be part of the
compound’s name.
• There is only one branch, and it contains one
carbon.
Naming Alkanes
• Four carbons are in the longest continuous chain,
so butane is the parent chain and will be part of the
compound’s name.
• There is only one branch, and it contains one
carbon.
Isomers
• Compounds that have the same formula but
different structures are called isomers.
• Butane and 2-methylpropane are known as
structural isomers.
• Each has the molecular formula C4H10, but they
have different structural formulas
because the
carbon chains
have different
shapes.
Properties of Alkanes
• Properties are affected by the structure or
arrangement of atoms present in a molecule.
• Another factor that affects properties of alkanes is
chain length.
• In general, the more carbons present in a straightchain alkane, the higher its melting and boiling
points.
• A property shared by all alkanes is their relative
unreactivity.
Properties of Alkanes
• Because alkanes don’t have any polar bonds, they
undergo only a small number of reactions and will
dissolve only those
organic compounds that
are nonpolar or that have
low polarity, such as oils
and waxes.
Four ways of representing molecules
Unsaturated Hydrocarbons
• A hydrocarbon that has one or more double or
triple bonds between carbons is called an
unsaturated hydrocarbon.
• Two carbon atoms can share one, two, or
I three pairs of electrons.
• An saturated hydrocarbons, carbon atoms
share only one pair, whereas in unsaturated
hydrocarbons, the carbon atoms
participating in double or triple bonds share
two or three pairs.
Unsaturated Hydrocarbons
Alkenes
• A hydrocarbon in which one or more double
bonds link carbon atoms together is called an
alkene.
• Alkenes are named using the root names of the alkanes,
with the -ane ending changed to -ene.
• The simplest alkene is ethene,
two carbons linked in a chain.
, which contains
Alkenes
• What is the name of the compound that has the
following structure?
• This compound has four carbons in a chain with
one double bond, so butene will be part of its
name.
Alkenes
• Numbering the carbons starting on the left
side of the compound gives the first carbon
that is part of the double bond the position
number of one.
• Thus, this compound is 1-butene.
Alkenes
• The formation of a double bond prevents the carbons on
each side of the bond from rotating with respect to each
other.
• If the two groups attached to either carbon are different,
the alkene can have two different geometric structures.
• These structures are geometric isomers.
Alkenes
• In the isomer called
cis-2-butene, the
hydrogen atoms and—
CH3 groups are on the
same side of the
double bond.
Organic Chemistry: Basic Concepts
Topic
24
Alkenes
• In trans-2-butene, the
hydrogen atoms and—CH3
groups are on opposite
sides of the double bond.
Alkenes
• Alkenes are more reactive than alkanes because
the two extra electrons in the double bond are not
held as tightly to the carbons as are the electrons
in a single bond.
• Alkenes readily undergo synthesis reactions in
which smaller molecules or ions bond to the atoms
on either side of the double bond.
Alkenes
• An unsaturated alkene can be converted into a
saturated alkane by adding hydrogen to the double
bond.
• This reaction is called
hydrogenation.
Alkynes
• Another type of unsaturated hydrocarbon, called
an alkyne, contains a triple bond between two
carbon atoms.
• Alkynes are named using the alkane root name for
a given carbon chain length and changing the -ane
ending to -yne.
• Ethyne, known more commonly as acetylene, is the
most important commercial alkyne.
• Melting and boiling points of alkynes increase
with increasing chain length, just as they did
for alkanes and alkenes. Alkynes have physical
and chemical properties similar to those of
alkenes; their melting points are higher than
those of alkanes, and they undergo synthesis
reactions.
Aromatic Hydrocarbons
• Another group of unsaturated hydrocarbons has distinctive,
six-carbon ring structures.
• The simplest compound in this group is benzene, with the
molecular formula C6H6.
• Benzene contains six carbons joined together in a flat ring.
Aromatic Hydrocarbons
• The name aromatic hydrocarbon is used to
describe a compound that has a benzene ring or
the type of bonding exhibited by benzene.
• Aromatic hydrocarbons were originally named
because most of them have distinctive aromas.
• Although it contains double bonds, benzene does
not share most of the properties of alkenes. It is
unusually resistant to hydrogenation,
• Whereas most alkenes readily become
hydrogenated. To account for this inertness,
chemists have suggested that the extra electrons
are shared equally by all six carbons in the ring
rather than being located between specific
carbon atoms.
Benzene
• In this hexagon, each corner represents a carbon
atom.
• The circle in the middle of
the structure represents the
cloud of six electrons that are
shared equally by the six
carbon atoms in the
molecule.
Sources of Organic Compound
• Most hydrocarbons come from fossil fuels,
especially petroleum, but also natural gas and
coal, Other important sources include wood
and the fermentation products of plant
materials.
• Natural gas contains large quantities of
methane, along with smaller amounts of
alkanes up to about five carbons in length.
• One property is boiling point. Distillation is a
technique used to separate substances that have
different boiling points. In the petroleum
industry, huge towers are used to distill
petroleum into its component liquids. Inside the
tower, many plates provide multiple surfaces on
which repeated vaporization-condensation cycles
take place. Repeated cycles provide for more
efficient separations and allow fractions
containing only one or a few different compounds
to be isolated. This method of separation is called
fractional distillation.
cracking
The hydrocarbons in the fractions coming off the
lower parts of the tower are mostly large alkanes.
These can be converted into smaller, more useful
alkanes and alkenes by a process called cracking.
Cracking uses catalysts or high temperatures in
the absence of air to break down or rearrange
large hydrocarbons. Cracking is also used to
increase the yield of natural gas by producing
small alkenes from larger molecules. The cracking
of propane produces some methane and ethene,
in addition to propene and hydrogen.
• Another process that uses heat, pressure, and
catalysts to convert large alkanes into other
compounds is called reforming. It is used to
form aromatic hydrocarbons.
Coal
Coal is formed from the remains of plants that
became buried underwater and were subjected
• to increasing pressures as layers of mud built up.
Coal is composed primarily of carbon but also
contains many mineral impurities. It is used
• mostly as a fuel and as a source of aromatic
hydrocarbons. Coal must be obtained from
underground or surface mines,