Fundamentals of General, Organic,
and Biological Chemistry
5th Edition
Chapter Twelve
Introduction to Organic
Chemistry: Alkanes
James E. Mayhugh
Oklahoma City University
2007 Prentice Hall, Inc.
Outline
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12.1 The Nature of Organic Molecules
12.2 Families of Organic Molecules: Functional Groups
12.3 The Structure of Organic Molecules: Alkanes and Their Isomers
12.4 Drawing Organic Structures
12.5 The Shapes of Organic Molecules
12.6 Naming Alkanes
12.7 Properties of Alkanes
12.8 Reactions of Alkanes
12.9 Cycloalkanes
12.10 Drawing and Naming Cycloalkanes
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12.1 The Nature of Organic
Molecules
► Organic chemistry: The study of carbon compounds.
► Carbon is tetravalent; it always forms four bonds. In
methane, carbon is connected to four hydrogen atoms.
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Organic molecules have covalent bonds. In ethane,
the bonds result from the sharing of two electrons,
either between C and C atoms or between C and H
atoms.
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► Organic molecules contain polar covalent bonds when
carbon bonds to an electronegative element on the
right side of the periodic table.
► In chloromethane, the Cl atom attracts electrons more
strongly than C, resulting in a polar bond. The C has a
partial (+) charge, and Cl has a partial (-) charge.
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► Carbon forms multiple covalent bonds by sharing
more than two electrons with a neighboring atom.
► In ethylene, the two carbon atoms share four
electrons in a double bond; in acetylene the two
carbons share six electrons in a triple bond.
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► When carbon is bonded to four atoms, the bonds are
oriented toward the four corners of a regular
tetrahedron with carbon in the center.
► This is shown using normal lines for bonds in the
plane of the page, dashed lines for bonds receding
behind the page, and wedged lines for bonds coming
out of the page:
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12.2 Families of Organic Molecules:
Functional Groups
► The structural features that allow us to class
compounds together are called functional groups. A
functional group is a group of atoms within a larger
molecule that has a characteristic structure and
chemical behavior.
► The chemistry of an organic molecule is primarily
determined not by its size and complexity, but by the
functional groups it contains. A given functional
group tends to undergo the same reactions in every
molecule of which it is a part.
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► The first four families in Table 12.1 are hydrocarbons,
organic compounds that contain only C and H.
► Alkanes have only single bonds. Alkenes contain a C-C
double-bond; alkynes contain a C-C triple-bond; and
aromatic compounds contain a six-membered ring of
carbon atoms with three alternating double bonds.
► The next four families contain only single bonds and
have a carbon atom bonded to an electronegative atom.
► Alkyl halides have a carbon–halogen bond; alcohols
have a C-O bond; ethers have two C’s bonded to the
same O; and amines have a C-N bond.
► Aldehydes, ketones, carboxylic acids, anhydrides,
esters, and amides all contain a C=O double bond.
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12.3 The Structure of Organic
Molecules: Alkanes and Their Isomers
► Alkane: A hydrocarbon that has only single bonds.
► The rule for hydrocarbons is that each carbon must
be bonded to at least one other carbon forming the
“backbone” of the compound. The hydrogens are on
the periphery. There is only one possible
combination of three carbons with eight hydrogens.
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► Straight-chain alkane: An alkane that has all its
carbons connected in a row.
► Branched-chain alkane: An alkane that has a
branching connection of carbons.
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Constitutional isomers: Compounds with the same
molecular formula but different connections among
their atoms.
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►The number of possible alkane isomers grows rapidly
as the number of C atoms increases.
►Different constitutional isomers are completely
different chemical compounds with different
structures, different physical properties, and potentially
different physiological properties.
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12.4 Drawing Organic Structures
Condensed structure: A shorthand way of drawing
structures in which C-C and C-H bonds are
understood rather than shown.
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12.5 The Shapes of Organic Molecules
The two parts of a molecule joined by a C-C single
bond are free to spin around the bond, giving rise to
an infinite number of possible three-dimensional
geometries, or conformations.
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► As long as any two structures show identical
connections between atoms, they represent identical
compounds no matter how the structures are drawn.
Such molecular structures are called conformers.
► To see that the following two structures represent the
same compound rather than two isomers, picture one
of them flipped right to left so that the red groups are
on the same side.
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12.6 Naming Alkanes
► The system of naming now used is one devised by
the International Union of Pure and Applied
Chemistry, IUPAC.
► In the IUPAC system for organic compounds, a
chemical name has three parts: prefix, parent, and
suffix.
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► Straight-chain alkanes are named by counting the
number of C atoms and adding the family suffix -ane.
► The first four parent names have historical origins, the
rest are named from Greek numbers.
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► Alkyl group: The part of an alkane that remains
when a hydrogen atom is removed.
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► The situation is more complex for larger alkanes.
► There are two different three carbon alkyl groups,
there are four different four carbon alkyl groups.
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► The butyl groups are n-butyl, sec-butyl, isobutyl, and
tert-butyl. The prefix sec- stands for secondary, and
the prefix tert- stands for tertiary, referring to the
number of other carbon atoms attached to the branch
point.
► There are four possible substitution patterns, called
primary, secondary, tertiary, and quaternary.
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► Branched-chain alkanes can be named by following
four steps:
► STEP 1: Name the main chain. Find the longest
continuous chain of carbons, and name the chain
according to the number of carbon atoms it contains.
The longest chain may not be immediately obvious
because it is not always written on one line; you may
have to “turn corners” to find it.
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► STEP 2: Number the carbon atoms in the main
chain. Begin at the end nearer the first branch point:
► STEP 3: Identify the branching substituents, and
number each according to its point of attachment to
the main chain:
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►If there are two substituents on the same carbon, assign
the same number to both. There must always be as many
numbers in the name as there are substituents.
► STEP 4: Write the name as a single word, using
hyphens to separate the numbers from the different
prefixes and commas to separate numbers if necessary.
If two or more different substituent groups are present,
cite them in alphabetical order.
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If two or more identical substituents are present, use
one of the prefixes di-, tri-, tetra-, and so forth, but
do not use these prefixes for alphabetizing purposes.
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12.7 Properties of Alkanes
► Odorless or mild odor; colorless; tasteless; nontoxic
► Nonpolar; insoluble in water but soluble in nonpolar
organic solvents; less dense than water
► Flammable; otherwise not very reactive
► The first four alkanes are gases at room temperature
and pressure, alkanes with 5–15 carbon atoms are
liquids; those with 16 or more carbon atoms are
generally low-melting, waxy solids.
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The boiling and melting points for the straight-chain
alkanes increase with molecular size.
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12.8 Reactions of Alkanes
► Alkanes do not react with acids, bases, or most other
common laboratory reagents. Their only major
reactions are with oxygen (combustion) and with
halogens (halogenation).
► Combustion: A chemical reaction that produces a
flame, usually because of burning with oxygen.
► Carbon dioxide and water are the products of
complete combustion of any hydrocarbon, and a
large amount of heat is released.
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► Halogenation: The replacement of an alkane
hydrogen by a chlorine or bromine in a process
initiated by heat or light.
► This reaction, like many organic reactions, yields a
mixture of products:
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12.9 Cycloalkanes
► Cycloalkanes contain a ring of carbon atoms.
► Cyclopropane and cyclobutane are less stable and
more reactive than other cycloalkanes since their
bond angles are considerably compressed.
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► Both cyclopentane and cyclohexane rings have
nearly ideal bond angles and are therefore stable.
► Cyclic and acyclic alkanes are similar in many of
their properties.
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12.10 Drawing and Naming
Cycloalkanes
► Even condensed structures become awkward when
we work with large molecules that contain rings.
► A more streamlined way of drawing structures is
often used in which cycloalkanes are represented
simply by polygons.
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In line structures, a C is located at every intersection,
and the number of H atoms necessary to give each C
four covalent bonds is understood. Methylcyclohexane,
for example, looks like this in a line structure:
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► Cycloalkanes are named by a straightforward
extension of the rules for naming open-chain
alkanes. In most cases, only two steps are needed:
► STEP 1: Use the cycloalkane name as the parent.
That is, compounds are named as alkyl-substituted
cycloalkanes rather than as cycloalkyl-substituted
alkanes. If there is only one substituent on the ring, it
is not even necessary to assign a number because all
ring positions are identical.
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STEP 2: Identify and number the substituents.
Start numbering at the group that has alphabetical
priority, and proceed around the ring in the direction
that gives the second substituent the lower possible
number.
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Chapter Summary
► Organic compounds contain C atoms and many are
joined in long chains by a combination of single, double
or triple bonds.
► Organic compounds can be classified according to their
functional groups. A functional group is a group of
atoms that has characteristic structure and reactivity.
► Constitutional isomers are compounds with the same
formula but different connections among atoms.
► Structural formulas show all atoms and bonds; not all
bonds are drawn in condensed structures, the carbon
skeleton is represented by lines and the locations of C
and H atoms are understood in line structures.
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Chapter Summary Cont.
► Compounds that contain only C and H are called
hydrocarbons, and hydrocarbons that have only single
bonds are called alkanes.
► A straight-chain alkane has all its C’s connected in a
row, a branched-chain alkane has a branching
connection of atoms somewhere along its chain, and a
cycloalkane has a ring of C atoms.
► Straight-chain alkanes are named by adding -ane to a
parent name that tells how many C atoms are present.
► Branched-chain alkanes are named by using the longest
continuous chain of C atoms for the parent and then
identifying the alkyl groups present as branches off the
main chain.
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Chapter Summary Cont.
► Substituent groups on the main chain are located by
numbering the C’s in the chain so that the substituents
have the lowest number. Cycloalkanes are named by
adding cyclo- as a prefix to the name of the alkane.
► Alkanes are soluble in nonpolar organic solvents, have
weak intermolecular forces, and are nontoxic.
► Their principal chemical reactions are combustion, a
reaction with oxygen that gives carbon dioxide and
water, and halogenation, a reaction in which hydrogen
atoms are replaced by chlorine or bromine.
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End of Chapter 12
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