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Chapter 10
An Introduction to Organic
Chemistry:
The Saturated Hydrocarbons
Denniston
Topping
Caret
5th Edition
10.1 The Chemistry of Carbon
Why are there so many organic compounds?
1. Carbon forms stable, covalent bonds with other
carbon atoms
• Consider three allotropic forms of elemental carbon
– Graphite in planar layers
– Diamond is a three-dimensional network
– Buckminsterfullerene is 60 C in a roughly spherical
shape
Why are there so many
organic compounds?
2. Carbon atoms form stable bonds with
other elements, such as:
–
–
–
–
•
Oxygen
Nitrogen
Sulfur
Halogen
Presence of these other elements
confers many new physical and
chemical properties on an organic
compound
Why are there so many
organic compounds?
3. Carbon atoms form
double or triple bonds
with:
–
–
–
•
Other carbon atoms
(double & triple)
Oxygen (double only)
Nitrogen (double & triple)
H2C
CH2
H2C O
H2C NH
double bonds
These combinations act HC
to produce a variety of
organic molecules with HC
very different properties
CH
N
triple bonds
Why are there so many
organic compounds?
4. Carbon atoms can be arranged with these
other atoms; is nearly limitless
–
–
–
Branched chains
Ring structures
Linear chains
CH3CH2CH2CH3
•
H2
C
H2C
H2C
CH2
CH2
Two organic compounds may even have the
same number and kinds of atoms but
completely different structures and thus,
different properties
– These are called isomers
Isomers
• Many carbon compounds exist in the
form of isomers
• Isomers are compounds with the same
molecular formula but different structures
• An isomer example: both are C4H10
but have different structures
– Butane
– Methylpropane
Isomers
All have the same molecular formula: C4H8
Important Differences Between
Organic and Inorganic Compounds
• Bond type
–Organics have covalent bonds
•
Electron sharing
–Inorganics usually have ionic bonds
•
Electron transfer
• Structure
–Organics
•
•
Molecules
Nonelectrolytes
–Inorganics
•
•
Three-dimensional crystal structures
Often water-soluble, dissociating into ions electrolytes
Important Differences Between
Organic and Inorganic Compounds
• Melting Point & Boiling Point
–Organics have covalent bonds
•
Intermolecular forces broken fairly easily
–Inorganics usually have ionic bonds
•
Ionic bonds require more energy to break
• Water Solubility
–Organics
•
Nonpolar, water insoluble
–Inorganics
•
Water-soluble, readily dissociate
Comparison of Major Properties of
Organic and Inorganic Compounds
Bonding Characteristics
and Isomerism
• One reason for the power of carbon is that it
can form 4 covalent bonds
• It appears to have only 2 available electrons
• Carbon can hybridize its orbitals to move 2
electrons out of it 2s orbital
Hybrid Orbitals
• Each carbon-hydrogen
bond in methane arises
from an overlap of a C(sp3)
and an H(1s) orbital
• 4 equivalent sp3 orbitals
point toward the corners of
a regular tetrahedron
• The 4 sp3 hybrid orbitals of
carbon combine with the
1s orbitals on 4 H to
produce methane – CH4
Families of Organic Compounds
• Hydrocarbons contain only carbon
and hydrogen
• They are nonpolar molecules
– Not soluble in water
– Are soluble in typical nonpolar organic
solvents
• Toluene
• Pentane
Families of Organic Compounds
• Hydrocarbons are constructed of chains
or rings of carbon atoms with sufficient
hydrogen atoms to fulfill carbon’s need
for four bonds
• Substituted hydrocarbon is one in which
one or more hydrogen atoms is replaced
by another atom or group of atoms
Division of the Family
of Hydrocarbons
Hydrocarbon Saturation
• Alkanes are compounds that contain only
carbon-carbon and carbon-hydrogen single
bonds
– A saturated hydrocarbon has no double or
triple bonds
• Alkenes and alkynes are unsaturated
because they contain at least one carbon to
carbon double or triple bond
Cyclic Structure of Hydrocarbons
• Some hydrocarbons are cyclic
– Form a closed ring
– Aromatic hydrocarbons contain a benzene
ring or related structure
Common Functional Groups
10.2 Alkanes
• The general formula for a chain alkane is
CnH2n+2
– In this formula n = the number of carbon atoms
in the molecule
• Alkanes are saturated hydrocarbons
– Contain only carbon and hydrogen
– Bonds are carbon-hydrogen and carbon-carbon
single bonds
Formulas Used in
Organic Chemistry
• Molecular formula - lists kind and number of each
type of atom in a molecule, no bonding pattern
• Structural formula - shows each atom and bond in a
molecule
• Condensed formula - shows all the atoms in a
molecule in sequential order indicating which atoms
are bonded to which
• Line formula - assume a carbon atom at any location
where lines intersect
– Assume a carbon at the end of any line
– Each carbon in the structure is bonded to the correct
number of hydrogen atoms
The Tetrahedral Carbon Atom
(a) Lewis dot structure
(b) The tetrahedral shape around the carbon atom
(c) The tetrahedral carbon drawn with dashes and
wedges
(d) The stick drawing of the tetrahedral carbon
atom
(e) Ball and stick model of methane
Drawing Methane and Ethane
H
H
H
in plane
H
C
C
109.5
H
H
behind plane
H
H
H in front of plane H
o
Staggered form of ethane
Comparison of Ethane and
Butane Structures
Names and Formulas of the First Ten
Straight-Chain Alkanes
Structural Isomers
• Constitutional/Structural Isomers differ in how
atoms are connected
– Two isomers of butane have different physical
properties
– The carbon atoms are connected in different
patterns
CH3
CH3 CH2 CH2 CH3 CH3 CH CH3
Butane
Bp –0.4 oC
Mp –139 oC
Isobutane
Bp –12 oC
Mp –145 oC
Comparison of Physical Properties of
Five Isomers of Hexane
Compare the basic linear structure of hexane
– All other isomers have one or more carbon atoms
branching from the main chain
– Branched-chain forms of the molecule have a much
smaller surface area
• Intermolecular forces are weaker
• Boiling and melting points are lower than straight chains
Physical Properties of
Organic Molecules
1.
2.
3.
4.
5.
6.
7.
Nonpolar
Not water soluble
Soluble in nonpolar organic solvents
Low melting points
Low boiling points
Generally less dense (lighter) than water
As length (molecular weight) increases, melting
and boiling points increase as does the density
Properties of Alkanes
200
150
100
Temperature
50
0
0
1
2
3
4
5
6
7
8
-50
-100
-150
-200
-250
Number of Carbons in Chain
9
10
Melting Point
Boiling Point
Properties of Alkanes
• Most of the alkanes are hydrophobic:
water hating
• Straight chain alkanes comprise a
homologous series: compounds of the same
functional class that differ by a –CH2- group
• Nonpolar alkanes are:
– Insoluble in water (a highly polar solvent)
– Less dense than water and float on it
Alkyl Groups
H
H C H
H
H
H C or CH3
H
• An alkyl group is an alkane with one hydrogen
atom removed
• It is named by replacing the -ane of the alkane
name with -yl
• Methane becomes a methyl group
Alkyl Groups
• All six hydrogens on ethane are equivalent
• Removing one H generates the ethyl group
• All 3 structures shown at right are the same
HH
HCCH
HH
CH3 CH2
CH2 CH3
C2H5
Names and Formulas of the First
Five Alkyl Groups
Alkyl Group Classification
• Alkyl groups are classified according to the
number of carbons attached to the carbon
atom that joins the alkyl group to a molecule
• All continuous chain alkyl groups are 1º
• Isopropyl and sec-butyl are 2º groups
Iso- Alkyl Groups
• Propane: removal of a hydrogen generates
two different propyl groups depending on
whether an end or center H is removed
CH3 CH2 CH3
CH3CH2CH2
n-propyl
CH3CH CH3
isopropyl
Sec- Alkyl Groups
• n-butane gives two butyl groups depending
on whether an end (1º) or interior (2º) H is
removed
CH3 CH2 CH2 CH3
CH3 CH2 CH2 CH2 CH3 CH CH2 CH3
n-butyl
sec-butyl
Structures and Names of Some
Branched-Chain Alkyl Groups
More Alkyl Group Classification
• Isobutane gives two butyl groups
depending on whether a 1o or 3o H is
removed
CH3
CH
CH
CH
3
3
o
o
1 C
CH3
CH3 CH CH2
isobutyl
3 C
CH3
CH3 C CH3
t-butyl
Nomenclature
• The IUPAC (International Union of
Pure and Applied Chemistry) is
responsible for chemical names
• Before learning the IUPAC rules for
naming alkanes, the names and
structures of eight alkyl groups must be
learned
• These alkyl groups are historical names
accepted by the IUPAC and integrated
into modern nomenclature
Carbon Chain Length and Prefixes
IUPAC Names for Alkanes
1. The base or parent name for an alkane is
determined by the longest chain of
carbon atoms in the formula
–
–
–
The longest chain may bend and twist, it is
seldom horizontal
Any carbon groups not part of the base chain
are called branches or substituents
These carbon groups are also called alkyl
groups
IUPAC Names for Alkanes
• Rule 1 applied
– Find the longest chain in each molecule
• A=7
B=8
CH3
A
CH2CH2CH CH2CH3
B CH3
CH2
CH3CH2CH2
CH2
CH3
CH3CH2CH CH2CH CH3
IUPAC Names for Alkanes
2. Number the carbon atoms in the
chain starting from the end with the
first branch
–
If both branches are equally from the
ends, continue until a point of
difference occurs
IUPAC Names for Alkanes
Number the carbon atoms correctly
• Left: first branch is on carbon 3
• Right: first branch is on carbon 3 (From
top) not carbon 4 (if number from right)
1
CH
3
6
4
5
2
CH
2
CH3
7
8
CH2CH2CH3
CH2CH2CH CH2CH3 CH3CH CH2CH CH2CH3
3
6
CH2
7
CH3
2
1
3
4
5
this branch would be on C-4
if you started at correct C-8
IUPAC Names for Alkanes
3. Write each of the branches/substituents in
alphabetical order before the base/stem
name (longest chain)
– Halogens usually come first
– Indicate the position of the branch on the main
chain by prefixing its name with the carbon
number to which it is attached
– Separate numbers and letters with a hyphen
– Separate two or more numbers with commas
IUPAC Names for Alkanes
CH3
CH2
CH3
CH3 CH2 CH CH2 CH CH3
Name : 4-ethyl-2-methylhexane
IUPAC Names for Alkanes
• Hyphenated and number prefixes are
not considered when alphabetizing
groups
– Name the compound below
– 5-sec-butyl-4-isopropylnonane
CH3
CH3
CH CH2 CH3
CH CH CH CH2 CH2 CH2 CH3
CH2 CH2 CH3
CH3
IUPAC Names for Alkanes
• When a branch/substituent occurs more
than once
– Prefix the name with
• di
• tri
• tetra
– Then list the number of the carbon branch for
that substituent to the name with a separate
number for each occurrence
• Separate numbers with commas
• e.g., 3,4-dimethyl or 4,4,6-triethyl
IUPAC Names for Alkanes
Name
CH3
CH2CH3
CH3CH CH CH2CH CH2CH3
CH3
5-ethyl-2,3-dimethylheptane
ethyl>dimethyl
Practice: IUPAC Name
Name
1
CH
CH
CH
CH
3
3
3
2
CH
CH2
2
4
5
6 CH CH
CH3C3 CH
CH
C
3
2
2
2
7
8
9
10
CH3
CH
CH
CH
CH
2
2
2
3
6-ethyl-6-isobutyl-3,3-dimethyldecane
10.3 Cycloalkanes
• Cycloalkanes have two less hydrogens than
the corresponding chain alkane
– Hexane=C6H14; cyclohexane=C6H12
• To name cycloalkanes, prefix cyclo- to the
name of the corresponding alkane
– Place substituents in alphabetical order before
the base name as for alkanes
– For multiple substituents, use the lowest
possible set of numbers; a single substituent
requires no number
Cycloalkane Structures
Cyclopropane
Cyclobutane
Cyclohexane
Type of Formula:
Structural
Condensed
Line
Naming a Substituted Cycloalkane
Name the two cycloalkanes shown below
• Parent chain
• Substituent
• Name
6 carbon ring
cyclohexane
1 chlorine atom
chloro
Chlorocyclohexane
5 carbon ring
cyclopentane
a methyl group
methyl
Methylcyclopentane
cis-trans Isomers in Cycloalkanes
• Atoms of an alkane can rotate freely around the
carbon-carbon single bond having an unlimited
number of arrangements
• Rotation around the bonds in a cyclic structure is
limited by the fact that all carbons in the ring are
interlocked
– Formation of cis-trans isomers, geometric isomers, is a
consequence of the lack of free rotation
• Stereoisomers are molecules that have the same
structural formulas and bonding patterns, but
different arrangements of atoms in space
– cis-trans isomers of cycloalkanes are stereoisomers
whose substituents differ in spatial arrangement
cis-trans Isomers in Cycloalkanes
• Two groups may be on the same side (cis) of the imagined
plane of the cycloring or they may be on the opposite side
(trans)
• Geometric isomers do not readily interconvert, only by
breaking carbon-carbon bonds can they interconvert
10.4 Conformations of Alkanes
• Conformations differ only in rotation about carboncarbon single bonds
• Two conformations of ethane and butane are shown
– The first (staggered form) is more stable because it allows
hydrogens to be farther apart and thus, the atoms are less
crowded
Two Conformations of Cyclohexane
Chair form (more stable)
A
A
E
E
Boat form
A
E
A
H
H
H
A E
E
E
A
H
H
H
E=equitorial
A=axial
H
H
H
H
H
H
10.5 Reactions of Alkanes
• Alkanes, cycloalkanes, and other
hydrocarbons can be:
– Oxidized (by burning) in the presence of excess
molecular oxygen, in a process called
combustion
– Reacted with a halogen (usually chlorine or
bromine) in a halogenation reaction
Alkane Reactions
The majority of the reaction of alkanes are
combustion reactions
– Complete CH4 + 2O2 CO2 + 2H2O
Complete combustion produces
– Carbon dioxide and water
– Incomplete 2CH4 + 3O2
2CO + 4H2O
• Incomplete combustion produces
– Carbon monoxide and water
– Carbon monoxide is a poison that binds
irreversibly to red blood cells
Halogenation
Halogenation is a type of substitution reaction, a
reaction that results in a replacement of one group
for another
– Products of this reaction are:
• Alkyl halide or haloalkane
• Hydrogen halide
– This reaction is important in converting unreactive
alkanes into many starting materials for other products
– Halogenation of alkanes ONLY occurs in the presence
of heat and/or light (UV)
H
H + Br2
heat or
light
Br
H
+HBr
Petroleum Processing
Fraction
Boiling Pt Range ºC
Carbon size Typical uses
Gas
-164-30
C1-C4
Heating, cooking
Gasoline
30-200
C5-C12
Motor fuel
Kerosene
175-275
C12-C16
Fuel for stoves, diesel
and jet engines
Heating oil
Up to 375
C15-C18
Furnace oil
Lubricating 350 and up
oil
C16-C20
Lubrication, mineral
oil
Greases
Semisolid
C18-up
Lubrication,
petroleum jelly
Paraffin
(wax)
Melts at 52-57
C20-up
Candles, toiletries
Pitch / tar
Residue in boiler
High
Roofing, asphalt
paving