Organic Chemistry
Classification of Hydrocarbons
24.1
Hydrocarbons
•
•
Hydrocarbons are organic compounds that
contain only hydrogen and carbon
Types of hydrocarbons include




Alkanes
Alkenes
Alkynes
Aromatic
3
Alkanes
•
•
Alkanes have only carbon to carbon single
bonds
Every carbon has four chemical bonds either
to hydrogen or another carbon atom
4
Alkane Structures
•
•
Alkanes have only carbon to carbon single bonds.
Each time a carbon atom is added to the chain there
are also two hydrogen atoms added.
5
Alkane Structures
• With carbon chains that are four carbon atoms or
longer there are multiple ways to arrange the carbon
chains.
6
Alkanes
Alkanes have the general formula CnH2n+2 where n = 1,2,3,…
• only single covalent bonds
• saturated hydrocarbons because they contain the
maximum number of hydrogen atoms that can bond with the
number of carbon atoms in the molecule
• a homologous series (Compounds that have the same general
formula but differing lengths of carbon chains form a homologous series)
CH4
C2H6
C3H8
methane
ethane
propane
24.2
•
•
•
•
Naming Organic
Compounds
Originally compounds were named based on
their source or use
Many organic compounds were given common
names which are still in use
However many ambiguities resulted
With the large number of organic compounds,
a method for systematically naming them is
very important
8
IUPAC Names
•
•
•
The International Union of Pure and Applied
Chemists (IUPAC) developed a system for
naming organic compounds.
This system eliminated many of the
ambiguities that plagued earlier naming
systems
Common names for many substances are still
widely used
9
Naming Hydrocarbons
using the IUPAC System
A series of prefixes are used to designate the
number of carbon atoms in a carbon chain
meth
1C
hex
6C
eth
2C
hept
7C
prop
3C
oct
8C
but
4C
non
9C
pent
5C
dec
10 C
10
Naming Alkanes
•
•
For straight chain hydrocarbons. The prefix indicates
the number of carbon atoms.
The suffix ane is added to designate that the
compound is an alkane
11
Alkane Nomenclature
24.2
Naming Alkanes with
branched chains
•
•
For branched chain hydrocarbons, identify the longest
consecutive (straight) chain first. Then name the side
chains or branches.
The name of the branches end in “yl” and go before
the name of the straight chain
-methylpropane
dimethylpropane
methylbutane
13
Alkane Nomenclature
1. The parent name of the hydrocarbon is that given to the
longest continuous chain of carbon atoms in the molecule.
CH3
CH3
1
CH2
2
CH2
3
CH
4
CH2
5
2. An alkane less one hydrogen
atom is an alkyl group.
24.2
CH4
methane
CH3
methyl
CH2
6
CH3 4-methylheptane
7
Alkane Nomenclature
3. When one or more hydrogen atoms are replaced by other
groups, the name of the compound must indicate the
locations of carbon atoms where replacements are made.
Number in the direction that gives the smaller numbers for
the locations of the branches.
CH3
CH3
1
CH
2
CH2
3
CH2
4
CH3
5
CH3
2-methylpentane
CH3
1
CH2
2
CH2
3
CH
4
CH3
5
4-methylpentane
24.2
Alkane Nomenclature
4. Use prefixes di-, tri-, tetra-, when there is more than one
alkyl branch of the same kind.
CH3
1
CH3
CH3
CH
CH
2
3
CH2
CH2
4
CH3
5
6
2,3-dimethylhexane
CH3
CH3
1
CH
2
C
3
CH2
4
CH2
5
CH3
6
CH3
3,3-dimethylhexane
24.2
Alkane Nomenclature
5. Use previous rules for other types of substituents. Branches
are placed alphabetically in the compound name.
CH3
1
Br
NO2
CH
CH
2
3
CH3
4
2-bromo-3-nitrobutane
Br
CH2
1
NO2
CH2
2
CH
3
CH3
4
1-bromo-3-nitrobutane
24.2
Structural isomers are molecules that have the same
molecular formula but different structures
24.2
How many structural isomers does pentane, C5H12, have?
H
H
H
H
H
H
C
C
C
C
C
H
H
H
H
H
H
H
CH3 H
C
C
H
CH3 H
n-pentane
H
H
H
CH3 H
H
C
C
C
C
H
H
H
H
2-methylbutane
H
C
H
2,2-dimethylpropane
24.2
Alkenes
•
•
•
Alkenes have one (or more) carbon to carbon
double bonds
Since there are fewer hydrogen atoms in
alkenes as a result of the double bond,
alkenes are referred to as unsaturated.
Alkanes on the other hand have the maximum
number of hydrogen atoms. They are referred
to as saturated.
20
Alkenes
•
•
•
Alkenes have one (or more) carbon to carbon
double bonds
When there are 4 or more carbon atoms in a chain,
the location of the double bond is indicated by a
number.
Numbering the location of the double bond(s) takes
precedence over the location of side chains
1 butene
2-butene
methylpropene
21
Alkene Structures
•
Like alkanes, alkenes can have branched or
consecutive chains. In the larger alkenes there are
also multiple locations for the C=C. Hence multiple
structural isomers are possible.
Straight chain. The
double bond is
between the first and
second carbon
Straight chain. The
double bond is
between the second
and third carbon
Branched
chain
22
Alkynes
•
•
Alkynes have one (or more) carbon to
carbon triple bonds
Since there are fewer hydrogen atoms in
alkynes as a result of the triple bond,
alkynes like alkenes are referred to as
unsaturated.
23
Alkynes
•
•
When there are 4 or more carbon atoms in a chain,
the location of the triple bond is indicated by a
number.
Numbering the location of the triple bond(s) takes
precedence over the location of side chains
24
Alkyne Structures
•
Like alkanes and alkenes, alkynes can have branched
or consecutive chains. In the larger alkenes there are
also multiple locations for the C=C. Multiple structural
isomers are possible. The branch cannot originate on
one of the carbons making up the triple bond
Straight chain. The
triple bond is
between the first and
second carbon
Straight chain. The
triple bond is
between the second
and third carbon
Branched chain. The triple
bond can occur in one of the
branches but branches cannot
be attached to any carbon in
25
the triple bond
Ring Structures
• Hydrocarbons that exist in chains are
known as aliphatic hydrocarbons
• The ends of a chain may be joined to
form a ring structure.
• These compounds are known as
cyclic structures
26
Saturated Cyclic
Hydrocarbons
• A number of the smaller alkanes
exist as cyclic structures including
– Cyclopropane
– Cyclobutane
– Cyclopentane
– Cyclohexane
27
Saturated Cyclic
Hydrocarbons
Cyclic alkanes
have the general
formula
CnH2n. The
additional C-C
bond results in
the loss of two
hydrogen atoms
28
Unsaturated Cyclic
Hydrocarbons –
• A few cyclic
hydrocarbons may
contain C=C double
bonds. Two of the
most common are
cyclopentene and
cyclohexene shown
in the diagram at the
left
29
Cyclic Hydrocarbons –
Condensed structures
•
•
Writing structural
formulas for cyclic
hydrocarbons can be
tedious. These short
form structures are
commonly used.
Each vertex represents
a carbon atom and it is
implied that there are
enough H atoms on
each vertex to make
four bonds
30
Aromatic Structures
• The benzene ring is a
common structure in organic
molecules
• It consists of 6 carbon atoms
and 6 hydrogen atoms.
• One would predict that there
should also be 3 C=C bonds
in a benzene ring
31
Aromatic Structures
• Further investigation reveals
that the double bonds are not
distinct in benzene. Rather it
is a resonance hybrid.
• Either of these structures
could be used to represent
benzene.
32
Aromatic Structures
• Research shows that
there are no differences
in the C to C bonds in
benzene.
• The current view of
benzene holds that there
are 6 C-C single bonds
and 3 pairs or 6
delocalized electrons
33
Aromatic Structures
• The structure of benzene
is shown as either of these
two structures, or as a
circle in a hexagon which
depicts that the electrons
are delocalized
34
Fused Aromatic Structures
• Aromatic hydrocarbons
are not limited to a single
ring
• The fused ring structure
shown is Naphthalene
35
Naming Compounds With
Functional Groups
•Various functional groups have unique suffixes that designate
the functional group.
•The functional group takes precedence in numbering the carbon
chain.
•Branches to the carbon chain are named in the usual manner.
alcohols
“ol”
Amides
“amide”
Aldehydes
“al”
Amines
“amine” or amino as a prefix
Ketones
“one”
Ethers
Ethoxy as prefix
Acids
“oic”
halohydrocarbons
Fluoro, bromo, chloro or
iodo
Esters
“oate”
36
Functional Group Chemistry
Alcohols contain the hydroxyl functional group and have the
general formula R-OH.
24.4
Alcohol Classification
Alcohols may be classified as
- Primary
- Secondary
- Tertiary
Depending on whether the carbon atom
that is attached to the –OH group is
surrounded by one, two or three other
carbon atoms
Primary Alcohols
A primary alcohol has only one carbon atom attached
Secondary Alcohols
A secondary alcohol has 2 carbon chains attached to the group on which
the –OH resides
Tertiary Alcohols
Functional Group Chemistry
O
Aldehydes and ketones contain the carbonyl ( C
functional group.
O
)
• aldehydes have the general formula R C H
O
• ketones have the general formula R C R’
O
O
H C H
H C CH3
formaldehyde
or methanal
acetaldehyde
or ethanal
O
H3C C CH3
acetone
or propanone
24.4
Functional Group Chemistry
Carboxylic acids contain the carboxyl ( -COOH ) functional
group.
methanoic acid
ethanoic acid
butanoic acid
benzoic acid
24.4
Functional Group Chemistry
Esters have the general formula R’COOR, where R is a
hydrocarbon group.
O
CH3COOH + HOCH2CH3
CH3 C O CH2CH3 + H2O
ethyl acetate
24.4
Alkane Reactions
Combustion
CH4 (g) + 2O2 (g)
CO2 (g) + 2H2O (l) DH0 = -890.4 kJ
Halogenation - Also known as free radical substitution.
•Many organic molecules undergo substitution reactions.
•In a substitution reaction one atom or group of atoms is
removed from a molecule and replaced with a different atom
or group.
Example:
Cl2 + CH4  CH3Cl + HCl
24.2
Three Basic Steps in a
Free Radical Mechanism
• Chain initiation
The chain is initiated (started) by UV light breaking a
chlorine molecule into free radicals.
Cl2  2Cl.
• Chain propagation reactions
These are the reactions which keep the chain going.
CH4 + Cl.  CH3. + HCl
CH3. + Cl2  CH3Cl + Cl .
• Chain termination reactions
These are reactions which remove free radicals from
the system without replacing them by new ones.
2 Cl.  Cl2
CH3. + Cl.  CH3Cl
CH3. + CH3.  CH3CH3
46
Free Radical Mechanism-The
Initiation Step
• The ultraviolet light is a source of energy that causes
the chlorine molecule to break apart into 2 chlorine
atoms, each of which has an unpaired electron
• The energies in UV are exactly right to break the
bonds in chlorine molecules to produce chlorine
atoms.
47
Homolytic Fission
• Free radicals are formed if a bond splits
evenly - each atom getting one of the
two electrons. The name given to this is
homolytic fission.
48
Free Radical Propagation
• The productive collision happens if a chlorine radical hits a
methane molecule.
• The chlorine radical removes a hydrogen atom from the
methane. That hydrogen atom only needs to bring one electron
with it to form a new bond to the chlorine, and so one electron is
left behind on the carbon atom. A new free radical is formed this time a methyl radical, CH3 .
49
Free Radical Propagation II
•
•
•
•
If a methyl radical collides with a chlorine molecule
the following occurs:
CH . + Cl  CH Cl + Cl.
3
2
3
The methyl radical takes one of the chlorine atoms
to form chloromethane
In the process generates another chlorine free
radical.
This new chlorine radical can now go through the
whole sequence again, It will produce yet another
chlorine radical - and so on and so on.
50
Termination Steps
• The free radical propagation does not go
on for ever.
• If two free radicals collide the reaction is
terminated.
2Cl.  Cl2
CH3. + Cl .  CH3Cl
CH3 . + CH3.  CH3CH3
51
Exercise
• Write the steps in the free radical mechanism for the
reaction of chlorine with methyl benzene. The overall
reaction is shown below. The methyl group is the
part of methyl benzene that undergoes attack.
52
Solution
• Initiation
Cl2  2Cl.
• Propagation

Termination
2Cl.  Cl2
53
Alkene Reactions
Cracking
C2H6 (g)
Pt
catalyst
CH2
CH2 (g) + H2 (g)
Addition Reactions
CH2
CH2 (g) + HBr (g)
CH3
CH2
CH2 (g) + Br2 (g)
CH2Br
CH2Br (g)
CH2Br (g)
24.2