Chapter 2
ALKANES
Classification of Hydrocarbons
• Hydrocarbons contain only carbon and hydrogen.
• A hydrocarbon that contains only single bonds is a
saturated hydrocarbon or alkane (Chapter 2).
• Representing Organic Compounds
Molecular formula shows kind and number of atoms, not how they are
arranged
C4H10
Structural formulas show all atoms with bonds.
Condensed formulas list all the atoms in order implying how they are
bound together:
CH3CH2CH2CH3 or CH3 (CH2) 2CH3
Ball and stick model
Line –angle formula
Each line represents a bond and
each vertex and terminus
represents a carbon atom
Root Name
Methethpropbutpenthexheptoctnondec-
# of Carbons
1
2
3
4
5
6
7
8
9
10
CH3CH2CH2CH3 = BUTANE
• Alkanes with 4 or more carbons can be straight chained
(normal) or branched.
C—C—C—C—C
normal alkane
C
|
C—C—C
|
C
branched alkane
•Constitutional Isomers: Compounds that have identical
molecular formulas, but different arrangement of atoms
(structural formulas).
Molecular Formula C4H10
Constitutional Isomerism
• Problem: draw structural formulas
for the five constitutional isomers of
molecular formula C6H14
2
1
6
4
3
5
S i x c arb on s i n an
u n b ran ch e d ch ai n
4
1
2
3
2
5
1
4
3
4
2
5
Fi ve c arb on s i n a ch ai n ;
o n e carb o n as a b ran ch
1
3
4
2
1
3
Fo u r carb o n s i n a ch ai n ;
tw o carb o n s as b ran ch e s
• Naming Alkanes
– The IUPAC method consists of:
• Alkyl group: a substituent group derived from an
alkane by removal of a hydrogen atom.
• commonly represented by the symbol R-.
• named by dropping the -ane from the name of the
parent alkane and adding the suffix -yl.
N am e
m e th y l
C on d e n s e d
S tru c tu ra l F o rm u l a
- CH 3
e th y l
- CH 2 C H 3
p ro p y l
- CH 2 C H 2 C H 3
i s o p ro p y l
N am e
i s o b u ty l
- CH
s e c- b u ty l
- CH
CH
- CH CH 3
CH
CH 3
t er t - b u ty l
b u ty l
- CH 2 C H 2 C H 2 C H 3
C on
S tru
- CC
CH
Naming Alkanes
• The ending –ane signifies the alkane family.
Step 1: Identify and name the longest carbon chain.
This gives the root and ending.
Step 2: Number the longest carbon chain to give the
lowest number to any carbon to which a group is
attached(substituent).
CH3
CH3
|
|
Example:
CH2 — CH2 — CH — CH3
5 CH
CH3
|
|
Example:
CH2 — CH2 — CH — CH3 (pentane)
3
4
3
2
1
Step 3: Locate and name the attached alkyl group.
Naming Alkanes, cont.
Step 4: Combine the longest chain and the branches
into the name. Use a hyphen to connect the number to
the name.
5 CH
CH3
|
|
CH2 — CH2 — CH — CH3
3
Example:
4
3
1
2-methylpentane
CH 3
CHCH 3
2
4
5
3
2
1
• Draw structural,
condensed and
line-angle formula of
3-ethylhexane
IUPAC Names
5. If the same substituent occurs more than once:
• Number the parent chain from the end that gives the
lower number to the substituent encountered first.
• Indicate the number of times the substituent occurs by
a prefix di-, tri-, tetra-.
• Use a comma to separate position numbers.
CH 3
CH 3
CH 3 CH 2 CHCH 2 CH CH 3
6
4
5
2
3
2,4-D i m e th y l h e xan e
(n ot 3,5-d im e th y l h e xan e )
1
IUPAC Names
6. If there are two or more different substituents:
• list them in alphabetical order.
• number the chain from the end that gives the lower
number to the substituent encountered first.
• If there are different substituents in equivalent
positions on opposite ends of the parent chain, give
the substituent of lower alphabetical order the lower
number.
CH 3
CH 3 CH 2 CHCH 2 CHCH 2 CH 3
2
1
4
3
CH 2 CH 3
3-E th y l -5-m e th y l h e p tan e
(n o t 3-m e th y l -5-e th y l h e p tan e )
6
5
7
IUPAC Names
7. Do not include the prefixes di-, tri-, tetra- in
alphabetizing;
• alphabetize the names of substituents first, and then
insert these prefixes
CH 3 CH 2 CH 3
CH 3 CCH 2 CHCH 2 CH 3
1
6
4
2
3
5
CH 3
4-Eth yl -2,2-d im e th y l h e xan e
(n ot 2,2-d im e th y l -4-e th y l h e xan e )
Cycloalkanes
• Cyclic hydrocarbon: a hydrocarbon that contains
carbon atoms joined to form a ring.
• Cycloalkane: a cyclic hydrocarbon in which all
carbons of the ring are saturated.
• Cycloalkanes of ring sizes ranging from 3 to over 30
carbon atoms are found in nature.
• Five-membered (cyclopentane) and six-membered
(cyclohexane) rings are especially abundant in nature.
Cy cl o p e n tan e
C yc l oh e xan e
Cycloalkanes
• Nomenclature
• To name a cycloalkane, prefix the name of the
corresponding open-chain alkane with cyclo-, and
name each substituent on the ring.
• If there is only one substituent on the ring, there is no
need to give it a location number.
• If there are two or more substituents, number the ring
beginning with the substituent of lower alphabetical
order.
Example:
CH2CH3
1
CH3
3
2
1-ethyl-3-methylcyclopentane
Physical Properties
• The most important physical property of
alkanes and cycloalkanes is their almost
complete lack of polarity.
– The electronegativity difference between carbon
and hydrogen is 2.5 - 2.1 = 0.4 on the Pauling
scale.
– Given this small difference, we classify a C-H bond
as nonpolar covalent.
– Alkanes are nonpolar compounds and have only
very weak interaction between molecules.
Methane
molecules
Oil
molecules
Physical Properties
• Melting and boiling points
– Boiling points of alkanes are lower than those of
almost any other type of compound of the same
molecular weight.
– In general, both boiling and melting points of
alkanes increase with increasing molecular weight.
Physical Properties
• Solubility: a case of “like dissolves like”.
– Alkanes are not soluble in water; they are unable
to form hydrogen bonds with water.
– Alkanes are soluble in each other.
– Alkanes are also soluble in other nonpolar organic
compounds.
Alkane Reactions
• Alkanes are the least reactive of all organic compounds.
• The most significant reaction of alkanes is combustion.
• Combustion- A chemical reaction that produces CO2, H2O and a
flame due to burning with oxygen.
• Many alkanes are used this way – as fuels.
Methane – natural gas
Propane – used in gas grills
Butane – lighters
Gasoline – a mixture of hydrocarbons
Complete Combustion (in the presence of adequate Oxygen)
CH 4
+ 2 O2
CO 2 + 2 H 2 O + 212 k c al /m ol
M e th an e
CH 3 CH 2 CH 3 + 5 O 2
Prop an e
3 CO 2 + 4 H 2 O + 530 k cal / m o l
Incomplete Combustion (where there is not enough Oxygen
available)
Reactions
• Reaction with halogens (halogenation)
• Halogenation of an alkane is a substitution reaction.
heat
or light
C H 4 + C l2
Methane
CH 3 Cl
+ Cl2
CH3 Cl +
HCl
Chloromethane
(Methyl chloride)
h e at
CH 2 Cl2
+
HCl
D i c h l o ro m e th an e
(M e th y l e n e ch l o ri d e )
Cl2
CH 2 Cl2
Cl2
CHCl3
CCl4
h e at
h e at
T ri ch l o rom e th an e
T e trach l o rom e th an e
(C h l o ro f orm )
(Carb o n te trach l o ri d e )
The Chlorofluorocarbons
• Chlorofluorocarbons (CFCs)
• manufactured under the trade name Freon
• CFCs are nontoxic, nonflammable, odorless, and
noncorrosive.
• Among the CFCs most widely used were CCl3F (Freon11) and CCl2F2 (Freon-12).
• CFCs were used as;
• heat-transfer agents in refrigeration systems.
• industrial cleaning solvents to prepare surfaces for
coatings and to remove cutting oils from millings.
• propellants for aerosol sprays.