Alkanes
• Alkanes are fully saturated hydrocarbons
- hydrocarbons have only C’s and H’s
- saturated = all single bonds (max. # of H’s)
• Alkanes have the general formula CnH2n+2
- example: C4H10
• They can be straight-chained or branched
- example: CH3CH2CH2CH3 vs. (CH3)2CHCH3
• Alkanes, and all other organic compounds, are
named according the the IUPAC system
(International Union of Pure and Applied
Chemistry)
Naming Straight-Chain Alkanes and Alkyl Groups
• The names of all alkanes end in -ane
• The number of carbons in a straight-chain alkane is
indicated by putting a prefix before the -ane
• Examples:
CH4 = methane
CH3-CH3
= ethane
CH3-CH2-CH3
= propane
• In branched alkanes the substituents (groups attached
to the carbon chain) are called alkyl groups
• An alkyl group = an alkane with one H removed
• Alkyl groups are named by changing the ending of the
alkane name to -yl
• Example: CH3-CH2- = ethyl
Classification of Carbons in Alkanes
• Carbons can be classified by how many other C’s
are attached to them:
- No C’s = methyl
CH4
-1C
= primary (1)
CH3-CH3
- 2 C’s
= secondary (2) CH3-CH2-CH3
- 3 C’s
= tertiary (3)
CH3-CH(CH3)2
- 4 C’s
= quaternary (4) C(CH3)4
Naming Branched Alkanes
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First find the longest chain of C’s (parent)
Number the C’s in parent chain (begin at end
nearest 1rst branch point)
Identify substituents and number by point of
attachment to parent chain
Write full name
Example:
CH3-CH(CH3)-CH2-CH3 = 2-methylbutane
If there is more than one of a substituent a prefix is
used and a number is given for each substituent
Example:
CH3-CH2-C(CH3)3
= 2,2-dimethylbutane
Conformations of Alkanes
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Because alkanes have only single bonds they have
free rotation about those bonds
This allows for different spatial arrangements of
the atoms called conformations
Conformations are not the same as isomers
Some conformations are more stable than others,
so the compound spends more time in those
For ethane(CH3-CH3):
- when all H’s are lined up as you sight down the
C-C bond, called eclipsed, this is highest energy
- when none of the H’s are lined up, called
staggered, this is lowest energy
Haloalkanes
•
Haloalkanes have one or more halogens replacing
hydrogen on an alkane
• The halogens are numbered and named as
substituents
-F
=
fluoro
- Cl
=
chloro
- Br
=
bromo
-I
=
iodo
• If more than one halogen is present, they are
named in alphabetical order
• Example:
CH3-CH(Br)-CH(Cl)-CH3 = 2-bromo-3-chlorobutane
Cycloalkanes
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Carbons can also bond together to form rings
Rings with only C’s, H’s and single bonds are
called cycloalkanes
Cycloalkanes have the general formula CnH2n
The smallest is cyclopropane (C3H6)
- cyclopropane is an unstable molecule
- it’s forced to have bond angles of 120between
the C’s, while they would normally be 109 (the
carbons each have 4 electrons groups and should
be tetrahedral)
The only cycloalkanes with little or no strain are
cyclopentane (C5H10) and cyclohexane (C6H12)
Most cycloalkanes are not flat because they prefer
tetrahedral geometry
Naming Cycloalkanes
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Cycloalkanes are named by adding cyclo- to the
parent alkane name
Substituents are numbered when there is more than
one of them
They are numbered starting at the one that gives
the others the lowest numbers
Or, they are numbered alphabetically when there
are only two, or both directions are equal
Cl
=
2-chloro-1,4-dimethylcyclohexane
Cis and Trans Isomers
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Because cycloalkanes do not have free rotation
around the carbons, it matters on which side of the
ring a substituent is relative to other substituents
Two substituents on the same side (top or bottom)
of the ring are called Cis
Two substituents on opposite sides of the ring are
called Trans
Cis and Trans isomers are stereoisomers; they have
the same molecular formula, and the atoms are
connected in the same order but are arranged in a
different spacial orientation
Physical Properties of Alkanes and Cycloalkanes
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Alkanes are nonpolar and are not soluble in water
They have low melting and boiling points due to
their weak intermolecular forces (dispersion)
They are also less dense than water
Alkanes are mostly obtained from crude oil
- the crude oil is fractionated based on boiling pt.
- heavier fractions are often “cracked”, put under
high heat and pressure, to obtain more gasoline
Alkanes are used extensively as fuels of various
types (for heating, cooking, driving, etc.)
Reactions of Alkanes and Cycloalkanes
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Because they are nonpolar, and their covalent
bonds are strong, alkanes and cycloalkanes are
relatively unreactive
The two types of reactions that they do undergo
are combustion and halogenation
In combustion reactions, alkanes react with
oxygen to form carbon dioxide, water and heat
CH4 + 2O2  CO2 + 2H2O + Heat
In halogenation reactions, halogens replace one or
more H’s on an alkane (a substitution reaction)
CH4 + Cl2 (+ light or heat)  CH3Cl + HCl
Mechanism of Alkane Halogenation
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These halogenations are radical reactions
Radicals are molecules (or atoms) that have one or more unpaired
electrons (the half-headed arrows represent movement of single electrons
1. Initiation:
Cl
+
Cl
Light
Cl
+
H
H
2. Propagation:
H
C
H
+
H
Cl
H
+
Cl
H
Cl
C
+
Cl
Cl
H
H
H
3. Termination:
H-Cl
H
H
C
+
C
H
H
Cl
H
C
H
H
+
Cl
H
C
H
Cl