Unit 2
Alkanes and Chemical Reactions
Alkanes
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Nomenclature
Physical Properties
Reactions
Structure and Conformations
Cycloalkanes
 cis-trans isomerism
 ring strain
 conformations
Alkanes
 All C atoms are sp3.
 Since there are no multiple bonds,
alkanes are saturated hydrocarbons.
 General formula for noncyclic alkanes
is CnH2n+2.
 The noncyclic alkanes are a
homologous series, differing only by
the number of methylene -CH2- groups.
Nomenclature
 We will learn the IUPAC (systematic)
names of the alkanes.
 Below is the template you will use to
build the name of ANY organic
compound.
stereomain
functional
substituents
unsaturation
isomerism
chain
group
Nomenclature of Alkanes
 Rule #1: Name the longest continuous
chain (main chain) of C atoms.
 When there are two chains of equal length,
use the chain with the greatest number of
substituents.
methane
CH4
hexane
CH3(CH2)4CH3
ethane
CH3CH3
heptane
CH3(CH2)5CH3
propane
CH3CH2CH3
octane
CH3(CH2)6CH3
butane
CH3(CH2)2CH3
nonane
CH3(CH2)7CH3
pentane
CH3(CH2)3CH3
decane
CH3(CH2)8CH3
Nomenclature of Alkanes
 Rule #2: Number the main chain
from the end nearest a
substituent.
Nomenclature of Alkanes
 Using the line-angle structure can
be helpful in naming a compound.
Nomenclature of Alkanes
 Rule #3: Name the substituent groups
as alkyl groups.
 Give the location as the number of the main
chain C atom to which the substituent is
attached.
alkyl substituents you should know
methyl -CH3
t-butyl
-C(CH3)3
ethyl
isopropyl
-CH(CH3)2
propyl -CH2CH2CH3
sec-butyl
-CH(CH3)CH2CH3
butyl
isobutyl
-CH2CH(CH3)2
neopentyl
-CH2C(CH3)3
-CH2CH3
-CH2(CH2)2CH3
Alkyl Substituents
isopropyl
isobutyl
t-butyl
sec-butyl
neopentyl
Nomenclature of Alkanes
 Rule #4: If two or more substituents
are present, list them in alphabetical
order.
 If more than one of the SAME substituent is
present, use di-, tri-, tetra-, etc.
 Do not alphabetize a hyphenated
beginning. E.g., t-butyl is alphabetized as
“b”, but isobutyl is “i”.
 Do not alphabetize a numerical prefix. E.g.,
dimethyl is alphabetized as “m”.
Nomenclature of Alkanes
 5-ethyl-2-methyl-4-propylheptane
Nomenclature of Alkanes
 Additional rule:
 If each end of the longest chain has a
substituent the same distance from the
end, start with the end nearer to the
second branch point.
Cl
CH3
C H 3 C H C H C H 2C H C H 3
CH3
3-chloro-2,5-dimethylhexane
Nomenclature of Alkanes
 Complex substituents
 Find the longest chain from the point of
attachment.
 The C at the point of attachment is C #1.
 Name as you would an alkane, but the ending
is -yl.
 Put the entire name in parentheses.
1-ethyl-2-methylpropyl
(1-ethyl-2-methylpropyl)cyclopentane
Nomenclature of Alkanes
 Draw the line-angle structure for
the following compounds:
 3, 3-dimethylpentane
 4-sec-butyl-2-methyloctane
 1, 2-dichloro-3-methylheptane
Isomers
 Two or more compounds that
have the same molecular formula
but different arrangements of
atoms.
 Two classes of isomers
 Structural isomers
 Molecules with different bonding
patterns.
 Stereoisomers (Unit 3)
Structural Isomers
CH3CH2CH2CH3
C4H10
butane
CH3
CH3 CH
CH3
C4H10
isobutane
IUPAC name is
methylpropane
Structural Isomers
Draw all of the structural isomers of C6H14.
Physical Properties of Alkanes
 Used as fuels, solvents, lubricants
 Nonpolar
 hydrophobic (water-hating)
 Densities of n-alkanes ≈ 0.7 g/mL
 Intermolecular force is dispersion
(van der Waal’s), so bp’s increase
with increasing surface area, as do
mp’s of n-alkanes.
Physical Properties of Alkanes
 Boiling points
 Straight-chain compounds will have the
highest bp’s.
 The more branched the chain, the more
compact the molecule, and the lower the bp
(due to smaller dispersion attractions).
Physical Properties of Alkanes
 Melting points of branched alkanes
 The effect of branching on melting point is
more difficult to predict. For a given number
of carbons, the more compact molecule will
have the higher melting point. (Because it
packs better.)
Sources of Alkanes
 Refining of crude oil
 distillation followed by catalytic
cracking (hydrocracking)
 Natural gas
 70% methane, 10% ethane, 15%
propane (plus small amounts of other
compounds)
Reactions of Alkanes
 The most significant reaction of
alkanes is combustion.
CH3(CH2)8CH3 + 15.5 O2(g)  10CO2(g) + 11H2O(l)
 Catalytic cracking
 different from hydrocracking
 Halogenation
Structure and Conformations of
Alkanes
 C atoms are sp3 hybridized and
bond angles are 109.5°.
 Sigma bonds (σ bonds)
 end-to-end overlap
 rotation possible
 conformations differ only in
dihedral angle
 conformations shown by Newman
projections
Newman Projections
 Pick a C-C bond and look
along that axis.*
* Here’s where those modeling kits come in handy.
Newman Projections of Ethane
Conformations
Torsional Strain
 The eclipsed conformation has a
higher energy than the other
conformations, so there is some
resistance to rotation.
 This resistance is called torsional
strain (or steric strain).
 Ethane at room temperature has
more than enough energy to
overcome this resistance.
Newman Projections of Butane
Conformations
anti is the
lowest energy
conformation.
Conformations of Butane
 More distinct conformations are
possible.
 The anti conformation has the
lowest energy.
 This is why we draw C skeletons as
zigzag lines.
 Gauche conformations are
responsible for “kinks” in the C
chain.
Steric Hindrance
 The totally eclipsed conformation of
butane has a higher energy than its
other conformations
 due to torsional strain, aka steric strain,
and sometimes called steric hindrance.
 The H atoms of the methyl groups are
actually in each other’s way in the totally
eclipsed conformation.
 However, butane at room temperature
has enough energy to rotate through
the totally eclipsed conformation.