Transcript Unit 2 – Alkanes and Chemical Reactions

Unit 2 – Alkanes and Chemical
Reactions


Structure and Stereochemistry of Alkanes
 Nomenclature of alkanes and cycloalkanes
 Physical Properties
 Conformational Analysis
The Study of Chemical Reactions
 Kinetics and Thermodynamic Quantities
 Free Radical Halogenation
 Reactive Intermediates and Transition States
Hydrocarbons

The simplest organic compounds are the
hydrocarbons:
 organic compounds that contain only carbon
and hydrogen

four general types:
 alkanes
 alkenes
 alkynes
 aromatic hydrocarbons
Hydrocarbons

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Alkanes are often called saturated hydrocarbons
 they contain the maximum number of
hydrogen atoms per carbon atom.
Alkenes, alkynes, and aromatic hydrocarbons
are called unsaturated hydrocarbons
 they contain fewer H atoms than an alkane
with the same number of carbon atoms
H H
H C
C H
H H
H
H
C
H
C
H
H C
C H
Alkanes

You must know the names and formulas for the
10 simplest alkanes:
CH4
CH3CH3
CH3CH2CH3
CH3CH2CH2CH3
CH3CH2CH2CH2CH3
CH3CH2CH2CH2CH2CH3
methane
ethane
propane
butane
pentane
hexane
Alkanes

You must know the names and formulas for
the 10 simplest alkanes:
CH3CH2CH2CH2CH2CH2CH3
CH3CH2CH2CH2CH2CH2CH2CH3
CH3CH2CH2CH2CH2CH2CH2CH2CH3
CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3
heptane
octane
nonane
decane
Alkanes

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The alkanes form a homologous series with a
general molecular formula of CnH(2n+2)
Homologous series:
 a series of compounds in which each member
differs from the next member by a constant
unit
 Alkanes differ from each other by -CH2-
Alkanes
Example: Which of the following are alkanes:
C2H6, C3H6, C5H12, C4H8
Example: What is the formula for an alkane with
12 carbons?
Alkanes

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The previous alkanes are straight-chain alkanes:
 all of the carbon atoms are joined in a
continuous chain
 also called “normal” alkanes (n-alkanes)
Alkanes containing 4 or more carbons can also
form branched alkanes
 one or more of the carbon atoms form a
“branch” or side-chain off of the main chain
Alkanes

An example of a straight chain alkane:
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
C5H12
CH3CH2CH2CH2CH3
pentane
Examples of branched alkanes:
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C5H12
CH3CHCH2CH3
CH3
2-methylbutane
CH3
CH3CCH3
CH3
2,2-dimethylpropane
Alkanes
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The three structures shown previously for C5H12
are structural (constitutional) isomers:
 compounds with the same molecular formula
but different bonding arrangements
Structural isomers have different properties:
 different melting points
 different boiling points
 often different chemical reactivity
Alkane Nomenclature CH3CH2CH2CH2C

Organic compounds can be named either using
common names or IUPAC names. CH CHCH CH
3
CH33CH2CH22CH
CH
CH22CH
CH33
CH33CHCH22CH
CH
CH33
CH3
CH
3
2-methylbutane
CH3
CH
or 3
CH
CCH
3CCH3
isopentane
CH
3
3
2
pentane CH3
CH3
CH3CCH3
CH3
2,2-dimethylpropane
or
neopentane
3
Alkane Nomenclature
The common name for any alkane containing a
CH3 group on the second carbon in the chain is
“isoalkane.”
CH3
CH3 CH R
“iso”
CH3
CH3CHCH2CH2CH3
Isohexane (6 C total)
CH3
CH3CH CH3
Isobutane
(4 C total)
Alkane Nomenclature

Most of the time, organic chemists use the
IUPAC names for organic compounds.

LEARN THE RULES FOR EACH CLASS OF
COMPOUNDS WE DISCUSS.
Alkane Nomenclature

To name an alkane:
 Find the longest continuous chain of carbon
atoms and use the name of that chain as the
base name of the compound:
 the longest chain is often NOT written in a
straight line
3
1
2
CH3CHCH2CH3
CH2CH2CH2CH3
4
5
6
7
Base name:
heptane
Alkane Nomenclature

Number the carbon atoms in the longest chain
starting at the end of the chain closest to a
substituent
 a group attached to the main chain that has
taken the place of a hydrogen atom on the
main chain
3
A substituent
1
2
CH3CHCH2CH3
CH2CH2CH2CH3
4
5
6
7
Alkane Nomenclature

Name and give the location of each substituent.
 Common substituents:
 Halo group
 a halogen atom
 “Halo” groups are named using “halo”:
 Cl
chloro
 Br
bromo
 I
iodo
 F
fluoro

Nitro group
 -NO2
Alkane Nomenclature

Common substituents:
 alkyl group
 A group that is formed by removing an H
atom from an alkane

the alkyl group attaches to the main chain
at the carbon that has lost its H
CH3CH2CHCH2CH2CH3
CH2CH3
correct
CH3CH2CHCH2CH2CH3
CH3CH2
incorrect
Alkane Nomenclature

Alkyl groups are named by replacing the “ane”
ending of the parent alkane with the “yl” ending.
H
CH4
H C
methyl
H
CH3CH3
CH3CH2CH3
CH3CH2
ethyl
CH3CH2CH2
propyl
CH3CHCH3
isopropyl
Alkane Nomenclature
CH3CH2CH2CH2
butyl
CH3CH2CH2CH3
butane
CH3CH2CHCH3
CH3
CH3
CH3CHCH3
isobutane
CH3CHCH2
CH3
CH3CCH3
sec-butyl
isobutyl
t-butyl
Alkane Nomenclature
CH3
CH3CCH3
CH3
CH3
CH3CCH2
neopentyl
CH3
neopentane
phenyl
benzene
Alkane Nomenclature
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Complex alkyl substituents
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Use the longest alkyl chain of the substituent as
the base name of the substituent
Number the substituent chain with the “head
carbon” as carbon 1
List substituents on the alkyl chain with the
appropriate numbers
Use parentheses around the name of the group
CH2CH3
CHCHCH3
CH3
a (1-ethyl-2-methylpropyl) group
Alkane Nomenclature
Methyl group
3
1
2
CH3CHCH2CH3
CH2CH2CH2CH3
4
5
6
7
3-methylheptane
Note: Separate numbers from letters using a
hyphen. Separate groups of numbers using
commas.
Alkanes

Alkane Nomenclature:
 When two or more substituents are present,
list them in alphabetical order:



isopropyl before methyl
t-butyl or sec-butyl before chloro
When more than one of the same substituent
is present (i.e. two methyl groups), use
prefixes to indicate how many. Give the
location of each as well.
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Di = two
Tri = three
Tetra = four
Penta = five
Hexa = six
Know these.
Note: Ignore these
prefixes when alphabetizing.
Alkane Nomenclature
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Additional rules:
 When there are two “longest” chains of equal
length, use the chain with the greater
number of substituents.
CH3
CH3 CH
CH
CH3
CH2
CH3 CH
CH
CH2
CH3 CH
CH3
CH3
incorrect
CH3
CH
CH2
CH
CH2
CH3 CH
CH3
CH3
correct
CH3
Alkane Nomenclature

Additional rules:
 If each end of the longest chain has a
substituent the same distance from the end,
start with the end nearer to the second
substituent.
1
2
Cl
CH3
CH3CHCHCH2CHCH3
3
CH3
4
5
6
correct
3-chloro-2,5-dimethylhexane
5
CH3
Cl
CH3CHCHCH2CHCH3
6
4
CH3
3
2
incorrect
1
Alkanes
Example: Name the following compounds:
Br
CH3CHCHCH3
CH2CH3
Alkanes
Example: Name the following compound:
CH3
CHCH3
CH3CCH2CH2NO2
CH2CH3
CH(CH3)2
CH3CHCHCH2CHCH3
C(CH3)3 CH2CH3
Alkanes

You must also be able to write the structure of
an alkane when given the IUPAC name.
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

Identify the main chain and draw the carbons
in it.
Identify the substituents (type and #) and
attach them to the appropriate carbon atoms
on the main chain.
Add hydrogen atoms to the carbons to make
a total of 4 bonds to each carbon
Alkanes
Example: Draw the structure for the following
compounds:
3, 3-dimethylpentane
4-sec-butyl-2-methyloctane
1,2-dichloro-3-methylheptane
2-nitropropane
Alkane Nomenclature
Example: Draw the structural isomers of hexane
(C6H14). Name each isomer.

Use a systematic approach to draw structural
isomers:
 Draw the unbranched isomer for the first
structure.

For other structures, remove 1 or more
carbons (and/or functional groups) from
the unbranched isomer and reposition to
make unique compounds
C C C
Types of Carbon Atoms H
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

H
C C H
Primary carbon (1o)
 a carbon bonded to
one other carbon
Secondary carbon (2o)
 a carbon bonded to
H
two other carbons
H
H
C C C
C C C
H
H
o
Tertiary carbon (3 )
H
H
C C
 a carbon bonded to
H
C C C
three other carbons
H C
C
C H
C
C C H
Physical Properties
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Solubility
 Alkanes are nonpolar
 hydrophobic
 do not dissolve in water
 soluble in nonpolar or weakly polar organic
solvents
Density:
 varies from ~0.5 - ~0.8 g/mL
 less dense than water (1.0 g/mL)
 Alkanes float on water
Physical Properties

Boiling Point
 In general, boiling point increases as the
molecular weight of the alkane increases
 larger molecules have greater surface area
and higher London dispersion forces of
attraction
 must be overcome for vaporization and
boiling to occur
Physical Properties
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Boiling Point (cont)
 Given the same total number of carbon
atoms:
BP (branched) < BP (n-alkane)

Branched alkanes are more compact.
 less surface area
 smaller London dispersion forces
 lower BP
Physical Properties
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Melting Points:
 In general, melting point increases as MW
increases
 irregular, sawtooth pattern
Physical Properties
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Melting Point:
 Alkanes with odd number of carbons have
lower than expected melting points (compared
to the previous alkane with an even number of
carbons)
 Even # carbons
 better packing in solid structure
 higher MP
 Odd # carbons
 do not pack as well
 lower MP
Physical Properties

Melting Points:
 Given the same total number of carbon
atoms:
MP (branched) > MP (n-alkane)
 branched alkanes have more compact
structure
 better packing
 higher MP
Sources & Uses of Alkanes
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Alkanes are derived primarily from petroleum
and petroleum by-products:
Refining via fractional distillation gives useful
mixtures of alkanes:
 C2 - C4
liquified petroleum gas
 C4 - C9
gasoline
 C8 - C16
kerosene
 C10 - C18
diesel
 C16+
heavy/mineral oil
Reactions of Alkanes
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Catalytic Cracking:
 converts alkanes into more valuable mixtures
of smaller alkanes and alkenes
alkane
D
smaller alkanes + alkenes
SiO2 or Al2O3
catalyst
C12H26
D
SiO2
+
Reactions of Alkanes
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Hydrocracking:
 converts higher alkanes into shorter alkanes
and eliminates N and S impurities
Alkane
D
shorter alkanes
H2, SiO2 or
Al2O3 catalyst
C12H26
D
H2, SiO2
+
Reactions of Alkanes

Combustion:
 a rapid, exothermic redox reaction that
converts hydrocarbons into carbon dioxide and
water
alkane + O2
CO2 + H2O
(unbalanced)
2 C6H14 + 19 O2
12 CO2 + 14 H2O
Reactions of Alkanes

Halogenation:
 a reaction in which a halogen atom is
substituted for a hydrogen atom on an alkane
alkane + X2
CH4 + Cl2
D or hu
hu
mixture of alkyl halides
CH3Cl + CH2Cl2 + CHCl3 + CCl4
unbalanced
Conformations of Alkanes
• The simplest alkane, CH4, is perfectly
tetrahedral:
•bond angle = 109.5
•C-H bond length = 1.09 A
•free rotation of the C-H bond
Conformations of Alkanes
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Ethane:
 Two carbons
3 hybrid orbitals form a
 overlapping sp
sigma bond
Conformations of Alkanes

The two methyl groups are relatively free to
rotate about the sigma bond between the two
carbon atoms



sigma bond maintains its overlap at all times
The different arrangements formed by rotation
around a single bond are called conformations.
Conformer:
 a specific conformation
 a “conformational isomer”
Conformations of Alkanes

Conformers are often drawn using Newman
projections:
 a way of drawing a molecule looking
straight down the bond connecting two
carbon atoms


H
H
H
front carbon atom is represented by
three lines joined together in a Y shape
back carbon is represented by a circle
with three bonds pointing out from it
H
H
H
Conformations of Alkanes
H
C
H
H
H
View
from
this
end
H
H
C C
H
H
H
H
H
H
=
H
H
H
H
3-D structure of
C
one conformer of
H
ethane
H
H
C
H
H
H
Newman
H
projection
H
Conformations of Alkanes

An infinite number of conformations are possible
for ethane and higher alkanes.
 The dihedral angle (q) can have an infinite
number of values
 angle between the C-H bonds on the front
and back carbons
C
H
H
H
H
q
H
H
Conformations of Alkanes
Important conformations for ethane:
Eclipsed
conformation
Staggered
conformation
Skew
conformation
Molecules are constantly rotating through all
possible conformations.
Conformations of Alkanes



The conformation of ethane changes constantly
at room temperature.
Conformations may have different energies.
 Lowest energy conformer is most favored.
 Highest energy conformer is least favored.
Conformational analysis:
 the study of the energies of different
conformations
 helps predict which conformation are favored
and which reaction may occur
Conformations of Alkanes
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
Staggered conformation of ethane:
 lowest energy
most favored
 electron clouds in the C - H bonds
separated as much as possible
Eclipsed conformation of ethane:
 highest energy
least favored
 electron clouds of C - H bonds are closest
together
Conformations of Alkanes

As ethane rotates from the staggered
conformation towards the eclipsed conformation:

potential energy increases due to torsional
strain
 resistance to rotation or twisting about a
bond
Conformations of Alkanes
Conformation of Alkanes


Butane:
 4 carbon chain held together by end-to-end
overlap of sp3 hybrid orbitals on the carbon
atoms
 tetrahedral geometry around each carbon
 free rotation about any C - C bond
 many conformers of differing energies are
possible
Newman projections of butane are drawn by
looking down the central C2 - C3 bond.
Conformations of Alkanes
Totally
eclipsed
(0o)
Gauche
(60o)
Eclipsed
(120o)
Anti
(180o)
Conformation of Alkanes


Totally eclipsed conformer of butane:
 highest energy due to steric hinderance
between the methyl groups
Steric hinderance:
 interference between two bulky groups that
are close enough together so that their
electron clouds repel each other
Conformations of Alkanes
Conformations of Alkanes



Eclipsed conformer of butane:
 second highest in energy due to repulsion of
the methyl group on one carbon and the
hydrogen atom on the other
All staggered conformers (gauche and anti) of
butane are lower in energy than any of the
eclipsed conformers.
Anti conformer of butane:
 lowest energy because methyl groups are
furthest apart
Conformations of Alkanes