Transcript Slide 1

ORGANIC CHEMISTRY
CHM 207
CHAPTER 2:
ALKANES
NOR AKMALAZURA JANI
TOPICS
 Nomenclature
 Structure
 Reactions:
- free radicals substitution
- combustion
 Industrial source and uses of aliphatic hydrocarbons
- petroleum and natural gas
- petroleum fractions: cracking and reforming and
their uses.
• General formula:
CnH2n+2, where n = 1, 2, ….
• Only single covalent bonds are present
• Known as saturated hydrocarbons because contain the
maximum number of hydrogen atoms that can bond
with the number of carbon atoms present.
• Can be assumed to be sp3-hydridized
Structures of the first four alkanes
Homologous Series
 Definition:
A series of compounds in which each
member differs from the next by a specific
number and kind of atoms.
 Alkanes: Differ only at number of (CH2)
 Series of compounds that has the same
functional group.
INITIAL NAMES OF THE HOMOLOGOUS SERIES
Number of carbon atoms, n
1
2
3
Name
Meth
Eth
Prop
4
5
6
But
Pent
Hex
7
8
Hept
Oct
9
10
Non
Dec
NAMING ALKANES
• Alkyl groups are used to name organic compounds.
• The general formula of an alkyl group is CnH2n+1.
• The letter “R” is often used in formulas to represent any
of the possible alkyl groups.
R= CnH2n+1 (any alkyl group)
R = CH3 — methyl group
R = CH3CH2 — ethyl group
IUPAC RULES
International Union of Pure and Applied Chemistry
RULE 1. Select the longest continuous chain of carbon
atoms as the parent compound.
 Consider all alkyl groups attached to it as branch chains or
substituents that have replaced hydrogen atoms of the
parent hydrocarbon. If two chains of equal length are
found, use the chain that has the larger number of
substituents attached to it.
 The alkane’s name consists of the parent compound’s
name prefixed by the names of the alkyl groups attached
to it.
This structure has 2 chains.
This chain has 6 carbon atoms.
1
2
3
4
5
6
CH3
CH2
CH
CH2
CH2
CH3
CH3
This chain has 4 carbon atoms.
1
CH3
2
CH2
3
CH
CH3
4
CH2
CH2
CH3
This is the longest continuous chain.
Select this chain as the parent compound.
1
CH3
2
CH2
3
CH
CH3
4
CH2
5
CH2
6
CH3
1
CH3
2
CH2
3
CH
4
CH2
5
CH2
6
CH3
CH3
This is a methyl group.
It is a branch chain and can be considered to have
replaced a hydrogen on the parent compound.
1
CH3
2
CH2
3
CH
4
CH2
5
CH2
6
CH3
CH3
The name of the compound is 33-methylhexane.
RULE 2. Number the carbon atoms in the parent carbon
chain starting from the end closest to the first carbon
atom that has an alkyl group substituted for a hydrogen
atom.
– If the first subsitutent from each end is on the samenumbered carbon, go to the next substituent to
determine which end of the the chain to start
numbering.
If the chain is numbered left to right, the
isopropyl group is on carbon 5.
1 2
3
isopropyl
group
4
5
6
7
8
If the chain is numbered right to left, the isopropyl group
is on carbon 4.
Use right to left numbering so that the isopropyl
group is on the lowest numbered carbon.
4-isopropyloctane
8 7
isopropyl
group
6
5
4
3
2
1
RULE 3.
Name each alkyl group and designate its position on the
parent carbon chain by a number (e.g., 2-methyl means
group attached to C-2).
5
4
3
2
1
2-isopropyl pentane
RULE 4. When the same alkyl-group branch chain appears more than
once, indicate this repetition by a prefix (di-, tri-, tetra- and so forth)
written in front of the alkyl group name (e.g. dimethyl indicates two
methyl groups).
–The numbers indicating the alkyl-group positions are separated by
a command and followed by a hyphen and are placed in front of the
name (e.g., 2,3-dimethyl).
5
The methyl group
appears twice
4
3
2
1
2,3-dimethylpentane
RULE 5.
When several different alkyl groups are attached to the
parent compound, list them in alphabetical order (e.g.
ethyl before methyl in 3-ethyl-4-methyloctane). Prefixes
are not included in alphabetical ordering (ethyl comes
before dimethyl).
1
2
3
4
5
6
7
methyl
ethyl
3-ethyl-4-methyloctane
8
• Alkanes can have many different types of
substituents.
• For example:
NO2 Br
1
2
CH3 CH
3
4
CH
CH3
5
CH3
3-bromo-2-nitropentane
CYCLIC HYDROCARBONS
A hydrocarbon that contains carbon atoms joined to
form a ring.
Cycloalkanes – all carbons of the ring are saturated
cyclopropane cyclobutane
cyclopentane
cyclopropane
cyclobutane cyclopentane
cyclohexane
cyclohexane
NOMENCLATURE OF
CYCLOALKANES
Similar to that alkanes. For examples:
CH3
CH3
=
methylcyclopentane
4 32
5 61
CH2CH3
1-ethyl-3-methylcyclohexane
CYCLIC HYDROCARBONS
When the acyclic portion of the molecule contains more
carbon atoms than the cyclic portion (or when it contains
an important fuctional group), the cyclic portion is
named as a cycloalkyl substituent.
Example:
H C C CH2 CH2 CH2
4-cyclopropyl-3-methyloctane
5-cyclobutyl-1-pentyne
ISOMERISATION
Structural isomers:
- Molecules that have the same molecular formula, but
different structure
Three isomers of pentane (C5H12)
STRUCTURE ISOMERS FOR ALKANES
NAME
MOLECULAR FORMULA
TOTAL OF ISOMERS
Methane
CH4
1
Ethane
C2H6
1
Propane
C3H8
1
Butane
C4H10
2
Pentane
C5H12
3
Hexane
C6H14
5
Heptane
C7H16
9
Octane
C8H18
18
Nonane
C9H20
35
Decane
C10H22
75
PHYSICAL PROPERTIES OF
ALKANES
Solubilities and densities
Boiling points
Melting points
SOLUBILITIES AND DENSITIES OF ALKANES
1)
Solubilities:

The C-H bond having only a very weak dipole moment.

Alkanes are weak polar molecules and considered as non-polar
molecules.

Soluble in non-polar solvents such as benzene and weak nonpolar organic solvents such as dimethyl ether (CH3-O-CH3).

Insoluble in water:
- alkanes are non-polar and do not form hydrogen bonds with
water molecules.
- described as ‘hydrophobic’ (water hating).
2) Densities:
- alkanes have densities around 0.7 g/mL, compared to
density of water (1.0 g/mL).
- alkanes is less dense than water and insoluble in water.
- water combined with alkanes will form two phase with the
alkanes on the top.
oil
water
BOILING POINTS AND MELTING POINTS OF
ALKANES
BOILING POINTS OF ALKANES
Effect of relative molecular mass on boiling point.
- the first four chain alkanes are gases.
- alkanes from C5H12 to C18H38 are liquids at room
temperature because their melting point are lower than
28oC (301K).
- alkanes above C18H38 are solids at room temperature.
- the boiling points of straight chain alkanes increase
steadily with relative molecular mass (due to increasing
forces of attraction between molecules).
* A larger molecule, with greater surface area and greater
van der Waals attractions, boils at higher temperature *
Boiling points of the straight chain of alkanes
Effect of branching on boiling point.
- Branched chain alkanes boils at a lower temperature
(more volatile) than the straight chain alkane with the
same number of carbon atoms.
- Examples: hexane boils at 68.7oC, 3-methylpentane (one
branch) boils at 63.3oC and 2,3-dimethylbutane (two
branches) boils at 58oC.
- Reason: the branched chain alkanes are more compact
(nearly spherical), have smaller surface area, smaller van
der Waals forces of attraction and boils at lower
temperature.
MELTING POINTS OF ALKANES
 The melting points increase with increasing of molecular weight.
 Alkanes with even numbers of carbon atoms pack better into a
solid structure, so higher temperatures are needed to melt them
(high melting point).
 Alkanes with odd numbers of carbon atoms do not pack as well,
and melt at lower temperatures (low melting points).
 Branched chain alkanes melts at a higher temperature than nalkanes (straight alkanes) with same numbers of carbon atoms.
- Reason: 3D-structure of branched alkanes are more compact,
pack more easily into solid structure and melt at higher
temperatures.
Formula: C6H14
H3C
CH CH2 CH2 CH3
H3C
H3C
CH3
CH CH
H3C
bp 60oC
o
mp -154 C
CH3
CH3
H3C C CH2 CH3
CH3
bp 58oC
bp 50oC
mp -135oC
mp -98oC
Melting points increase, boiling points decrease
Shape of the molecule become more highly branched and compact
UNREACTIVITY OF ALKANES
• Alkanes is chemically inert to most reagents.
• For example, acids, alkalis, and oxidising agents such as potassium
manganate (VII) or potassium dichromate (VI) do not react with
alkanes.
• Alkanes reacts with oxygen and halogens in suitable conditions.
• Why alkanes has low reactivity?
- lack of electron-deficient or electron-rich sites on the alkanes
molecules.
- polar molecules, positive and negative ions (such as H+ and OH-),
do not react with alkanes because the C-H bond is weak polar and
C-C bond is non-polar.
REACTIONS OF
ALKANES
COMBUSTION
(WITH OXYGEN)
HALOGENATION
(FREE RADICAL
REACTIONS)
REACTIONS OF ALKANES
• COMBUSTION
- Alkanes burn in a plentiful supply of air or oxygen to produce
water and CO2 only.
- for example:
C3H8 + 5 O2 → 3CO + 4H2O
- In a limited supply of air, combustion of alkanes
produces carbon monoxide and water
C2H6 + 5 O2 → 2CO + 3H2O
2
- In a very limited supply of air, alkanes burn to form
carbon as one of the product.
C2H6 + 3 O2 → 2C + 3H2O
2
REACTIONS OF ALKANES
• HALOGENATION OF ALKANES
- At RT, alkanes do not react with chlorine or bromine in the
dark.
- if the mixture of alkanes and chlorine or bromine is heated at
high temparature (300-400oC), or irradiated by ultraviolet
light, the hydrogen atoms in the alkanes are successively
replaced by chlorine or bromine atoms to produce a mixture
of products (halogenated alkanes).
Equations for the reactions of methane with chlorine:
CH4 + Cl2
CH3Cl + HCl
CH3Cl + Cl2
CH2Cl2 + HCl
CH2Cl2 + Cl2
CHCl3 + HCl
CHCl3 + Cl2
CCl4 + HCl
(1)
(2)
• Equation 1: reaction with limited supply of chlorine and
excess of methane. Tha major product is chloromethane.
• Equation 2: reaction with the excess of chlorine. The major
product is tetrachloromethane.
* Bromine reacts with alkanes in the same way of chlorine,
but iodine do not react well with alkanes *
• This reaction is called a substitution reaction (an atom or
a group atom in an organic compound is replaced by
another atom or a group of atoms).
• Involves a halogen - called halogenation
• If the halogen is chlorine – called chlorination.
• If the halogen is bromine – called bromination.
* Condition of reaction:
light or heat (high temperature) or ultraviolet radiation
(provides energy that is absorbed by reactant molecules
to produce free radicals).
MECHANISM OF FREE RADICAL
SUBSTITUTION REACTIONS
3 STEPS
INITIATION
STEP
PROPAGATION
STEP
TERMINATION
STEP
MECHANISM OF FREE RADICAL
SUBSTITUTION REACTIONS
1) INITIATION STEP
- homolytic fission
Cl
Cl
ΔH = +242 kJMol-1
hv
Cl
Cl
chlorine atoms
(free radicals)
hv = radiation energy
= movement of single electron (radical)
2) PROPAGATION STEPS
- Free radical species produce another free radical
species.
- Free radicals is highly reactive.
Cl
H
H C H
H
methane
HCl + CH3
methyl radical
-
Methyl radical propagates a chain reaction as the
methyl free radical then reacts with another
chlorine molecule to form chloromethana and a
chlorine free radical.
CH3
methyl
radical
-
Cl2
CH3Cl + Cl
chloromethane
Chlorine free radical produced then react with
another methane molecule and the cycle is
repeated.
3) TERMINATION STEPS
- The reaction stops when two free radicals collide
and combine.
- highly exothermic.
Cl
Cl
Cl2
CH3
Cl
CH3Cl
CH3
CH3
H3C CH3
(by product)
- If a large excess of methane is used, CH3Cl is obtained
as the main organic product.
- In excess of chlorine, the propagation steps may
proceed with the reaction between a chlorine free
radical with chloromethane to produce
dichloromethane.
- The reaction may continue to produce
trichloromethane and finally tetrachloromethane.
CH3Cl
Cl
CH2Cl
HCl
CH2Cl
Cl2
CH2Cl2
Cl
dichloromethane
Cl
CHCl2
HCl
CHCl2 Cl2
CHCl3
Cl
CH2Cl2
trichloromethane
CHCl3
CCl3
Cl
Cl2
CCl3
CCl4
HCl
Cl
tetrachloromethane
FREE RADICAL SUBSTITUTION REACTIONS OF ETHANE
Br
hv
Br
CH3CH3
CH2CH3
Br
Br2
Br
CH2CH3
HBr
CH3CH2Br Br
Br
Br
Br2
CH3CH2
Br
CH3CH2Br
CH3CH2
CH3CH2
Br
initiation step
propagation step
termination step
CH3CH2CH2CH3
Further propagation steps can take place until CBr3CBr3 is finally produced
FREE RADICAL SUBSTITUTION REACTIONS OF PROPANE
Br
CH3CHCH3
Br2
o
2 alkyl radical
Br
CH3CHCH3
Br
2-bromopropane
CH3CH2CH3
MAIN PRODUCT
Br
CH3CH2CH2
Br2
1o alkyl radical
CH3CH2CH2Br
Br
1-bromopropane
• 2-bromopropane is the main product because:
- it is easier for halogen free radical to abstract a hydrogen atom
from a 2o carbon atom than 1o carbon atom.
H
H CH
methyl radical
H
R C H
primary
radical
R
R CH
R
R C R
secondary tertiary
radical
radical
stability increases
INDUSTRIAL SOURCE AND USES
OF ALIPHATIC HYDROCARBON
• Three major sources of alkanes are the fossil fuels which are
natural gas, petroleum and coal.
i) Natural gas
- consists of 90-95% methane, 5-10% ethane, and a mixture
of other relatively low boiling alkanes, which are propane,
butane and 2-methylpropane.
- used primarily as a fuel to heat buildings and generate
electricitya as well as starting material for the production of
fertilizers.
ii) Petroleum
- Petroleum: a thick, viscous liquid mixture of literally
thousands of compounds, mostly are hydrocarbons, formed
from the decomposition of ancient marine plants and
animals.
- Uses:
a) fuel for automobiles, aircraft and train.
b) provide most of the greases and lubricants required for
the machinery of highly industrialized society.
c) petroleum with natural gas provides 90% of organic raw
materials for the synthesis and manufacture of synthetic
fibers, plastics, detergents, drugs and dyes.
• Petroleum refining:
- The process whereby the petroleum is separated into its
components along with the separation of impurities.
- The refining is done by fractional distillation. Each
hydrocarbon component with its own boiling point
separates out neatly when the petroleum is heated.
- The fractions are further treated to convert them into
mixtures of more useful saleable products by various
methods such as cracking and reforming.
Fractional distillation
• Crude oil enters a refinery and then goes to distillation units where it is
heated to temperatures as high as 370 to 425 oC and separates into
fractions.
• Volatile components (low boiling point) will come out first.
• Common names of fractions and their uses:
i) Gases boiling below 20oC
- taken off at the top of distillation column.
- mixture of low-molecular-weight hydrocarbons, mainly propane,
butane and 2-methylpropane.
- substances can be liquefied under pressure at RT.
- uses: liquefied petroleum gas (LPG) is a convenient source of gaseous
fuel for home heating and cooking.
ii) Naphthas, bp 20-200 oC
- mixture of C5 to C12 alkanes and cycloalkanes, small
amount of benzene, toluene, xylene and aromatic
hydrocarbons.
- light naphtha fraction (bp 20-150oC) is a source of
straight-run gasoline.
- uses: motor fuel, source of raw materials for the
organic chemical industry.
iii) Kerosene, bp175 to 275 oC
- mixture of C9 to C15 hydrocarbons
- uses: heat, jet fuel
iv) Fuel oil (diesel), bp 250 to 400 oC
- mixture of C15 to C18 hydrocarbons
- motor fuel
v) Lubricating oil and heavy fuel oil distill (above 350oC)
- mixture of C16-C30 hydrocarbons
- uses: heating, lubrication
vi) Asphalt
- black, tarry residue remaining after removal of the other
volatile fractions
- C35 and above
Fractional distillation
Reforming process
i)


Cracking:
A process whereby a saturated hydrocarbon is
converted into an unsaturated hydrocarbon and
hydrogen.
Ethane is cracked by heating in furnace at 800 to 900 oC.
CH3CH3
ethane
800-900oC
thermal cracking
H2C CH2
ethene
H2
ii) Catalytic reforming:
- A process for increasing the octane number of
naphthas.
- It involves isomerization of alkanes, dehydrogenation of
cyclohexanes to aromatic hydrocarbons, isomerization
and dehydrogenation of alkylcyclopentanes, and
dehydrocyclization of alkanes.
- Example:
CH3CH2CH2CH2CH2CH3
hexane
catalyst
-H2
catalyst
-H2
cyclohexane
benzene
Octane number / octane rating
* The quality of gasoline as a fuel for internal combustion engines.
* Fuel that have high octane number, has very good antiknock
properties (the fuel / air mixture burns smoothly in the combustion
chamber).
* 2,2,4-trimethylpentane (isooctane) has very good antiknock
properties with octane number 100 compared to heptane which has
poor antiknock properties (octane number 0).
* If other fuel has octane number 90, means that the knock
properties of the fuel is same as those mixture of 10% heptane and
90% isooctane.