ORGANIC CHEMISTRY CHM 207

1
ORGANIC CHEMISTRY CHM 207
CHAPTER 2 ALKANES
NOR AKMALAZURA JANI
2
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.

3

• General formula
• CnH2n2, 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

4
Structures of the first four alkanes
5
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.

6
INITIAL NAMES OF THE HOMOLOGOUS SERIES
Number of carbon atoms, n Name
1 Meth
2 Eth
3 Prop
4 But
5 Pent
6 Hex
7 Hept
8 Oct
9 Non
10 Dec
7
NAMING ALKANES

• Alkyl groups are used to name organic compounds.

• The general formula of an alkyl group is CnH2n1.

• The letter R is often used in formulas to
  represent any of the possible alkyl groups.

R CnH2n1 (any alkyl group) R CH3 methyl
group R CH3CH2 ethyl group
8
(No Transcript)
9
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 alkanes name consists of the parent
  compounds name prefixed by the names of the
  alkyl groups attached to it.

10
This structure has 2 chains.
11
(No Transcript)
12
This is a methyl group.
It is a branch chain and can be considered to
have replaced a hydrogen on the parent compound.
13
1
2
4
5
6
3
14
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
  same-numbered carbon, go to the next substituent
  to determine which end of the the chain to start
  numbering.

15
If the chain is numbered left to right, the
isopropyl group is on carbon 5.
isopropyl group
16
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
isopropyl group
17

• 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).

18
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 thename (e.g.,
2,3-dimethyl).
19

• 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).

20
1
2
4
5
3
6
7
8
3
4
21

• Alkanes can have many different types of
  substituents.
• For example

22
CYCLIC HYDROCARBONS

• A hydrocarbon that contains carbon atoms joined
  to form a ring.
• Cycloalkanes all carbons of the ring are
  saturated

23
NOMENCLATURE OF CYCLOALKANES

• Similar to that alkanes. For examples

24
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

25
ISOMERISATION

• Structural isomers
• Molecules that have the same molecular formula,
  but different structure

Three isomers of pentane (C5H12)
26
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
27
PHYSICAL PROPERTIES OF ALKANES

• Solubilities and densities
• Boiling points
• Melting points

28
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 non-polar 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).

29

• 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.

30
oil
water
31
BOILING POINTS AND MELTING POINTS OF ALKANES
32
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

33
Boiling points of the straight chain of alkanes
34

• 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.

35
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 n-alkanes (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.

36

• Formula C6H14

Melting points increase, boiling points decrease
Shape of the molecule become more highly branched
and compact
37
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.

38
(No Transcript)
39
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
40
- In a limited supply of air, combustion of
alkanes produces carbon monoxide and water
- In a very limited supply of air, alkanes burn
to form carbon as one of the product.
41
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).

42
Equations for the reactions of methane with
chlorine

• 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
43

• 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).

44
MECHANISM OF FREE RADICAL SUBSTITUTION REACTIONS
45
MECHANISM OF FREE RADICAL SUBSTITUTION REACTIONS

• INITIATION STEP
• - homolytic fission

?H 242 kJMol-1
46

• 2) PROPAGATION STEPS
• Free radical species produce another free radical
  species.
• Free radicals is highly reactive.

47
- 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.

• Chlorine free radical produced then react with
  another methane molecule and the cycle is
  repeated.

48
3) TERMINATION STEPS - The reaction stops when
two free radicals collide and combine. - highly
exothermic.
49

• - 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.

50
(No Transcript)
51

• FREE RADICAL SUBSTITUTION REACTIONS OF ETHANE

Further propagation steps can take place until
CBr3CBr3 is finally produced
52
FREE RADICAL SUBSTITUTION REACTIONS OF PROPANE
MAIN PRODUCT

• 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.

53
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.

54

• 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.

55

• 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.

56
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.

57

• 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

58

• 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

59
Fractional distillation
60
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.

61

• 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

62
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.