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General formula CnH2n+2
are organic compounds that consist only of the elements carbon (C) and hydrogen (H)
atoms are linked together exclusively by single bonds
all carbons are sp3 (tetrahedral) and all bond angles are 109.5o
Greek numerical prefix denoting the number of carbons and the suffix "-ane".
It forms Homologous series - a series of compounds in which each member differs from the next member by a constant amount, members are called homologs)
The first four members of the series (in terms of number of carbon atoms) are named as follows: methane, CH4 ethane, C2H6 propane, C3H8
1. Petroleum – the principal source of alkanes; are the end products of anaerobics decay of plants and animals for several years.
2. fossil fuel, coal – secondary sources of alkanes
3. Natural gas – contains more volatile alkanes( low molecular weight); contains 3% higher alkanes.
Petroleum ConstituentsUses Fraction Distillation Temp. Carbon Number
Heating Natural Gas Below 20 degrees C1-C4
HeatingSolvents for many organic materials of low polarity
Petroleum EtherLigroin (light naphtha)
60-10040-205
C5-C6C6-C7
Internal combustion engine
Natural Gasoline 40-205 C5-C10 & cycloalkanes
HeatingTractor and jet
Kerosine 175-325 C12-C18 & aromatics
HeatingDiesel oil
Gas oil( furnace oil) Above 275 C12 & higher
Petroleum wax, petroleum jelly(Vaseline)
Lubricating oil Non-volatile liquid Long chain attached to cyclic structure
Roof and roads Asphalt and petroleum *coke
Nonvolatile solids Polycyclic structure
*coke – paraffin base crude oil; complex HC having a high C:H ratio; fuel in the manufacture of C electrode for the electrochemical industries.
Crude oil contains hundreds of different hydrocarbons mixed together. To obtain useful products, the process of fractional distillation is used. The following diagram shows a schematic of a fractional distillation column. Longer hydrocarbon chain lengths have progressively higher boiling points, so they can all be separated by distillation. Crude oil is heated and the different chains are separated by boiling temperatures.
Refinery and tank storage facilities, like this one in Texas, are needed to change the hydrocarbons of crude oil to many different petroleum products.
A. Melting point and boiling point Melting (blue) and boiling (pink)
points of the first 14 n-alkanes in °C.
For simple straight-chain alkanes, boiling and melting points generally increase with increasing chain length. ( 20-30 degrees rises in boiling point for each carbon that is added to the chain)
B. Forces of attraction Van der waals repulsion
C. Conductivity Alkanes do not conduct electricity, nor
are they substantially polarized by an electric field
D. Molecular geometry molecular structure of the alkanes
directly affects their physical and chemical characteristics
E. Bond lengths and bond angles An alkane molecule has only C – H
and C – C single bonds F. Conformation free rotation about the C-C single
bonds two conformations, also known as
rotamers Staggered Eclipsed
In general, alkanes show a relatively low reactivity, because their C bonds are relatively stable and cannot be easily broken. Unlike most other organic compounds, they possess no functional groups.
react only very poorly with ionic or other polar substances
Nonpolar to slightly polar
Solubility; soluble in nonpolar solvents like benzene, ether, chloroform and insoluble in water and other highly polar solvents.
Density; increasing Carbon chain increasing density but tends to level off at about 0.8, thus all alkanes are less dense than water.
acid dissociation constant (pKa) values of all alkanes are above 60
Classes of Carbon and Hydrogen atoms
1o Carbon - primary carbon is attached to only one other C atoms
2o Carbon - secondary carbon is attached to two other C atoms
3o Carbon - tertiary carbon is attached to three other C atoms
Preparation of Alkanes
smaller alkanes can be obtained in pure form by fractional distillation of petroleum and natural gas.
1. Hydrogenation of alkenesCnH2n H2, Pt/Pd/Ni CnH2n+2
Example: CH3 CH CH2
H2, PtCH3 CH2 CH3
2. Reduction of Alkyl Halidesa. Using Grignard Reagent Stronger acid
etherRX+ Mg RMgX
H2ORH + Mg(OH)X
alkyl halides Grignard Reagent weaker acid
alkyl magnesium halides
CH3 C
CH3
CH3
Cl Mgether
CH3 C
CH3
CH3
MgCl H2O CH3 C
CH3
CH3
H+ + Mg(OH)Cl
CH3 CH2 CH
Br
CH3
MgCH3 CH2 CH
MgBr
CH3
OH2 CH3 CH2 CH
H
CH3
sec-butyl bromide sec-butyl magnesium bromide n-butane
b. Reduction by metal and acid
RX + Zn + H+
RH + Zn+ + X -
CH3 CH Cl Mg CH3 CH2CH3+Cl
Zn, H+
+ Zn+ + Cl
CH3CH2 CHCH3
Br
Zn, H CH3CH2CH2CH3
sec-butyl bromide n-butane
3. Coupling of RX with organometallic compounds
RXLi
RLiCuX
R2CuLi R'X R-R'+1o, 2o, 3o alkyl lithium Lithium dialkylcopper alkyl halides(1o)
CH3CH2 CHCH3
Cl
Li CuI(CH3CH2 CH
CH3
)2CuLi
CH3CH3CH2CH2CH2Br
CH3CH2CH(CH3)(CH2)4CH3
sec-butyl chloride n-Pentyl bromide
CH3CH2 ClLi
CH3CH2LiCuI (CH3CH2)2CuLi CH3(CH2)5CH2Br CH3(CH2)7CH3+
ethyl chloride ethyllithium Lithium diethylcopper n-Heptyl bromide n-nonane
4. Wurtz Reactions ( used to produce symmetrical alkanes)
Na is used.
2CH3 C
CH3
CH3
ClNa
CH3 C
CH3
CH3
C
CH3
CH3
CH3
Symmetrical alkane
Reactions of Alkanes1. Halogenation(free radical
substitution)
CH3CH2CH3240 - 400240 - 400240 - 400
Cl2CH3 C
H
Cl
CH3 + CH3CH2CH2Cl
CH3 C
H
CH3
CH2CH3Cl2
LIGHTCH3 C
Cl
CH3
CH2CH3 + ClCH2C
H
CH3
CH2CH3
+ CH3 C
H
CH3
CH2CH2Cl + CH3 C
H
CH3
C
Cl
H
CH3
+
H
CH2Cl
CH2CH3CH3C
In this reaction, a halogen atom abstracts a hydrogen atom from an alkane.
Reaction occurs slowly in the dark but rapidly in sunlight.
The rate at which a hydrogen atom is replaced by a halogen depends upon its position in the molecule.
The positions are dependent on the position of the carbon to which hydrogen is attached.
If the hydrogen atom s attached to a primary carbon (1˚), the H-atom is said to be a primary H-atom. If it is attached to a secondary carbon (2˚), the H-atom is a secondary H-atom, and so on and so forth.
The secondary H-atom is more rapidly replaced by a halogen compared with primary H-atom.
A. Chlorination A chlorine atom abstracts a hydrogen
atom from an alkane. It is known that the chlorination of an
alkane, promoted by sunlight or artificial ultraviolet light takes place by free radical chain reaction.
There are 3 fundamental stages in this reaction: initiation, propagation and termination.
B. Bromination The mechanism for bromination is
similar. When the alkane is methane, traces
of ethane are found in the final mixture of products. This provides evidence for a mechanism involving a methyl radical.
It would be formed from combining two methyl radicals: H3C. + .CH3 ==> H3C-CH3
Reactivity: Cl2 > Br2
Ease of abstraction of H : 3o> 2o > 1o >CH3-X
2. Combustion or complete oxidation A reaction wherein alkanes burn in air or
oxygen which then forms the products CO2 and H2O.
The general formula for combustion is: flame
CnH2n + 2 + excess O2 n CO2 + (n+1) H2O + heat
An insufficient supply of oxygen leads to the production of soot, formaldehyde, or other products.
The heat of combustion of alkanes increases with chain length, simply because there is more C and H to burn along longer chain.
When alkane hydrocarbons are heated to a high temperature (450-900oC, with/without superheated steam) they are thermally decomposed or 'cracked' to form mainly alkanes of lower C number, alkenes of equal or smaller C number and hydrogen.
When the temperature is high enough, the kinetic energy of the particles is sufficient to cause bond fission on collision, and this initiates a free radical chain reaction.
alkanes may be converted into nitro derivatives by heating the hydrocarbon in the vapor state with vapors of nitric acid at a temperature of about 420˚.
A hydrogen atom is replaced by a nitro (NO2) group.
Physical Properties1. Hybridization of carbon is sp3, with 109.5o angle.
Cycloalkanes will experienced angle bending strain in the formation of a ring due to compression of the tetrahedral bond angle.
2. Nonpolar molecules.3. Forces of attraction is van der waals4. Soluble in nonpolar solvents like CCl4, CHCl3,
benzene, ether.5.They are more reactive than straight chain alkane6. They have higher boiling points, melting points
and densities
Shapes/conformationchair conformationboat conformationtwisted boat conformation
Note: These three are free of angle-bending strain.
The most stable conformation is the chair conformation because it is free from torsional strain, angle-bending strain and steric strain.
Boat conformation will experienced torsional strain, there is also van der waals strain due to crowding of the flagpole hydrogen which lies only 1.83A apart.
COMPARISON IN TERMS OF REACTIVITY As stability of cycloalkanes decreases
the addition reaction increases.
Undergo cycloaddition
Strain in cyclopropane and cyclobutane1. Angular strain- deviation from tetrahedral
geometry or angle of 109.5 degrees. Compression of bond angles Poor, overlap of angle(109.5o) 2. Torsional strain - deviation from staggered
conformation. Exist anytime C-H bonds are eclipsed Due to eclipsing of bonds on neighboring
atoms All H’s eclipsed
3. Steric Strain the presence of van der waals repulsion Electronic repulsion that occurs when
two atoms or groups are forced together
Due to repulsive interactions when two atoms approach each other to closely
Van der Waals repulsion
Natural Sources of CycloalkanesPetroleum and coal
Preparation of cycloalkanes
1. Hydrogenation
+ 3H225 atm
Ni, Pt, Pd
hydrogenation
2. Conversion of some open-chain compounds into a compound that contains a ring, a process of cyclization.
H2C
CH2Cl
CH2Cl
Zn, NaI H2C
CH2ZnCl
CH2Cl
H2C
H2C
OH
+ 3H2
15atm
OHNi, 150 ~ 200oC
3. Cycloaddition – reaction in which molecule are added together to form a ring.
C
CH3
CH3
C
CH3
CH3
+ :CH2
:CH2H2C N N H2C C O + N2 or COor
diazomethane ketene
heat
carbene