Lecture (3)
3-Properties of alkanes:
A-Physical properties:
(1)-All the alkanes with four carbon atoms or fewer are gases at room
temperature.
(2)-The unbranched alkanes pentane through heptadecane are liquids, while
those of higher molecular weight are solids.
(3)-Alkanes with branched chains have lower boiling points than their
unbranched isomers, because they are more compact and have a smaller
surface area. The melting point and boiling points rise with increasing in
molecular weight.
(4)-Alkanes are soluble in non-polar solvents such as benzene, ether and
chloroform and are insoluble in water.
B-Chemical properties:
(1)-Halogenation:
Replacement (substitution) of one or more hydrogen atoms by halogen
atoms, when a mixture of methane and chlorine is heated above 100 °C or
irradiated with light of suitable wave length,(ultraviolet) methyl chloride is
produced.
The reaction is represented as:
Heat or U.V
 CH4 (gas) +CL2 (gas)
CH3CL (gas) +HCL (gas)
Methane chlorine
methylchloride
If an excess of chlorine gas is presented, the reaction can proceeded further:
Heat or U.V
 CH3CL (gas) +CL2 (gas)
CH2CL2 (liq) +HCL (gas)
Dichloromethane
Heat or U.V
 CH2CL2 (liq) +CL2 (gas)
CH2CL2 (liq) +HCL (gas)
Trichloromethane (chloroform)

i-e:
Heat or U.V
CHCL3 (liq) +CL2 (gas)
CCL4 (liq) +HCL (gas)
Tetrachloromethane
Heat or U.V
CH4+CL2
CH3CL+CH2CL2+CHCL3+CCL4+HCL
Thus the products depend on the concentration of reactants and reaction
times.
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Mechanism of chlorination (free radical mechanism):
A reaction which proceeds via formation of free radical is a chain reaction.
Chain reaction: (3 steps)
A-Initiation:
 CL2+energy (U.V)
CL•+CL•
B-Propagation:
 CL• + CH4
 CH3•+CL2
CH3• + HCL
CH3CL+ CL•
C-Termination:
Reaction stops when it does not produce radicals.
 CH3•+ CH3•
 CH3•+ CL•
 CL•+ CL•
CH3CH3
CH3CL
CL2
In general:
A-Initiation: increase the number of radicals.
B-Propagation: leaves the net number of radicals unchanged.
C-Termination: Decreases the net number of radicals.
Examples:
1-CH3CH3 + CL2
heat or U.V
2-CH3CH2CH3 + CL2
CH3CH2CL (Ethyl Chloride)
heat or U.V CH3 CH CH3 +CH3CH2CH2CL
CL
Iso-propyl chloride (55%) n-Propyl chloride (45%)
2-chloro Propane
1-chloro Propane
NOTE: reactivity of halogen F2>CL2>Br2 (iodination does not take place)
(2)-combustion reactions (reaction with oxygen):
Alkanes are converted into carbon dioxide and water in the present of
sufficient of oxygen for complete combustion, incomplete combustion leads
to carbon monoxide or sometimes carbon as carbon black.
CnH2n+2
+ excess
O2
n CO2+ (n+1) H2O+heat
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e.g:
CH3CH2CH3 + 5O2
3 CO2 + 4 H2O + heat
Propane
oxygen
The combustion of alkanes is exothermic reaction, meaning that it gives
off heat, and is principle source of energy in our society.
(3)-Thermal Cracking (Pyrolysis):
We define Pyrolysis as thermal decomposition of organic compounds in
the absence of oxygen.Thermal craking long chain alkanes are converted
into small chain alkanes.
heat, pressure, catalyst
CnH2n+2
mixture of smaller chain compounds long chain
alkanes
4-Preparation of alkanes:
(1)-Hydrolysis of grinard reagent
Dry +
H2O
RX +Mg
R (Mg-X)
R-H + Mg (OH) X
Ether
grinard reagent
e.g.:
Dry
CH3I +Mg
Ether
H2O
CH3-MgI
CH4 + Mg (OH) I
grinard reagent
(2)-Coupling of alkyl halides with grinard reagent
RMgX + R′ X
Grinard reagent
(Alkyl magnesium halide)
e.g.:
CH3CH2MgI + CH3I
R-R′ + MgX
CH3CH2CH3 + MgI2
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(3)-Reduction of alkyl halides using metal and acid:
Zn/H+
R-H + Zn++X-
R-X
e.g.:
Zn/H+
CH3CL
Methyl chloride
CH4 + ZnCl2
(4)-Hydrogenation of alkenes
In the presence of catalyst (Pt, Pd or Ni)
Pt, Pd or Ni
CnH2n + H2
CnH2n+2
e.g.:
heat, pressure
CH2=CH2 + H2
Ethene
Ni
CH3CH3
Ethane
5-Cyclic Alkanes:
Besides forming chains, carbon atoms also form rings , the simplest of the
cyclic alkanes is cyclopropane (C3H6) “GENERAL FORMALA
CnH2n”.Cyclopropane form equivalent triangle(60º)bond angles.The
cyclopropane molecule is much more reactive than straight chain propane
.Cyclobutane (C4H8) form square (88 º) and is also quite reactive , while
cyclopentane (C5H10) and cyclohexane (C6H12) are quite stable .
The cyclic alkanes are often represented by the following structures.
∆ □ ⌂
 Naming Cyclic Alkanes:
The nomenclature of cycloalkanes follows the same rules as the other
alkanes except that the root name is preceded by the prefix cyclo. The ring is
numbered to yield the smallest substituent numbers possible.
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EXERCISE:
Name each of the following cyclic alkanes:
CH2CH3
* CH3-CH-CH3
*
CH2CH2CH3
CH3
Solution:
*The Six- Carbon cyclohexane ring is numbered as follows:
CH3-CH- CH3
(Isopropyl group)
1
6
2
5
3
CH3
4
4
1-isopropyl-3-methyl cyclohexane
1
(Methyl group)
CH2CH3
1-ethyl -2-propyl cyclobutane
*
3
2
CH2CH2CH3
 Cis – Trans Isomerism in Cycloalkanes:
The behavior of cycloalkanes is similar to that of open chain of alkanes.
Both are non polar and chemically inert to most reagents. However , there
are some important difference . One difference is that cycloalkanes are less
flexible .Open chain alkanes have nearly free rotation around their C-C
bonds, cycloalkanes are more constrained.
For example, because of the cyclic structure cyclopropane has a top and
bottom sides and there are two 1,2 –dimethylecyclopropane isomers: One
with two methyl groups on the same sides of the ring (Cis) , and the other is
the methyl on opposite side (Trans).
Both isomers cannot be converted into the other without breaking bonds.
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CH3
CH3
CH3
H
H
H
H
H
H
H
CH3
H
Cis-1,2-dimethylcyclopropane
Trans-1,2-dimethylcyclopropane
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Lecture (4)
Revisions and sheets lecture
Sheet 1
A`-Give the Isomers of:
1-C4H10
2-C5H12
3-C5H12
B-Give IUPAC names for the following alkanes:
1- The three isomers of C2H5
CH3
2- CH3CH2CHCHCH3
CH2CH3
CH3
CH3
3- CH3CHCH2CHCH3
CH3
CH2CH3
4- CH3-C-CH2CH2CHCH3
CH3
C-Name the following using common method:
1- CH3CH2CH2CH2CH3
CH3
2- CH3CHCH3
CH3
3- CH3CHCH2CH3
4- CH3CH2CH224
D-Draw Structures corresponding to the following IUPAC names.
* 3,4- dimethylnonane
* 3-ethyl-4,4-dimethylheptane
* 2,2-dimethyl-4-propyloctane
* 2,2,4-trimethylpentane
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Sheet 2
1- Draw Cis 1-4-dimethylcyclohexane
2- Draw Cis- 1-chloro-3-methylcyclopentane
3- Draw both Cis and Trans isomers of 1,2-dibromocyclobutane.
4- Draw Cycloalkane with5 carbons that contain :
 four methyl groups
 Ethyl&methyl groups
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Sheet 3
1-Tell how many hydrogen are bonded to each carbon, and give the molecular
formula of carvone.
2-Convert the following skeletal structures in to molecular formulas, and tell how
many hydrogen are bonded to each carbon.
A) - pyridine
N
B)-cyclo-hexanone
C)-Indole
N
H
3-Propose structures for the following molecular formula:
A)-C4H8
B)-C3H6O
C)-C4H9CL
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