Properties of Alkanes/ Alkenes/ Alkynes (10th Carbon and its compounds)
Properties
of Alkanes/ Alkenes/ Alkynes
(a) Alkanes are gas (C1to C4) ; liquid (C5to
C17) and rest are solid at room temperature.
(b) The melting and boiling point of Alkanes increases with
increase in number of C – atom in Alkanes.
Alkane
|
Formula
|
Boiling point [°C]
|
Melting point [°C]
|
State (at 20 °C)
|
Methane
|
CH4
|
-162
|
-182
|
gas
|
Ethane
|
C2H6
|
-89
|
-183
|
gas
|
Propane
|
C3H8
|
-42
|
-188
|
gas
|
Butane
|
C4H10
|
0
|
-138
|
gas
|
Pentane
|
C5H12
|
36
|
-130
|
liquid
|
Hexane
|
C6H14
|
69
|
-95
|
liquid
|
Heptane
|
C7H16
|
98
|
-91
|
liquid
|
Octane
|
C8H18
|
126
|
-57
|
liquid
|
Nonane
|
C9H20
|
151
|
-54
|
liquid
|
Decane
|
C10H22
|
174
|
-30
|
liquid
|
Undecane
|
C11H24
|
196
|
-26
|
liquid
|
Dodecane
|
C12H26
|
216
|
-10
|
liquid
|
Hexadecane
|
C16H34
|
281
|
18
|
liquid
|
Icosane
|
C20H42
|
343
|
37
|
solid
|
Triacontane
|
C30H62
|
450
|
66
|
solid
|
Tetracontane
|
C40H82
|
525
|
82
|
solid
|
Pentacontane
|
C50H102
|
575
|
91
|
solid
|
Hexacontane
|
C60H122
|
625
|
100
|
solid
|
Source:
http://en.wikipedia.org/wiki/Alkane
|
Reason: Alkanes experience inter-molecular van der Waals forces.
Stronger inter-molecular van der Waals forces give rise to greater boiling
points of alkanes.
A
straight-chain alkane will have a boiling point higher than a branched-chain
alkane due to the greater surface area in contact.
Note: In Chemistry the
Van der Waals forces include attractions and repulsions between atoms,
molecules, and surfaces, as well as other intermolecular forces.
(c) The density
of the alkanes usually increases with increasing number of carbon atoms
Alkane
|
Formula
|
Density
|
Pentane
|
C5H12
|
0.626 (liquid)
|
Hexane
|
C6H14
|
0.659 (liquid)
|
Heptane
|
C7H16
|
0.684 (liquid)
|
Octane
|
C8H18
|
0.703 (liquid)
|
Nonane
|
C9H20
|
0.718 (liquid)
|
(d)
Solubility: Alkanes are generally insoluble in polar
compound like water but dissolve in organic solvents (Non polar compound) like
benzene. The liquid alkanes are good solvents for many other covalent
compounds.
Reason: Alkanes do not conduct electricity,
nor are they polarized by electricity. For this reason they do not form
hydrogen bonds and are insoluble in polar solvents such as water.
(e)
Reactivity:
Alkanes are stable and less reactive than alkenes
Reason: This is because saturated
hydrocarbons contain only single bonds which are very stable and difficult to
break. On the other hand, unsaturated hydrocarbons contain pie bonds, which can
be easily broken as they are more strained.
(f)
Combustion:
Alkanes are generally good combustible material due presence of the good
percentage of Hydrogen.
Since
Alkanes burn with non smoky and non sooty flame due to presence of the good
percentage of Hydrogen, It is used as a fuel like LPG and CNG
2CH4
|
+
|
2O2
|
---------®
|
CO2
|
+
|
H2O
|
+
|
Heat and light
|
2C2H6
|
+
|
7O2
|
---------®
|
4CO2
|
+
|
6H2O
|
+
|
Heat and light
|
C2H4
|
+
|
3O2
|
---------®
|
2CO2
|
+
|
2H2O
|
+
|
Heat and light
|
2C2H2
|
+
|
5O2
|
---------®
|
4CO2
|
+
|
2H2O
|
+
|
Heat and light
|
2CH3 CH2OH
|
+
|
3O2
|
----------®
|
2CO2
|
+
|
3H2O
|
+
|
Heat and light
|
However,
if the supply of air or oxygen is not sufficient for complete combustion,
carbon monoxide is formed. Carbon monoxide (CO) is highly poisonous.
2CH4
|
+
|
3O2
|
-----------®
|
2CO
|
+
|
4H2O
|
2CH4
|
+
|
3O2
|
-----------®
|
2CO
|
+
|
4H2O
|
2C4H10
|
+
|
9O2
|
-----------®
|
8CO
|
+
|
10H2O
|
Q. What is
the difference between Oxidation and combustion?
Answer:
Combustion is the complete oxidation of organic compound into carbon dioxide
and water molecules in presence of oxygen gas while oxidation is the addition
of oxygen in a organic compound or with an element the loss of electron from an
atom or ion is also oxidation.
Hence,
all Oxidation reactions are not combustion reaction but all combustion
reactions are Oxidation. Oxidation reaction does not involve heat where as combustion
reactions do.
(g) Substitution reaction:
Alkanes do not undergo addition reaction due to strong van
der wall force but take part in substitution reaction:
Alkanes
undergo substitution reaction
because hydrogen attached to carbon easily replaced by atom more reactive than
hydrogen like halo atom Cl , Br ,I etc.
|
|
|
Sunlight
|
|
|
|
|
CH4
|
+
|
Cl2
|
---------
|
CH3Cl
|
+
|
HCl
|
|
|
|
|
|
|
|
|
|
|
|
|
Sunlight
|
|
|
|
|
CH4Cl
|
+
|
Cl2
|
----------
|
CH2Cl2
|
+
|
HCl
|
|
|
|
|
|
|
|
|
|
|
|
|
Sunlight
|
|
|
|
|
CH2Cl2
|
+
|
Cl2
|
----------
|
CHCl3
|
+
|
HCl
|
|
|
|
|
|
|
|
|
|
|
|
|
Sunlight
|
|
|
|
|
CHCl3
|
+
|
Cl2
|
-----------®
|
CH3Cl
|
+
|
HCl
|
|
If chlorine present in excess, then reaction does not stop in
between but keep on reacting till it form carbon tetra chloride.
(h) Addition
reaction:
Unlike
alkane, generally alkene Undergo addition reaction with hydrogen gas
in the presence of Palladium or nickel at (473 k) catalyst and gives saturated hydrocarbon.
Example:
Ethene
Undergo addition reaction with hydrogen gas in the presence of
Palladium or nickel at (473 k) catalyst and gives saturated hydrocarbon Ethane
|
|
|
Ni
|
|
|
CH2 = CH2
|
+
|
H2
|
-----------®
|
CH3 - CH3
|
|
|
|
|
473 k
|
|
|
Ethyne
Undergo addition reaction with hydrogen gas in the presence of Palladium or nickel at (473 k) catalyst
and gives saturated hydrocarbon Ethane . This reaction is known as Hydrogenation
Reaction.
|
|
|
Pd
|
|
|
CH2 º
CH2
|
+
|
2H2
|
-----------
|
CH3 - CH3
|
|
This
addition reaction of hydrogen is helpful in converting vegetable Oil into
saturated fat like vanaspati gee. This reaction is commonly called
Hydrogenation of Oil. This test helps to distinguish between saturated and saturated fats.
Note:
Butter contain saturated compound where as cooking oil contain unsaturated
compound. If Alkaline KMno4 added to both, pink color of KMno4 disappear
in cooking oil but remain in pink in butter.
|
|
|
Ni
|
|
||
Vegetable
Oil
|
+
|
H2
|
-----------
|
CH3 - CH3
|
||
Bromine
and chlorine react with alkene to form vicinal dihalides (iodine does not undergo
addition reaction under normal conditions).
|
|
|
CCl4
|
|
|
|
|||||||||
CH2 = CH2
|
+
|
Cl2
|
---------®
|
CH2
|
-
|
CH2
|
|||||||||
|
|
|
|
I
|
|
I
|
|||||||||
|
|
|
|
Cl
|
|
Cl
|
|||||||||
Ethene
|
|
|
|
1,2- Dichloroethane
|
|||||||||||
|
|
|
CCl4
|
|
|
|
|
|
|||||||
CH2 = CH2
|
+
|
Br2
|
---------®
|
CH3
|
-
|
CH
|
-
|
CH2
|
|||||||
|
|
|
|
|
|
I
|
|
I
|
|||||||
|
|
|
|
|
|
Br
|
|
Br
|
|||||||
Ethene
|
|
Bromine water
|
|
1,2-Dibromoethane
|
|||||||||||
Bromine
water test:
Solution of Bromine in water is reddish brown in colour but 1, 2-Dibromoethane
formed from the addition reaction is colourless. This helps us to identify
unsaturated carbon compound. If an organic compound is unsaturated Solution of Bromine
in water became colourless.
Note: In Alkanes,
the four valencies of carbon atom are saturated and they have
stable s (sigma) bonds.
Any nuleophile comes; one hydrogen atom is replaced to accommodate the
nucleophile.
CH4 +
Cl2 ® CH3Cl
+ HCl
In case of Alkene or Alkyne, Carbon atom has double or triple bonds. In
this, one is stable s (sigma) bond
and the rests are unstable (or weak) p bonds. When
nucleophile approaches to the carbon atom that has double or triple bonds, they
easily break to accommodate it.
e.g.,
H2C=CH2 + Cl2 ® H2ClC-CH2Cl.
(i) Cracking (or
pyrolysis): Higher alkanes
undergo thermal decomposition to give lower alkanes. This process is called
pyrolysis or cracking. In this process, vapour of higher alkanes is passed
through a hot metal tube (5000C – 7000C). Propane on
cracking gives,
|
|
D
|
|
C3H6 + H2
|
||
C3H8
|
-----------
|
|
||||
|
|
CH4 + C2H4
|
||||
Cracking of hexane gives
butane and ethane.
|
||||||
|
|
D
|
|
|||
C6H14
|
-------------
|
C4H10 +
C2H4
|
||||
Q. What's
the difference between cracking and Pyrolysis?
Pyrolysis: The decomposition of a compound on
heating in absence of air is known as pyrolysis.
Cracking: is the breakdown of large organic
compounds by use of a catalyst and low temperature to form fewer different
compounds
Q. In cracking will the decomposed hydrocarbon
always be a saturated and a unsaturated hydrocarbons? Can they both be either
unsaturated or saturated hydrocarbons?
Cracking
is defined as the process of breaking or decomposition of large hydrocarbons
into smaller compounds. This process is widely used in the petroleum industry.
Usually, we get a mixture of alkane and alkenes by the process of cracking of hydrocarbons.
This is because the bonds in the hydrocarbons can be cleared in any way. As a
result, we will not get alkane and alkene exclusively but a mixture of both.
(j)
Oxidation:
The chemical reaction in which oxygen added to substance is called oxidation of
substance. The substance which donate oxygen to other substance during chemical
reaction is called oxidizing agent like K2Cr2O7,
HNO3 and KMnO4 . Alkane on
oxidation produces heat and light known as combustion.
(a)
Ethene on
treating with alkaline KMnO4 oxidized to Ethylene Glycol
|
H
|
|
H
|
|
H
|
H
|
|
|||||
|
I
|
|
I
|
Alk. KMnO4
|
|
I
|
I
|
|
||||
H -
|
C
|
=
|
C
|
-
|
H
|
+ H2O
|
+ O
|
----------
|
H –
|
C –
|
C -
|
H
|
|
|
|
|
|
|
|
|
|
|
I
|
I
|
|
|
|
|
|
|
|
|
|
|
|
OH
|
OH
|
|
Ethene
|
|
|
|
|
|
Ethylene Glycol
|
(b)
Ethyne on
treating with alkaline KMnO4 oxidized to Oxalic Acid
|
|
Alk. KMnO4
|
COOH
|
H – C º C – H
|
+ 4O
|
---------
|
I
|
|
|
COOH
|
|
Ethyne
|
|
Oxalic Acid
|
(c) Ethanol on treating with alkaline KMnO4 or acidified K2Cr2O7 oxidized to Ethanoic Acid.
During
oxidation of Ethanol alkaline KMnO4 losses its purple colour .
|
Alk. KMnO4 + Heat
|
|
CH3 CH2 OH + 4O
|
-------------------®
|
CH3COOH
|
Ethanol
|
|
Ethanoic Acid
|
Ethanol
is highly inflammable and catches fire readily. This is why during oxidation it
should never be heat directly on a burner. it should be heated on a water bath.
The
ionization energy of an atom or molecule describes the amount of energy
required to remove an electron from the atom or molecule in the gaseous state.
X
+ energy → X+ + e- Know
your chemistry knowledge
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