Alkanes
Alkanes
Alkanes
2 types of hydrocarbons:
Aliphatic hydrocarbons
Aromatic hydrocarbons
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3 groups of aliphatic hydrocarbons:
Alkanes
Alkenes
Alkynes
Aromatic hydrocarbons:
Hydrocarbons with benzene rings.
They are also known as arenes.
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Saturated hydrocarbons:
Alkanes
Cycloalkanes
Unsaturated hydrocarbons:
Alkenes
Alkynes
Arenes
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Saturated hydrocarbons.
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6
The prefix of the name indicates the number
of carbon atoms.
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Steps in naming branched
chain alkanes:
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Methyl
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4. Name the hydrocarbons.
▪ Write the name (substituent and
parent HC) as one word. Ex:
methylhexane (NOT methyl hexane).
▪ Specify the locations of substituents
using the numbers assigned to the
carbon atoms of the main chain.
▪ If more that one type of substituent
is present, they are named in
alphabetical order (ethyl before
methyl).
▪ If two or more identical substituents 2,2,4-trimethylhexane
are present, use the prefixes di-, tri-
tetra- etc.
▪ Use hyphens to separate numbers
from words and commas to separate
numbers.
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Names of alkyl groups:
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12
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Prefixes for substituents groups:
1 Mono-
2 Di-
3 Tri-
4 Tetra-
5 Penta-
6 Hexa-
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Combustion of alkanes
Halogenation of alkanes
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Combustion of alkanes:
Alkanes burn in a plentiful supply or air or oxygen to
produce water and carbon dioxide only (complete
combustion).
y y
C x H y (x )O2 xCO 2 H 2O
4 2
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In a limited supply of oxygen, combustion of alkanes
produces carbon monoxide and water.
5
C2 H 6 ( g ) O 2 ( g ) 2CO(g) 3H 2O(g)
2
3
C2 H 6 ( g ) O 2 ( g ) 2C(s) 3H 2O(g)
2 18
Halogenation of alkanes:
At room temperature, alkanes do not react when
mixed with chlorine or bromine in the dark.
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Ex: Reactions of methane with chlorine:
CH 4 Cl 2 CH 3Cl HCl............(1)
CH 3Cl Cl 2 CH 2 Cl 2 HCl
CH 2 Cl 2 Cl 2 CHCl3 HCl
CHCl3 Cl 2 CCl 4 HCl...........(2)
If, excess methane and limited chlorine; major product is
chloromethane (1).
If, excess chlorine and limited methane; major product is
tetrachloromethane (2).
Bromine reacts with alkanes in the same way.
Iodine does not react well with alkanes.
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The reaction in which an atom or group of atoms in an
organic compound is replaced by another atom or group
of atoms is called a substitution reaction.
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Mechanism of free radical substitution reaction:
The reaction between alkane and halogen gas
proceeds via a free radical mechanism.
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Free radical mechanism between methane and
chlorine gas:
▪ Initiation:
▪ Homolytic fission of chlorine molecules to produce chlorine
radicals (atoms).
▪ The splitting of a chlorine molecule by absorption of a photon
(hv).
hv
+
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▪ Propagation:
▪ Reaction of a free radical species (highly reactive) to
produce another species of free radical.
▪ When chlorine radicals collide with methane molecules,
they remove hydrogen atoms from methane to form
hydrogen chloride and methyl radicals.
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▪ The production of methyl radicals propagates a chain reaction as
the methyl free radical then reacts with another chlorine
molecule to form chloromethane and a chlorine free radical.
▪ The chlorine free radical produced can then react with another
methane molecule and the cycle is repeated.
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▪ Termination:
▪ The reaction stops when two free radicals collide and
combine.
Cl Cl Cl 2
CH 3 Cl CH 3Cl
CH 3 CH 3 CH 3 CH 3 (by product)
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▪ If methane is in excess, the major product is chloromethane.
(CH3Cl)
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CH 3Cl Cl CH 2 Cl HCl
CH 2 Cl Cl 2 CH 2 Cl 2 Cl
dichloromethane
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