Alkanes

Alkanes 1
 Hydrocarbons are organic compounds that

contain only carbon and hydrogen.
 2 types of hydrocarbons:
 Aliphatic hydrocarbons
 Aromatic hydrocarbons
2
 3 groups of aliphatic hydrocarbons:
 Alkanes
 Alkenes
 Alkynes
 Aromatic hydrocarbons:
 Hydrocarbons with benzene rings.
 They are also known as arenes.
3
 Saturated hydrocarbons:
 Alkanes
 Cycloalkanes
 Unsaturated hydrocarbons:
 Alkenes
 Alkynes
 Arenes
4
 Saturated hydrocarbons.
 All C-C bonds in alkanes are single bonds.
 Open chain alkanes = CnH2n+2 (n≥1)
 Cycloalkanes are cyclic alkanes in which the

carbon atoms are arranged in a ring; C2H2n (n≥3)
5
6
 The prefix of the name indicates the number
of carbon atoms.
 The suffix “ane” indicates that it is an alkane.
7
 Steps in naming branched
chain alkanes:
1. Identify and name the parent HC

(longest unbranched carbon
chain)
▪ If 2 different chains of equal length
are present, choose the one which
has more alkyl groups attached to
the main chain (page 115).
8
Methyl
2. Name the alkyl groups

(substituents).
▪ Alkyl groups are named by
replacing the “ane” suffix of
the alkane with “yl”.
3. Number the carbon atoms

in the main chain.
▪ The carbon atoms are
numbered from the end of
the chain which gives the
substituents the lowest
possible number.
9
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.
10
Names of alkyl groups:
11
12
13
Prefixes for substituents groups:
1 Mono-
2 Di-
3 Tri-
4 Tetra-
5 Penta-
6 Hexa-
14
15
 Combustion of alkanes
 Halogenation of alkanes
 Free radical substitution reactions
16
 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
17
 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
 In a very limited supply of air, alkanes burn to form

carbons as one of the products.
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.
 If the mixture is heated to a high temperature (300 –

400 °C) or irradiated by UV light, the hydrogen atoms
in the alkane are successively replaced by chlorine or
bromine atoms to produce a mixture of products
(halogenated alkanes).
19
 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.
20
 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.
 A substitution reaction that involves halogen is known as

halogenation.
 If the halogen is chlorine, the reaction is called

chlorination.
 If the halogen is bromine, the reaction is called

bromination.
21
 Light or heat is needed to initiate the reactions
between halogens and alkanes.
 UV radiation provides the energy needed by the

reactant molecules to produce free radicals.
22
 Mechanism of free radical substitution reaction:
 The reaction between alkane and halogen gas
proceeds via a free radical mechanism.
 3 steps involved in this reaction:

▪ Initiation
▪ Propagation
▪ Termination
23
 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
+
24
▪ 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.
25
▪ 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.
26
▪ 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)
27
▪ If methane is in excess, the major product is chloromethane.
(CH3Cl)
 The propagation steps may proceed with reaction between a

chlorine free radical with chloromethane to produce
dichloromethane. (CH2Cl2)
 The reaction may continue to produce trichloromethane

(CHCl3) and finally tetrachloromethane (CCl4).
28
CH 3Cl  Cl  CH 2 Cl  HCl
 CH 2 Cl  Cl 2  CH 2 Cl 2  Cl 
dichloromethane
CH 2 Cl 2  Cl  CHCl2  HCl

 CHCl2  Cl 2  CHCl3  Cl 
trichloromethane
CHCl3  Cl  CCl 3  HCl

 CCl 3  Cl 2  CCl 4  Cl 
tetrachloromethane
29

Alkanes

Uploaded by

Copyright:

Available Formats

Alkanes

Uploaded by

Document Information

Original Description:

Copyright

Available Formats

Share this document

Share or Embed Document

Sharing Options

Did you find this document useful?

Is this content inappropriate?

Copyright:

Available Formats

Alkanes

Uploaded by

Copyright:

Available Formats

Alkanes 1

 Hydrocarbons are organic compounds that

 All C-C bonds in alkanes are single bonds.

 Open chain alkanes = CnH2n+2 (n≥1)

 Cycloalkanes are cyclic alkanes in which the

 The suffix “ane” indicates that it is an alkane.

1. Identify and name the parent HC

2. Name the alkyl groups

3. Number the carbon atoms

 Free radical substitution reactions

 In a very limited supply of air, alkanes burn to form

 If the mixture is heated to a high temperature (300 –

 A substitution reaction that involves halogen is known as

 If the halogen is chlorine, the reaction is called

 If the halogen is bromine, the reaction is called

 UV radiation provides the energy needed by the

 3 steps involved in this reaction:

 The propagation steps may proceed with reaction between a

 The reaction may continue to produce trichloromethane

CH 2 Cl 2  Cl  CHCl2  HCl

CHCl3  Cl  CCl 3  HCl

You might also like