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Alkane 11
1. Alkanes
• Introduction
• Alkanes are aliphatic saturated hydrocarbons.
• They are represented by the general formula CnH2n+2
• Where ‘n’ is the no. of carbon atoms.
• They contain C-C single bonds.
• They are also called as paraffin’s(means less affinity)
• Because they do not undergo chemical reactions easily.
2. • Structural formula of a compound gives the exact arrangement
of atoms various elements in a molecule.
• It can be represented in the different ways.
• E.g. Methane CH4
Alkanes.swf
CH4 Methane.swf
3. Structural formula
• Condensed formula
• In this – (dash) representing covalent bonds are omitted and
identical groups attach to ‘C’ atoms indicated by subscript.
• E.g. Hexane C6H14
4. Structural formula
• Bond line formula
• A molecule can be represented by lines representing C-C bond in
zig - zag manner. In this terminal denotes – CH3 group and line
junction indicates – CH2 group.
• E.g. n-hexane C6H14
9. Isomerism
• The compounds having same molecular formula
but different structural formulae are called
isomers of each other.
• This phenomenon is called as isomerism.
• First three alkanes do not show isomerism.
• From butane and onwards alkanes show
isomerism.
• Isomers have different physical and chemical
properties.
11. Types of ‘C’ atoms
• There are four types of ‘C’ atoms
Primary (10
) C atom: which is attached to one ‘C’ atom.
Secondary (20
) C atom: which is attached to 2 more ‘C’ atoms.
Tertiary (30
) C atom: which is attached to 3 more ‘C’ atoms.
Quaternary (40
) C atom: which is attached to 4 more ‘C’ atoms.
12. Confirmations of Ethane
• Conformation : The phenomenon of different
spatial arrangements of atoms that can be
converted into one – another by free rotation of
atoms about C – C single bonds is called
conformation.
• And the compounds are called conformers or
rotamers.
• These conformational isomers interconvert
rapidly and cannot be isolated easily.
• Ethane (S).swf
13. Conformations of ethane
• Ethane (C2H6) has two carbon atoms joined by single covalent
bond. Each carbon atom has three hydrogen atoms.
• Considering free rotation of one of the carbon atom around the C
– C bond axis, infinite number of spatial arrangements of
hydrogen atoms (attached to one carbon atom with respect to
the hydrogen atoms attached to another carbon atom) are
possible.
• These are called conformational isomers.
• As they all have nearly the same energy, they can change from
one form to another freely.
14. Conformations of ethane
• Thus two extreme arrangements are considered:
• viz. Staggered and
• Eclipsed conformations.
• a. Staggered conformation: In this arrangement hydrogen
atoms attached to two carbon atoms are as far apart as possible.
• b. Eclipsed conformation: In this arrangement hydrogen
atoms attached to two carbon atoms are as close as possible.
• The intermediate conformations during rotation are called skew
conformation.
15. Representation of conformation
• Sawhorse projection of ethane:
In this representation, C – C bond is viewed from
an oblique angle which indicates spatial
arrangements by showing all C – H bonds.
Ethane (staggered) - sawhorse projection.swf
Ethane (eclipsed) - sawhorse projection.swf
16. Representation of conformation
• Newmann projection of ethane:
In this representation, C – C bond is viewed
directly end – on and represents two carbon
atoms by a circle.
• Ethane (staggered) - newman projection.swf
• Ethane (eclipsed) - newman projection.swf
17. Alkyl Group
• An alkyl group is a free radical obtained by
homolytic fission of C – H bond in alkane.
18. Alkyl groups are classified
1. Primary alkyl group : In this alkyl group, valency
on the primary carbon atom is available.
Example: methyl radical
2.Secondary alkyl group : In this alkyl group valency
of a secondary carbon atom is available.
Example: isopropyl radical,
3.Tertiary alkyl group : In this alkyl group, valency
of a tertiary carbon atom is available.
Example: tert – butyl radical,
19. IUPAC nomenclature of alkanes
• The rules of IUPAC nomenclature which you have
studied in chapter 12 are applied.
• Common names of first 4 alkanes are used in
IUPAC nomenclature.
• Methane, Ethane, Propane and Butane
• All normal chain alkanes are considered as parent
alkanes.
• Common nomenclature system.swf
23. IUPAC nomenclature of alkanes
1. Locate the longest continuous chain of carbon atoms; this
chain determines the parent name for the alkane. We
designate the following compound, for example, as a hexane
because the longest continuous chain contains six carbon atoms:
The longest continuous chain may not always be obvious from
the way the formula is written. Notice, for example, that the
following alkane is designated as a heptane because the longest
chain contains seven carbon atoms:
24. IUPAC nomenclature of alkanes
• 2. Number the longest chain beginning with the end of
the chain nearer the substituent. Applying this rule, we
number the two alkanes that we illustrated previously
in the following way:
25. IUPAC nomenclature of alkanes
3. Use the numbers obtained by application of rule 2 to
designate the location of the substituent group. The parent
name is placed last, and the substituent group, preceded by
the number designating its location on the chain, is placed
first. Numbers are separated from words by a hyphen. Our
two examples are 2-methylhexane and 3-methylheptane,
respectively:
26. IUPAC nomenclature of alkanes
4. When two or more substituents are present, give each
substituent a number corresponding to its location on the
longest chain. For example, 4-ethyl-2-methylhexane:
The substituent groups should be listed alphabetically (i.e.,
ethyl before methyl).* In deciding on alphabetical order,
disregard multiplying prefixes such as “di” and “tri.”
5. When two substituents are present on the same carbon
atom, use that number twice:
27. IUPAC nomenclature of alkanes
6. When two or more substituents are identical, indicate this
by the use of the prefixes di-, tri-, tetra-, and so on. Then
make certain that each and every substituent has a number.
Commas are used to separate numbers from each other:
7. When two chains of equal length compete for selection as
the parent chain, choose the chain with the greater number
of substituents:
28. IUPAC nomenclature of alkanes
8. When branching first occurs at an equal distance from either
end of the longest chain, choose the name that gives the
lower number at the first point of difference:
29. Methods of preparation of alkanes
• From unsaturated hydrocarbons by hydrogenation
• Decarboxylation of Na – salt of fatty acids
• From alkyl halides by reduction
• Wurtz synthesis
30. Methods of Preparation of Alkane
Alkanes are prepared by the following methods:
1. Reduction Reactions:
Alkanes can be prepared by the reduction of various organic
compounds as follows:
31. From unsaturated hydrocarbons by hydrogenation
• Alkenes and alkynes are unsaturated hydrocarbons.
When mixture of alkenes/alkynes and H2(g) passed over
Raney Ni catalyst at 473K to 573K form corresponding
alkane by addition reaction.
Raney Ni
2 2 2 3 3
Ethene Ethane
CH CH H CH CHD
= + -¾¾¾¾®
Raney Ni
2 3 3
Ethyne Ethane
CH CH 2H CH CHD
+ -º ¾ ¾ ¾ ¾®
32. Decarboxylation of Na – salt of fatty acids
• When anhydrous Na- salt of fatty acid is heated with
soda lime (mixture of NaOH + CaO) forms alkane by
decarboxylation. Alkane containing one atom less than
acid is obtained.
2 5 2 6 2 3
CaO
sodium propionate ethane
C H COONa NaOH C H Na COD+ +¾¾¾®
33. From alkyl halides by reduction
• Alkyl halides when treated with reducing agent like Zn
– Cu couple and alcohol form corresponding alkane.
3 4
Zn Cu
alcohol
methaneMethyl bromide
CH Br 2H CH HBr-
D
+ +¾¾¾¾®
2 5 2 6
Zn Cu
alcohol
Ethyl bromide Ethane
C H Br 2H C H HBr-
D
+ +¾¾¾¾®
34. Wurtz synthesis
• When alkyl halide is treated with sodium metal
in the presence of dry ether as solvent gives
higher alkanes.
Dry
2 5 2 5 4 10ether
Ethyl bromide Butane
C H Br 2Na Br C H C H 2NaBr+ + +¾¾¾®
36. Physical Properties of Alkane
1. In normal alkanes, as the no. of C atoms
increases, melting point and boiling point
increase due to increase in intermolecular
forces.
2. Branched alkanes have lower boiling points than
straight chain.
More the no. of branches lower is the boiling
point because increased branching increases
surface area and decreases intermolecular
forces.
3. Alkanes are insoluble in H2O, but soluble in non
polar solvents like C6H6, CHCI3 etc.
4. First 4 are gases, C5 to C17 are liquids, remaining
are solids.
37. Chemical properties of alkane
• 1. Halogenation
Alkanes react with halogens like chlorine or
bromine to form alkyl halide in presence of diffused
sunlight or U.V. light or by heating.
Chlorination of methane:
• In this reaction one by one H atoms replaced by
Chlorine atoms to form mixture of products.
• Alkyl halide is obtained by limiting supply of
chlorine.
39. Chlorination of methane
• Mechanism of halogenations – Chlorination of
CH4. It is a chain reaction, involves a series of
steps. Each step generates a reactive species that
brings about the next step.
i. Chain initiation step –
ii. Chain Propagation step –
iii. Chain termination step –
40. Chlorination of methanei. Chain initiation step –
• CI2 molecule absorbs energy; bond breaks
homolytically to give chlorine free radicals.
41. Chlorination of methane
ii. Chain Propagation step – Chlorine free radical is
highly reactive. It abstracts a H atom of CH4 and
forms Methyl free radical and HCI.
• Methyl free radical attacks chlorine to form CH3CI
and Chlorine free radical.
• These steps are repeated many times. Overall
reaction is
42. Chlorination of methane
iii. Chain termination step – After some time, reaction
stops due to combination of free radicals.
may further get chlorinated.
43. Bromination, Nitration of alkanes
Bromination is carried out in presence of AlBr3
3
2 6 2 2 5
AlBr
Ethyl bromide
C H Br C H Br HBrD+ +¾¾¾®
Nitration of Alkanes: In this reaction one H
atom of alkane is replaced by nitro ( - NO2 )
group.
423 to 698 k
2 6 3 2 5 2 2
Ethane Conc.nitricacid nitroethane
C H HNO C H NO H OD
+ +¾¾¾¾®
44. Pyrolysis of alkanes
• The thermal decomposition of alkanes in
absence of air to give lower alkanes, alkenes and
H2 is called as pyrolysis. It takes place as follows.
Dehydrogenation : It involves breaking of C-H bond
in alkanes to form alkenes by dehydrogenation.
3 3 2 2 2
etheneethane
CH CH CH CH HD
- = +¾¾®
Cracking : It involves breaking of C - C bonds and
C - H bonds to form lower alkanes and alkenes.
Cracking.swf
3 2 3 2 2 4
ethene MethanePropane
CH CH CH CH CH CHD
- - = +¾¾®
45. Pyrolysis of alkanes
Cracking : It involves breaking of C - C bonds and
C - H bonds to form lower alkanes and alkenes.
3 2 3 2 2 4
ethene MethanePropane
CH CH CH CH CH CHD
- - = +¾¾®
Aromatization : Alkanes containing more than 5 ‘C’
atoms get cyclised to benzene and it’s homologues
on heating under 10 to 20 atm. at 773K in presence
of Cr2O3.
46. Combustion
• Alkane when heated in air, combine with oxygen to
give CO2(g) and water vapour.
• It is a exothermic reaction because large amount of
heat is evolved.
( ) ( ) ( ) ( )24 g 2 g 2 g g
Methane
CH 2 O 2 H O heatCOD
+ + +¾¾®
47. Uses of alkanes
1) As fuels e.g. LPG, CNG, Petrol, Diesel etc.
2) Liquid alkanes used as solvents.
3) C17 to C20 as lubricants.
4) C21 to C30 as a lubricant base for preparation of
cosmetics and candles.
5) As a source of Hydrogen.
6) Incomplete combustion gives carbon black for
manufacture of printing ink, polishes, black
pigments etc.
Uses of alkanes.swf
