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Introduction
The compounds having only C, and H-atoms
Concept Ladder
are known as Hydrocarbon. Natural gas, Petroleum
and coal are major sources of Hydrocarbon. Alkanes are saturated
acyclic compounds i.e. they
Alkanes have single bonds only.
y The general formula of alkanes is Cn H2n + 2.
y The main sources of alkanes include
petroleum, natural gas and coal.
y Alkanes are also termed as paraffins due to
their almost inert nature or reduced activity.
y Bond length of C–C is 1.54 Å and that of C–H
bond is 1.11 Å. Rack your Brain
y Alkanes gives a C–C bond energy of 80 kcal
per mole and for C–H bond it is calculated Why alkanes are known as
as 97 kcal per mole. Paraffins?
y They are non-polar in nature.
y Alkanes show chain, position, and
conformational isomerism.
e.g , ,
y Liquefied Petroleum Gas (LPG) is composed Concept Ladder
of propane, butane, and isobutane.
CaO added, makes sure
Methods of Preparation of Alkanes that the reaction occurs
By decarboxylation of sodium salts of fatty acids at relatively higher
When R–COOH heated with Sodalime (4 : 1 temperature and hence
Mixture of CaO + NaOH) alkane is formed. Here complete decarboxylation
alkane has one C– atom less than R–COOH as occurs.
one CO2 molecule is eliminated.
NaOH ∆
R – COOH
–H O
→ R – COONa
NaOH+CaO
→ R – H + Na2CO3
2
R–may be ® CH3 , C2 H5 , C6 H5 etc
∆
2HCOONa
CaO
→ H2 + Na2CO3
Hydrocarbon
1.
, y Here CaO is used to make the reaction more
mild and to keep NaOH dry. It also helps in
easy fusion.
Rack your Brain
By Sabatier and Senderen Reaction
Why we need CaO in the above
y Here, catalysts used are Raney Ni [An alloy of
reaction?
Ni–Al] and Pd–Pt (Adam’s catalyst).
y Temperature range suited for reaction is 300–
400°C.
y In case of Raney Nickel reaction is possible at
room temperature also.
H H
For Example : CH3 – C ≡ CH
2
Ni
→ CH3 –
= CH CH2
2
Ni
→ CH3 − CH2 − CH3
Concept Ladder
From the reduction of R–X Addition of H2 to the
y Here, the reducing agent used is usually Zn + unsaturated compounds
NaOH or HCl or Zn–Cu/acid, Na/alcohol. to form alkanes is called
LiAlH4 , NaBH4 , (Ph)3 SnH etc reduction.
Zn/HCl
R – X + H – H → R – H + HX
Mechanism
Zn ® Zn+2 + 2e–
R – X + e– ® R° + X– Rack your Brain
R:– + C2 H5 OH → R – H + C2 H5 O–
∆ ,150°C
R – I + HI → R – H + I2 ↑ How raney Ni is different from
Ni?
LiAlH
Hydrocarbon
or
(Ph),SnH
R–X
H2
→R – H + HX
2.
,y Order of Reduction reaction is RI > R – Br >
R – Cl > R – F. LiAlH4 is effective in case of 1° Rack your Brain
and 2° halides while (Ph)3SnH is effective for
all types of halides. Why 3° alkyl halides undergo
dehydrohalogenation with
LiAlH4?
Using Wurtz Reaction
y In case of Wurtz reaction, alkyl halide is
treated with sodium metal in presence of dry
ether to give alkanes.
Dry ether
y R – X + 2Na → R – R + 2NaX
y If ‘R’ group attached is same, then one alkane Concept Ladder
is formed.
2Na
HCl or Zn–Cu/acid –
e.g 2CH3Cl
Ether
→ C2H6 + 2NaCl reduction occurs by
y If ‘R’ group attached is different, then it leads Transfer of Electrons.
to formation of three alkanes. Zn/NaOH – reduction
R – X + 2Na + X – R '
Dry ether
→ R '– R+ R '– R '+ R – R ' occurs with Nascent
–NaX
Hydrogen
For example
The mechanism involved in the reaction is :
• •
R – X + Na
→ R+ NaX
• • Concept Ladder
R+ R
→R – R
Here disproportion is also possible. Wurtz reaction is used to
C2H5• + C2H5• → C2H4 + C2H6
prepare alkanes containing
even number of carbon
y In case of tertiary–halide reaction is not possible
atoms.
as major product is according to dehydro-
halogenation which means reaction holds only
Hydrocarbon
for a 1° or 2° halide.
3.
, y In place of Na, activated Cu, Ag, Fe can also
be used. Rack your Brain
y It is good for symmetrical alkanes (C2H6, C4H10)
and CH4 can not be obtained.
Name an alkane which can not
be prepared by wurtz reaction?
y In Frankland reaction, alkyl halide is treated Previous Year’s Question
with Zn in a closed tube to prepare alkanes.
R – X + Zn + X – R
→ R – R + ZnX2 Which of the following alkane
cannot be made in good yield by
y Rest of process is same as given above in Wurtz reaction? [NEET]
Wurtz reaction. (1) n-Hexane
(2) 2,3-Dimethylbutane
Corey–house synthesis: (3) n-Heptane
This method is suitable for preparation of (4) n-Butane 2
alkanes possessing odd number of carbon
atoms also (un-symmetrical alkanes).
Li Cul R'–X
R – X
Ether
→ R – Li → R2CuLi → R – R '+ R – Cu + LiX
–Li X
The process can be illustrated in following
example:
Concept Ladder
Corey House reaction is
limited to a 1° alkyl halide
and the alkyl groups in the
Dialkyl Lithium Cuprate
may be 1°, 2° or 3°.
Here R–X must be used for better yield.
Kolbe Synthesis
Hydrocarbon
4.
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