2. Preparations of alkanes-
Sabateier and senderens reactions-
Decarboxylation reaction
Reduction of alkyl halides
Reduction of alcohols, aldehydes,ketones and
carboxylic acids
Wurtz reaction
Hydrolysis of Grignard reagents
Kolbe’s method
3. 1) Sabatier-and Senderens Reactions-
In this type of reaction unsaturated hydrocarbons( like-
alkenes; alkynes) are treated with hydrogen in the presence of
nickel at 200-300˚C than same no. of alkane are produced.
Example- Ni
200-300˚C
Alkene + 𝑯 𝟐 Alkane
𝑪𝑯 𝟐 = 𝑪𝑯 𝟐 + 𝑯 𝟐 Ni (200-300˚C) 𝑪𝑯 𝟑- 𝑪𝑯 𝟑
Ethene Ethane
4. Important point of this reaction-
Palladium and platinum can also be used
in this reaction in place of nickel but
doesn’t use because they are expensive.
This method is used in the industry
because large amount of alkynes and
alkanes are obtained from cracking of
petroleum.
6. 2) Decarboxylation of carboxylic acid
When the sodium salts of carboxylic acid is heated
strongly with sodalime ( NaOH+ CaO) than carbon
dioxide and alkane are produced.
This process is also known as Decarboxylation
reaction.
Example-
R-COONa + NaOH RH + 𝑵𝒂 𝟐 𝑪𝑶 𝟑
𝑪𝑯 𝟑 𝑪𝑶𝑶𝑵𝒂 + 𝑵𝒂𝑶𝑯 𝑪𝑯𝟒 + 𝑵𝒂 𝟐 𝑪𝑶 𝟑
Sodium acetate Methane
7. Important point of this reaction-
In this reaction one carbon less than the reactant is
formed in the product.
9. 3) Reduction of alkyl halides-
When alkyl halides are treated with nascent hydrogen they
get converted into corresponding alkanes.
Example-
R-X + 2H R-H + HX
Alkyl halide Alkane
𝑪𝑯 𝟑I + 2H 𝑪𝑯 𝟑- H + H-I
Methyl iodide Methane
10. Important point of this reaction-
This reaction occurs best with iodides and bromides.
Chlorides and fluorides are less reactive.
This method is used to give pure hydrocarbons of
alkanes.
11. Questions-
What is Lucas reagent.
Give example of various reducing agent
which can be used in this reaction.
12. 4) Reduction of Alcohol; Aldehyde; Ketone and
Carboxylic acid
When aldehyde; ketone; carboxylic acid and alcohol
are treated with hot concentrated hydriodic acid in
combination with red phosphorus reduces oxygenated
compounds to alkanes.
Example- P
(150˚C)
R-OH + 2HI R-H + 𝑰 𝟐 + 𝑯 𝟐O
Alcohol Hydriodic Alkane
acid
16. Important point of this reaction-
Hydriodic acid is an effective reducing agent.
This method is used to produce higher alkanes from
carboxylic acid obtained from hydrolysis of fats.
17. 5) Wurtz Reaction-
When two molecules of alkyl halide are treated with
sodium metal in presence of dry ether than higher
alkanes are produced this type of reaction is named as
wurtz reaction.
Example- dry ether
2R-X + 2Na R-R + 2NaX
Alkyl halide Alkane
dry ether
2𝐶𝐻3Br + 2 Na 𝐶𝐻3 𝐶𝐻3 + 2 Na Br
Methyl bromide Ethane
19. Free Radical Mechanism-
In this it involves the attack of a sodium atom on alkyl
group like methyl bromide and form a methyl free radical.
Than 2 methyl free radical combine to give ethane.
dry ether
2𝐶𝐻3Br + 2 Na 𝐶𝐻3 𝐶𝐻3 + 2 Na Br
Methyl bromide Ethane
Mechanism- Step1- 𝐶𝐻3Br + Na . 𝐶𝐻3 + Na Br
Methyl free radical
Step2- 2𝐻3 C. 𝐶𝐻3 𝐶𝐻3
20. Ionic Mechanism-
This involves the intermediate formation of metal alkyls. In this
methyl bromide react with 2 sodium atoms to produce methyl sodium
as an intermediate and sodium bromide. When this methyl sodium
react with another molecule of methyl bromide than ethane is
produced.
Example- 2𝐶𝐻3Br + 2 Na 𝐶𝐻3 𝐶𝐻3 + 2 Na Br
Methyl bromide Ethane
Mechanism Step1- 𝐶𝐻3Br + Na 𝐶𝐻3 𝑁𝑎++ N𝑎+ 𝐵𝑟 −
Methyl sodium
Step2- 𝐶𝐻3 𝑁𝑎+
+𝐶𝐻3Br 𝐶𝐻3 𝐶𝐻3 + N𝑎+
𝐵𝑟 −
Ethane
21. Important point of this reaction-
When different alkyl halides are taken in this reaction than it produces
a mixture of alkyl halides.
The order of reactivity of alkyl halide- RCl< RBr< RI.
Other metal that can be used in place of sodium are zinc and finely
divided silver.
Limitations - Only used for symmetrical alkanes production.
Mixtures of alkanes cannot be separated because of
less difference in their boiling point.
22. 6) Hydrolysis of Grignard Reagents-
Alkylmagnesium halides ( Grignard Reagent) are obtained by heating
alkyl halides with magnesium metal in anhydrous ether. These on
treatment with water decomposes to give alkanes containing the same
number of carbon atoms as the original alkyl halide.
Example- dry ether
R-X + Mg R-Mg-X
Alkylmagnesium halide ( Grignard Reagent)
R-Mg-X + H-OH R-H + (MgOH)+ 𝑿−
Water Alkane
23. 7) Kolbe’s method-
When a concentrated solution of sodium or potassium salt of a
carboxylic acid is electrolysed, a higher alkane is formed.
Example- electrolysis
2RCOONa + 2𝑯 𝟐O R-R + 2𝑪𝑶 𝟐+ 2NaOH+ 𝑯 𝟐
Sodium carboxylate At anode At cathode
2𝑪𝑯 𝟑COONa + + 2𝑯 𝟐O 𝑪𝑯 𝟑 𝑪𝑯 𝟑 + 2𝑪𝑶 𝟐+ 2NaOH+ 𝑯 𝟐
Sodium acetate Ethane
Anode Cathode
24. Important point of this reaction-
This reaction is also used for symmetrical
alkanes.
25. “ Action is the foundation key to all
success’’.
Thank You
