Preparation of Alkenes for Pharmacy

1. Preparation of
alkenes
For B-Pharm students
Presented By-
Sandhya Punetha

2. Preparations of alkenes-
1) Dehydration of alcohol
2) Dehydrohalogenation of alkyl halides
3) Dehydrogenation of vicinal dihalides
4) Partial hydrogenation of alkenes
5) Pyrolysis of alkanes
6) Pyrolysis of carboxylic esters
7) Pyrolysis of quaternary ammonium hydroxide
8) Electrolysis of salts of dicarboxylic acids

3. 1) Dehydrogenation of alcohols-
 Dehydrogenation means removal of a molecule of
water from a reactant molecule.
 When an alcohol is heated in the presence of
sulphuric acid, a molecule of water is eliminated and
an alkene is formed.
 Example- 𝑯 𝟐 𝑺𝑶 𝟒
R-CH-𝑪𝑯 𝟑 R-CH=C𝑯 𝟐 + 𝑯 𝟐O
H OH Alkene
Alcohol

4. IMPORTANT FACTS-
 The order of reactivity of alcohol is-
Tertiary alcohol > secondary alcohol> primary alcohol.
 This reaction follows saytzeff rule.
 Saytzeff rule states that when alternatives exist hydrogen is
preferentially eliminated from that carbon atom with fewer
number of hydrogen atoms.
 Example- 𝑯 𝟐 𝑺𝑶 𝟒
𝑪𝑯 𝟑- CH-CH-𝑪𝑯 𝟑 𝑪𝑯 𝟑- CH=CH-𝑪𝑯 𝟑
H OH 2-Butene
2-Butanol
 Dehydrating agents are- Phosphorus pentaoxide ( 𝑷 𝟐 𝑶 𝟓)
Phosphoric acid ( 𝑯 𝟑 𝑷𝑶 𝟒)

5. QUESTIONS-
 Complete the reaction-
𝐻2 𝑆𝑂4
𝐶𝐻3- 𝐶𝐻2- CH- 𝐶𝐻2 ˀˀˀˀˀ
H OH
 Give examples of dehydrating agents.

6. 2) Dehydrohalogenation of alkyl halide
 Dehydrohalogenation involves the removal of the
elements of hydrogen halides (H-X) from adjacent
carbon atoms.
 When alkyl halide is heated with an alcoholic
solution of sodium or potassium hydroxide a
molecule of hydrogen halide is eliminated and an
alkene is formed.
 Example-
alcohol
R-C𝑯 𝟐-𝑪𝑯 𝟐X + KOH R-CH=C𝑯 𝟐 + KX+𝑯 𝟐O
Alkyl halide Alkene

7. IMPORTANT FACTS-
 Reactivity of alkyl halides-
Tertiary > Secondary> Primary
 Halogens reactivity-
Iodine> Bromine> Chlorine> Fluorine

8. QUESTION-
 COMPLETE THE REACTION-
alcohol
𝐶𝐻3 𝐶𝐻2Br + NaOH
Ethyl bromide
alcohol
𝐶𝐻3 𝐶𝐻2 𝐶𝐻2Br + NaOH

9. 3) Dehalogenation of Vicinal dihalides
 Dehalogenation involves the removal of a halogen
molecule ( X-X) from a reactant molecule.
 A compound having two halogen atoms of adjacent
carbon atoms is called a vicinal dihalide.
 The treatment of vic- dihalides with zinc dust using
ethyl alcohol as solvent results in dehalogenation
and an alkene is formed.
H H ethanol
R - C – C- H + Zn R-CH=C𝑯 𝟐 + Zn𝑩𝒓 𝟐
Br Br Alkene
Vic- dihalide

10. IMPORTANT FACTS-
 This reaction is used for separating an alkene
from non-olefinic substrates and also in the
purification of alkenes.

11. 4) By Partial Hydrogenation of Alkynes-
 Alkynes can be hydrogenated using Raney Nickel or
finely divided Platinum or Palladium to give alkenes.
 Under these conditions alkenes are further reduced
to alkanes.
 It is possible to stop the reaction if mild catalyst
such as palladium on calcium carbonate partially
deactivated by lead acetate (Lindlar’s catalyst) is
used. Lindlar’s catalyst
 Eg- R-CΞC-H + 𝑯 𝟐 R-CH=𝑪𝑯 𝟐
Alkyne Alkene

12. 5) By Pyrolysis or Cracking of Alkanes
 Higher molecular weight alkanes when heated to high
temperature (500-700˚𝐂) 𝐝𝐞𝐜𝐨𝐦𝐩𝐨𝐬𝐞𝐬 𝐭𝐨 𝐠𝐢𝐯𝐞𝐬
lower molecular weight alkenes, alkanes and
hydrogen.
600˚C
 Eg- 𝑪𝑯 𝟑 − 𝑪𝑯 𝟑 𝑪𝑯 𝟐 = 𝑪𝑯 𝟐 + 𝑪𝑯 𝟒 + 𝑯 𝟐
Ethane Ethene Methane

13. 6) By Pyrolysis of Carboxylic Esters-
 Esters particularly acetates when heated to a high
temperature ( 450-550˚C) break down to give
carboxylic acid and an alkene.
 Eg-
O 450-550˚
R-C-OC𝑯 𝟐C𝑯 𝟐R’ R’CH=C𝑯 𝟐+ RCOOH
Carboxylic ester Alkene carboxylic ester

14. 7) Pyrolysis of Quaternary Ammonium
Hydroxide-
 This reaction is also known as Hofmann Degradation
Reaction.
 The quaternary ammonium hydroxides when heated
at 100-200˚ decompose to give a tertiary amine and
alkene.
 Eg-
[RC𝑯 𝟐 𝑪𝑯 𝟐 𝑵+(𝑪𝑯 𝟑)3]𝑶−H RCH=C𝑯 𝟐+ N (𝑪𝑯 𝟑)3+𝑯 𝟐 𝑶
Alkene Tertiary amine

15. 8) By Electrolysis of Salts of
Dicarboxylic acids-
 Alkenes are formed by electrolysis of aqueous
solutions of potassium salts of dibasic acids of the
succinic acid series.
 Eg- 𝑪𝑯 𝟐COOK 𝑪𝑯 𝟐COO-
𝑪𝑯 𝟐COOK 𝑪𝑯 𝟐COO- + 𝟐𝑲+
Potassium succinate Succinate ion
At anode- 𝑪𝑯 𝟐COO- 𝑪𝑯 𝟐
𝑪𝑯 𝟐COO- - 2electron 𝑪𝑯 𝟐 + 2C𝑶 𝟐

16.  At Cathode-
𝟐𝑲+
+ 2electron 2K
2K + 𝑯 𝟐O 2KOH + 𝑯 𝟐

17. “When you focus on good, the good gets
better”
THANK YOU……