Book Cover Organic Chemistry 4e Carey
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Chapter 9: Alkynes



Summary | Alkynes | Preparation of Alkynes | Addition Reactions of Alkynes | Self Assessment | Quiz |


Addition Reactions of Alkynes

  

Summary

  1. increased nucleophilicity of the starting p system (CºC  vs  C=C), and
  2. stability of any intermediates (for example carbocations)


Reactions:

Hydrogenation of Alkynes

Reaction Type: Electrophilic Addition

Summary

Related reactions
CATALYTIC HYDROGENATION
Step 1:
Hydrogen gets absorbed onto the metal surface.
schematic diagram of catalytic hydrogenation of an alkyne
Step 2:
Alkyne approaches the H atoms absorbed on the metal surface.
Step 3:
Cº reacts with the H atoms on the surface forming the two new C-H s bonds generating the alkene.

 

Dissolving Metal Reduction of Alkynes

Reaction Type: Addition

Summary

Related reactions:
MECHANISM FOR THE REDUCTION OF ALKYNES WITH Na / NH3
Step 1:
Sodium transfers an electron to the alkyne giving a radical anion.
mechanism for the dissolving metal reduction of an alkyne
Step 2:
The radical anion removes a proton from the ammonia in an acid / base reaction.
Step 3:
A second atom of sodium transfers another electron to the alkyne giving an anion.
Step 4:
The anion removes a proton from the ammonia in an acid / base reaction.

 

Reaction of Alkynes with Hydrogen Halides

hydrohalogenation of alkynes
hydrohalogenation of alkyne with excess reagent
Reaction type: Electrophilic Addition

Summary

addition of HBr to propyne
addition of HBr to propyne under radical conditions
Why does this reaction have the opposite regiochemistry ? the Br radical adds first to give the more stable secondary radical, then the H adds.
Related Reactions
MECHANISM FOR REACTION OF ALKYNES WITH HBr

Step 1:
An acid/base reaction. Protonation of the alkyne to generate the more stable carbocation.  The p electrons act pairs as a Lewis base.
addition of HBr to C=C
Step 2:
Attack of the nucleophilic bromide ion on the electrophilic carbocation creates the alkenyl bromide.
Step 3:
In the presence of excess reagent, a second protonation occurs to generate the more stable carbocation.
Step 4:
Attack of the nucleophilic bromide ion on the electrophilic carbocation creates the geminal dibromide.

Hydration of Alkynes

hydration of alkynes
Reaction type: Electrophilic Addition

Summary

MECHANISM FOR REACTION OF ALKYNES WITH H3O+
Step 1:
An acid / base reaction. Protonation of the alkyne to generate the more stable carbocation.  The p electrons act pairs as a Lewis base.
addition of H2O to alkynes
Step 2:
Attack of the nucleophilic water molecule on the electrophilic carbocation creates an oxonium ion.
Step 3:
An acid / base reaction. Deprotonation by a base generates the alcohol and regenerates the acid catalyst forming an unstable enol.
Step 4:
An acid / base reaction. Reprotonation by the acid catalyst occurs on the carbon. The oxygen atom electrons help facilitate this process generating an oxonium ion.
Step 5:
Another acid / base reaction. Deprotonation of the oxonium ion creates the ketone. Steps 4 and 5 show the acid catalyzed tautomerization of the enol to the ketone.

Halogenation of Alkynes

halogenation of alkynes
Reaction type: Electrophilic Addition

Summary

This CHIME image shows the structure of the bromonium ion formed during the reaction of ethyne with Br2
Note how the Br  is attached to both the C atoms. 
What is the charge on the bromine ? +1
When the Br- nucleophile attacks, it will attack from the least hindered face, the side opposite to the Br in this intermediate. 
Make sure you look at the spacefilling model to show this.
MECHANISM FOR REACTION OF ALKENES WITH HALOGENS
Step 1:
The p electrons act as a nucleophile, attacking the bromine, displacing a bromide ion but forming a cyclic bromonium ion as an intermediate.
addition of Br2 to an alkyne
Step 2:
Attack of the nucleophilic bromide from the side away from the bromonium center in an SN2 like fashion opens the cyclic bromonium ion to give overall anti addition.

Ozonolysis of Alkynes

ozonolysis of alkynes
Reaction type: Electrophilic Addition

Summary

  • Overall transformation :  CºC to 2 x CO2H
  • Ozonolysis implies that ozone causes the alkyne to break (-lysis)
  • Reagents : ozone followed by aqueous work-up.
  • Note that each of the CC bonds in the CºC becomes a CO bond:
 
 What would be the products of the ozonolysis reactions of: 
(a) ethyne ? 2 molecules of methanoic acid, HCO2H
(b) 1-butyne ? two carboxylic acids, CH3CH2CO2H and HCO2H

(c) 2-butyne ? 2 molecules of CH3CO2H
(d) cyclooctyne ? a dicarboxylic acid, HO2C(CH2)6CO2H, octandioic acid

Related Reactions



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