Alkynes:

An Introduction to Organic Synthesis

Goals

n      After this chapter you should be able to:

n       Describe bonding in alkynes

n       Name alkynes

n       Apply alkene reactions to alkynes

n       Apply enol tautomerism in synthesis

n       Complete and partial reduction of alkynes

n       Use oxidative cleavage in synthesis

n       Use acetylide anion in synthesis

n       Show steps to synthesize various molecules using the reactions learned thus far. 

 

Alkynes

n      Carbon-carbon triple bond

n       sp hybridized

n       Linear

n       One s and two p bonds

 

Alkyne Nomenclature

n      Lowest number to multiple  bond

n      More than one multiple bond

n       -diyne, -triyne etc. 

n       Compounds with double and triple = enynes

n       Double bond gets lowest number when equal choice

n      As substituent groups are named as –ynyl gtoups

n       CH₃C�ßC-             1-propynyl group

n       HC�ßCCH₂-          2-propynyl group

Preparation of Alkynes:

n      Elimination reactions of dihalides

 

 

Reactions of Alkynes

n      Addition of HX – Markovnikov

 

 

Bromine addition to alkynes

n      Bromine adds one at a time

 

 

Addition Reaction

Mechanisms

n      Mechanism is the same as alkene

 

 

Addition Reaction Mechanisms

n      Bromine addition same as for alkenes

 

 

Hydration  of Alkynes

n      Reaction is analogous to oxymercuration reaction for alkenes but forms an enol.

 

 

Hydration: Keto-Enol Tautomerism

 

 

 

 

 

Hydroboration - Oxidation

n      Internal Alkyne

 

 

 

 

 

 

 

Hydroboration - Oxidation

n      Terminal Alkyne

 

 

 

 

 

Reduction of Alkynes H₂/Pd

n      Mechanism is the same for H₂/Pd as for alkenes but readily goes to the alkane

n       Syn addition 

 

Reduction of alkynes  with Lindlar Catalyst Stops at Alkene

 

n      Syn addition

 

 

 

 

Reduction of Alkynes with Li-NH₃

n      Anti addition

 

 

 

Oxidation of Alkynes Cleavage

n      Mechanism is the same as for alkene

 

 

 

 

 

 

Alkyne Acidity

n      Acetylide Anions formed from sodium amide  -  Na⁺NH₂^-

 

 

Alkylation of Acetylide Anions

n      Once formed acetylide ions can act as nucleophiles in SN₂ type reactions.

n       What’s a leaving group to do…

+ Br-

 

 

n      But leave…. As Br^- in this case.

 

 

 

 

Relative Reactivity

Leaving Groups

Tosylate(TsO^-), Water(H₂O), Ammonia(NH₃)              Excellent

Iodide(I^-)                                                                               Very Good

Bromide(Br^-)                                                                         Good

Chloride(Cl^-)                                                                          Fair

Fluoride(F^-)                                                                            Poor

Hydroxide(HO^-), Amide(NH₂^-), Alkoxide(RO^-)           Almost Unreactive

 

Introduction to Organic Synthesis

n      It is generally best to work backward from products to reactants.

n      Use methods that do not affect other parts of the molecule.

n      Know how functional groups can be made.

n      Keep in mind the starting material.

n      Not all roads lead to Rome.         

Synthesis one

 

 

 

 

 

 

 

Synthesis two

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Synthesis three

 

 

 

 

 

 

 

 

 

 

 

Synthesis four

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Synthesis five