thereacons of$alkynes$ - los angeles mission college · pdf file•...
TRANSCRIPT
The Reac(ons of Alkynes
An Introduc+on to Mul+step Synthesis
• Hydrocarbons that contain carbon-‐carbon triple bonds
• Acetylene, the simplest alkyne is produced industrially from methane and steam at high temperature
• Our study of alkynes provides an introduc+on to organic synthesis, the prepara+on of organic molecules from simpler organic molecules
Alkynes
Nomenclature of Alkynes An alkyne is a hydrocarbon that contains a carbon–carbon triple bond.
General formula: CnH2n–2 (acyclic) CnH2n–4 (cyclic)
General hydrocarbon rules apply with “-‐yne” as a suffix indica+ng an alkyne Numbering of chain with triple bond is set so that the smallest number possible is assigned to the first carbon of the triple bond
Naming Alkynes
A Double Bond and a Triple Bond
Double and Triple are Tied
When the same number is obtained for both, the double bond gets the lower number.
Priority of Func+onal Groups Suffixes
• Treatment of a 1,2-‐dihaloalkane with KOH or NaOH produces a two-‐fold elimina+on of HX
• Vicinal dihalides are available from addi+on of bromine or chlorine to an alkene
• Intermediate is a vinyl halide
Prepara+on of Alkynes: Elimina+on Reac+ons of Dihalides
• Carbon-‐carbon triple bond results from sp orbital on each C forming a sigma bond and unhybridized pX and py orbitals forming π bonds.
• The remaining sp orbitals form bonds to other atoms at 180º to C-‐C triple bond.
• The bond is shorter and stronger than single or double • Breaking a π bond in acetylene (HCCH) requires 202 kJ/mole (in
ethylene it is 269 kJ/mole)
Electronic Structure of Alkynes
The Structure of Alkynes
The triple bond is composed of a sigma bond and two π bonds.
Alkynes (Like Alkenes) Undergo Electrophilic Addi+on Reac+ons
Forma+on of a π-‐Complex
A primary vinylic ca+on is less sable than a primary alkyl ca+on.
The First Step is Addi+on of an Electrophile
Rela+ve Carboca+on Stability
• Addi+on of H-‐X to alkyne should produce a vinylic carboca+on intermediate – Secondary vinyl
carboca+ons form less readily than primary alkyl carboca+ons
– Primary vinyl carboca+ons probably do not form at all
• Nonethelss, H-‐Br can add to an alkyne to give a vinyl bromide if the Br does not end up on a primary carbon
Addi+on of HX to Alkynes Involves Vinylic Carboca+ons
Mechanism for Electrophilic Addi+on
Forma+on of the More Stable Transi+on State Accounts for the Regioselec+vity
Evidence for Forma+on of a π-‐Complex
Evidence for the forma+on of a π-‐complex
A Second Addi+on Reac+on Can Occur
Addi+on to a Terminal Alkyne is Regioselec+ve
The Second Electrophilic Addi+on Reac+on is Also Regioselec+ve
An Asymmetrical Internal Alkyne Forms Two Products
A Symmetrical Internal Alkyne Forms One Product
• Addi+on reac+ons of alkynes are similar to those of alkenes
• Intermediate alkene reacts further with excess reagent
• Regioselec+vity according to Markovnikov
Reac+ons of Alkynes: Addi+on of HX and X2
• Ini+al addi+on gives trans intermediate • Product with excess reagent is tetrahalide
Addi+on of Bromine and Chlorine
Addi+on of Cl2 and Br2
• Addi+on of H-‐OH as in alkenes – Mercury (II) catalyzes Markovnikov oriented addi+on – Hydrobora+on-‐oxida+on gives the an+-‐Markovnikov product
Hydra+on of Alkynes
Mercury(II)-‐Catalyzed Hydra+on of Alkynes
• Alkynes do not react with aqueous pro+c acids
• Mercuric ion (as the sulfate) is a Lewis acid catalyst that promotes addi+on of water with Markovnikov orienta+on
• The immediate product is a vinylic alcohol, or enol, which spontaneously transforms to a ketone or to an aldehyde in the event that acetylene is employed.
• If the alkyl groups at either end of the C-‐C triple bond are not the same, both products can form and this is not normally useful
• If the triple bond is at the first carbon of the chain (then H is what is ahached to one side) this is called a terminal alkyne
• Hydra+on of a terminal alkyne always gives the methyl ketone, which is useful
Hydra+on of Unsymmetrical Alkynes
Mechanism for Catalysis by Hg2+
• Isomeric compounds that can rapidily interconvert by the movement of a proton are called tautomers and the phenomenon is called tautomerism
• Enols rearrange to the isomeric ketone by the rapid transfer of a proton from the hydroxyl to the alkene carbon
• The keto form is usually so stable compared to the enol that only the keto form can be observed
Keto-‐enol Tautomerism
• BH3 (borane) adds to alkynes to give a vinylic borane • Oxida+on with H2O2 produces an enol that converts to the
ketone or aldehyde • Process converts alkyne to ketone or aldehyde with orienta+on
opposite to mercuric ion catalyzed hydra+on
Hydrobora+on/Oxida+on of Alkynes
9-‐BBN or R2BH
R2BH should be used with terminal alkynes.
R2BH can be used with internal alkynes.
Hydrobora+on–Oxida+on
Terminal alkynes form aldehydes.
Internal alkynes form ketones.
• Hydrobora+on/oxida+on converts terminal alkynes to aldehydes because addi+on of water is an+-‐Markovnikov
• The product from the mercury(II) catalyzed hydra+on converts terminal alkynes to methyl ketones
Comparison of Hydra+on of Terminal Alkynes
• Addi+on of H2 over a metal catalyst (such as palladium on carbon, Pd/C) converts alkynes to alkanes (complete reduc+on)
• The addi+on of the first equivalent of H2 produces an alkene, which is more reac+ve than the alkyne so the alkene is not observed
Reduc+on of Alkynes
Addi+on of Hydrogen Forms an Alkane
Stopping at the Alkene
• Addi+on of H2 using chemically deac+vated palladium on calcium carbonate as a catalyst (the Lindlar catalyst) produces a cis alkene
• The two hydrogens add syn (from the same side of the triple bond). This is called syn addi+on.
Conversion of Alkynes to cis-‐Alkenes
Why Cis?
The catalyst delivers the hydrogens to one side of the triple bond.
• Anhydrous ammonia (NH3) is a liquid below –33 ºC – Alkali metals dissolve in liquid ammonia and func+on as reducing
agents
• Alkynes are reduced to trans alkenes with sodium or lithium in liquid ammonia
• The reac+on involves a radical anion intermediate
Conversion of Alkynes to trans-‐Alkenes
An2 Addi+on
Mechanism for Forma+on of a Trans Alkene
Why Trans?
• Strong oxidizing reagents (O3 or KMnO4) cleave internal alkynes, producing two carboxylic acids
• Terminal alkynes are oxidized to a carboxylic acid and carbon dioxide
• Neither process is useful in modern synthesis – were used to elucidate structures because the products indicate the structure of the alkyne precursor
Oxida+ve Cleavage of Alkynes
Rela+ve Electronega+vi+es of Carbon
A Hydrogen Ahached to an sp Carbon is the Most Acidic
Need a Strong Base to Remove the Hydrogen
HO– is Not Strong Enough
• Terminal alkynes are weak Brønsted acids (alkenes and alkanes are much less acidic )
• Reac+on of strong anhydrous bases with a terminal alkyne produces an acetylide ion
• The sp-‐hydbridiza+on at carbon holds nega+ve charge rela+vely close to the posi+ve nucleus
Alkyne Acidity: Forma+on of Acetylide Anions
• Acetylide ions can react as nucleophiles as well as bases (see Figure 9-‐6 for mechanism)
• Reac+on with a primary alkyl halide produces a hydrocarbon that contains carbons from both partners, providing a general route to larger alkynes
Alkyla+on of Acetylide Anions
• Reac+ons only are efficient with 1º alkyl bromides and alkyl iodides
• Acetylide anions can behave as bases as well as nucelophiles • Reac+ons with 2º and 3º alkyl halides gives
dehydrohalogena+on, conver+ng alkyl halide to alkene
Limita+ons of Alkya+on of Acetylide Ions
• Organic synthesis creates molecules by design • Synthesis can produce new molecules that are needed as
drugs or materials • Syntheses can be designed and tested to improve efficiency
and safety for making known molecules • Highly advanced synthesis is used to test ideas and methods,
answering challenges • Chemists who engage in synthesis may see some work as
elegant or beau2ful when it uses novel ideas or combina+ons of steps – this is very subjec+ve and not part of an introductory course
An Introduc+on to Organic Synthesis
• In order to propose a synthesis you must be familiar with reac+ons – What they begin with – What they lead to – How they are accomplished – What the limita+ons are
• A synthesis combines a series of proposed steps to go from a defined set of reactants to a specified product – Ques+ons related to synthesis can include par+al informa+on
about a reac+on of series that the student completes
Synthesis as a Tool for Learning Organic Chemistry
• Compare the target and the star+ng material
• Consider reac+ons that efficiently produce the outcome. Look at the product and think of what can lead to it
• Read the prac+ce problems in the text
Strategies for Synthesis
How Can the Compound Be Prepared From the Star+ng Material?
Designing a Synthesis
Designing a Synthesis
Retrosynthetic Analysis and Synthesis
Designing a Synthesis
Designing a Synthesis
Designing a Synthesis
Designing a Synthesis
Designing a Synthesis
Synthesis Problem
Prepare n-‐octane from 1-‐pentyne.
The best strategy to approach this problem is to use acetylide coupling: