Alkynes

Structure

sp hybridization

Acidity of Terminal Alkynes

Other strong bases that will ionize the terminal alkyne:

Not KOH

Stronger base

Weaker base

Important Synthetic Method: Dehydrohalogenation

1. Dehydrohalogenation…

An alkyl halide can eliminate a hydrogen halide molecule, HX, to produce a pi bond.

RCH=CHR + HX RCHXCH2RStrong base

Also, if we start with a vinyl halide and a very strong base (vinyl halides are not very reactive).

RCH=CHBr RCCHNaH

Or rewriting

RCHBrCH2R RCH=CHRbase

Recall that HX can be added to a double bond to make an alkyl halide. HX can also be removed by strong base, called dehydrohalogenation.

Preparation of alkene

Synthetic planning (Retrosynthesis)

CH3

prop-1-yne

Target molecule.

Br

H

CH3

NaNH2NaNH2

CH3

Br

H

H H

BrCH3

Br2

Trace the reactions sequence from the desired product back to ultimate reactants.

H

HH

H

CBut typical of synthetic problems side reaction occurs to some extent and must be taken into account.

Overall Sequence converts alkene alkyne

Work Backwards…..

Starting reactant

More Sythesis: Nucleophilic Substitution

Use the acidity of a terminal alkyne to create a nucleophile which then initiates a substitution reaction.

Note that we still have an acidic hydrogen and, thus, can react with another alkyl group in this way to make RCCR’

Alkyl halides can be obtained from alcohols

ROH + PX3 RX

Reactions: alkyne with halogen

No regioselectivity with Br2.

Stereoselective for trans addition.

RCCR + Br2 RBrC=CBrR

Reactions: Addition of HX

The expected reaction sequence occurs, formation of the more stable carbocation.

Markovnikov orientation for both additions.

Now for the mechanism….

MechanismThe expected reaction sequence occurs, formation of the more stable carbocation.

Addition of the second mole, another example of resonance.

Reactions: Acid catalyzed Hydration (Markovnikov).

Markovnikov addition, followed by tautomerism to yield, usually, a carbonyl compound.

Reactions: Anti Markovnikov Hydration of Alkynes, Regioselectivity

Similar to formation of an anti-Markovnikov alcohol from an alkeneStep 1, Internal Alkyne: addition to the alkyne with little or no regioselectivity issue.

Alternatively Asymmetric, terminal, alkyne if you want to have strong regioselectivity then use a borane with stronger selectivity for more open site of attack.

sia2BH

Less exposed site.

More exposed site.

overall: R R'

BH3 H2O2, NaOHRCH2CR'

o

R R'

H BStep 1 Step 2

Aldehyde not ketone.

Tautomerism, enol carbonyl

Overall…

Step 2, Reaction of the alkenyl borane with H2O2, NaOH would yield an enol. Enols are unstable and rearrange (tautomerize) to yield either an aldehyde or ketone.

H

OH

enol

H

B

H2O2

NaOHH

O

Hcatalyzed bybase or acid

either an aldehyde or a ketone

internal alkyne ketone (possibly a mixture, next slide)

Terminal alkyne aldehyde

ExamplesAs before, for a terminal alkyne.

Used to insure regioselectivity.

Get mixture of alkenyl boranes due to low regioselectivity.

But for a non-terminal alkyne frequently will get two different ketones

Reduction, Alkyne Alkene

You can use a reduced activity catalyst (Lindlar), Pd and Pb, which stops at the alkene. You obtain a cis alkene.

1. Catalytic Hydrogenation

Syn addition

If you use catalysts which are also effective for alkene hydrogenation you will get alkane.

Reduction - 2

2. Treatment of alkenyl borane with a carboxylic acid to yield cis alkene.

BH3

H B

hex-3-yne

CH3CO2H

3. Reduction by sodium or lithium in liquid ammonia to yield the trans alkene.

Instead of H2O2 / NaOH

Alkenyl borane

Plan a Synthetic SequenceRetrosynthesis

Synthesize butan-1-ol from ethyne. Work backward from the target molecule.

OH

butan-1-ol

Target molecule

Is read as “comes from”.

A big alkyne can be formed via nucleophilic substitution. This is the chance to make the C-C bond we need.

Do a “disconnect” here.

Br

bromoethane

ethyne

Catalytic Lindlar reduction

Catalytic reduction Lindlar

Addition of HBr.

Convert ethyne to anion and react with EtBr.

1. BH3

2. H2O2, NaOH

Now, fill in the “forward reaction” details

Major problem: make big from small. Be alert for when the “disconnect” can be done.

Ask yourself! Do we know how to join any two molecules together to yield an alcohol?

Not yet! So how can we get it?How about joining molecules to get an alkene? Not yet!! So how can we get an alkene?

Can we get an alkyne from smaller molecules?

YES!