ADDITION REACTIONS OF ALKYNESADDITION REACTIONS OF ALKYNES

ADDITION REACTIONS OF ALKYNESADDITION REACTIONS OF ALKYNES

C C C CC C

A carbon-carbon triple bond is like two carbon-carbondouble bonds.

Most double bond reagents will react twice.

C C two pi bonds

ALKYNES ARE LESS REACTIVE THAN ALKENESALKYNES ARE LESS REACTIVE THAN ALKENES

C C

C C

bonds are longerweakerless s character

bonds are shorterstrongermore s character

Shorter - Stronger - more S characterSSSRULE

(sp2)

(sp)

SIMPLE ADDITIONSSIMPLE ADDITIONS

DO IT TWICE !

C C HH3C CH3C

CCl

H

H

CH3C C

Cl

Cl

H

H

H

HCl

HCl

first mole

second mole

Follows the MarkovnikoffRule in both additions.

ADDITION OF ADDITION OF HClHCl TO AN ALKYNETO AN ALKYNE

Also HBr and HI …..

C C HH3C CH3C

CBr

Br

H

CH3C C

Br

Br

H

Br

Br

first mole

second mole

Br2

Br2

ADDITION OF BROMINE TO AN ALKYNEADDITION OF BROMINE TO AN ALKYNE

Both additions proceed througha bridged intermediate and bothadditions are stereospecific anti.

C C HH3C

Br

Br -

+

antiaddition

Also Cl2 and I2

HYDRATION OF ALKYNESHYDRATION OF ALKYNES

SOMETHING DIFFERENT

When you add water to a triple bond, it doesn’tadd twice - rather only once, usually giving aketone as the product.

ENOL-KETO TAUTOMERISM

C CHH3C

H OH

H

C CHH3CH

+

+

H2SO4H2O

ALKYNES ARE LESS REACTIVE THAN ALKENES

reaction is slow

A stronger electrophile than H3O+ is required …….

ADDITION OF WATERADDITION OF WATER

……. HgSO4 is added.

dilute

Water doesn’t add under the usual conditions (dil. H2SO4).

H+ Hg2+

MERCURY IS LIKE A MERCURY IS LIKE A ““BIG PROTONBIG PROTON””Hg2+ is a stronger electrophile than H+

C CHH3C C CHH3C

slower faster

So we use a mixture of H2SO4, H2O and HgSO4 as our reagents. ( HgSO4 = mercuric sulfate )

Hg2+ is not only a stronger Lewis acid, it is large and it forms a bridged ion.

C CHH3CH2SO4

H2OHgSO4/

Hg+2

C CHH3C

Hg2+

O H

H

C CHH3C

Hg+

O HH

+

O HH

C CHH3C

H

OH

H3C C

O

CH3

ADDITION OF WATER TO AN ALKYNEADDITION OF WATER TO AN ALKYNE

unstableenol

ketone

Follows the MarkovnikoffRule

exchanges for a proton

bridged ion,anti openingstronger

electrophile

ENOLS ARE UNSTABLE (ENOLS ARE UNSTABLE (USUALLYUSUALLY))

C C

O Hol

ene

discussedlater

ENOLS :( have -OH attached to a double bond)Think of this combination as unstable.

OH

Phenols are not “enols” and they arevery stable (benzene resonance).

NOTE :

ENOL ENOL -- KETO TAUTOMERISMKETO TAUTOMERISM

H3C C

O

CH2

H

C CH2H3C

O H

TAUTOMERISMTAUTOMERISMTAUTOMERS :

species inequilibriumthat differ inthe positionof a proton orother group.

enol

keto

ENERGY

Most enols are not favored,

unstable enol

To interconvert tautomers,a proton is transferred from oxygen to carbon.

ENOL KETO

they rapidly change to keto.

Mechanism, nextslide…..

Bond Energies:C=O 179 C=C 146O-H 111 C-H 106

+OH HH

C CH2H3C

OH

H3C CO

CH2

HC CH2H3CO

H

H

O H

H

+C CH2H3C

OH

H

C CH2H3C

O H

H

+

+

OH H

H+

CONVERSION OF AN ENOL TO A KETONECONVERSION OF AN ENOL TO A KETONE

ENOL

KETONE

( catalyzed by small amounts of acid )

H3C C

O

CH2

H

C CH2H3C

O H

:O-HH

H-O-HH

H-O-HH

.. ..

..+

+

OVERALL RESULT (OVERALL RESULT (NOT A STEPWISE MECHANISMNOT A STEPWISE MECHANISM))

ENOLENOL

KETOKETO

HYDROGENATION OF ALKYNESHYDROGENATION OF ALKYNES

HYDROGENATION OF ALKYNESHYDROGENATION OF ALKYNES

Alkynes react twice with hydrogen when a metal catalyst is present. The alkene is formed first, andthen the alkane.

CH3 C C CH3H2

PdC C

CH3CH3

H H syn addition

H2 /Pd

CH3CH2CH2CH3

cis

Since alkenes are more reactive than alkynes, it isdifficult to stop the reaction after the first addition.

.H.

H

.H

.H .

H.H.

H

.H

.H

RR

R R

. .ALKENE PICKS UPTWO HYDROGENS

MECHANISM OF CATALYTICMECHANISM OF CATALYTICHYDROGENATIONHYDROGENATION

H HR RR R

metal catalyst withadsorbed hydrogenatoms

syn addition

REVIEW

LINDLARLINDLAR

Converting an alkyne to a cis alkene.

LINDLAR CATALYSTLINDLAR CATALYSTThere is a special deactivated catalyst, Lindlar, that allows the hydrogenation of an alkyne to stop at the alkene, after the addition of only one mole of hydrogen.

Lindlar is a palladium catalyst, where the palladium isprecipitated onto particles of barium sulfate from a solution which has quinoline present.

Lindlar = Pd / BaSO4 / quinoline

The quinoline is thought to “poison”, or reduce theactivity, of the catalyst. Many sulfur and nitrogen-containing compounds act as poisons.

N

quinoline..

With Lindlar catalyst, only one mole of hydrogen will add to an alkyne. As with other catalysts, hydrogen-ations with Lindlar are syn stereospecific additions.

syn additionC C

CH3CH3

H H

H2CH3 C C CH3

Lindlarcatalyst

cis

CH3CH2CH2CH3Pd

H2CH3 C C CH3

(without Lindlar)

QUINOLINEQUINOLINE

QUINOLINE

QUINOLINE

POISONING MECHANISM IS UNKNOWN POISONING MECHANISM IS UNKNOWN

Quinoline binds to sites on the catalyst.Linear alkynes can approach and pick up hydrogens.Planar alkenes are blocked.

QUINOLINE

QUINOLINE

QUINOLINE C=CR R

R RQUINOLINE

QUINOLINE

R-C C-R

alkyne fitsalkene blocked

A POSSIBLE HYPOTHESIS IS GIVEN HERE

surface ofLindlar catalyst

bound quinoline

( probably over-simplified )

MAKING A MAKING A transtrans --ALKENEALKENE

Can you also convert an alkyne to a trans -alkene?

YES …...

This is a different type of reaction (not a catalytic hydrogenation)

and it converts an alkyne to a trans alkene.

anti additionC C

CH3

H

H

CH3

CH3 C C CH3

trans

Na

NH3 (liq)

SODIUM METAL IN LIQUID AMMONIASODIUM METAL IN LIQUID AMMONIA

Just keep in mind that it is possible to make trans alkenes as well as cis alkenes, however, a different reaction is used.

- 60 oC

CONTROLLING THE OUTCOMECONTROLLING THE OUTCOMEChemists are always trying to either discover ordesign pairs of reactions that allow one to control the outcome of a change…. especially regioselectivity or stereochemistry.

Consider:

HBr (peroxide free)

and now

alkyne to cis alkene(catalytic hydrogenation)

versus alkyne to trans alkene(sodium/ liquid ammonia)

Markovnikoff Anti-Markovnikoff

HBr (peroxidesversus

If the organic chemist didn’t have methods givingdifferent results, it would be difficult to do synthesis.

The task of a synthetic chemist is to make moleculeswhere each atom is arranged in the correct placeand with the correct stereochemistry.

There are 256 stereoisomers of cholesterol!

To synthesize (construct) a molecule of cholesterolin the lab, the chemist would have to arrange the atomsin the correct order with the correct types of bonds and control the outcome at all eight stereocenters or the molecule would not be cholesterol.

SYNTHESISSYNTHESIS