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