Electrophlic addition reactions of alkynes

1. 5/22/2012
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Session 27
Organic Chemistry, UNAM School of Medicine1
Electrophilic addition reactions ofElectrophilic addition reactions ofElectrophilic addition reactions ofElectrophilic addition reactions of
alkynesalkynesalkynesalkynes
Dr L.H.A. Prins (Ph.D.)
Dept. of Pharmacy
UNAM
Learning Outcomes
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By the end of this session, the student should understand:
Electrophilic addition of hydrogen to alkynes
Addition of hydrogen halides to alkynes
Addition of water to alkynes
Organic Chemistry, UNAM School of Medicine

2. 5/22/2012
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Reactivity of alkynesReactivity of alkynesReactivity of alkynesReactivity of alkynes
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Just like alkenes, alkynes are
e−-rich molecules
Have a cloud of e− completely
surrounding the σ bond
∴Alkynes = nucleophiles & they can react with electrophiles
Organic Chemistry, UNAM School of Medicine
ElectrophilicElectrophilicElectrophilicElectrophilic additionadditionadditionaddition ofofofof hydrogen (Hhydrogen (Hhydrogen (Hhydrogen (H2222)))) ----
HydrogenationHydrogenationHydrogenationHydrogenation
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Hydrogen (H2) adds to an alkyne in the presence of a metal catalyst
such as Pt (platinum) or Pd (palladium) in the same manner that it
adds to an alkene
Difficult to stop reaction at the alkene stage
Reason: H2 readily adds to alkenes in presence of these efficient
metal catalysts
∴Product of alkyne hydrogenation reaction = normally an alkane
Organic Chemistry, UNAM School of Medicine

3. 5/22/2012
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Electrophilic addition of hydrogen (HElectrophilic addition of hydrogen (HElectrophilic addition of hydrogen (HElectrophilic addition of hydrogen (H2222)))) ----
HydrogenationHydrogenationHydrogenationHydrogenation
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Example:
However, reaction can be stopped at alkene stage if a partially
deactivated metal catalyst is used
Most commonly used partially deactivated metal catalyst is Lindlar
catalyst
Organic Chemistry, UNAM School of Medicine
Electrophilic addition of hydrogen (HElectrophilic addition of hydrogen (HElectrophilic addition of hydrogen (HElectrophilic addition of hydrogen (H2222)))) ----
HydrogenationHydrogenationHydrogenationHydrogenation
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Addition of H2 to an internal alkyne in presence of Lindlar catalyst
produces a cis alkene
Example:
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ElectrophilicElectrophilicElectrophilicElectrophilic additionadditionadditionaddition ofofofof hydrogen halideshydrogen halideshydrogen halideshydrogen halides
(H(H(H(H----X)X)X)X) ---- HydrohalogenationHydrohalogenationHydrohalogenationHydrohalogenation
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If a reagent such as HCl is added to an alkyne - relatively weak ̟ bond
will break
Reason: ̟ e− are attracted to electrophilic proton (H+)
In 2nd step of reaction, the + charged carbocation intermediate
reacts rapidly with − charged chloride ion (Cl−)
∴ Like alkenes, alkynes undergo electrophilic addition
reactions & same electrophilic reagents that add to alkenes also add
to alkynes
Organic Chemistry, UNAM School of Medicine
ElectrophilicElectrophilicElectrophilicElectrophilic additionadditionadditionaddition ofofofof hydrogen halideshydrogen halideshydrogen halideshydrogen halides
(H(H(H(H----X)X)X)X) ---- HydrohalogenationHydrohalogenationHydrohalogenationHydrohalogenation
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Example:
Because product of hydrogen halide addition to an alkyne is an alkene, a
2nd electrophilic addition reaction can occur if excess hydrogen halide
(H-X) is present
Example:
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5. 5/22/2012
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ElectrophilicElectrophilicElectrophilicElectrophilic additionadditionadditionaddition ofofofof hydrogen halideshydrogen halideshydrogen halideshydrogen halides
(H(H(H(H----X)X)X)X) ---- HydrohalogenationHydrohalogenationHydrohalogenationHydrohalogenation
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If the alkyne is a terminal alkyne - H+ will add to sp carbon bonded to
the hydrogen
Reason: 2° ethenylic cation that results is more stable than 1° ethenylic
cation that would be formed if H+ added to the other sp carbon
Example:
Organic Chemistry, UNAM School of Medicine
2°°°° ethenylic carbocation 1°°°° ethenylic carbocation
ElectrophilicElectrophilicElectrophilicElectrophilic additionadditionadditionaddition ofofofof hydrogen halideshydrogen halideshydrogen halideshydrogen halides
(H(H(H(H----X)X)X)X) ---- HydrohalogenationHydrohalogenationHydrohalogenationHydrohalogenation
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Likewise, when 2nd equivalent of H-X adds to the = bond, the
electrophile (H+) adds to sp2 carbon bonded to greater # of
hydrogens
Example:
All of the above also correlates with Markovnikov’s rule
Organic Chemistry, UNAM School of Medicine

6. 5/22/2012
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ElectrophilicElectrophilicElectrophilicElectrophilic additionadditionadditionaddition ofofofof hydrogen halideshydrogen halideshydrogen halideshydrogen halides
(H(H(H(H----X)X)X)X) ---- HydrohalogenationHydrohalogenationHydrohalogenationHydrohalogenation
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Addition of a H-X to an internal alkyne forms 2 products
Reason: Initial addition of H+ can occur with equal ease to either of
the sp carbons
Example:
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ElectrophilicElectrophilicElectrophilicElectrophilic additionadditionadditionaddition ofofofof hydrogen halideshydrogen halideshydrogen halideshydrogen halides
(H(H(H(H----X)X)X)X) ---- HydrohalogenationHydrohalogenationHydrohalogenationHydrohalogenation
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However, if group attached to each sp carbon is the same, only 1
product is obtained
Example:
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ElectrophilicElectrophilicElectrophilicElectrophilic addition of Haddition of Haddition of Haddition of H2222OOOO ---- HydrationHydrationHydrationHydration
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Alkynes (like alkenes) also undergo acid-catalysed addition of H2O
Initial product of reaction = enol
Enol = molecule containing C–C = bond & an OH group bonded
to 1 of the sp2 carbons
The enol immediately rearranges to a ketone
Example:
Organic Chemistry, UNAM School of Medicine
ElectrophilicElectrophilicElectrophilicElectrophilic addition of Haddition of Haddition of Haddition of H2222OOOO ---- HydrationHydrationHydrationHydration
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A ketone & an enol differ only in the location of a = bond & a H
The ketone & enol are called keto-enol tautomers
Tautomers = Special kind of structural isomers that are in rapid
equilibrium
Readily interconvert by a chemical reaction called
tautomerisation
Keto tautomer is usually more stable than the enol tautomer
∴Keto tautomer predominates at equilibrium
Organic Chemistry, UNAM School of Medicine

8. 5/22/2012
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ElectrophilicElectrophilicElectrophilicElectrophilic addition of Haddition of Haddition of Haddition of H2222OOOO ---- HydrationHydrationHydrationHydration
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When H2O is added to internal alkyne that has identical groups
attached to each of sp carbon, 1 ketone is formed as the only product
Example:
If the 2 groups are not the same, 2 ketones are formed
Reason: Initial addition of H+ can happen to either of the sp carbons
Example:
Organic Chemistry, UNAM School of Medicine
H2O
H2SO4
H2O
H2SO4
ElectrophilicElectrophilicElectrophilicElectrophilic addition of Haddition of Haddition of Haddition of H2222OOOO ---- HydrationHydrationHydrationHydration
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Terminal alkynes - less reactive than internal alkynes toward
addition of H2O
Reaction will only take place if mercuric ion (Hg2+) is added to
mixture as a catalyst
Example:
Organic Chemistry, UNAM School of Medicine
H2O
H2SO4
Hg2+

9. 5/22/2012
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Quiz: Electrophilic addition to alkynes
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Quiz: Electrophilic addition to alkynes
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Organic Chemistry, UNAM School of Medicine

10. 5/22/2012
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Thank you
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END
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Quiz answers:
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11. 5/22/2012
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Quiz answers:
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