(27) session 27 electrophilic addition of alkynes

Electrophlic addition reactions of alkynes

1. 5/22/2012
1
Session 27
Organic Chemistry, UNAM School of Medicine
1
Electrophilic addition reactions of Electrophilic addition reactions of Electrophilic addition reactions of Electrophilic addition reactions of 
alkynesalkynesalkynesalkynes
Dr L.H.A. Prins (Ph.D.)
Dept. of Pharmacy
UNAM
Learning Outcomes
2
hBy the end of this session, the student should understand:
hElectrophilic addition of hydrogento alkynes
hAddition of hydrogen halidesto alkynes
hAddition of waterto alkynes
Organic Chemistry, UNAM School of Medicine

2. 5/22/2012
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Reactivity of alkynesReactivity of alkynesReactivity of alkynesReactivity of alkynes
3
nJust like alkenes, alkynesare
e
−
$rich molecules
nHave a cloud of e
−
completely
surrounding the σbond
n\Alkynes = nucleophiles& they can react with electrophiles
Organic Chemistry, UNAM School of Medicine
Electrophilic Electrophilic Electrophilic Electrophilic addition addition addition addition of of of of hydrogen (Hhydrogen (Hhydrogen (Hhydrogen (H
2222) ) ) ) ----
HydrogenationHydrogenationHydrogenationHydrogenation
4
nHydrogen (H
2) 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
nDifficult to stop reaction at the alkene stage
nReason:H
2readily adds to alkenesin presence of these efficient
metal catalysts
n\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 (H
2222) ) ) ) ----
HydrogenationHydrogenationHydrogenationHydrogenation
5
hExample:
hHowever, reaction can be stopped at alkene stage if a partially
deactivated metal catalyst is used
hMost 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 (H
2222) ) ) ) ----
HydrogenationHydrogenationHydrogenationHydrogenation
6
hAddition of H
2to an internal alkyne in presence of Lindlar catalyst
produces a cisalkene
hExample:
Organic Chemistry, UNAM School of Medicine

4. 5/22/2012
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Electrophilic Electrophilic Electrophilic Electrophilic addition addition addition addition of of of of hydrogen halides hydrogen halides hydrogen halides hydrogen halides 
(H(H(H(H----X)X)X)X)----HydrohalogenationHydrohalogenationHydrohalogenationHydrohalogenation
7
hIf a reagent such as HClis added to an alkyne$ relatively weak ̟ bond
will break
hReason: ̟ e
−
are attracted to electrophilic proton (H
+
)
hIn 2
nd
step of reaction, the +charged carbocationintermediate
reacts rapidly with −charged chloride ion (Cl
−
)
h\Like alkenes, alkynes undergo electrophilic addition
reactions& same electrophilic reagents that add to alkenes also add
to alkynes
Organic Chemistry, UNAM School of Medicine
Electrophilic Electrophilic Electrophilic Electrophilic addition addition addition addition of of of of hydrogen halides hydrogen halides hydrogen halides hydrogen halides 
(H(H(H(H----X)X)X)X)----HydrohalogenationHydrohalogenationHydrohalogenationHydrohalogenation
8
hExample:
hBecause product of hydrogen halide addition to an alkyneis an alkene, a
2
nd
electrophilic addition reaction can occur if excess hydrogen halide 
2Rv3(pis present
hExample:
Organic Chemistry, UNAM School of Medicine

5. 5/22/2012
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Electrophilic Electrophilic Electrophilic Electrophilic addition addition addition addition of of of of hydrogen halides hydrogen halides hydrogen halides hydrogen halides 
(H(H(H(H----X)X)X)X)----HydrohalogenationHydrohalogenationHydrohalogenationHydrohalogenation
9
hIf the alkyne is a terminal alkyne$H
+
will add to spcarbon bonded to
the hydrogen
hReason:2°ethenylic cationthat results is more stable than 1°ethenylic
cationthat would be formed if H
+
added to the other spcarbon
hExample:
Organic Chemistry, UNAM School of Medicine
2°°°°ethenylic carbocation1°°°°ethenylic carbocation
Electrophilic Electrophilic Electrophilic Electrophilic addition addition addition addition of of of of hydrogen halides hydrogen halides hydrogen halides hydrogen halides 
(H(H(H(H----X)X)X)X)----HydrohalogenationHydrohalogenationHydrohalogenationHydrohalogenation
10
hLikewise, when 2
nd
equivalent of H$Xadds to the =bond, the
electrophile (H
+
) adds to sp
2
carbon bonded to greater # of
hydrogens
hExample:
hAll of the above also correlates with Markovnikov’s rule
Organic Chemistry, UNAM School of Medicine

6. 5/22/2012
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Electrophilic Electrophilic Electrophilic Electrophilic addition addition addition addition of of of of hydrogen halides hydrogen halides hydrogen halides hydrogen halides 
(H(H(H(H----X)X)X)X)----HydrohalogenationHydrohalogenationHydrohalogenationHydrohalogenation
11
hAdditionof a H$Xto an internal alkyne forms 2 products
hReason: Initial addition of H
+
can occur with equal ease to either of
the spcarbons
hExample:
Organic Chemistry, UNAM School of Medicine
Electrophilic Electrophilic Electrophilic Electrophilic addition addition addition addition of of of of hydrogen halides hydrogen halides hydrogen halides hydrogen halides 
(H(H(H(H----X)X)X)X)----HydrohalogenationHydrohalogenationHydrohalogenationHydrohalogenation
12
hHowever, if group attached to each spcarbonis the same, only 1
product is obtained
hExample:
Organic Chemistry, UNAM School of Medicine

7. 5/22/2012
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Electrophilic Electrophilic Electrophilic Electrophilic addition of Haddition of Haddition of Haddition of H
2222O O O O ----HydrationHydrationHydrationHydration
13
hAlkynes(like alkenes) also undergo acid$catalysed addition of H
2O
hInitial product of reaction = enol
hEnol = molecule containing C–C = bond & an OH group bonded
to 1 of the sp
2
carbons
hThe enolimmediately rearranges to a ketone
hExample:
Organic Chemistry, UNAM School of Medicine
Electrophilic Electrophilic Electrophilic Electrophilic addition of Haddition of Haddition of Haddition of H
2222O O O O ----HydrationHydrationHydrationHydration
14
hA ketone& an enoldiffer only in the location of a =bond & a H
hThe ketone & enolare called grncvroci nhXnckrea
hTautomers = Specialkind of structural isomers that are in rapid
equilibrium
Readily interconvert by a chemical reaction called
tautomerisation
hKeto tautomer is usually more stable than the enol tautomer 
h\Keto tautomer predominates at equilibrium
Organic Chemistry, UNAM School of Medicine

8. 5/22/2012
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Electrophilic Electrophilic Electrophilic Electrophilic addition of Haddition of Haddition of Haddition of H
2222O O O O ----HydrationHydrationHydrationHydration
15
hWhen H
2Ois addedto internal alkyne that has identical groups 
attached to each of spcarbon, 1 ketone is formed as the only product
hExample:
hIf the 2 groups are not the same, 2 ketones are formed
hReason: Initial addition of H
+
can happen to either of the spcarbons
hExample:
Organic Chemistry, UNAM School of Medicine
H
2O
H
2SO
4
H
2O
H
2SO
4
Electrophilic Electrophilic Electrophilic Electrophilic addition of Haddition of Haddition of Haddition of H
2222O O O O ----HydrationHydrationHydrationHydration
16
hTerminal alkynes $less reactive than internal alkynestoward
addition of H
2O
hReaction will only take place if mercuric ion (Hg
2+
) is added to
mixture as a catalyst
hExample:
Organic Chemistry, UNAM School of Medicine
H
2O
H
2SO
4
Hg
2+

9. 5/22/2012
9
Quiz: Electrophilic addition to alkynes
17
1.w
Organic Chemistry, UNAM School of Medicine
Quiz: Electrophilic addition to alkynes
18
2.w
Organic Chemistry, UNAM School of Medicine

10. 5/22/2012
10
Thank you
19
END
Organic Chemistry, UNAM School of Medicine
20
hQuiz answers:
1.w
Organic Chemistry, UNAM School of Medicine

11. 5/22/2012
11
21
hQuiz answers:
2.w
Organic Chemistry, UNAM School of Medicine