(26) session 26 electrophilic addition of alkenes

Electrophilic addition reactions of alkenes

1. 5/15/2012
1
Session 26
Organic Chemistry, UNAM School of Medicine
1
Electrophilic addition reactions of Electrophilic addition reactions of Electrophilic addition reactions of Electrophilic addition reactions of 
alkenesalkenesalkenesalkenes
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 hydrogen halidesto alkenes
hAddition of waterto alkenes
hAddition of alcoholto alkenes
hAddition of hydrogento alkenes
Organic Chemistry, UNAM School of Medicine

2. 5/15/2012
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Reactivity of alkenesReactivity of alkenesReactivity of alkenesReactivity of alkenes
3
σDoublebond of alkenes – consist
of a s& a pbond
σ̟ bond is weaker than the σbond
σ\̟ bond is brokenwhen an
alkene undergoes a reaction
σAlkenes = e
−
)rich, because ̟ bond consists of a cloud of e
−
above
& below ̟ bond
σ\Alkenes = 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 halides hydrogen halides hydrogen halides hydrogen halides 
(H(H(H(H----X) X) X) X) ----HydrohalogenationHydrohalogenationHydrohalogenationHydrohalogenation
4
σAll alkene reactions have one feature in common: Delocalised ̟
e
−
of the C)C double bond are attracted to an electrophile (E)
σEach reaction starts with addition of E to one of the sp
2
carbonsof
the alkene & concludes with addition of a nucleophile (Nu) to the
other sp
2
carbonσ\End result = ̟ bond breaks & sp
2
carbons form new σbonds
with the E & Nu
Organic Chemistry, UNAM School of Medicine

3. 5/15/2012
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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
5
hExample:
hElectrophilic addition reaction of HBrto but)2)ene
hStep 1: Relatively slow addition of proton, (H
+
) (electrophile) to the
alkene(nucleophile) to form a carbocation intermediate
hStep 2:+ charged carbocationintermediate (electrophile) reacts
rapidly with − charged bromide ion (nucleophile)
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
6
hElectrophilic addition reactionof alkenesmake them an
important class of organic compounds, because they can be used to
synthesise a wide variety of other compounds
hProduct obtained depends on electrophile & nucleophile used in the
addition reaction
hIf electrophilic reagent that adds to an alkene is a hydrogen halide
(HF, HCl, HBr or HI) – reaction product will be a halo-alkane
(alkyl halide) 
Organic Chemistry, UNAM School of Medicine

4. 5/15/2012
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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
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hExamples:
hBecause alkenes above have the same substituentson both sp
2
carbons, it
does not make any difference which sp
2
carbon the E & Nu attach to
h\Same product will be obtained in either case
hBut what happens if alkene does not have the same substituents on both
sp
2
carbons?
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
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hExample:
hWhen reaction is carried out, only product isolated is tert“sadlni
chloride(product 1)
hStep 1 of reaction (addition of E(H
+
) to an sp
2
carbon ) = rate)
determining step!
h\The particular carbocationthat is formed faster determines final
product of reaction
Organic Chemistry, UNAM School of Medicine
HCl or
HCl

5. 5/15/2012
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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
hWhy is tert-butyl carbocation formed faster than isobutyl carbocation?
hAnswer lies in the stability of the carbocation
hCarbocation classification“according to # of alkyl substituents bonded
to +charged C
hCarbocation stability ­as the # of alkyl substituentsbonded to +
charged C ­
Organic Chemistry, UNAM School of Medicine
htert-butyl cation (a 3°carbocation)
hisobutyl cation (a 1°carbocation)
RegioselectivityRegioselectivityRegioselectivityRegioselectivityof electrophilic addition of electrophilic addition of electrophilic addition of electrophilic addition 
reactionsreactionsreactionsreactions
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hIn both cases:More stable 3°carbocationformed more rapidly than less
stable 2°carbocation
h\Major product of each reaction = one that results from forming the 3°
carbocation
Organic Chemistry, UNAM School of Medicine
Hl
Major productMinor product
Major productMinor product
HBr

6. 5/15/2012
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RegioselectivityRegioselectivityRegioselectivityRegioselectivityof electrophilic addition of electrophilic addition of electrophilic addition of electrophilic addition 
reactionsreactionsreactionsreactions
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hThe 2 products of each of these reactions are structural isomers of
one another
hReaction in which 2 or more structural isomers could be obtained as
products, but 1 of them predominates(obtained in larger amounts
than the other products), is called a regioselective reaction
hRegioselectivity= the preferential formation of one structural
isomer over the other
Organic Chemistry, UNAM School of Medicine
RegioselectivityRegioselectivityRegioselectivityRegioselectivityof electrophilic addition of electrophilic addition of electrophilic addition of electrophilic addition 
reactionsreactionsreactionsreactions
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hAddition of HBrto 2)penteneis not regioselective:
hAddition of H
+
to either sp
2
carbons produces a 2°carbocation, both
carbocation intermediates have the same stability, so both will be
formed equally easily
h\\\\Approximately equal amounts of the two alkyl halides will be
formed
Organic Chemistry, UNAM School of Medicine
HBr
2)bromopentane
50%
3)bromopentane
50%
2)pentene

7. 5/15/2012
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Markovnikov’s ruleMarkovnikov’s ruleMarkovnikov’s ruleMarkovnikov’s rule
13
hBy examining all addition reactions of hydrogen halides to alkenes we
have seen so far, we can devise a rule that applies to all electrophilic
addition reactions to alkenes:the electrophile (H
+
) adds to the sp
2
carbon that is bonded to the greater number of hydrogens
hThis rule is referred to as Markovnikov’s rule
Organic Chemistry, UNAM School of Medicine
HCl
H
+
will preferentially add to C)1& NOT to C)2!!
1
2
Electrophilic Electrophilic Electrophilic Electrophilic addition of Haddition of Haddition of Haddition of H
2222O O O O ----HydrationHydrationHydrationHydration
14
hWhen H
2Ois added to an alkene, no reactiontakes place
hReason: O–H bonds of H
2O are too strong –water is too weakly acidic –
to allow the H to act as E to start the reaction by adding to the
nucleophilic alkene
hIf an acid(ex. HCl or H
2SO
4) is added as a catalyst, a reaction will
occur
hReason:The acid provides an electrophile (H
+
)
hProductof the reaction is an alcohol
hHydration= addition of H
2O to any molecule
Organic Chemistry, UNAM School of Medicine

8. 5/15/2012
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Electrophilic Electrophilic Electrophilic Electrophilic addition of Haddition of Haddition of Haddition of H
2222O O O O ----HydrationHydrationHydrationHydration
15
hFirst 2 steps of mechanism for acid)catalysed addition of H
2Oto alkene )
essentially the same as first 2 stepsof mechanism for addition of
hydrogen halideto alkene. Results in formation of a protonated alcohol
hProtonated alcohols= very strong acids (proton donors)
h\In 3
rd
step of reaction, protonated alcohol loses a proton (H
+
) to form
the final product, an alcohol
Organic Chemistry, UNAM School of Medicine
Electrophilic Electrophilic Electrophilic Electrophilic addition of Haddition of Haddition of Haddition of H
2222O O O O ----HydrationHydrationHydrationHydration
16
hA H
+
(donated from HClor H
2SO
4) adds to alkene in 1
st
step& a
H
+
is
returned to reaction mixture in final step
h\H
+
is not consumed in the reaction
hThe acid acts as a catalyst in the reaction – this is an acid-
catalysed reaction
Organic Chemistry, UNAM School of Medicine

9. 5/15/2012
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Electrophilic Electrophilic Electrophilic Electrophilic addition of alcohol (Raddition of alcohol (Raddition of alcohol (Raddition of alcohol (R----OH)OH)OH)OH)
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hAlcohols react with alkenes in the same way that H
2O does
hAs with addition of H
2O, addition of alcohol requires an acid catalyst
hProductof reaction = ether
hMechanism for acid)catalysed addition of alcohol is essentially the
same as mechanism for acid)catalysed addition of H
2O
hOnly difference:Nu is ROH instead of H
2O
Organic Chemistry, UNAM School of Medicine
HCl
Electrophilic Electrophilic Electrophilic Electrophilic addition of alcohol (Raddition of alcohol (Raddition of alcohol (Raddition of alcohol (R----OH)OH)OH)OH)
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h%&'''i“Do not memorise the products of alkene addition 
reactions 
hKnow the reaction mechanism
hWhen presented with a reaction, ask yourself: “Which is the 
electrophile?” &“Which is the nucleophile?” 
Organic Chemistry, UNAM School of Medicine

10. 5/15/2012
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Electrophilic Electrophilic Electrophilic Electrophilic addition addition addition addition of of of of hydrogen (Hhydrogen (Hhydrogen (Hhydrogen (H
2222) ) ) ) ----
HydrogenationHydrogenationHydrogenationHydrogenation
19
hIn the presence of metal catalyst such as Pt(platinum) or Pd
(palladium), hydrogen (H
2) adds to =bond of alkene to form an
alkane
hWithout metal catalyst, reaction will not take place
hReason:H–H bond is very strong & energy barrier is enormous
hCatalyst ¯activation energy by breaking the H–H bond
hPt & Pd are used adsorbed
*
on carbon (Pt/C, Pd/C)
*
Adsorption= adhesion of atoms to a surface
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
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hExamples:
hAddition of H
2to an alkene is called alkene hydrogenation& since
the reaction requires a catalyst, it is specifically called a catalytic 
hydrogenation of an alkene
hDetails of mechanismfor catalytic hydrogenation are not yet completely
understood
Organic Chemistry, UNAM School of Medicine
H
2
Pd/C
H
2
Pt/C

11. 5/15/2012
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Quiz: Electrophilic addition to alkenes
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1.w
2.w
Organic Chemistry, UNAM School of Medicine
Quiz: Electrophilic addition to alkenes
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3.W
4.w
Organic Chemistry, UNAM School of Medicine

12. 5/15/2012
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Thank you
23
END
Organic Chemistry, UNAM School of Medicine