The document discusses electrophilic addition reactions of alkenes. It describes how alkenes react with electrophiles by breaking the pi bond to form new sigma bonds. Specific reactions covered include addition of hydrogen halides, water, alcohols and hydrogen. The mechanism and products of each reaction are explained. Markovnikov's rule for the regioselectivity of hydrogen halide additions is also covered.

1. 5/15/2012
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Session 26
Organic Chemistry, UNAM School of Medicine1
Electrophilic addition reactions ofElectrophilic addition reactions ofElectrophilic addition reactions ofElectrophilic addition reactions of
alkenesalkenesalkenesalkenes
Dr L.H.A. Prins (Ph.D.)
Dept. of Pharmacy
UNAM
Learning Outcomes
2
By the end of this session, the student should understand:
Electrophilic addition of hydrogen halides to alkenes
Addition of water to alkenes
Addition of alcohol to alkenes
Addition of hydrogen to alkenes
Organic Chemistry, UNAM School of Medicine

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Reactivity of alkenesReactivity of alkenesReactivity of alkenesReactivity of alkenes
3
Double bond of alkenes – consist
of a σ & a π bond
̟ bond is weaker than the σ bond
∴ ̟ bond is broken when an
alkene undergoes a reaction
Alkenes = e−-rich, because ̟ bond consists of a cloud of e− above
& below ̟ bond
∴Alkenes = nucleophiles & they can react with electrophiles
Organic Chemistry, UNAM School of Medicine
ElectrophilicElectrophilicElectrophilicElectrophilic additionadditionadditionaddition ofofofof hydrogen halideshydrogen halideshydrogen halideshydrogen halides
(H(H(H(H----X)X)X)X) ---- HydrohalogenationHydrohalogenationHydrohalogenationHydrohalogenation
4
All alkene reactions have one feature in common: Delocalised ̟
e− of the C-C double bond are attracted to an electrophile (E)
Each reaction starts with addition of E to one of the sp2 carbons of
the alkene & concludes with addition of a nucleophile (Nu) to the
other sp2 carbon
∴ End result = ̟ bond breaks & sp2 carbons form new σ bonds
with the E & Nu
Organic Chemistry, UNAM School of Medicine

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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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Example:
Electrophilic addition reaction of HBr to but-2-ene
Step 1: Relatively slow addition of proton, (H+) (electrophile) to the
alkene (nucleophile) to form a carbocation intermediate
Step 2: + charged carbocation intermediate (electrophile) reacts
rapidly with − charged bromide ion (nucleophile)
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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Electrophilic addition reaction of alkenes make them an
important class of organic compounds, because they can be used to
synthesise a wide variety of other compounds
Product obtained depends on electrophile & nucleophile used in the
addition reaction
If 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

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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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Examples:
Because alkenes above have the same substituents on both sp2 carbons, it
does not make any difference which sp2 carbon the E & Nu attach to
∴ Same product will be obtained in either case
But what happens if alkene does not have the same substituents on both
sp2 carbons?
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:
When reaction is carried out, only product isolated is tert-butyl
chloride (product 1)
Step 1 of reaction (addition of E (H+) to an sp2 carbon ) = rate-
determining step!
∴The particular carbocation that is formed faster determines final
product of reaction
Organic Chemistry, UNAM School of Medicine
HCl orHCl

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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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Why is tert-butyl carbocation formed faster than isobutyl carbocation?
Answer lies in the stability of the carbocation
Carbocation classification - according to # of alkyl substituents bonded
to + charged C
Carbocation stability ↑ as the # of alkyl substituents bonded to +
charged C ↑
Organic Chemistry, UNAM School of Medicine
tert-butyl cation (a 3° carbocation)
isobutyl cation (a 1° carbocation)
RegioselectivityRegioselectivityRegioselectivityRegioselectivity of electrophilic additionof electrophilic additionof electrophilic additionof electrophilic addition
reactionsreactionsreactionsreactions
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In both cases: More stable 3° carbocation formed more rapidly than less
stable 2° carbocation
∴ Major product of each reaction = one that results from forming the 3°
carbocation
Organic Chemistry, UNAM School of Medicine
Hl
Major product Minor product
Major product Minor product
HBr

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RegioselectivityRegioselectivityRegioselectivityRegioselectivity of electrophilic additionof electrophilic additionof electrophilic additionof electrophilic addition
reactionsreactionsreactionsreactions
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The 2 products of each of these reactions are structural isomers of
one another
Reaction 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
Regioselectivity = the preferential formation of one structural
isomer over the other
Organic Chemistry, UNAM School of Medicine
RegioselectivityRegioselectivityRegioselectivityRegioselectivity of electrophilic additionof electrophilic additionof electrophilic additionof electrophilic addition
reactionsreactionsreactionsreactions
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Addition of HBr to 2-pentene is not regioselective:
Addition of H+ to either sp2 carbons produces a 2° carbocation, both
carbocation intermediates have the same stability, so both will be
formed equally easily
∴∴∴∴ 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

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Markovnikov’s ruleMarkovnikov’s ruleMarkovnikov’s ruleMarkovnikov’s rule
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By 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 sp2
carbon that is bonded to the greater number of hydrogens
This 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
ElectrophilicElectrophilicElectrophilicElectrophilic addition of Haddition of Haddition of Haddition of H2222OOOO ---- HydrationHydrationHydrationHydration
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When H2O is added to an alkene, no reaction takes place
Reason: O–H bonds of H2O 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
If an acid (ex. HCl or H2SO4) is added as a catalyst, a reaction will
occur
Reason:The acid provides an electrophile (H+)
Product of the reaction is an alcohol
Hydration = addition of H2O to any molecule
Organic Chemistry, UNAM School of Medicine

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ElectrophilicElectrophilicElectrophilicElectrophilic addition of Haddition of Haddition of Haddition of H2222OOOO ---- HydrationHydrationHydrationHydration
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First 2 steps of mechanism for acid-catalysed addition of H2O to alkene -
essentially the same as first 2 steps of mechanism for addition of
hydrogen halide to alkene. Results in formation of a protonated alcohol
Protonated alcohols = very strong acids (proton donors)
∴ In 3rd step of reaction, protonated alcohol loses a proton (H+) to form
the final product, an alcohol
Organic Chemistry, UNAM School of Medicine
ElectrophilicElectrophilicElectrophilicElectrophilic addition of Haddition of Haddition of Haddition of H2222OOOO ---- HydrationHydrationHydrationHydration
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A H+ (donated from HCl or H2SO4) adds to alkene in 1st step & a
H+ is returned to reaction mixture in final step
∴ H+ is not consumed in the reaction
The acid acts as a catalyst in the reaction – this is an acid-
catalysed reaction
Organic Chemistry, UNAM School of Medicine

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ElectrophilicElectrophilicElectrophilicElectrophilic addition of alcohol (Raddition of alcohol (Raddition of alcohol (Raddition of alcohol (R----OH)OH)OH)OH)
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Alcohols react with alkenes in the same way that H2O does
As with addition of H2O, addition of alcohol requires an acid catalyst
Product of reaction = ether
Mechanism for acid-catalysed addition of alcohol is essentially the
same as mechanism for acid-catalysed addition of H2O
Only difference: Nu is ROH instead of H2O
Organic Chemistry, UNAM School of Medicine
HCl
ElectrophilicElectrophilicElectrophilicElectrophilic addition of alcohol (Raddition of alcohol (Raddition of alcohol (Raddition of alcohol (R----OH)OH)OH)OH)
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NB!!! - Do not memorise the products of alkene addition
reactions
Know the reaction mechanism
When presented with a reaction, ask yourself: “Which is the
electrophile?”&“Which is the nucleophile?”
Organic Chemistry, UNAM School of Medicine

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ElectrophilicElectrophilicElectrophilicElectrophilic additionadditionadditionaddition ofofofof hydrogen (Hhydrogen (Hhydrogen (Hhydrogen (H2222)))) ----
HydrogenationHydrogenationHydrogenationHydrogenation
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In the presence of metal catalyst such as Pt (platinum) or Pd
(palladium), hydrogen (H2) adds to = bond of alkene to form an
alkane
Without metal catalyst, reaction will not take place
Reason: H–H bond is very strong & energy barrier is enormous
Catalyst ↓ activation energy by breaking the H–H bond
Pt & Pd are used adsorbed* on carbon (Pt/C, Pd/C)
*Adsorption = adhesion of atoms to a surface
Organic Chemistry, UNAM School of Medicine
ElectrophilicElectrophilicElectrophilicElectrophilic additionadditionadditionaddition ofofofof hydrogen (Hhydrogen (Hhydrogen (Hhydrogen (H2222)))) ----
HydrogenationHydrogenationHydrogenationHydrogenation
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Examples:
Addition of H2 to an alkene is called alkene hydrogenation & since
the reaction requires a catalyst, it is specifically called a catalytic
hydrogenation of an alkene
Details of mechanism for catalytic hydrogenation are not yet completely
understood
Organic Chemistry, UNAM School of Medicine
H2
Pd/C
H2
Pt/C

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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

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Thank you
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END
Organic Chemistry, UNAM School of Medicine