2 Alkenes X2 HX Markovnikov

Reactions of Alkenes
(I)
Grade 11 Chemistry
Reactivity of alkenes
• Alkenes are much more reactive than alkanes, due to the
presence of the C=C double bond.
• Electrons in the π-bond are not as strongly attracted to the
nuclei of the C atoms, making the π-bond weaker, and easier to
break, than the σ-bond.
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Reactivity of alkenes
• The π-bond serves as a source of electrons. It has high electron
density. Thus alkenes are attacked by electrophiles (species that
accept a pair of electrons).
• The typical reaction of alkenes is electrophilic addition.
• In general,
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Electrophilic addition of alkenes
• The C atoms involved in the double bond are sp2 hybridized,
forming a trigonal planar shape about the atom.
• The fairly open shape makes them easy for incoming groups to
attack.
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Electrophilic addition of alkenes – reactions with hydrogen
halides
• In general,
Reagents: HX
Conditions: room
temperature in
absence of UV light
• Reactivity differs depending on the hydrogen halide used:
HI > HBr > HCl
HF reacts with alkenes only under pressure
Can you think why?
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Electrophilic addition of alkenes – reactions with hydrogen
halides
• E.g.
Reagents: HBr
Conditions: room
temperature in
absence of UV light
carbocation
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Electrophilic addition of alkenes – reactions with hydrogen
halides
Draw the mechanism for the reaction between propene and HBr.
Give the IUPAC name of the product(s) formed.
5 Minutes
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Electrophilic addition of alkenes – reactions with hydrogen
halides
Draw the mechanism for the reaction between propene and HBr.
Give the IUPAC name of the product(s) formed.
WHY?!
2-bromopropane is the major product.
Only a small amount of 1-bromopropane is formed.
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Markovnikov’s rule
The electrophile will preferentially bond to the C atom of the
alkene that is already bonded to the largest number of hydrogen
substituents.
The intermediate carbocations have different stabilities.
is less stable than
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Markovnikov’s rule
The electrophile will preferentially bond to the C atom of the
alkene that is already bonded to the largest number of hydrogen
substituents.
The intermediate carbocations have different stabilities.
Most stable
Least stable
• Alkyl groups stablise the carbocations by exerting a positive
inductive effect, by pushing electron density away from itself and
towards the positive charge.
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Electrophilic addition of alkenes – reactions with halogens
• In general,
Reagents: X2 in CCl4
Conditions: room
temperature
Observation:
decolourisation of
reddish-brown
bromine in CCl4
• Reactivity differs depending on the hydrogen halide used:
F2 > Cl2 > Br2 > I2
F2 reacts explosively, reaction with I2 is slow and incomplete
Can you think why?
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Electrophilic addition of alkenes – reactions with halogens
Step 2: Attack by Br – on
carbocation
Step 1: Formation of
carbocation
The electron cloud of πbond in ethene polarizes
an approaching Br2
molecule.
The positive end of the
molecule attacks ethene.
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Electrophilic addition of alkenes – reactions with halogens
For unsymmetrical alkenes, the intermediate formed will be the
more stable carbocation, according to Markovnikov’s Rule
More stable
Less stable
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Electrophilic addition of alkenes – reaction with steam
(hydration)
• In general,
Reagents: H2O with
conc. H2SO4 catalyst
Conditions: warm
• The conditions for the industrial hydration of ethene to form
ethanol is different from that at the laboratory level.
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Electrophilic addition of alkenes – reaction with steam
(hydration)
Step 1: Electrophilic attack by H+ (from H2SO4) to form
carbocation, followed by attack by HSO4-
For unsymmetrical alkenes, the
formation of carbocations will
follow Markovnikov’s Rule
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Electrophilic addition of alkenes – reaction with steam
(hydration)
Step 2: Warm with water to form alcohol
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Electrophilic addition of alkenes – reaction with steam
(hydration)
The industrial manufacturing of ethanol is an electrophilic addition
reaction of steam and ethene.
Reagents: steam
Conditions: 300 °C,
65 atm, H3PO4
catalyst
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Reduction of alkenes (hydrogenation)
• In general,
Reagents: H2
Conditions: Pt or Pd
catalyst, room
temperature
∆H = negative
OR Ni catalyst, 140°C
• The addition of H2 to alkenes is used in industry to vegetable oils
to margarine, which is harder and have higher boiling points.
Saturated oils (trans fats) are
linked to higher risks of coronary
heart diseases
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