Download Group B_reaction of alkynes

ALKYNES
Sem 1: 2012/2013
Khadijah Hanim bt Abdul Rahman
School of Bioprocess Engineering, UniMAP
Week 6:16/10/2012
[email protected]
LEARNING OUTCOMES
 Addition of Hydrogen Halides to Alkynes:
- DEFINE, REPEAT and APPLY the halogen and halide
addition reactions.
- EXPLAIN and REPEAT water addition reaction to alkynes:
mechanism and tautomerization.
- EXPLAIN and DISCUSS the halogen acidity at sp carbon
and formation of acetylide ion.
The addition of hydrogen halides and
the addition of halogens to an alkyne
 Terminal alkyne- the electrophilic addition reaction is
regioselective: the H+ adds to sp carbon that is bonded to
H.
Markovnikov’s rule is applied for HCl and HBr additions to
terminal alkynes
 The electrophile adds to the sp carbon that is bonded to the
H because the secondary vinylic cation- more stable than the
primary vinylic cation.
 The addition of a hydrogen halide to an alkyne can be
stopped after the addition of 1 equivalent of hydrogen halide
because- although alkyne is less reactive than alkene,
alkyne is more reactive than the halo-substituted
alkene.
 Halo-substituted alkene-less reactive: the substituent
withdraws electrons inductively (through the σ bond)decreasing the nucleophilic character of double bond.
 Although the addition of hydrogen halide to alkyne can be
stopped- a 2nd electrophilic addition reaction will take place
if excess hydrogen halides present.
 Product of 2nd electrophilic reaction= geminal dihalide (2
halogens on same carbon).
 When the excess hydrogen halide adds to double bond, the
electrophile (H+) adds to the C with greater H.
 The resulted carbocation is more stable- because Br can share
the +ve charge with carbon by overlapping 1 of its orbitals
that contain a lone pair with the 2p orbital of +vely charged
carbon.
 Mechanism for addition of hydrogen halide- intermediate as
vinylic cation- not completely correct.
 A secondary vinylic cation-as stable as a primary carbocationgenerally, primary carbocations are unstable to be formed.
 Instead, π-complex is formed as intermediate rather than a
vinylic cation is formed as intermediate.
 Why π-complex intermediate?
 From observation that many alkyne addition reactions-
stereoselective.
 Addition of a hydrogen halide to an internal alkyne- 2
geminal dihalides- the initial addition of the proton can occur
with equal ease to either of the sp carbons.
 The halogens Cl2 and Br2 also adds to alkynes
 Excess halogen- a 2nd addition reaction occurs.
 The solvent is CH2Cl2.
Exercises
 Give the major product of each of the following reactions:
- HC
CCH3 
HBr
excess
- CH3C
CCH2CH3 
HBr
excess
ADDITION OF WATER TO AN ALKYNE
 Alkynes undergo acid-catalyzed addition of water- product
enol.
 Enol- compound with carbon-carbon double bond and OH
group bonded to sp2 carbons.
 Enol immediately rearranges to ketone
 A carbon doubly bonded to an oxygen is called a carbonyl
 Ketone- compound with 2 alkyl groups bonded to a carbonyl
group
 Aldehyde- compound that has at least 1 H bonded to a
carbonyl group.
 A ketone and its corresponding enol- keto-enol tautomers
 Tautomers- isomers that are in rapid equilibrium.
 Interconversion of tautomers- tautomerization.
 Keto and enol tautomers- equilibrium in solution.
 Keto tautomer- more stable than enol- predominates.
O
OH
H
Rapid
H
H
H
H
Enol Tautomer
Enol Tautomer
(Less favored)
(More favored)
 Addition of water to an internal alkyne- the same group
attached to each of the sp carbons- forms a single ketone
product.
 2 groups are not identical- 2 ketones are formed.
 Terminal alkynes- less reactive towards the addition of water.
 Addition of water to a terminal alkyne will occur if mercuric
ion (Hg+) is added to acidic mixture.
 Hg+ - catalyst- increase the rate of the addition reaction.
MECHANISM FOR THE MERCURIC-IONCATALYZED HYDRATION OF AN ALKYNE
• Reaction of alkyne with Hg+ forms a cyclic mercurinium ion.
• water attacks the more substituted carbon of the cyclic intermediate
• the protonated OH group- very strong acid, loses a proton to form mercuric
enol- immediately rearranges to a mercuric ketone.
• Loss of mercuric ion forms an enol- rearranges to a ketone
THE ADDITION OF BORANE TO AN
ALKYNE: HYDROBORATION-OXIDATION
 Boron- electrophile (boron atom does not have complete




octet), H- is nucleophile.
1 mole BH3 reacts with 3 moles of alkyne to form 1 mole of
boron-substituted alkene.
When added reaction is over- aqueous NaOH and H2O2
added to reaction mixture.
Product- replacement of the boron by an OH group.
The enol that is formed immediately rearrranges to ketone.
Hydroboration-Oxidation of Terminal
Alkynes
 Addition of borane to terminal alkyne- Boron adds
preferentially to the sp carbon bonded to the hydrogen.
 Thus, the reaction follows the general rule for electrophilic
addition reaction: the electrophile (BH3) adds to the sp
carbon bonded to the greater no. of hydrogens.
 Boron-containing group is replaced by OH grouphydroboration-oxidation produces aldehyde
Formation of Ketone versus Aldehyde
 Hydroxyboration-oxidation of a terminal alkyne produces
an aldehyde (carbonyl group at the terminal carbon)
 The mercuric-ion-catalyzed addition of water to terminal
alkyne produces ketone (carbonyl group is not at terminal)
Exercises
 For the following alkynes, give products of
- Acid catalyzed addition of water
- Hydroboration-oxidation
1-butyne
b) 2-butyne
 There is only one alkyne that forms an aldehyde when it
undergoes the mercuric-ion-catalyzed addition of water.
Identify the alkyne.
a)
The addition of hydrogen to an alkyne
 Alkynes undergo catalytic hydrogenation- initial product-
alkene.
 Difficult to stop the reaction because hydrogen’s strong
tendency to add to alkenes in the presence of metal catalysts.
 Final product for hydrogenation reaction- alkane.
 Reaction can be stopped at alkene stage using partially
deactivated metal catalyst- Lindlar catalyst
 Lindlar catalyst- prepared by precipitating palladium on
calcium carbonate and treating it with lead (II) acetate and
quinoline.
 This treatment modifies the surface of palladium- more
effective in catalyzing the addition of H to a triple bond than
a double bond.
Lindlar Catalyst
 The components of
 Origin of the cis
Lindlar catalyst:
Palladium Metal
Calcium Carbonate
Lead Oxide
stereochemistry:
The catalyst
The substrate
The poison
Delivery of hydrogen atoms to
one side by the solid-phase
catalyst
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 The alkyne sits on the surface of metal catalyst and the
hydrogens are delivered to the triple bond from the surface
of the catalyst- both H are delivered to the same side of the
double bond.
 Only syn addition of H.
 Syn addition of H to an internal alkyne forms a cis alkene.
 Internal alkynes- converted to trans alkenes using sodium (or
lithium) in liquid ammonia.
 Reaction stops at alkene stage- Na reacts rapidly with triple
bonds than double bonds.
 Ammonia is gas at RT. Kept as liquid using dry ice/acetone
mixture.
Mechanism for conversion of an alkyne
to a trans alkene
• single
electron from s orbital of Na is transferred to sp carbon of alkyneradical anion- negative charge and unpaired electron.
• radical anion- strong base that it can remove a proton from ammonia.
Results in the formation of a vinylic radical- radical’s unpaired electron is
on vinylic carbon.
• another single-electron transfer from Na to the vinylic radical forms a
vinylic anion
• vinylic anion- strong base – removes proton from another molecule of
ammonia. Product- trans alkene.
A hydrogen bonded to an sp carbon is
‘acidic’
 Electronegativity of an atom depends on its hybridization.
 sp carbon is more electronegative than sp2 carbon, which is
more electronegative than sp3.
 Acidic compound- compound with most hydrogen attached
to the most electronegative atom.
 Ethyne is stronger acid then ethene and ethene is a stronger
acid than ethane.
Acidity of a Hydrogen Bonded to an sp
Hybridized Carbon
High s character, sp
orbital penetrates
nucleus.
Lower s character, sp3
orbital penetrates
nucleus to a lesser
degree
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 In order to remove a proton from acid, the base that removes
the proton must be stronger than base that is generated.
 Start with a stronger base than the base that will be formed.
 NH3 is a weaker acid than terminal alkyne, the conjugate base
of NH3 (NH2-) is a stronger base than the carbocation- called
acetylide ion- formed when a hydrogen is removed from sp
carbon of terminal alkyne.
 Therefore, an amide ion (-NH2) can be used to remove a
proton from a terminal alkyne to prepare an acetylide ion.
The stronger the acid, the weaker its conjugate base:
 In contrast, if hydroxide ion were used as base, the reaction
would strongly favor the reactants because- OH ion is weaker
base than acetylide ion that would be formed.
 Amide ion cannot remove a hydrogen bonded to an sp2 or an
sp3 carbon.
 Only a hydrogen bonded to an sp carbon is sufficiently acidic
to be removed by an amide ion.
 Hydrogen bonded to an sp carbon referred as ‘acidic’
hydrogen.
 ‘acidic’ property that differs the reactivity of terminal
alkynes from alkenes.
 It is more acidic than most other hydrogens, but it is much
less acidic than hydrogen of water molecule- water is weakly
acidic compound.
relative base strength
CH3CH2-
strongest
base
>
H2C
CH-
>
H2N-
>
HC
C-
>
HO-
>
F-
weakest
base
Exercises
 List the following compounds in order of decreasing acidity:
+
CH3CH2NH3
+
CH3CH=NH2
+
CH3C NH
 Draw the conjugate bases of the above compounds and list
them in order of decreasing basicity.
Synthesis using acetylide ions
 Reactions that form C-C bonds are important- to synthesis a
molecule with larger carbon skeletons.
 Reaction that forms a carbon-carbon bond is the reaction of
an acetylide ions with alkyl halide.
 Only primary alkyl halides or methyl halides should be used
in this reaction.
We can convert terminal alkynes to longer internal
alkynes by addition of alkyl halides
 The mechanism: Br is more electronegative than carbon and
as a result, the electrons in the C-Br bond are not shared
equally by 2 atoms.
 There is a partial +ve charge on carbon and a partial –ve
charge on bromine.
Known as an SN2
reaction
 The –vely charged acetylide ion (nucleophile) attracted to
the partially +vely charged carbon (electrophile) of alkyl
halide.
 As electron of acetylide ion approach the C to form new C-C
bond, they push out the Br and its bonding electrons- carbon
can only bond to 4 atoms.
 The reaction- alkylation reaction- attaches an alkyl group
to a species.
 Terminal alkynes can be converted into internal alkynes of
any desired length by choosing an alkyl halide of the
appropriate structure.
 Count the no of carbons in terminal alkyne and the no of
carbons in the product to see how many carbons are needed
in alkyl halide.
Exercise
 A chemist wants to synthesize 3-heptyne but cannot find any
1-pentyne. How else can 3-heptyne be synthesized?