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Slide 1
Slide 2
Khadijah Hanim bt Abdul Rahman School of Bioprocess
Engineering, UniMAP Week 6:20/10/2011 [email protected]
ALKYNES Sem 1: 2011/2012
Slide 3
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.
Slide 4
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.
Markovnikovs rule is applied for HCl and HBr additions to terminal
alkynes
Slide 5
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 bond)- decreasing the nucleophilic
character of double bond.
Slide 6
Although the addition of hydrogen halide to alkyne can be
stopped- a 2 nd electrophilic addition reaction will take place if
excess hydrogen halides present. Product of 2 nd electrophilic
reaction= geminal dihalide (2 halogens on same carbon).
Slide 7
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.
Slide 8
Mechanism for addition of hydrogen halide- intermediate as
vinylic cation- not completely correct. A secondary vinylic
cation-stable as a primary carbocation- generally, primary
carbocations are unstable to be formed. Instead, -complex is formed
as intermediate.
Slide 9
Why -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.
Slide 10
The halogens Cl 2 and Br 2 also adds to alkynes Excess halogen-
a 2 nd addition reaction occurs. The solvent is CH 2 Cl 2.
Slide 11
Exercises Give the major product of each of the following
reactions: - HC CCH 3 - CH 3 C CCH 2 CH 3 HBr excess HBr
excess
Slide 12
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.
Slide 13
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.
Slide 14
Mechanism involves a prototropic shift: Acid CatalyzedBase
Catalyzed
Slide 15
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.
Slide 16
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.
Slide 17
MECHANISM FOR THE MERCURIC-ION- CATALYZED 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
Slide 18
THE ADDITION OF BORANE TO AN ALKYNE: HYDROBORATION-OXIDATION
Boron- electrophile (boron atom does not have complete octet), H -
is nucleophile. 1 mole BH 3 reacts with 3 moles of alkyne to form 1
mole of boron-substituted alkene. When added reaction is over-
aqueous NaOH and H 2 O 2 added to reaction mixture. Product-
replacement of the boron by an OH group. The enol that is formed
immediately rearrranges to ketone.
Slide 19
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 group-
hydroboration-oxidation produces aldehyde
Slide 20
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)
Slide 21
Exercises For the following alkynes, give products of - Acid
catalyzed addition of water - Hydroboration-oxidation a) 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.
Slide 22
The addition of hydrogen to an alkyne Alkynes undergo catalytic
hydrogenation- initial product- alkene. Difficult to stop the
reaction because hydrogens strong tendency to add to alkenes in the
presence of metal catalysts. Final product for hydrogenation
reaction- alkane.
Slide 23
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.
Slide 24
23 Lindlar Catalyst The components of Lindlar catalyst:
Palladium Metal Calcium Carbonate Lead Oxide Origin of the cis
stereochemistry: The poison The substrate The catalyst Delivery of
hydrogen atoms to one side by the solid-phase catalyst
Slide 25
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.
Slide 26
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.
Slide 27
Mechanism for conversion of an alkyne to a trans alkene single
electron from s orbital of Na is transferred to sp carbon of
alkyne- radical 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- radicals
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.
Slide 28
A hydrogen bonded to an sp carbon is acidic Electronegativity
of an atom depends on its hybridization. sp carbon is more
electronegative than sp 2 carbon, which is more electronegative
than sp 3. 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.
Slide 29
28 Acidity of a Hydrogen Bonded to an sp Hybridized Carbon High
s character, sp orbital penetrates nucleus. Lower s character, sp 3
orbital penetrates nucleus to a lesser degree
Slide 30
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. NH 3 is a weaker
acid than terminal alkyne, the conjugate base of NH 3 (NH 2 - ) 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 ( - NH 2 ) 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:
Slide 31
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 sp 2 or an sp 3 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.
Slide 32
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 weakest base
strongest base
Slide 33
Exercises List the following compounds in order of decreasing
acidity: CH 3 CH 2 NH 3 CH 3 CH=NH 2 CH 3 C NH Draw the conjugate
bases of the above compounds and list them in order of decreasing
basicity. + + +
Slide 34
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
Slide 35
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. 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. Known as an S N 2 reaction
Slide 36
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.
Slide 37
Exercise A chemist wants to synthesize 3-heptyne but cannot
find any 1-pentyne. How else can 3-heptyne be synthesized?