1. ALKYNES Sem 1: 2011/2012 Khadijah Hanim bt Abdul Rahman
School of Bioprocess Engineering, UniMAP
Week 6:20/10/2011

2. 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.

3. 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

4. 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.

5. 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).

6. 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.

7. 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, π-complex is formed as intermediate.

8. 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.

9. The halogens Cl2 and Br2 also adds to alkynes
Excess halogen- a 2nd addition reaction occurs.
The solvent is CH2Cl2.

10. Exercises Give the major product of each of the following reactions:
HC CCH3 
CH3C CCH2CH3 
HBr
excess
HBr
excess

11. 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.

12. 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.

13. Mechanism involves a prototropic shift:
Acid Catalyzed
Base Catalyzed

14. 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.

15. 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.

16. 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

17. 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.

18. 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

19. 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)

20. Exercises For the following alkynes, give products of
Acid catalyzed addition of water
Hydroboration-oxidation
1-butyne
2-butyne
There is only one alkyne that forms an aldehyde when it undergoes the mercuric-ion-catalyzed addition of water. Identify the alkyne.

21. 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.

22. 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.

23. Lindlar Catalyst
The components of Lindlar catalyst: Palladium Metal Calcium Carbonate Lead Oxide
Origin of the cis stereochemistry:
The catalyst
The substrate
The poison
Delivery of hydrogen atoms to one side by the solid-phase catalyst

24. 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.

25. 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.

26. 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- 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.

27. 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.

28. 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

29. Start with a stronger base than the base that will be formed.
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:

30. 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.

31. ‘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

32. Exercises List the following compounds in order of decreasing acidity:
CH3CH2NH CH3CH=NH CH3C NH
Draw the conjugate bases of the above compounds and list them in order of decreasing basicity. + + +

33. 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

34. 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 SN2 reaction

35. 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.

36. Exercise
A chemist wants to synthesize 3-heptyne but cannot find any 1-pentyne. How else can 3-heptyne be synthesized?