Sections 7.57.7

1
Sections 7.5-7.7

• Lecture 19
• Chem 30A

2
Acidity

• The pKa of acetylene and terminal alkynes is
  approximately 25, which makes them stronger acids
  than ammonia but weaker acids than alcohols
  (Section 4.1).

3
Acidity

• Terminal alkynes can be converted to alkyne
  anions with sodium hydride or lithium
  diisopropylamide(LDA).
• Hydroxide ion is not a strong enough base to
  convert a terminal alkyne to an alkyne anion.

4
Alkylation of Alkyne Anions

• Alkyne anions are both strong bases and good
  nucleophiles.
• They participate in nucleophilic substitution
  reactions with alkyl halides to form new C-C
  bonds to alkyl groups they undergo alkylation.

5
Alkylation of Alkyne Anions

• Alkyne anion alkylation is used for the
  synthesis of terminal alkynes.
• Second Alkylation forms internal alkyne.

6
Preparation from Alkenes

• Treatment of a vicinal dibromoalkane with two
  moles of base results in two successive
  dehydrohalogenation reactions and formation of an
  alkyne.

7
Preparation from Alkenes

• In order to convert a terminal alkene to a
  terminal alkyne, 3 moles of base are required.

8
Preparation from Alkenes

• A side product of this synthetic scheme may be an
  allene, a compound containing adjacent
  carbon-carbon double bonds, CCC.

9
Allene

• Allene A compound containing a CCC group

10
Allenes

• Most allenes are less stable than their isomeric
  alkynes, and are generally only minor products in
  alkyne-forming dehydrohalogenation reactions.

11
Addition of X2

• Alkynes add one mole of bromine to give a
  dibromoalkene.
• Addition shows anti stereoselectivity.

12
Addition of X2

• The intermediate in bromination of an alkyne is a
  bridged bromonium ion.

13
Addition of HX

• Alkynes undergo regioselective addition of either
  1 or 2 moles of HX, depending on the ratios in
  which the alkyne and halogen acid are mixed.

14
Addition of HX

• 2 vinylic carbocation is preferred over a 1
  vinylic carbocation.
• Reaction of the vinylic cation with halide ion
  gives the product.

15
Addition of HX

• In the addition of the second mole of HX, the
  remaining pi bond reacts with HBr to form a
  carbocation.
• Forms Resonance-stabilized 2 carbocation.

16
Hydroboration

• Addition of borane to an internal alkyne gives a
  trialkenylborane.
• Addition is syn stereoselective.

17
Hydroboration

• To prevent dihydroboration with terminal alkynes,
  a sterically hindered dialkylborane is used
• Treatment of a terminal alkyne results in
  stereoselective and regioselective hydroboration.

18
Hydroboration

• Treating an alkenylborane with H2O2 in aqueous
  NaOH gives an enol.
• Enol A compound containing an OH group on one
  carbon of a carbon-carbon double bond.
• Keto forms generally predominate at equilibrium.
• Keto and enol forms are tautomers

19
Hydroboration
20
Addition of H2O hydration

• In the presence of sulfuric acid and Hg(II)
  salts, alkynes undergo addition of water.

21
Addition of H2O hydration

• Step 1 Attack of Hg2 on the triple bond
• Step 2 Attack of water opens the ring.

22
Addition of H2O hydration

• Step 3 Proton transfer to gives an organomercury
  enol.
• Step 4 Tautomerism gives the keto form.

23
Addition of H2O hydration

• Step 5 Proton transfer to the carbonyl oxygen
  gives an oxonium ion.
• Steps 6 and 7 Loss of Hg2 gives an enol
  tautomerism of the enol gives the ketone.