Topic 6: alkyne

Question 1

Describe the electronic structure of alkyne

Answer 1

• 2sp atoms > linear σ skelaton + 2 perpendicular π
• e- rich = good nucleophile

Question 2

How are alkynes prepared

Answer 2

• Treat vicinal dihalide = 1,2-dihaloalkane with strong base e.g. KOH/NaNH2

Question 3

What is the reaction alkynes with HX or X2

Answer 3

• Markovnikov’s addition for terminal C
• Goes from alkyne > haloalkene > haloalkane

Question 4

Describe the hydration of alkynes

Answer 4

• Forms aldehydes + ketones
  Step 1) Additionn of H2O = enol
  Step 2) Tautomerization rearrangement = carbonyl compound
• Double bond goes from C=C > O in -OH
• Give the H from -OH to other C
  2 TYPES:
  1) Mercury catalyzed = oxymercuration
• Markovnikov
  2) Hydroboration
• Anti-Markovnokiv

Question 5

Explain mercury catalyzed hydration

Answer 5

• 1 step process
• Reagents = HgSO4 + H2O + H2SO4
• If terminal = methyl ketone produced = C=O in position 2
• If not terminal = no Markovnoikov = 2 tautomerization = 2 different ketones

Question 6

Explain hydroboration of alkyne

Answer 6

1) BH3/THF
2) H2O2/OH
IF TERMINAL
- Anti-Markovnikov
- Tautomerization
- Makes aldehyde
IF NOT TERMINAL
- No Markovnikov = 2 different enols
- Tautomerization = 2 different ketones

Question 7

Explain hydrogenation of alkynes

Answer 7

• Adding H2 with metal catalyst
• Pt/C + Pd/C + PtO2
• Produces alkanes via alkenes

Question 8

Explain partial alkyne hydrogenation

Answer 8

• If want to stop reduce at alkene
• Either produce cis/trans alkene
  CIS:
• Lindlar catalyst = Pd/CaCO3/Pb
  TRANS:
• Reagent = Li/NH3 or Na/NH3 > NH3 liquid

Question 9

Describe oxidation of alkynes

Answer 9

• Ozonolysis OR KMnO4
• Produce carboxylic acids for both
• If CO2 produced = terminal alkyne

Question 10

Describe formation of acetylide anions

Answer 10

• Terminal alkynes = acidic
• Strong base to remove H+ = NaNH2/NH3
• C≡C-H > C≡C:-

Question 11

Why are acetylide anions important?

Answer 11

• Fully negtive C = difficult to firm
• C- allows attack + attach into +ve C= enkarging carbon skelaton = make larger compounds

Question 12

Explain the alkylation of acetylide anion

Answer 12

• Adding alkyl group to anion
• Using alkyl halide
• Remove halogen and connect carbons

Question 13

Explain the SN2 reaction of alkylation

Answer 13

• Steric hindrance when C: tries to attack central C in C-X
• If alkyl halide is tertiary of secondary = steric hindrance