Chapter 9: Alkynes

dehydrogenation

prepares alkynes

1. add 2 NaNH2 and H3O+

2. Br2 (or Cl2) and then 2KOH + ethanol

• 2Br and 2H are removed to form triple bond between carbons 

• Alkane = 2, alkene = 1

addition of HX

2 rounds w/ether

• C1 w/2x, C2 w/2H

• Mark., trans

addition of X2

2 rounds, each carbon gets 2 X

• trans (usually)

hydration: mercury(ii)-catalyzed

add H2SO4, H2O and HgSO4

• forms ketone

• terminal alkyne and ketone tamer are favored (ketone taumer rearrangement)

• Mark.

Oxygen double bonds to carbon next to R and CH becomes CH3 single bonded (looses triple bond)

hydration: hydroboration-oxidation of terminal alkynes

add 1. BH3 and 2. H2O2 to form aldehyde

• anti-Mark.

Oxygen double bonds to carbon on end, triple bond between carbons becomes single, 2 hydrogens add to other carbon

hydration: hydroboration-oxidation of internal alkynes

add 1. BH3 and 2. H2O2 to form methyl ketone

same result as mercury catalyzed oxidation

reduction: catalytic

add 2 H2 w/metal catalyst to form alkane

2 hydrogens add to each carbon, breaking double bond

reduction: alkyne to alkene

• 2 ways

H2 and Lindlar catalyst

• cis + syn stereo.

• One hydrogen bonds to each carbon, C=C

Li and NH3

• trans and anti stereo.

• One hydrogen bonds to each carbon, C=C

acetylide anions

add NaNH2 to form anion from terminal alkyne

• Hydrogen on carbon is lost and Na- adds

• By product is NH3

alkylation of acetylide anions

act as nucleophile to produce new alkyne

• add NaNH2 to form anion

(can add R’CH2Br to terminal alkyne to form internal alkyne)

• ____Br adds to other side of triple bond (terminal carbon)