Question

Explain why terminal alkynes give an aldehyde when the reagent is BH3 / H2O2/NaOH but methyl ketones when the reagent is Hg2+and H3O+,

Na/NH3 A reagent that gives a stereoselective product ( i.e. a trans alkene isomer)

Lindlar catalyst A reagent that gives a stereoselective product ( i.e. a cis alkene isomer)

Explain why terminal alkynes ( pKa ~ 25 ) are much more acidic than terminal alkenes ( pKa ~ 44)

Given a ketone or aldehyde write the corresponding enol form and vice versa ( i.e given an enol, write the keto tautomer)

Name or draw structures for simple to moderately complex alkynes.

Answer 1

Alkynes with BH3 and H2O2 undergo antimarkonikovs additons and forms aldehyde while with Hg^2+ they undergo Markonikov's additon and forms ketone

The reaction between Na in liq.NH3 and alkyne occurs via freeradical mechanism and forms an anion. This anion intermediate makes the groups far apart from each other to minimize the repulsions between bulky groups thus it give majorily trans product

The reaction of alkynes with Lindar catalyst occurs on the surface of PdCO3 and both Hs add on the same side of the double bond thus gives cis as major product

In alkynes, carbon is SP hybridized while in alkenes sp2 hybiridized. The SP hybridized carbon in alkynes is more electronegative due to more S-charcter(50%) and hence terminal Hs of alkynes are acidic