elect_addn_alkyne

Question 1

A four carbon alkyne (but-2-yne) reacts with excess hydrogen gas and a platinum catalyst. If the reaction goes to completion what must be the product?

Answer 1

The only product will be butane (a 4 carbon alkane).

Question 2

What reagents/conditions are required to change to the 4 carbon alkyne (but-2-yne) to (Z)-but-2-ene?

Answer 2

The reagents/conditions required to change to the 4 carbon alkyne (but-2-yne) to (Z)-but-2-ene are H₂ gas in the presence of Lindlar’s catalyst.

Question 3

What reagents/conditions are required to change to the 4 carbon alkyne (but-2-yne) to (E)-but-2-ene?

Answer 3

The reagents/conditions required to change to the 4 carbon alkyne (but-2-yne) to (E)-but-2-ene are Na or Li metal in the presence of liquid ammonia (-78^oC).

Question 4

The reaction of but-2-yne with excess HCl will result in the formation of which dichloride: 2,3-dichlorobutane or 2,2-dichlorobutane?

Answer 4

The reaction of but-2-yne with excess HCl will result in the formation of 2,2-dichlorobutane (i.e. the geminal (gem) dihalide).

Question 5

Will the reaction of but-2-yne with 1 mol equivalent Cl2/CCl4 result in cis-2,3-dichlorobut-2-ene or trans-2,3-dichlorobut-2-ene?

Answer 5

The reaction of but-2-yne with 1 mol equivalent Cl₂/CCl₄ will result in the formation of the trans isomer. This is because the chloronium ion must be attacked by the chloride nucleophile from the opposite side and this will result in an anti addition.

Question 6

Name the term for the thermodynamic process by which an enol spontaneously transforms to a ketone (or aldehyde).

Answer 6

The thermodynamic process by which an enol spontaneously transforms to a ketone (or aldehyde) is called tautomerization.

Question 7

Describe acid catalyzed addition of water to an alkyne in terms of regiochemistry.

Answer 7

Acid catalyzed addtion of water to an alkyne occurs via the most stable carbocation intermediate. Therefore, the water is added Markovnikov which will result in the formation of an enol, which will in turn tautomerize to the ketone functional group.

Question 8

Describe the conditions necessary for an anti-Markovnikov addition of an OH group to an alkyne.

Answer 8

An anti-Markovnikov addition to an alkyne requires two steps: 1) BH3,THF; 2) H2O2, NaOH, H2O. The resulting enol, with OH on the least substituted carbon, will tautomerize to the aldehyde functional group.