Chapter 7.6: Electrophilic Addition to Alkynes

Question 1

What is A carbocation in which the positive charge is on one of the carbons of a carbon-carbon double bond?

Answer 1

vinylic carbocation

Question 2

Descirbe the mechanism of the addition of hydrogen halides to an alkyne`

Answer 2

 Step 1: Make a new bond between a nucleophile (pi bond) and an electrophile—add a proton
• Proton transfer from HBr to the alkyne gives a vinylic carbocation; the more stable 2° vinylic carbocation is formed in preference to the less stable 1° vinylic carbocation
 Step 2: Make a new bond between a nucleophile and an electrophile
• Reaction of the vinylic carbocation (an electrophile) with bromide ion (a nucleophile) gives the vinylic bromoalkene

Question 3

What is the stereochemistry of the addition of bromine or chlorine to an alkyne?

Answer 3

• The major product corresponds to anti addition of the two bromine atoms

Question 4

Define vinylic carbocation

Answer 4

A carbocation in which the positive charge is on one of the carbons of a carbon-carbon double bond

Question 5

Describe the mechanism of the addition of bromine or chlorine to an alkyne

Answer 5

o Addition of one mole of Br2 to an alkyne gives a dibromoalkene
 Addition of bromine to alkynes follows much the same type of mechanism as it does for addition to alkene, namely formation of a bridged bromonium ion intermediate, which is then attacked by bromide ion from the face opposite that occupied by the positively charged bromine atom
o Alkynes similarly undergo addition of Cl2, although less stereoselectively than with Br2

Question 6

What is the regiochemistry of the addition of hydrogen halides to an alkyne?

Answer 6

Markovnikov’s Rule