Chapter 7.6: Electrophilic Addition to Alkynes
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What is A carbocation in which the positive charge is on one of the carbons of a carbon-carbon double bond?
vinylic carbocation
Descirbe the mechanism of the addition of hydrogen halides to an alkyne`
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
What is the stereochemistry of the addition of bromine or chlorine to an alkyne?
• The major product corresponds to anti addition of the two bromine atoms
Define vinylic carbocation
A carbocation in which the positive charge is on one of the carbons of a carbon-carbon double bond
Describe the mechanism of the addition of bromine or chlorine to an alkyne
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
What is the regiochemistry of the addition of hydrogen halides to an alkyne?
Markovnikov’s Rule