Alkyne Reactions Flashcards
Addition of one mole of Br2 or Cl2 is ____ stereoselective.
anti
Addition of bromine and chlorine:
Anti addition of halogen to an alkyne gives an _________. Addition of a second ole of halogen gives a ________.
(E)-dihaloalkene, tetrahaloalkane
Addition of HX is _______.
regioselective
Addition of HX _____ follow Markovnikov’s rule.
does
Steps of mechanism for addition of HX to an alkyne.
Step 1: make a new bond between a nucleophile (pi bond) and an electrophile - add a proton
Step 2: make a new bond between a nucleophile and electrophile
Describe step 1 of addition of HBr to an alkyne: make a new bond between a nucleophile and an electrophile
Proton transfer from HBr to the alkyne gives a vinylic carbocation.
Describe step 2 of the addition of HBr to an alkyne: 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
Describe step 1 for the addition of a second mole of HX:
The reaction of the electron pair of the remaining pi bond with HBr to form a carbocation
The sterically hindered (sia)2BH gives ________ reaction with 1-alkynes leading to ______.
regioselective, aldehydes
Alkynes react with water in the presence of H2SO4 and Hg(II) salts to give ______ via an ______ intermediate.
ketones, enol
Hydroboration of an internal alkyne is ____ stereoselective. Oxidation of the resulting __________ by H2O2/NaOH gives an ____ that is in equilibrium with a ________.
syn, trialkenylborane, enol, ketone
Hydroboration of a terminal alkyne using a hindered __________ followed by oxidation of the resulting ________ with H2O2/NaOH and the keto-enol tautomerism gives an _______.
dialkylborane, trialkenylborane, aldehyde
Acid-catalyzed addition of water in the presence of Hg(II) salts is ________.
regioselective
Acid-catalyzed hydration of alkynes is analogous to ___________.
oxymercuration of alkenes
For terminal alkenes, addition of water through acid catalyzed hydration ______ follow Markovnikov’s rule.
does
What are the steps of acid-catalyzed hydration of an alkyne?
Step 1: make a new bond between a nucleophile (pi bond) and an electrophile
Step 2: Make a new bon between a nucleophile and an electrophile
Step 3: take a proton away
Step 4: Keto-enol tautomerism
Step 5: Add a proton
Step 6 and 7: break a bond to give stable molecules or ions followed by keto-enol tautomerism
What are the reagents in acid catalyzed hydration of an alkyne?
HgSO4, H2SO4, H2O
What are the reagents in hydroboration-oxidation of an alkyne?
BH3 or (sia)2BH, H2O2, NaOH
Describe step 1 of acid-catalyzed hydration of an alkyne: make a new bond between a nucleophile and an electrophile
Attack of the C-C triple bond on the Hg2+ (an electrophile) gives a bridged mercurinium ion intermediate
Describe step 2 of acid-catalyzed hydration of an alkyne: make a new bond between a nucleophile and an electrophile
Attack of water (a nucleophile) on the bridged mercurinium ion intermediate (an electrophile) from the side opposite the bridge opens the three-membered ring.
Describe step 3 of acid-catalyzed hydration of an alkyne: take a proton away
Proton transfer to solvent gives an organomercury enol
Describe step 5 of acid-catalyzed hydration of an alkyne: add a proton
proton transfer to the carbonyl group of the ketone gives an oxonium ion
Describe step 6 and 7 of acid-catalyzed hydration of an alkyne: break a bond to give stable molecules and ions followed by keto-enol tautomerism
Loss of Hg2+ from the oxonium ion gives the enol form of the final product. Tautomerism of the enol gives a ketone
Reaction of an alkyne with two moles of H2 under moderate pressure in the presence of a transition metal catalyst at room temperature gives an ______.
alkane
Catalytic reduction of an alkane in the presence of the Lindlar catalyst is ____ stereoselective; Lindlar reduction of an internal alkyne gives a ___ alkene.
syn, cis
What are the transition metal catalysts used in catalytic reduction to yield an alkane?
palladium (Pd), platinum (Pt), or nickel (Ni)
Hydroboration of an alkyne followed by protonolysis converts an alkyne to a _________.
cis-alkene
Treating a trialkenylborane with a ______ results in ________ replacement of boron by hydrogen.
carboxylic acid such as acetic acid, stereoselective
Alkali metal reduction is ______: ____ addition of hydrogens to an internal alkyne gives a ____ alkene.
stereoselective, anti, trans
What are the metals used in alkali metal reduction?
Na or Li
What is the solvent in alkali metal reduction?
NH3(l)
What are the steps of reduction of an alkyne by sodium in liquid ammonia?
Step 1: one-electron reduction
Step 2: add a proton
Step 3: one-electron reduction
Step 4: add a proton
Describe step 1 of reduction of an alkyne by sodium in liquid ammonia
A one-electron reduction of an alkyne gives an alkenyl radical anion
Describe step 2 of reduction of an alkyne by sodium in liquid ammonia: add a proton
The alkenyl radical anion (a very strong base) abstracts a proton from a molecule of ammonia (under these conditions, a weak acid) to give an alkenyl radical
Describe step 3 of reduction of an alkyne by sodium in liquid ammonia:
A one-electron reduction of the alkenyl radical gives an alkenyl anion. The trans-alkenyl anion is more stable than its cis isomer
Desribe step 4 of reduction of an alkyne by sodium in liquid ammonia: add a proton
A second proton-transfer reaction completes the reduction and gives the trans-alkene
