Alkyne Reactions

Question 1

Addition of one mole of Br2 or Cl2 is ____ stereoselective.

Answer 1

anti

Question 2

Addition of bromine and chlorine:
Anti addition of halogen to an alkyne gives an _________. Addition of a second ole of halogen gives a ________.

Answer 2

(E)-dihaloalkene, tetrahaloalkane

Question 3

Addition of HX is _______.

Answer 3

regioselective

Question 4

Addition of HX _____ follow Markovnikov’s rule.

Answer 4

does

Question 5

Steps of mechanism for addition of HX to an alkyne.

Answer 5

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

Question 6

Describe step 1 of addition of HBr to an alkyne: make a new bond between a nucleophile and an electrophile

Answer 6

Proton transfer from HBr to the alkyne gives a vinylic carbocation.

Question 7

Describe step 2 of the addition of HBr to an alkyne: make a new bond between a nucleophile and an electrophile

Answer 7

Reaction of the vinylic carbocation (an electrophile) with bromide ion (a nucleophile) gives the vinylic bromoalkene

Question 8

Describe step 1 for the addition of a second mole of HX:

Answer 8

The reaction of the electron pair of the remaining pi bond with HBr to form a carbocation

Question 9

The sterically hindered (sia)2BH gives ________ reaction with 1-alkynes leading to ______.

Answer 9

regioselective, aldehydes

Question 10

Alkynes react with water in the presence of H2SO4 and Hg(II) salts to give ______ via an ______ intermediate.

Answer 10

ketones, enol

Question 11

Hydroboration of an internal alkyne is ____ stereoselective. Oxidation of the resulting __________ by H2O2/NaOH gives an ____ that is in equilibrium with a ________.

Answer 11

syn, trialkenylborane, enol, ketone

Question 12

Hydroboration of a terminal alkyne using a hindered __________ followed by oxidation of the resulting ________ with H2O2/NaOH and the keto-enol tautomerism gives an _______.

Answer 12

dialkylborane, trialkenylborane, aldehyde

Question 13

Acid-catalyzed addition of water in the presence of Hg(II) salts is ________.

Answer 13

regioselective

Question 14

Acid-catalyzed hydration of alkynes is analogous to ___________.

Answer 14

oxymercuration of alkenes

Question 15

For terminal alkenes, addition of water through acid catalyzed hydration ______ follow Markovnikov’s rule.

Answer 15

does

Question 16

What are the steps of acid-catalyzed hydration of an alkyne?

Answer 16

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

Question 17

What are the reagents in acid catalyzed hydration of an alkyne?

Answer 17

HgSO4, H2SO4, H2O

Question 18

What are the reagents in hydroboration-oxidation of an alkyne?

Answer 18

BH3 or (sia)2BH, H2O2, NaOH

Question 19

Describe step 1 of acid-catalyzed hydration of an alkyne: make a new bond between a nucleophile and an electrophile

Answer 19

Attack of the C-C triple bond on the Hg2+ (an electrophile) gives a bridged mercurinium ion intermediate

Question 20

Describe step 2 of acid-catalyzed hydration of an alkyne: make a new bond between a nucleophile and an electrophile

Answer 20

Attack of water (a nucleophile) on the bridged mercurinium ion intermediate (an electrophile) from the side opposite the bridge opens the three-membered ring.

Question 21

Describe step 3 of acid-catalyzed hydration of an alkyne: take a proton away

Answer 21

Proton transfer to solvent gives an organomercury enol

Question 22

Describe step 5 of acid-catalyzed hydration of an alkyne: add a proton

Answer 22

proton transfer to the carbonyl group of the ketone gives an oxonium ion

Question 23

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

Answer 23

Loss of Hg2+ from the oxonium ion gives the enol form of the final product. Tautomerism of the enol gives a ketone

Question 24

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 ______.

Answer 24

alkane

Question 25

Catalytic reduction of an alkane in the presence of the Lindlar catalyst is ____ stereoselective; Lindlar reduction of an internal alkyne gives a ___ alkene.

Answer 25

syn, cis

Question 26

What are the transition metal catalysts used in catalytic reduction to yield an alkane?

Answer 26

palladium (Pd), platinum (Pt), or nickel (Ni)

Question 27

Hydroboration of an alkyne followed by protonolysis converts an alkyne to a _________.

Answer 27

cis-alkene

Question 28

Treating a trialkenylborane with a ______ results in ________ replacement of boron by hydrogen.

Answer 28

carboxylic acid such as acetic acid, stereoselective

Question 29

Alkali metal reduction is ______: ____ addition of hydrogens to an internal alkyne gives a ____ alkene.

Answer 29

stereoselective, anti, trans

Question 30

What are the metals used in alkali metal reduction?

Answer 30

Na or Li

Question 31

What is the solvent in alkali metal reduction?

Answer 31

NH3(l)

Question 32

What are the steps of reduction of an alkyne by sodium in liquid ammonia?

Answer 32

Step 1: one-electron reduction
Step 2: add a proton
Step 3: one-electron reduction
Step 4: add a proton

Question 33

Describe step 1 of reduction of an alkyne by sodium in liquid ammonia

Answer 33

A one-electron reduction of an alkyne gives an alkenyl radical anion

Question 34

Describe step 2 of reduction of an alkyne by sodium in liquid ammonia: add a proton

Answer 34

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

Question 35

Describe step 3 of reduction of an alkyne by sodium in liquid ammonia:

Answer 35

A one-electron reduction of the alkenyl radical gives an alkenyl anion. The trans-alkenyl anion is more stable than its cis isomer

Question 36

Desribe step 4 of reduction of an alkyne by sodium in liquid ammonia: add a proton

Answer 36

A second proton-transfer reaction completes the reduction and gives the trans-alkene