week 4: alkynes

Question 1

explain why alkynes are good nucleophiles

Answer 1

because alkynes have two pi-bonds, they have high electron density (electron rich)

Question 2

which is more reactive, alkenes or alkynes?

Answer 2

alkynes are more reactive

Question 3

how are alkynes prepared?

Answer 3

• alkynes are formed through vicinal dihalides (ex: Br2)
• to preprare alkynes, we treat the vicinal dihalide with a strong base (such as KOH or NaNH2)

Question 4

what are the different types of alkyne reactions?

Answer 4

• addition of HX or X2 (Markovnikov addition)
• hydration of alkynes
• hydrogenation of alkynes
• oxidative cleavage of alkynes (ozonolysis, KMnO4)
• alkylation of terminal alkynes (formation of acetylide anions)

Question 5

describe Markovnikov addition of alkynes

Answer 5

• can ONLY be done to terminal alkynes (due to presence of hydrogens)
• add H to alkyne carbon attached to most hydrogens
• add X (ex: Cl) to other carbon (most substituted)

Question 6

describe the addition of Br2 to an INTERNAL alkyne

Answer 6

• no Markovnikov addition
• once bromine approaches nucleophilic alkyne, it is polarized
• add each Br to each carbon (forming a trans conformation)

Question 7

what is the difference between the hydration of alkenes and alkynes?

Answer 7

• the hydration of alkenes produces alcohols
• the hydration of alkynes forms aldehydes and ketones

Question 8

what are the two steps of alkyne hydration reactions?

Answer 8

(1) addition of water (H-OH) forms enol (alkene + alcohol)
(2) tautomerization forms a carbonyl compound (ketone or aldehyde)

Question 9

why does an enol have to undergo tautomerization?

Answer 9

• because it is an unstable molecule
• they are in an equilibrium with a more favorable carbonyl group

Question 10

out of the two methods of alkyne hydration, which follows Markovnikov addition, and which follows anti-Markovnikov?

Answer 10

• oxymercuration: Markovnikov
• hydroboration/oxidation:
  anti-Markovnikov

Question 11

describe oxymercuration of alkynes

Answer 11

• one step process (unlike alkenes)
• reagents: HgSO4, H2O, H2SO4
• terminal alkyne, methyl ketone forms (C=O in position 2)
• internal alkyne, 2 ketones are formed (if 2 R groups are different)

Question 12

describe hydroboration of alkynes

Answer 12

• two step process
• reagents same as alkenes: BH3/THF & H2O2
• terminal alkyne, an aldehyde will form
• internal alkyne, same as oxymercuration, 2 separate ketones will form

Question 13

describe hydrogenation of alkynes

Answer 13

• addition of H2 (reduction reaction) in the presence of a metal catalyst
• catalysts include palladium or platinum powders

Question 14

what is the result of a hydrogenation reaction of alkynes? explain

Answer 14

• production of alkanes
• due to the strong reactivity of alkynes and their high electron density

Question 15

what is partial alkyne hydrogenation?

Answer 15

wanting to reduce an alkyne into an alkene (not proceed to alkane formation)

Question 16

what are the two ways to undergo alkyne partial hydrogenation?

Answer 16

• production of cis alkene
• production of trans alkene

Question 17

what is the condition for alkyne partial hydrogenation? why?

Answer 17

• alkyne has to be INTERNAL
• if alkyne was terminal, it would already be attached to a H, and you would add another, so the end carbon would have 2 Hs, and if the groups are the same you cannot create cis- and trans- alkenes

Question 18

describe the processes to produce cis and trans alkenes from an alkyne

Answer 18

cis alkene:
- use H2 gas and the Lindlar catalyst
- the alkene will not be able to become reduced to an alkane

trans alkene:
- use alkali metals dissolved in liquid ammonia as reagents: such as Li/NH3 and Na/NH3

Question 19

what are the two different processes involved in oxidative cleavage of alkynes?

Answer 19

• ozonolysis:

Question 20

compare products formed by oxidative cleavage reactions in alkenes and alkynes

Answer 20

alkenes:
- O3 - produces aldehydes/ketones
- KMnO4 - produces ketones/carboxylic acids

alkynes:
- BOTH procedures produce carboxylic acids

Question 21

describe oxidative cleavage for alkynes

Answer 21

• the triple bond is cleaved (split)
• the alkyne carbons are fully oxidized to carboxylic acid groups

Question 22

what is the main difference between terminal alkynes and internal alkynes during oxidative cleavage?

Answer 22

terminal alkynes form CO2
- after cleavage, the 2nd carbon is oxidized to CO2 gas

Question 23

what is the formation of CO2 gas in oxidative cleavage seen as?

Answer 23

a diagnostic test for the position of the triple bond

Question 24

compare the products formed from oxidative cleavage of terminal alkenes and terminal alkynes

Answer 24

terminal ALKENE:
KMnO4 –> form CO2
ozonolysis –> form aldehyde

terminal ALKYNE:
both form CO2

Question 30

list the products for each:
(1) terminal alkyne, oxymercuration
(2) internal alkyne, oxymercuration
(3) terminal alkyne, hydroboration
(4) internal alkyne, hydroboration
(5) internal alkyne, hydrogenation, Lindlar catalyst
(6) internal alkyne, hydrogenation, Na/NH3 or Li/NH3
(7)