week 4: alkynes Flashcards
explain why alkynes are good nucleophiles
because alkynes have two pi-bonds, they have high electron density (electron rich)
which is more reactive, alkenes or alkynes?
alkynes are more reactive
how are alkynes prepared?
- 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)
what are the different types of alkyne reactions?
- 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)
describe Markovnikov addition of alkynes
- 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)
describe the addition of Br2 to an INTERNAL alkyne
- no Markovnikov addition
- once bromine approaches nucleophilic alkyne, it is polarized
- add each Br to each carbon (forming a trans conformation)
what is the difference between the hydration of alkenes and alkynes?
- the hydration of alkenes produces alcohols
- the hydration of alkynes forms aldehydes and ketones
what are the two steps of alkyne hydration reactions?
(1) addition of water (H-OH) forms enol (alkene + alcohol)
(2) tautomerization forms a carbonyl compound (ketone or aldehyde)
why does an enol have to undergo tautomerization?
- because it is an unstable molecule
- they are in an equilibrium with a more favorable carbonyl group
out of the two methods of alkyne hydration, which follows Markovnikov addition, and which follows anti-Markovnikov?
- oxymercuration: Markovnikov
- hydroboration/oxidation:
anti-Markovnikov
describe oxymercuration of alkynes
- 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)
describe hydroboration of alkynes
- 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
describe hydrogenation of alkynes
- addition of H2 (reduction reaction) in the presence of a metal catalyst
- catalysts include palladium or platinum powders
what is the result of a hydrogenation reaction of alkynes? explain
- production of alkanes
- due to the strong reactivity of alkynes and their high electron density
what is partial alkyne hydrogenation?
wanting to reduce an alkyne into an alkene (not proceed to alkane formation)
what are the two ways to undergo alkyne partial hydrogenation?
- production of cis alkene
- production of trans alkene
what is the condition for alkyne partial hydrogenation? why?
- 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
describe the processes to produce cis and trans alkenes from an alkyne
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
what are the two different processes involved in oxidative cleavage of alkynes?
- ozonolysis:
compare products formed by oxidative cleavage reactions in alkenes and alkynes
alkenes:
- O3 - produces aldehydes/ketones
- KMnO4 - produces ketones/carboxylic acids
alkynes:
- BOTH procedures produce carboxylic acids
describe oxidative cleavage for alkynes
- the triple bond is cleaved (split)
- the alkyne carbons are fully oxidized to carboxylic acid groups
what is the main difference between terminal alkynes and internal alkynes during oxidative cleavage?
terminal alkynes form CO2
- after cleavage, the 2nd carbon is oxidized to CO2 gas
what is the formation of CO2 gas in oxidative cleavage seen as?
a diagnostic test for the position of the triple bond
compare the products formed from oxidative cleavage of terminal alkenes and terminal alkynes
terminal ALKENE:
KMnO4 –> form CO2
ozonolysis –> form aldehyde
terminal ALKYNE:
both form CO2
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)
