Reactions From Alkynes Flashcards
Describe the hydrogenation reaction to get an alkene
This is the reaction of an alkyne with H2 gas and a Lindlar catalyst to form an alkene
What is a lindlar catalyst and why is the Pd poisoned?
Lindlar is CaCO3 and Pdo
It is poisoned to reduce the catalytic activity of Pd to prevent further hydrogenation to an alkane
Is the hydrogenation of an alkyne syn or anti addition? And what is the stereochemistry of the product
It is syn addition as H2 adds to the same face and so the stereochemistry is Z
This is because H2 and C triple bond come together on surface of catalyst to hydrogens add to same side
Apart from hydrogenation, how else can an alkene be formed from an alkyne
Sodium and liquid ammonia reduction Sodium metal dissolves in liquid ammonia to make electrons This this is a radical reaction NH3 adds H to compound Electrons make . Into negative charges
What is the stereochemistry of the alkene from sodium and liquid ammonia reduction?
Is it E stereochemistry due to steric strain
What is Markovnikov’s rule?
This states where atoms in HX add to unsymmetrical alkene
X prefers more substitutes carbon
H goes to the less substituted carbon
- creates more stable intermediate
(Substituted means more groups have replaced hydrogen)
- hydrogen likes its pals
What does the term geminal mean?
This indicates that both the halogen atoms are on the same carbon
Describe the addition of Br2 on an alkyne
The bromine adds anti addition
What are the two ways H-X can add to an alkyne
Through radical mechanism and ionic mechanism
What are the differences in the products form by ionic addition of HX compared to radical addition of HX?
In ionic addition, the X is added to the most substituted carbon atom
In radical addition, the H is added to the most substituted carbon
These form the most stable intermediates
How can you form methyl ketones from terminal alkynes?
React with H2O, H2SO4, heat and HgSO4
The addition of the OH is on the most substituted carbon (as the mechanism is ionic)
Tautomerise the alkene to get a ketone
Describe the stereochemistry of coupling reactions?
The stereochemistry is retained as you are essentially adding the other reactant to the end of the starting reactant
What is Sonogashira coupling?
This is the coupling of terminal acetylene with an aryl or unyl halide
Pd catalyst with a base and Cu-I co catalyst
The two reactants basically just add together
Stereochemistry is retained
What is meant by vinyl halide and aryl halide
A vinyl halide is an alkene with a halide attached
An aryl halide is an aromatic ring with a halide attached
