Hydrogenation Flashcards
How do you control catalytic hydrogenation?
Using different metals catalysts
What do additives do?
Adds more selectivity
What reaction conditions do you need to look at in catalytic hydrogenation?
Temperature and pressure
What type of process is catalytic hydrogenation?
Chemoselective
What are the pros of using H2?
There is no waste as you use both H+
Atom efficient
Catalytic at 0.00000001 mol%
What are the cons of LiAlH4/NaBH4?
B/Al waste
H+/OH pollution released
Only use 1/6 atoms
What is the process of hydrogenation of alkenes?
H2 absorbed onto a metal surface
Alkene coordinates metal
There is a stepwise addition of H2 to the alkene
What type of addition is hydrogenation of alkenes? What does it depend on?
Usually syn-addition
Catalyst, conditions and substrate
What is syn addition?
Both hydrogens are on the same face of the molecule
Why does syn addition take place?
Takes place on a surface so it cant rotate for second addition
What happens to the melting point when alkenes are removed?
The melting point decreases
What type of process is the hydrogenation of alkynes?
two step process
Alkyne to alkene
Alkene to alkane
In the hydrogenation of alkynes, can you stop at the intermediate Z-alkene?
No
What are the three components of a lindlar catalyst?
Pd
CaCO3
Pb(OAc)2
How is a lindlar catalyst structured?
Palladium on calcium carbonate which is then poisoned with Pb(OAc)2
What is the function of CaCO3 in a lindlar catalyst?
Replaces carbon as the support to release the product alkene more easily
What is the purpose of Pb(OAc)2 in the lindlar catalyst?
Poison preventing the over reduction
What does the lindlar catalyst do?
Allows the reaction to stop at the intermediate Z alkene
If an amine has a hydroxyl group attached too, which group reacts to form the imine? Why?
NH2
The lone pair is more available as it is less electronegative, it has a higher pKa
How is an imine formed?
RNH2 + C=O
Condensation reaction
What is formed when an imine goes through a hydrogenation reaction?
Amine
What type of reaction is hydrogenation and why?
Chemoselective
C=N is a weaker bond than C=O
When you remove phenyl groups, what does a benzylamine become?
Amine
When you remove phenyl groups, what does a benzylether become?
Alcohol
When you remove benzyl groups, what does the co-ordination of arena to Pd surface do?
Restrict the process to just benzyl groups
What is formed when you hydrogenate nitro groups?
Amines
What are the most common metals used for the hydrogenation of nitro groups?
Palladium and platinum
What might you need to add when you hydrogenate the nitro groups? Why?
Add a weak acid
It prevents the product amine poisoning the catalyst
What is Raney Nickel?
RaNi
Made up of Ni and H2
What is the structure of RaNi like?
Has a large surface area, finely divided form of nickel
Honeycomb structure
What is RaNi made from?
nickel-aluminium alloy
What is the function of the concentrated NaOH in the formation of RaNi?
It dissolves the aluminium
In de-sulfurisation, which bonds do you hydrogenate?
The two C-S bonds
Why do you hydrogenate the two C-S bonds?
They are weaker than the C-O bonds
What does RaNi not reduce? Why?
Alkenes
Chemoselective reduction
What is the easiest functional group to hydrogenate?
Acyl Chloride
Which functional groups are considered easy to hydrogenate and above an alkene?
Acyl chloride
Nitro
Alkyne
Aldehyde
Which functional groups are considered hard to hydrogenate and are below an alkene?
Ketone Benzyl Cyanide Ester Conjugated system
What is the hardest functional group to hydrogenate?
A conjugated system
