Reduction Flashcards
What is reduction?
Gain of hydrogen
What is H-?
Always a base
Is H- hard or soft? Why?
Hard
High charge density
What are the two types of hydride?
From X-H bond
From NaH splitting
What are the common hydride reducing agents?
NaBH4
LiAlH4
Which hydride reducing agent is less reactive?
NaBH4
What is classed as dry?
50ppm water
How must LiAlH4 be used?
Dry
Why are esters, amides and carboxylic acids not as well attacked by nucleophiles?
They have their negative charge delocalised across the molecules
What determines the reactivity of a carbonyl molecule?
The size of delta plus
In NaBH4, what is the nucleophile?
The B-H bond
What is a hydride?
Hydrogen plus a pair of electrons
Which reduction is chemoselective?
NaBH4
Reduces some molecules, aldehydes/ketones, but not others, esters/amides
Why cant you kick out bromine from an alkane with a hydride but you can break a carbonyl?
There is only slight polarisation of C-Br
There is a larger positive charge on carbon on carbonyl due to the electronegative oxygen
How many hydrides are used in the reduction of esters?
2
How many hydrides are used to reduce an aldehyde/ketone?
1 hydride
How many hydrides are used to reduce amides?
2 hydrides
How many hydrides are used to reduce a carboxylic acid?
3
Which reducing agent is good to use and why?
NaBH(tBuO)3
Very unlikely to have any hydrides left
Why is it important to have no hydrides left?
They will react with water
How does LiAlH4 react with water?
Reacts violently
Fires
What is LiAlH4 usually used in?
Anhydrous ethers
Et2O, THF
What are the steps to reduce an ester to an alcohol?
Ester to aldehyde
Aldehyde to alcohol
In the reduction of amides, what leaving group is kicked out?
O-AlH3
What gas is released in the reduction of acids?
H2
What are the steps of the LiAlH4 reduction of acids?
Formation of lithium carboxylate; ROOLi
Reduction of C=O group
Loss of aluminium salt - formation of aldehyde
Reduction of aldehyde
Acid hydrolysis of Al-alkoxide gives alcohol
