REDUCTION OF ALDEHYDES AND
KETONES
This page looks at the reduction of aldehydes and ketones by
two similar reducing agents - lithium tetrahydridoaluminate(III)
(also known as lithium aluminium hydride) and sodium
tetrahydridoborate(III) (sodium borohydride).
Background to the reactions
The reducing agents
Despite the fearsome names, the structures of the two reducing
agents are very simple. In each case, there are four hydrogens
("tetrahydido") around either aluminium or boron in a negative
ion (shown by the "ate" ending).
The "(III)" shows the oxidation state of the aluminium or boron,
and is often left out because these elements only ever show the
+3 oxidation state in their compounds. To make the names
shorter, that's what I shall do for the rest of this page.
Note: It isn't important as far as the current page is concerned,
but if you want to understand more about oxidation states
(oxidation numbers), you will find them explained if you follow this
link.
Use the BACK button on your browser to return to this page.
The formulae of the two compounds are LiAlH4 and NaBH4.
Their structures are:
In each of the negative ions, one of the bonds is a co-ordinate
covalent (dative covalent) bond using the lone pair on a hydride
ion (H-) to form a bond with an empty orbital on the aluminium or
boron.
Note: Follow this link if you aren't happy about co-ordinate
covalent (dative covalent) bonding.
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The overall reactions
The reduction of an aldehyde
You get exactly the same organic product whether you use
lithium tetrahydridoaluminate or sodium tetrahydridoborate.
For example, with ethanal you get ethanol:
Notice that this is a simplified equation - perfectly acceptable to
UK A level examiners. [H] means "hydrogen from a reducing
agent".
In general terms, reduction of an aldehyde leads to a primary
alcohol.
Note: If you aren't sure about types of alcohol it is essential to
follow this link before you go on. You only need to read the
beginning of that page.
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The reduction of a ketone
Again the product is the same whichever of the two reducing
agents you use.
For example, with propanone you get propan-2-ol:
Reduction of a ketone leads to a secondary alcohol.
Reaction details
Using lithium tetrahydridoaluminate (lithium aluminium
hydride)
Lithium tetrahydridoaluminate is much more reactive than
sodium tetrahydridoborate. It reacts violently with water and
alcohols, and so any reaction must exclude these common
solvents.
The reactions are usually carried out in solution in a carefully
dried ether such as ethoxyethane (diethyl ether). The reaction
happens at room temperature, and takes place in two separate
stages.
In the first stage, a salt is formed containing a complex
aluminium ion. The following equations show what happens if
you start with a general aldehyde or ketone. R and R' can be
any combination of hydrogen or alkyl groups.
The product is then treated with a dilute acid (such as dilute
sulphuric acid or dilute hydrochloric acid) to release the alcohol
from the complex ion.
The alcohol formed can be recovered from the mixture by
fractional distillation.
Note: You can see why UK A level examiners are happy with
the equations showing H in square brackets!
On the other hand, you may well be expected to know that the
reaction is done initially in solution in ethoxyethane followed by
treatment with acid.
The practical details above are simplified to what you are likely to
need in a theory exam. In practice, there is an additional step for
safety reasons. In particular, if you added dilute acid to the
reaction without first removing any excess of LiAlH4, there is an
explosion risk because of the violent reaction between the
excess LiAlH4 and water in the dilute acid.
Some un-dried ethoxyethane is added first before you add the
acid. (Remember that the initial reaction is carried out in carefully
dried ethoxyethane.) Traces of water in this react with the excess
LiAlH4. Because there are only traces present, the reaction is
controllable.
Using sodium tetrahydridoborate (sodium borohydride)
Sodium tetrahydridoborate is a more gentle (and therefore safer)
reagent than lithium tetrahydridoaluminate. It can be used in
solution in alcohols or even solution in water - provided the
solution is alkaline.
I have a problem over describing the reaction conditions,
because it seems to be used in so many different ways.
Practical details found from various university sites vary widely,
and don't necessarily agree with what theoretical sources say!
In what follows, I am choosing one out of many different
methods. I have chosen this one largely because I think I
understand what is going on!
Solid sodium tetrahydridoborate is added to a solution of the
aldehyde or ketone in an alcohol such as methanol, ethanol or
propan-2-ol. Depending on which recipe you read, it is either
heated under reflux or left for some time around room
temperature. This almost certainly varies depending on the
nature of the aldehyde or ketone.
At the end of this time, a complex similar to the previous one is
formed.
In the second stage of the reaction, water is added and the
mixture is boiled to release the alcohol from the complex.
Again, the alcohol formed can be recovered from the mixture by
fractional distillation.
Note: Experimental variants on this use sodium hydroxide
solution or dilute acids instead of water in the second stage.
Another variation treats the original aldehyde or ketone with the
sodium tetrahydridoborate dissolved in sodium hydroxide
solution, adding acid at the end to destroy excess sodium
tetrahydridoborate. Unfortunately, none of the experimental
sources I have looked at explain what is happening in the second
stage in any detail.
The complication doesn't actually stop here! The boron complex
formed at the end of the first stage is almost certainly more
complicated than this. At least a part of the organic bits attached
to the boron will probably come from whatever alcohol molecules
are used as the solvent for the reaction.
I have included this detail (even though UK A level students don't
need it) because I object to giving simplifications that you might
have to completely unlearn in the future if you do a Chemistry
degree.
You will find a simplified (for UK A level purposes) mechanism for
the reaction by following this link. This mechanism is simplified to
the point of being wrong, so if you are working outside of the UK
A level system, don't bother to look at it!
This will take you to another part of the site. Use the BACK
button on your browser to return to this page if you want to
continue to explore aldehyde and ketone reactions.