Posted by: Mark Foreman | February 2, 2010

LiAlH4 vs NaBH4

Several students wanted to know why lithium aluminium hydride is such a stronger reducing agent than sodium borohydride. Rather than having to keep saying the answer I thought I would show you here the reasons behind the answer.

We need to consider a series of four reducing agents, if you want to know how to make dot and cross diagrams of the anions then please click here otherwise keep on reading.

LiAlH4, here is a picture of the anion and the cation.

A picture of the anion and the cation in LiAlH4

Reduces esters, ketones, aldehydes, alkyl halides, epoxides and amides

LiBH4, here is a picture of the anion and cation.

The anion and the cation in LiBH4

Reduces esters, ketones and aldehydes

NaBH4, here is a picture of the anion and the cation.

The anion and the cation in sodium borohydride

Reduces ketones and aldehydes

NaB(CN)H3, here is a pciture of the anion and the cation.

The anion and the cation in sodium cyanoborohydride

Weaker version of NaBH4

So looking at this list we have a set of four reagents, in terms of strength the order is

LiAlH4 > LiBH4 > NaBH4 > Na(BH3CN)

We now need to consider why the order exists.

1. The cation.

The lithium is better able to act as a lewis acid than the sodium, the lewis acid bonds to the carbonyl oxygen. This in turn increases the amount of positive charge density on the carbonyl carbon. As the reduction is favoured by an increase in the positive character of the carbonyl carbon a change from sodium to lithium will make the reducing agent better. For more details please read here.

2. The electronegativity of the atom at the centre of the EH4 anion.

The more electronegative the atom is the less electron density will be on the hydrides, the less electron density of the hydrides the less able they are to act as nucleophiles to reduce the carbonyl.

Pauling electronegativity of Al is 1.61

Pauling electronegativity of B is 2.04

The two factors combined to make LiAlH4 a stronger reducing agent than NaBH4.

3. I would also like to point out that as we go down the groups in the p block the hydrides become less and less stable. This is because the hydrogen to whatever atom bond becomes weaker. For example phosphine (PH3) is more stable than arsine (AsH3) which in turn is more stable than SbH3. This general reduction in the strength of the bond to the hydrogen may also help to increase the reactivity of LiAlH4 when it is compared with LiBH4.

The weakening of the bonding to hydrogen is not an isolated example, the carbon whatever bonds also become weaker as you go down a group in the p block. For example consider the alkyls

CEt4, SiEt4, GeEt4, SnEt4 and PbEt4.

While CEt4 (3,3-diethylpentane) is a very stable saturated hydrocarbon (alkane) the lead compound (tetraethyl lead) is unstable when it is heated. It breaks down with ease to form lead atoms and ethyl radicals. It used to be used in motor fuel (petrol) as an additive increase the octane rating but for public health reasons it has been withdrawn from use in petrol. The other ethyl compounds will have stabilities which will be partway between the alkane and the tetraalkyl lead. I reason that as the central atom becomes more heavy the bonds between the whatever and the ethyl will become weaker.

I have noticed that this blog entry seems to have taken on a life of its own, what started with a student asking “why” has resulted in my most popular work on my organic chemistry teaching blog. If you like my explanation of why one reducing agent is stronger than the other then please spread the good news by giving people the URL to this blog post.

Now before I go I would like to point out that lots of fun chemistry other than the reduction of aldehydes and ketones can be done using sodium borohydride. For example the reaction of nickel chloride with sodium borohydride is a good way to make nanoparticles of nickel which are embedded in a boron oxide matrix. This makes one of my favourite hydrogenation cataylsts.

If you like this post maybe you should check out the “joy of organic chemistry” where I have put up some more stuff.

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Responses

  1. hey, thanks so much for your explanation. it certainly is concise. (:

  2. interesting but i needed the dot and cross diagram of lithium aluminum hydride

    • Well when I get time I will go and do a dot and cross diagram of LiAlH4

  3. Thanks, Mark! Your Post makes chemistry easier. Never have seen such a nice and concise scheme explaining these reagents before.

    Any idea about how to reduce a conjugated carboxamide (possibly requires LiAlH4) in the presence of a several keto groups (conjugated in the same and in other electron systems) ? What makes (de)protection with glycol difficult is the fact that the molecule doesn’t like acids…

    • Maybe you should consider using DIBAL-H (Diisobutyl aluminium hydride) as this reacts more quickly with the electron rich functional groups.

      Please name the exact compound you are being troubled by, and I may have a go at writing a blog entry on it. Please note that while I am willing to help students out, I hope that you are not trying to get me to do your homework or problem sheets for you.

  4. sichere wette
    Hey, can I use your post on my blog with a linkback?

  5. AWESOME! it’s soooo easy to understand now!

  6. Sir i am finding organic chemistry a bit difficult could you help me about how i should study ; my study schedule for organic chemistry is learning all the reaction and their mechanisms but still i do not know how to attempt a question
    any question given to me does not look like i have not been taught the theory for it but i am not able to attempt.
    Sir , i am an iit aspirant so if you could help me as soon as possible the better it would be for me
    Thank You sir.

  7. Thank you very much. Cleared much of my concepts now :D

  8. I am fully satisfied by this explanation

  9. Thanks sooo much! This really helped put a reason behind something I have been using a lot!

  10. gud explanation its realy helped

  11. […] seems that my readers love sodium borohydride and they want to know why¬†it is a more selective reducing agent than lithium aluminium hydride. This page is devoted to the […]

  12. Good explanation. May I know which software do you use to produce those nice diagrams?

    • I used chemdraw for the diagrams, sometimes I copy the pictures into paint and then add extra bits

  13. Hi! I chanced upon your post whilst looking up reducing powers of these reagents. I’d like to ask about point 1. I looked at the mechanism for reduction of carbonyl compounds and noticed that for NaBH4, the sodium cation will coordinate with the oxygen of the carbonyl group when a H atom has been added. However for LiAlH4, it is the aluminium that coordinates to said oxygen, not the lithium. Though, I can understand if both lithium and aluminium can coordinate to the oxygen.. So, does point 1 in your post still stand? Is it right to say that aluminium can coordinate more strongly to the oxygen, than lithium? Since aluminium is more positive and more electronegative than lithium..

    • Point 1 still stands, the first step is for the lithium to bind to the carbonyl oxygen. Next the AlH4 anion attacks the carbonyl carbon, and finally the carbonyl oxygen will bind to the aluminium. When I get the chance I will draw the mechanism.

      I respectfully suggest you look at http://www.cardiff.ac.uk/chemy/staffinfo/poc/Problem%20Sets/PS280508.pdf and do these problems from Wales. Maybe you will change your point of view after doing these questions.

  14. […] recently one of my readers did not believe me regarding¬†the mechanism of the lithium aluminium hydride reduction of a carbonyl, but I have dug up strong evidence which I hope will be of interest to you […]

  15. Gallium is not less electronegative than Aluminium according to both the Pauling and Allen scale. I think it was the rule of thumb that going down a group leads to a lower electronegativity that may have been the fault here.

    Lange’s handbook of chemistry 15th edition gives values of 1.61 (Al) and 1.81 (Ga) for Pauling.

    Allen gives 1.613 (Al), 1.753 (Ga)
    J. Am. Chem. Soc.1989, 111, 9003-9014

    A very informative explanation otherwise. Thank you.

    • Good point well spotted, I just assumed that the increased size of the atoms would decrease the electronegativity of the atoms. I will ammend the blog post soon.


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