 In this video, we are going to briefly look at two very important reducing agents that help us synthesize alcohols from carbonyl compounds. And these are Lithium-Aluminium Hydride and Sodium-Borohydride. The reactive species in these two reagents is the Hydride-Ion or H-Ion. It is this H-Ion that reacts with the carbonyl carbon and gives us the alcohol as the final product. Now unlike the simpler hydrides like Lithium-Hydride or Sodium-Hydride, these reducing agents are called complex hydrides because it is a complex of a metal and hydrid ions. Here you have Lithium-Aluminium bonded to hydrogen atoms whereas in Sodium-Borohydride you have Sodium and Boron atoms bonded to hydrogen atoms. Unlike the ionic hydrides, the hydrogen atoms here are covalently bonded to the central atoms and because of the electronegativity difference between them, they carry a partial negative charge. And this arrangement helps the hydride ions to act as better nucleophiles while decreasing its basicity. Now some of you might wonder, aluminium has a formal negative charge. So how is it that the hydrogen atom in the form of hydride acts as a nucleophile and not aluminium? Well, it turns out that this is an example of a situation where a formal charge does not correctly give us an idea about the electron density. You see, aluminium is much more electro-positive than hydrogen. And because of this electronegativity difference, aluminium gets a partial positive charge and hydrogen atoms get a partial negative charge. And as a result of this, the electron density remains predominantly on the hydrogen atoms but aluminium overall has got a formal negative charge. You can calculate the formal charge for aluminium using the formal charge formula but we won't go into the details of how to calculate that in this video. Needless to say, the same logic can be extended to sodium borohydride as well. So when these reagents take part in reactions, it is not the electrons on the aluminium or the boron atom that attacks the carbonyl compound but the electrons in these, ALH bond and the BH bonds. So it is the electrons in this bond that actually carries out the nucleophilic attack. Now when you compare these two reagents, you can see that lithium aluminium hydride is much more reactive or a much stronger reducing agent than sodium borohydride. And this is because the electronegativity difference between aluminium and hydrogen is much greater than that between boron and hydrogen. So that means hydrogen atoms here carry more negative charge as compared to the hydrogen atoms in the BH4- complex. This makes it much more reactive and honestly much more challenging to work with when you use it in the laboratories. Lithium aluminium hydride is so strong that it reacts with almost everything in its vicinity. It reacts explosively with water and alcohols generating hydrogen gas. Whereas sodium borohydride being less reactive is much more selective in nature. And needless to say, a lot more easier to handle in the laboratories. This is why sodium borohydride reduces only aldehydes and ketones to primary alcohols and secondary alcohols respectively. On the other hand, lithium aluminium hydride being highly reactive and less selective will reduce aldehydes, ketones, carboxylic acids and even esters. Now it reduces aldehydes, acids and esters to primary alcohols whereas ketones to secondary alcohols. So this also shows how the two reagents differ in their reactivity or selectivity.