 So, today we will be talking about the nucleophilic addition reactions of aldehydes and ketones. Before we begin, let's just recall what nucleophiles really were. Nucleophiles are actually negatively charged or neutral electron rich species that are craving for a nucleus, right? Or we can also say that these are electron rich species that are craving for a positive or a partially positive center or an electrophile. So there's a nucleophile that is going to add onto an aldehyde or ketone. But where do aldehydes and ketones have an electrophilic or a partial positive or positively charged center? Let's find out. Looking at this given carbonyl compound, it could be an aldehyde or a ketone. We can see how there's an electronegativity difference between the carbon and oxygen atom. And since oxygen is more electronegative, it will pull the electron density towards itself which tells us how the oxygen has a partial negative charge while carbon has a partial positive charge. So this is our electrophilic center where the nucleophile is going to attack. But how? Let's find out. Let's take an example of a negatively charged nucleophile attacking for aldehyde. So the nucleophile tends to attack the carbonyl center. But if it just does that, then the carbon will be forming about five bonds. Is it possible? Nah, carbon will not form five bonds. So what really happens is when the nucleophile attacks the carbonyl carbon, this electron density of the pi bond moves to the more electronegative oxygen atom and we get something that looks like this, which is an intermediate. But there's a problem here. I can see how the nucleophile attacked and I got the intermediate. But why is there a reversible sign here? It's because what if this oxygen says that no, no, no, I want to form a double bond with the carbonyl center. In that case, somebody has to leave. Hydrogens don't leave, right? That's what we have studied in the previous units. They don't take the negative charge on themselves and leave as that's not stable for them. So the nucleophile would be sent back to where it belonged. But I don't want that. I'm on my reaction to complete. So I would have to make sure that I make this intermediate stable by protonating it. So it becomes a neutral species. And that's what I do. Yeah, I finally got the product. So let's just quickly summarize what really happened during the entire process. The nucleophile attacked the carbonyl carbon. The pi electron density moved to the more electronegative oxygen atom. There was a protonation that happened and we finally got the final product. Let's dig a little deeper now. Let's start talking about the hybridization change during this reaction. As we have studied previously, carbonyl compounds are sp2 hybridized as the carbon center forms three sigma bonds. If I look carefully at the intermediate, it forms four sigma bonds. So it's sp3 hybridized and also the final product has four sigma bonds around the central atom. So it is also sp3 hybridized. So if somebody asks me what is the hybridization shift really, I would tell them that if you go from reactant to intermediate, there's a change in hybridization from sp2 to sp3. But when the intermediate finally gives the final product, the hybridization does not really change. And if we talk about reactant to product, there's a hybridization shift from sp2 to sp3. Let's dig a little further. One might ask that, okay, I get it. There's a negatively charged thing that has attacked and all this has happened. But what if there's a neutral thing that's attacking? Usually we see how a negatively charged species is a stronger nucleophile as compared to a neutral one. So a strong nucleophile can totally attack a carbonyl compound easily and do whatever we just asked it to do, right? But what about a weak one? Does it need assistance? It might, right? It might need help. If I work with a neutral nucleophile, how can I help it? I can help it by doing that in an acidic medium. This acidic medium helps to increase the electrophilicity of the carbonyl center. Let's see how. Out of carbon and oxygen, oxygen is more electronegative. It also has lone pair of electrons. So as soon as it sees a proton or an hydrogen ion in the solution, it goes on to grab it, right? And we get a protonated carbonyl compound. But how is it increasing the electrophilicity? Well, oxygen is an electronegative atom. Why would it like a positive charge on itself? So what really happens is, these pi electrons move to the oxygen atom and yay, there you go. There is my sweet electrophilic center, right there and the weak nucleophile can now easily attack it. Let's see how. So we saw how we got to this point, right? What happens now? The alcohol molecule can easily attack, whoops, there's a positive charge on the electronegative oxygen atom again. What now? Well, do you remember how the hydrogen ion was taken from the solution initially? It's time to return it. So in this video, we actually saw how nucleophiles attacked and attached themselves to the carbonyl compounds. If they were negatively charged nucleophiles, they simply attacked, they stayed, the intermediate got protonated and we got the product. While if they are neutral nucleophiles, we try and help them a little by using them in an acidic solution. This helps increase the electrophilicity or the positive character of the carbonyl so they can easily attack.