 In this video, we will be talking about SN1 reactions or nucleophilic substitution reaction unimolecular. We will talk about the mechanism. We will also talk about what do we mean when we say a step is a rate determining step and also talk a little about the factors that affect the rate of this reaction. So let's go. Whenever we talk about nucleophilic substitution reactions what we essentially mean is a nucleophile y- would come and replace some x- right? For that to happen, x- needs to be more stable than y- right? Else, y- won't be able to replace it. X- would attack back and get its place back. And how can we determine this? Using the electronic effects right? So whatever knowledge we have gathered from the electronic effects unit, we can use that here and apply that to understand whether x- is more stable than y- or not. Coming back to the substitution, there are two ways it can happen. Either rx bond breaks first, x- leaves, there's a carbocation formed and y- attacks later. Or y- attacks and x- leaves simultaneously. It all happens in one step. Today, we'll be focusing on this one. Let's look at the mechanism of an SN1 reaction now. The bond between the carbon and the leaving group breaks. Leaving group takes away the set of electrons of the bond and we get a carbocation and x- as a result. This carbocation is then attacked by the attacking nucleophile and we get the required product. So the reaction happens in two steps. In step one, the carbocation forms. In the step two, the carbocation is attacked. So do these two steps occur at the same pace? Or is one of them quicker and one of them slower? In the laboratory chemists tried various ways to understand which of the two steps was quicker and the result was the second step was faster than the first one. The breaking of the cx bond was slower and the slowest step is called the rate determining step. What do I mean when I say rate determining step? Let's take an example to understand this. Let's say I'm a water bottle manufacturer and I make water bottles in a way that there are about a thousand caps produced per hour while just 50 bottles produced per hour. If I want to increase the rate of production, what should I focus on? I should be increasing the number of bottles that have been made. There are a lot of caps made anyway per hour. What should I be focusing on? I should be focusing on increasing the number of bottles made per hour so that my net production per hour increases. So what did I actually focus on? I focused on the slowest step. So whenever I want to increase the rate of a reaction, I will have to focus on the slowest step. I will have to think about ways to increase the rate of that step so that the rate of the overall reaction increases. Let's recall a little about the rate of the reaction from chemical kinetics. The rate of a reaction is equal to the rate constant multiplied by the product of concentration of the reactants raised to the power something that may or may not be equal to the stoichiometric coefficient. Rate is equal to k, rate constant, multiplied by the concentration of one reactant raised to some power multiplied by the concentration of the second reactant raised to some power. If the reaction occurs in a single step or is an elementary reaction, these powers become equal to the stoichiometric coefficients. And the order of the reaction is the sum of the powers to which these concentrations of reactants are raised. Let's now apply this concept to our mechanism of SN1 reaction. Since the slowest step is the rate determining step, the rate of the reaction would be directly proportional to the concentration of the substrate only since the nucleophile has not attacked in the first step, right? And it's raised to the power 1 which is its stoichiometric coefficient. What would be the order of this reaction? The order of this reaction is 1. Now, since this equation of rate has only and only the substrate involved, so the rate depends on the concentration of the substrate while it is independent of the strength or the concentration of attacking nucleophile. So, what matters to me? What should I be focusing on while talking about the rate of an SN1 reaction? Clearly, the nucleophile hasn't attacked in the rate determining step. So, it doesn't matter. The substrate matters to me. It's concentration and also the strength of this Cx bond because for the rate to be higher, I want this bond to break quickly, right? And therefore, the carbocation stability and the x-minus stability also matter to me because if they are stable individually, the bond will break quickly and the rate will be higher.