hello everybody my name is Iman welcome back to my YouTube channel today we're continuing our lecture on analyzing organic reactions we made it to objective two now almost all reactions in organic chemistry they can be divided into one of two groups oxidation reduction reactions or nucle nucleophile electrophile reactions nucleophiles electrophiles and leaveing groups they are particularly important to the reactions of alcohols and carbon containing compounds which we'll look at in depth in later chapters but what we want to do for this objective specifically is Define these terms nucleophile electrophile and leing group and specifically focus on nucleophilic substitution reactions so first we're going to start with nucleophiles nucleophiles are defined as nucleus nucleus loving species with either lone pairs or piie bonds that can form new bonds to electrophiles all right now you may have noted that nucleophilicity and basicity appear to have similar definitions and this is true there is however a distinction between the two nucleophile strength it's based on the relative rates of reaction with common electrophiles and is therefore a kinetic property base strength however is related to the equilibrium position of a reaction and it's therefore a thermodynamic property all right now keeping that in mind we can see a couple of examples of nucleophiles right here all right we see that some common examples of nucleophiles are the following all right annion things like BR bromine bromide all right or hydroxide or cyanide all right they're anion so they have this negative charge associated with them all right molecules or compounds with pi bonds also sometimes make good or strong nucleophile so here we see carbon carbon double bond triple bond all right and we see here a Benzene ring alternating carbon carbon double bonds all right another example of of of nucleophiles atoms with lone pairs all right here we have water ammonia all right as examples of some nucleo files now nucleophilicity is determined by four major factors all right four major factors these are one charge all right nucleophilicity increases with increasing electron density all right with increasing electron density more negative charge all right a second major factor is electro negativity nucleophilicity decreases as electr negativity increases because these atoms are less likely to share electron density a third factor is steric hindrance so think bulkier molecules are going to be less nucleophilic and then last but not least solvents all right protu solvents can hinder nucleophilicity by protonating the nucleophile or through hydrogen bonding all right so these are four major factors that determine nuclear filicity now something to ask all right something that you might be thinking about already is this fourth major factor that we just mentioned solvents all right what are the solvent effects right what what is a solvent effect in regards to nucleophilicity well the solvent consideration is actually worth spending a bit more time on all right in polar prodct solvents nucleophilicity increases down the periodic table in polar AIC solvents nucleophilic nucleophilicity increases up the periodic table all right and we can see examples of both produ solvents all right and AIC solvents but really I want to talk about big picture here nucle Ile nucleophiles and leaving groups are ionic and so polar solvents are required and then there's two types of polar solvents pric and aoic in polar protic solvents we said nucleophilicity increases down the periodic table in polar aoic solvents nucleophilicity increases up the periodic table the halogens are a really good example of this effect all right it's there are really good examples of the effects of the solvent on nucleophilicity in a pric solvent all right in pric solvents all right here is the rule nucleophilicity decreases in the order all right we have Florine chlorine bromine and iodide all right because the rule was that in proteic solvents nucleophilicity increases down the periodic table all right which is why here all right iodine is the strongest nucleophile in aoic solvent and this is because the protons in solution will be attracted to the nucleophile all right fluoride is the conjugate base of HF a weak acid as such it will form bonds with the protons in solution and be less able to access the electrophile to react with iodide on the other hand is the conjugate base of hi which is a strong acid as such it is less effective by the protons in solution and can react with the electrophile now we haven't talked about sn2 reactions just yet all right but something to keep in mind because we will talk about sn2 reactions is that they are better off and faster in polar AIC solvents because they won't solvate both the positive and negative charges only the positive charges and this leaves the negative charge free to react so in other words the nucleophile is left unstable and wanting to react now the trend for nucleophilicity and a produ solvents is different than it was for product solvents all right we said for a produ solvents nucleophilicity decreases in the this order all right decreases in this order all right the rule was that in a prodct solvent nucleophilicity increases up the periodic table that's why an AIC solvent fluide all right is a stronger nucleophile than iodide so you notice the opposite Trend between which solvent you use and the trend for which is a stronger nucleophile all right fantastic now something that we won't be concerned with or we won't use our non-polar solvents with this kind of reaction because we need our nucleophile to dissolve all right so that is nucleophiles all right the next thing we want to talk about now is electrophiles all right electrophiles are defined as electron loving species with a positive charge or a positively polarized atom that accepts an electron pair when forming new bonds with a nucleophile all right now this definition might bring to mind Lewis acids the distinction as with nucleophiles and bases above is that electrophilicity is a kinetic property whereas acidity is a thermodynamic property practically however electrophiles will almost always act as leis acids in reactions a greater degree of positive charge increases electrophilicity so a carbo cation is more electrophilic than a carbonal carbon for example all right here you can see some comparison between electrophilicity all right so here all right you have this positive charge associated with a carbon whereas here you have just a partial positive charge associated with a carbon all right this formal charge on the carbon makes it a better electrophile all right than just this formal charge all right all right fantastic now something else to keep in mind here all right is that the nature of the leaving group influences electrophilicity in species without empty orbitals so better leaving groups make it more likely that a reaction will happen if empty orbitals are present an incoming nucleophile can make a bond with the electrophile without displacing the leaving group all right now electrophilicity and acidity they're effectively identical properties when it comes to reactivity just as alcohols alahh and ketones carboxilic acids and their derivatives act as acids all right they also act as electrophiles and can make good targets for nucleophiles all right fantastic so just to reiterate all right a greater degree of positive charge increases electrophilicity so a carbocation like we see here is more electrophilic than a carbonal carbon all right Additionally the nature of the leaving group all right influences the El influences electrophilicity in species without empty orbitals so better leaving groups make it more likely that a reaction will happen if empty orbitals are present and un incoming nucleophile can make a bond with the electrophile without displacing the leaving group all right which is why this is a better electrophile than this now we've defined nucleophiles we've defined electrophiles the thing that's next to discuss is leaving groups all right leaving groups are the molecular fragments that retain the electrons after heterolysis all right heterolytic reactions are essentially the opposite of coordinate coent Bond formation a bond is broken and both electrons are given to one of the two products the best leaving groups will be able to stabilize the extra electrons weak bases are more stable with an extra set of electrons and therefore they make good leaving groups by this Logic the conjugate bases of strong acids like iodide bromide chloride they tend to make good leing groups leing groups ability can be augmented by things like resonance or by inductive effects from electron withdrawing groups because these help delize and stabilize negative charge now things like alkaa and hydrogen ions will almost never serve as leaving groups because they form very reactive strongly basic anion we can think of leaving groups and nucleophiles as serving opposite functions all right and in substitution reactions which we'll talk about here shortly the weaker the base all right the leaveing group all right the weaker base is replaced by the stronger base all right the weaker base the leing group is replaced by the strong stronger base than nucleophile all right so keeping that in mind with these definitions we can start talking now about nucleophilic substitution reactions nucleophilic substitution reactions are perfect examples for demonstrating nucleophile electrophile reactions in both sn1 and sn2 reactions a nucleophile forms a bond with a substrate carbon and leaving group leaves we're actually going to start with sn2 reactions all right we're going to start with sn2 reactions and what we're going to how we're going to learn sn2 uh reactions is through a love sto story all right we're going to learn sn2 reactions through a love story it starts all right a strong nucleophile meets a nice electrophilic carbon attached to a good leaving group this could for example be a hogen let's say a hogen all right this is the start of a love story in this fairy tale the strong and handsome nucleophile attacks or as one would say hits on the electrophilic carbon attached to a good leaving group let's say a hallogen now we can see that here nucleophile all right hits on the electrophilic carbon that's an electrophilic carbon here and X denotes a hogen that's going to be our leaving group all right so our nucleophile attacks the electrophilic carbon all right that's attached to our leaving group which we denote as X after the electrophilic carbon gets hit on by the nucleophile she's going to dump her H hallogen boyfriend whatever leing group that may be all right and it gets with the nucleophile instead now the thing to know is that this happens all in one step all right this kind of reaction happens all in one step and we know this because the energy diagram for this kind of reaction which we demonstrate here all right demonstrates one transition state all right and there are no intermediates to suggest more steps all right and so that is the sn2 love story you have a nucleophile that hits on right or Attacks An electrophilic carbon that has that is attached to a leing group all right that electrophilic carbon dumps that leaving group all right and it forms a bond now with the nucleophile and this all happens in one step now the follow-up question that you might ask is well why is this reaction called sn2 well the S stands for substituted n stands for nucleophile and there is a two because this reaction is bimolecular meaning this steps this step involves two chemical entities for biomolecular reactions that means the rate equation is written as rate is equal to the rate constant multiplied by the concentration of two species the leaving group all right the the the the I should say not just the leaving group but the molecule the electrophilic carbon molecule all right the alkal haly for example if your leaving group is a hallogen all right multiplied by the concentration of your nucleophile all right the reaction rate is dependent on the concentration of both chemical entities okay let's restate and finalize some points on sn2 now all right sn2 bimolecular nucleophilic substitution reaction all right it contains only one step in which the nucleophile attacks The Compound at the same time as the leaving group leaves because this reaction has only one step we call it a concerted reaction all right and we call this reaction bimolecular because the single rate limiting step involves two molecules all right fantastic now one thing to talk about for sn2 all right is what if our molecule what if this molecule that we're dealing with instead of looking like that it was was let's see we're going to draw it out all right let's say that it was um chyal all right let's say that our molecule had a chyal center now since in our story we just said that this was an alkal halide because halogens make great leing groups let's ask what if our alkal halide has a chyal center all right so if our carbon had four different groups attached to it so let's say chlorine ch3 had this ethyl group and a hydrogen all right and then we had a nucleophile all right so that makes this carbon a chyal carbon all right if our nucleophile comes in and attacks this chyro carbon what happens how do we deal with that all right how do we deal if our alkal haly has a chyal center all right what happens if that attack site is chyal now in sn2 reactions the nucleophile it actively displaces this is the leaving group in what's called a backside attack for this to occur the nucleophile must be strong and the substrate cannot be sterically hindered all right therefore the less substituted the carbon the more reactive it is to sn2 reactions and the way that this reaction is set to proceed is with inversion of configuration all right so the strong and handsome nucleophile is going to attack from the backside and you get a change in the chyal Center that's in to the opposite configuration so if our hallogen here all right was on a dash I'm just going to hypothetically speak here all right I'm just going to hypothetically speak here and say that um this chyro carbon has an R configuration all right just randomly speaking here if it gets attacked by a nucleophile and the nucleophile replaces the leaving group because the Chlor because the chloride ion leaves all right this would have an inversion of configuration all right and so now now your molecule now your chyro centers no longer R after you have an sn2 reaction happen it will now be S you get that inversion of configuration all right if it's R it becomes s if it's s it becomes R after the backside attack occurs for an sn2 reaction all right and the backside attack happens because it's easier for the nucleophile to approach in that way all right this has to do with molecular orbitals that are associated with the molecule as a whole it allows for efficient overlap between the the the homo of the nucleophile and the lumo of the electrophile when a backside attack occurs we don't have to worry about that language or that explanation here but I I thought I would provide it for those of us who have taken um organic chemistry and are familiar maybe a little bit even with molec orbitals all right that is sn2 we're going to end the video here in the next video we're going to talk about sn1 reactions and continue with our chapter let me know if you have any questions comments concerns down below other than that good luck happy studying and have a beautiful beautiful day future doctors