so today what I'd like to do is take just a couple of minutes to finish up some ideas from chapter one and we'll be talking about elular geometry and particularly on and then what I'd like to do is to move on to chapter two where we'll be discussing acids and bases and particularly their Rel reltionship to organic chemistry and just as in chapter one where we took Concepts that were familiar from General chemistry but gave them an organic flavor and began to think about in the case of chapter one structure and bonding in organic molecules we're going to do the same with acids and bases and begin to get to see the relationship to reactivity in organic chemistry and very end of chapter 2 we'll begin talking about actually even throughout we'll begin talking about this very Central concept of curved arrow notation that I talked about well let's take a moment to finish up our discussion of molecular geometry and we got a little bit bit of this when we were talking about orbitals and sp3 hybridized orbitals and so forth and I just want to go through very explicitly and let's start start with sp3 hybridized atoms so sp3 hybridized atoms have about 109° roughly a tetrahedral arrangement of lians and lone pears let's say about 109 degree Bond angles and a tetral arrangement ofs so we started by talking about methane before pardon we started by talking about methane before so that's where we'll begin right now here's methane we have an sp3 hybridized carbon and remember we're going to think about the arrangement of ligans the substituents on that carbon so we would describe the geometry of methane as tetraedro by that I mean you have foure substituents around carbon and if you look at all four of those substituents they form the corners of a tetrahedron in fact in methane all of the bond angles are equal they're all 109.5° is what the geometry of a tetron dictates if we move across the periodic table from carbon next to nitrogen then to oxygen we'll skip Florine that's just going to be linear we get next ammonia and I'm going to draw the L pair of electrons on top like so here's our hydrogen here's another hydrogen and here's another hydrogen if we just look at ammonia and forget about the lone Paar for a moment oops NH3 and we just consider the arrangement of atoms that you see in other words you don't really see the long pair in fact often when we draw ammonia you sorted write NH3 and forget about it we'd say that ammonia makes a pyramid structure where the nitrogen sits on top of a pyramid of three hydrogens as the base so we would describe it as paramal molecular geometry even though the arrangement of lians and lone pairs collectively is tetrahedral the lone pairs of electrons take up a little bit more space than the pairs of electrons that are in a bond that makes sense the electrons in the lone pair are sort of spread out in a bond they're more focused between the two atoms as a result that crunches down ever so slightly on the hydrogen nitrogen hydrogen bond angles and so the hydrogen nitrogen hydrogen bond angles are just a little bit under 109 ° they're [Music] 107.3 de the exact numbers aren't too important in in organic chemistry they'll sometimes make a little bit of a difference but you can think of it as 109 if you if you don't want to remember separate numbers as we continue to move across the periodic table then we get to oxygen and hence to water and I'll draw the two hydrogens on water in the plane of the page and try to represent this molecule in three dimensionality by trying to show one long pair coming out off the oxygen and the other lone pair going back and so if we just look at the atoms if we just consider the hydrogen oxygen and hydrogen we say this molecule is bent the trend of the lone pairs of electrons taking up a little bit more space continues so the hydrogen oxygen hydrogen bond angle is crunched down just a little bit more 104.5 de all of this falls into something that you've learned in general chemistry probably who's heard the name VPR Theory or ve and shell electron pair repulsion Theory so this is just a review of BPR Theory as applied to types of structures in organic chemistry and of course what you'll see is as we move on from molecules like water which are in the realm of inorganic chemistry to molecules like methanol for dimethyl ether which are in the realm of organic chemistry the geometri stay the same in other words if instead of having a hydrogen here we had a methyl group making methanol ch30 we would have a Ben geometry around oxygen and if instead of having a hydrogen here we had another metal group making dimethyl ether ch3 o ch3 we would also have a big geometry all right we've seen some SP2 hybridized atoms before SP2 hybridized atoms have about 120° Bond angles and a trigonal planer arrangement of lians and lone paars we looked at ethene last time and we were concerned with an orbital base model of the structure of ethylene we remember that we said the molecule ethylene is [Applause] [Applause] planer like so each of the carbons is SP2 hybridized and the carbon hydrogen carbon Bond angle is about 120° as is the hydrogen carbon hydrogen angle let's take a look at another organic species let's take a look at the methyl carbocation ch3 Plus [Applause] [Applause] what's the first thing you notice about the the veilance state of the methyl carbocation doesn't have a complete OPP te that carbon is very unhappy in fact you will never see a naked methyl carbocation in reactions in this class you will in some cases in the gas phas you may for example under special conditions in Mass spectrometry you will however see other carbocations where instead of having a hydrogen you will have carbon atoms on the central carbon and in fact in this week's discussion section you'll see the turb carbocation which like the methyl carbocation doesn't have a complete onch but is a little bit less unstable so the methyl carbocation is also planer trigonal we have an SP2 hybridized carbon we have three lians around the carbon no lone pairs of electrons 120° Bond angles hydrogen carbon hydrogen methyl carbocation has a vacant p orbital meaning that it has a p orbital with no electrons in it that p orbital is orthogonal to the plane so I'm now drawing the methyl carbocation like so and here's our vacant p [Applause] orbital question so we have electr in bonds with hyrogen and then El zero electrons so you you bring in carbon three which has four electrons three hydrogen atoms that would give you seven veence electrons but you take away one you have six and a net positive charge so and in fact we drew the species Let's see we drew Boron no we drew a a fluorinated carbon well if you think about boron and you take a boron compound with three Li on it you also have a vacant p orbital on Boron so the carbon basically has been reduced to coping with not having enough electrons take one more example we saw last time for Malahide maybe we saw it the previous time exact same situation as ethylene the carbon is SP2 hyro ized and the oxygen also is SP2 hybridized you have two lone pairs of electrons on the oxygen of formalde all right last bit of geometry I want to talk about s hybridized atoms SP hybridized atoms have approximately 180° Bond angles and a linear arrangement of linging and lone pairs okay let's take a look at a few examples we talked about acetylene before H C triple bond CH the carbons in acetylene or S [Applause] hybridized you have 180° hydrogen carbon hydrogen bond angle on both sides take a look at another molecule see a new functional group here let's take a look at aceton nital [Music] ch3 oops CN I'll drawn the lone pair of electrons as I said the name of this molecule is commonly referred to as aeton nital the carbon and the nitrogen are SP hybridized and the arrangement of the bond angle about the central carbon the carbon carbon nitrogen Bond angle is 180° let's take a look at one more molecule the molecule alen or one 12 two [Music] [Applause] propod Allen is the common name for this molecule you'll learn more about nomenclature when we get to systematic names are more about the systematic n nomenclature when we get to systematic names don't sweat for right now the question was would you rather we know iopac or common names so the iopac name of a molecule like acetic acid is ethanolic acid you'd probably never hear anyone in class talk about ethanolic acid it just wouldn't roll off the tongue but you might see it particularly if you have a substituent on it but then you'd recognize it you'd say oh wait yeah I know propenoic acid and eoic acid so right now don't sweat go with the flow on some of the more more common terms we're getting all right alen's a very funny looking molecule we haven't seen a molecule quite like this with a carbon double bonded to another carbon and on both sides but of course we've seen a structure like this in carbon dioxide with carbon double bonded to an oxygen on one side and double bonded to an oxygen on the other side now we have a little bit of a chance to think about hybridization the central carbon is SP hybridize and the outer two carbons are SP2 hybridized and the carbon carbon carbon Bond angle is 180° uh what makes it less than 180 or more than 180° well we saw exactly the situation with Lan pairs where you can have unequal substituents a lone pair on one side takes up a little bit more space now in the case of 180 you're not really going to have that circumstance coming up here but for example in F Malahide if the hydrogen carbon hydrogen angle was a hair under well that wouldn't even be a good example here so basically lians lone pairs can take up just a little bit more than ligans there are also cases where there are some really interesting variations in hybridization and this is the stuff that makes chemists scratch their heads so for example I talked about carbon dioxide if we concatenate and add another double bond here to an oxygen that molecule actually has a little Bend to it and that's the sort of thing that makes chemists at a very advanced level say why is this we have simple rules and then we see there are some exceptions and the exceptions are often the fascinating thing that get people at an advanced level well be Beyond sof organic chemistry to get excited about a res all right well let us move on to chapter 2 now and I want us to begin talking about acids and bases in organic chemistry she the acids and bases are Central to organic chemistry both in the generation of reactive species species that we call electrophiles nucleophiles and also as reactive species in their own right so this is why it's very important for us to think about acids and bases and as I said we'll use this also as a springboard for learning curved arrow notation okay [Applause] [Music] [Applause] for for throughout the course of this week we're going to start to see what this means and then of course because this concept is so Central it will come back to us again and again we're going to see for example that what we've known of in general chemistry as Lis acids species that react by accepting a pair of electrons are very similar in many cases the same as what organic chemists call electrophiles things that want electrons species that we think of as Louis bases in general chemistry species that react by sharing a pair of electrons are What organic chemists called nucleophiles I should close my parentheses and in many cases we're going to simply see protons coming on protons coming off in organic chemistry let's start there are number of different models correct models for acids and bases let's start with the model you certainly learned first in general chemistry and in high school chemistry what's often referred to as a bron acid or Bron Lowry acid the definition of a bronzee acid is very simple in intuitive a bronstad acid is a proton donor conversely brunstad base is a proton acceptor and so we think of a general brunede reaction as a bron stead acid and a bronstad base react together to give the conjugate acid of the Bron base another acid and the conjugate base of the Bron acid another base what do I mean let's take HCL hydrogen chloride hydrogen chloride is a gas when you dissolve it in water they react you have an equilibrium that equilibrium happens to lie very far to the right you form h3o+ and cl minus in this reaction the HCL acts as an acid it donates a proton to water the water thus acts as a base we form the conjugate acid of water h3o+ so I'll say that this is an acid and we form the conjugate base of HCL namely chlorine so I'll write that this is a base thought we can write this reactions an equilibrium as I said it happens to lie far to the right in other words if I wanted to amplify on this I could draw longer Arrow on top and a shorter arrow on the bottom but we can also write the reverse equilibrium chloride anion plus h3o+ is in equilibrium with HCL and H2 in that case we'd be saying a base plus an acid is in equilibrium with the conjugate acid and the conjugate base let's start to think about the mechanism of this reaction and the flow of electrons and now we're going to use our organic chemist notation of the curved Arrow so I'm going to now draw the water very explicitly I'll draw all bonds and long pairs of electrons so we can see how Bonds were made and how bonds are broken I'll do the same for hydrogen chloride remembering to draw in my three L pairs of electrons around chorine the water takes the proton off of hydrogen chloride electrons flow from the lone pair of electrons on water to the proton to the Bron dead acid we can't have four electrons around that hydrogen so electrons get pushed back onto chlorine you'll notice how I draw my current arrows I always start at a source of electrons a long pair or a bond and I always end up on an atom and when all is said and done the sharing of this pair of electrons with the hydrogen results in a new Bond we have a formal positive charge on oxygen positive charge on oxygen and now we get our chloride anion we have four long pairs on the chide and now as I hinted before or what's really powerful about this type of way of thinking is we can see all sorts of analogies for example if instead of using water in this reaction instead of adding hydrogen chloride gas to liquid water to get aquous hydrochloric acid which is a solution of hydronium ion and chloride in water If instead I had done the same reaction with methanol ch3 o ch3 we can predict what would happen methanol looks a lot like water the only difference is we have a methyl group instead of a hydrogen we still have an oxygen it's still sp3 hybridized we still have a lone pair of electrons and the other component in this equation is the same so it shouldn't be a great leap to be able to predict that we should see a similar reaction in which electrons flow from the lone pair on methanol to the proton on we push electrons back on to Chloride we set up an equilibrium we get protonated methanol and chloride anion for let's try another example and again I'll show the comparison between Concepts that we get in general chemistry and Concepts that we get organic chemistry let's take the ammonium ion nh4+ and we'll allow it to react with the hydroxide anion again we have an equilibrium the ammonium ion acts as a proton donor in other words as an acid the hydroxide anion acts as a a proton acceptor in other words as a base the hydroxide an pulls a proton off of the ammonium ion and we can use the same concept of the curved Arrow to demonstrate this we have electrons flow from the hydroxide an pair to the proton on ammonium we push the electrons from the bond onto the nitrogen [Music] atom now we generate ammonia is our lone pair ammonia is the conjugate base of the ammonium ion and we generate water water is the conjugate acid of hydroxide on let's try the same reaction with an organic compound we'll try this reaction with methyl ammonium ion [Applause] [Applause] and we'll still use hydroxide [Applause] anion now it should be very easy with all of these examples to predict what hydroxide an does pull the proton off of the methyl ammonium ion we push the electrons onto nitrogen we have an [Applause] equilibrium we get water and methylamine [Applause] you'll often hear of this reaction referred to as making the free base of an organic amine you can have an amine hydrochloride salt for example generated by reaction of an amine with hydrochloric acid when you add a base like sodium hydroxide you'll pull that proton off the ammonium ion to generate the free am you've heard of this term making the free base from an AM Soul where have you heard of it cocaine cocaine exactly and when you hear of cocaine being free based what that means is using a base like sodium bicarbonate to generate the free base of cocaine the physiologically active form so not only is free base a noun but by popular usage it has ended up becoming a ver all right now and in our next lecture we're going to be discussing the degree of acidity position of equilibria and in order to measure the degree of acidity or to characterize to describe the degree of acidity we'll often use a term PKA PKA is a fancy way of saying negative log of the acidity constant ka ka is the acidity constant or the acid dissociation [Applause] constant and all that means is if you have an acid we'll call it ha and it's you put it into water you set up an equilibrium of dissociation an equilibrium between the hydronium ion and the conjugate base a [Applause] minus our equilibrium constant remember how you get that that's just the concentration of products product the product that multiplied out of the concentration of products over the concentration of reactants ignoring anything that's constant concentration like solvent so in other words KA for acid dissociation equals the concentration of hydronium ion times the concentration of the conjugate annion divided by concentration of ha but simply that's the pH youd get of a one molar solution where half of it is AJ and half of it is a minus so if you take half a mole of sodium acetate and half a mole of acetic acid dissolve it to make a liter of solution and measure the pH you'll get about 4.56 the pka of a c acid a strong acid is fully dissociated a weak acid is partially dissociated and what I call a very weak acid is essentially unmeasurably dissociated so one thing to keep in mind is PKA is a log scale what does that mean that means a change in of one PKA unit corresponds to a change of 10 fold in acid dissociation constant a change of two in PKA unit or a difference of two PKA units corresponds to a factor of two of 100 in acidity constant so if at one end of the scale we take a look in at HCL a very strong acid HCL has a PKA [Music] of7 in other words that means that Ka equal 10 the 7 at the other end of the scale the weakest acid and I'll use that term very Loosely that you would ever see would be something something like methane or another alkane the pka of methane is 50 positive 50 Ka = 10^ 50 what that means is if I dissolved a mole of methane and a liter of water you would not have even a single molecule of the methane that's dissociated it is such a weak acid it is in the context of General chemistry only an acid in name in that you could write an equation but the equilibrium of methane and water to give methyl anion would be completely to the left you would see no dissociation so in other words what we're saying here is that HCL is 10 to the 57 I can't even WRA my head around such a big number 10 to 57 times more powerful acid than methane [Applause] thoughts or [Applause] questions ah a great question why are we moving the lone pair of electrons from the base to the acid so we're beginning to think about organic reactivity we're beginning to think about the making and breaking of bonds and the making and breaking of bonds is all about the flow of electrons when we form a bond we make a new shared pair of electrons when we break a bond we give up in here the and showing the curved Arrow shows the flow that makes all right we will pick up next time talking about acid strength and look at various acids of varing strengths