41 Circle the strongest acid determine the effect associated with each example think about why one molecule is stronger in acidity than the other molecule is it because of the inductive effect hybridization electronegativity electron delocalization or atomic size so for the first example we're comparing nh2 with PH2 on empiric table we have these elements below nitrogen is phosphorus below oxygen is sulfur and then here are the halogens so in this example we're comparing nitrogen and phosphorus so when we're looking at two elements in the same column either we're looking at the electronegativity or atomic size now because these elements are directly attached to the hydrogen atomic size will be more relevant the element that is bigger will be able to stabilize the negative charge on the conjugate base atomic size increases as you go down phosphorus is bigger than nitrogen so this is going to be more acidic because the conjugate base is going to be better stabilized by a bigger phosphorus atom phosphorus will be in a better position to handle that negative charge than nitrogen so this is the correct answer and the reason is due to atomic size now let's look at number two here we have an amino acid which is glycine and here we have acetic acid we know the pka for acidic acid is 4.75 for the amino acid the carboxylic acid has the pka of around 2.2 2.3 the NH3 that's like 9 to 10. so this one is clearly way more acidic than acetic acid now what is the reason why the amino acid is more acidic than a carboxylic acid so here we have a nitrogen atom and that nitrogen is not directly attached to the hydrogen so atomic size doesn't apply here the reason why this carboxylic acid is more acidic is because nitrogen is electronegative and it can pull electrons through the sigma bond by means of the inductive effect so it's not just electronegativity but more specifically the inductive effect but electronegativity does play a role in that example so the primary reason is the inductive effect the secondary reason is electronegativity so because nitrogen is electronegative it can pull electrons by means of the inductive effect but if I had a test if this was a test I would pick inductive effect over over electronegativity if the inductive effect wasn't there then I would pick electronegativity number three so here we're comparing two hydrogen atoms one is on a double bond the other is on a single Bond so the hybridization is different here we have a hydrogen on an SP2 carbon here it's on an sp3 carbon we know the alkene is more acidic than the alkane so that's going to be the answer the pka for an alkene is 44. for now Kane it's about 50 to 60. so the answer for this is this is the strongest acid or the stronger acid rather but the reason is due to hybridization here this is the inductive effect and this is atomic size now for the fourth example we're comparing sulfur and phosphorus when you compare in two different elements in the same row there's only one reason it could be and that is electronegativity sulfur is more electronegative than phosphorus electronegativity increases to the right so this is going to be the answer so we're going to put en for the reason now for number five we're looking at the alpha hydrogens the alpha hydrogen next to a ketone has a pka of 19. this Alpha hydrogen between two ketones that are 1 3 apart this is much less it's nine now the question is why is this so much more acidic what is the answer is it the inductive effect is it hybridization it's not hybridization because the hydrogens they're both next to and SP2 carbon so it's not going to be hybridization is it electron delocalization is it atomic size it's not atomic size because we don't have any different elements all the elements are the same it's not electronegativity the elements are the same so is it the inductive effect or electron delocalization the inductive effect you're pulling electrons through the sigma Bond but electron delocalization that's when you that's when you start moving the pi electrons if we were to take off the hydrogen we would have lone pair and this lone pair can move into the carbonyl group so it's delocalized so the reason why this is relatively acidic compared to an alkane is because we can put the negative charge on an oxygen that's why the pka is so much different here but in this example we could put the negative charge on two oxygen atoms even though the carbonyl groups they do exert an inductive effect on hydrogen that's not the major reason that's not the strongest reason the strongest effect has to do with electron delocalization so the answer is primarily electron delocalization the secondary answer is the inductive effect because these are electronegative electron with charm groups so you do have some electronegative atoms here you could put electronegativity as a third reason but it's a minor reason because both of these have the same electronegative atoms but this one has two of them so the reason why this one is more acidic the primary reason is electron delocalization which is equivalence the same resonance stabilization of the conjugate base you could put the negative charge on two oxygen atoms the secondary reason is inductive effect because these are electronegative atoms and they could pull electrons through the sigma Bond but that's just not the primary reason so this is the primary reason secondary and you could make an argument that hey we got some electronegative oxygen atoms so that will contribute to the acidity as well now I do have one more example and here it is so which one is the stronger acid looking at the indicated hydrogen atoms which hydrogen is more acidic and what's the reason so this is just a regular alkane hydrogen which is going to be 50 to 60. now this hydrogen is not directly attached to the double bond however it's on the allylic position and that makes it special so this is going to be around 40 to 50. if we take off this hydrogen if we draw the conjugate base the negative charge is localized on the primary carbon if we take off this hydrogen the negative charge can delocalize among two carbon atoms so we can draw resonance structures it's better to have a negative charge over two carbon atoms as opposed to one so this is going to be the stronger acid because the conjugate base is relatively more stable than this one so the answer has to do with electron delocalization or resonance stabilization of the conjugate base