the acidity of alkynes now in the last lesson we just got done naming alkynes we'll proceed on this is the one physical property we really want to talk about that's unique to alkynes and and it turns out it's going to be relevant later on in the chapter after this we're just going to cover all the different chemical reactions that either make or use alkynes and this acidity is going to be directly relevant to one of those now this lesson's part of my new organic chemistry playlist i'll be releasing these lessons weekly throughout the 2020-21 school year so if you don't want to miss one or any of my future playlists subscribe to the channel click the bell notification you'll be notified every time i release a new lesson all right so the city of alkines and we're going to talk about a terminal alkyne where the carbon carbon triple n comes at the end of a molecule so it's this hydrogen here that's attached to an sp hybridized carbon that is going to be of relevance here so we're not really going to address internal alkynes in this part it's just the terminal alkynes are going to have some relevance here we learned back in the chapter on acid bases that a terminal kind has a pka in the 25 to 26 range and and so in this case that means it's actually not very acidic so but it's significantly more acidic than an alkane or an alkene so for an alkane your acidity you know pka is in the 50 range for an alkene your pka is in the 44 range so this at pkf26 much lower makes it significantly more acidic than an alkane or alkene but the truth is it's still way less acidic than say like water which is not a good acid so it's not a great acid but it's acidic enough that we can potentially deprotonate this if you're going to try and deprotonate a typical alkene or alkane it's just really not going to happen but we do have a way to deprotonate an alkyne so if we look classic strong base you learned about in gen chem would have been like sodium hydroxide so if we add sodium hydroxide to this so all right nr here you'll find that no reaction it turns out it is not strong enough so again this is not the most acidic thing in the world again much more so than an alkene or alkane but not very acidic and we can kind of see why here if we kind of show this reaction here you could be like plus hydroxide and if you actually did form products here you'd form the deprotonated alkyne which will give a special name here a little bit and you'd form water and so our acid on this side's got a pkf26 but water has got a pka of like 15.4 and so we can see that the equilibrium here is going to lie towards the weaker acid in base and a pkf-26 is a much weaker acid than a pka of 15.4 and that's why this is not going to work this thing is way less acidic than water and the equilibrium lies so far back towards the products that again essentially we just say that no reaction happens here so if you're going to want to deprotonate a terminal alkyne you're going to have to have a new strong base new and improved that's significantly stronger than sodium hydroxide fortunately we have one and we referenced this back also in our chapter on acids and bases and it turns out this new one here is sodium amide nanh2 this is an ionic compound it's got a sodium that's a positive ion and then an amide anion and it's this amide anion that is the super duper strong base here and so notice having a negative charge on oxygen and hydroxide not as basic as having a negative charge on the less electronegative nitrogens that's the key here and so if we look at this reaction we'll now effectively shift this equilibrium so far towards the products it turns out that this reaction effectively goes to completion and we'll form this lovely conjugate base of our terminal alkyne and he gets a name here we call this an acetylide ion cool we'll find out these satellite ions are really good nucleophiles they're great for sn2 reactions which will be relevant towards the end of this chapter super helpful for synthesis purposes as we'll see all right so why does this reaction work whereas again the top one did not well again if we kind of write in our other reactant here it's really the amide ion so we get the amide ion and then your product over here would therefore be ammonia so and the key difference here is that ammonia's got a pka not a 15.4 but somewhere in the ballpark of like 37 38 and so now all of a sudden he's the weaker acid as compared to the terminal alkyne with a pka of 26 and he's much higher pka by a fair amount and so the equilibrium is going to lie so far to the right that it effectively happens virtually 100 percent so again if you use sodium hydroxide not going to cut it if you use sodium amide virtual 100 to completion now one thing you should note here i just wrote it as amide here but again the reagents nanh2 and so sometimes you'll see the product drawn in and they'll draw the counter sodium ion right here as well i just want to make sure you've seen it both ways whether you draw include the sodium counter ion or not your acetylide ion is the result here and that's the big take home so again you want to deprotonate a terminal alkyne we've got a new and improved favorite strong base to carry this out the old ones like sodium hydroxide not going to work now if you found this lesson helpful would you consider giving me a like and a share a couple of the best things you can do to promote the channel and if you've got questions feel free to leave them in the comment section below if you are looking for practice problems on all kinds if you're interested at all in my brand new organic chemistry one rapid review if you're studying for finals check out my premium course on chadsprep.com