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Current time:0:00Total duration:11:22

Introduction to reaction mechanisms

Video transcript

let's think about how hydrogen bromide might react with this thing right here and let's think about what this would be called we have one two three four five carbons it has a double bond right here if we start numbering at this end because that's where the double bond is then this would be pent this is pent the double bond starts at the number one carbon pent 1 and it's obviously an alkene it has a double bond so it's pent 1 in sometimes this is called 1-pentene 1-pentene either way so let's think about how these two characters might react with each other in some type of solvent usually when a solvents not specified it's usually water or alcohol if this was water then this would actually be we would have a solution of hydrobromic acid but let's not worry about that right now let's just worry about how these two these two characters might react with each other now the first thing we might look at is this hydrogen bromide right there and as you get more experience there you'll say well you know bromine is much more electronegative than hydrogen it likes to hog electrons much more and if that's if that's a completely foreign concept to you I'll do a little bit of a review this is a periodic table electronegativity increases from the bottom left to the top right of the periodic table electro electronegativity and they electronegativity and really that's just a fancy way of saying how much does an atom like to hog electrons so for example fluorine loves to hog electrons and calcium doesn't like the hog electrons so much so when if we think about hydrogen bromide hydrogen's way up here on the left side of the periodic table bro means all the way to the right bromine is much more electronegative so in this situation since bromine is more electronegative it will hog the electrons in this bond and since it's hogging the electrons in this bond you'll have a partial negative charge on the bromine end you'll have a partial negative charge on the bromine end and you'll have a partial positive charge on the hydrogen end and whenever you do these reactions actually is useful to draw all of the valence electrons so bromine right here bromine right here it has an atomic number of 30 five which means it has 35 electrons in its neutral state and we don't have any charge here so it's in a neutral state but you can look at its group it's in group number seven if you count from here one two three four five six seven or it has seven valence electrons so let me draw that seven electrons in its outermost shell let me draw them right here so one two three four five six and then the seventh is in the bond with the hydrogen hydrogen obviously has one electron hydrogen has one electron it's right there so these two guys are bonding with each other and they both allow each other to pretend that they're part of a pair and that's what gives it a lower energy level or makes a little more stable that's why the bond forms in the first place but anyway let's think about what might happen here this guy is really electronegative so maybe he wants to hog this maybe he wants to take this electron away this green this green hydrogen electron maybe that happens maybe that happens but that can't look let me just draw it out so maybe that happens so he takes that electron that green electron where it's just getting closer and closer if there's only some other place that this hydrogen could get an electron from then this guy could just go to the bromine which is what bromine really wants to happen because it's so electronegative so where can the hydrogen get an electron to replace the one that it's about to lose well we have a double bond here and maybe one of these carbons lose an electron and in future videos we'll talk more which one is more likely to give up the electron at this stage of the reaction but just for simplicity let's say let's assume let's assume that this carbon right here this carbon right here gives up an electron where its valence electrons and just as a bit of review here's carbon on the periodic table it has 6 it has 6 protons and 6 electrons in its stable state to are sitting in its first shell and then the other for its valence electrons and you could see that it's in group 4 right 1 2 3 4 we could ignore these for now and usually you're not going to be dealing with well we'll ignore those for now it has 4 valence electrons most of what you're going to deal with you just have to count the group numbers like this we won't worry too much about all of the metals and all right now now it's four valence electrons and you see it right here it has one two three four valence electrons it has two and it's one shell so it actually has six but you only draw the four out there so what we're saying is this green electron can go to the bromine as long as he can take as long as the hydrogen can take an electron maybe from this carbon right here so let's draw that so this this electron right here is going to go to the hydrogen and when that electron goes to the hydrogen simultaneously that will allow this electron to go to bromine obviously they won't be this far when it happens there would be some type of collision that would have to occur in just the right way with you know just the right level of energy for this to occur maybe the hydrogen is getting really close to this part right at the right moment when this electron is being sucked away from the bromine so this has a partial positive charge attracted the electron that electron goes over there it won't always happen but this is a potential reaction mechanism but this whole step happens at once this electron goes from this character to the hydrogen at the same time as hydrogen loses its electron to the to the bromine so what's the what's the what's going to happen right after this step what's going to happen right after this step so right after that happens what will everything look like well we'll have the bromine will have gained the electron so it's now a bromide ion so let me draw it like this so it had its original seven valence electrons 1 2 3 4 5 6 7 and now it just stole an electron from the hydrogen it was able to swipe an electron off the hydrogen that's the electron it swiped off the hydrogen and now what will this thing that was 1-pentene or pen 20 what will this look like now let me draw it so we have a carbon and a hydrogen and a hydrogen and then you have a carbon and then a hydrogen and then you have the rest of the chain right over here you have the rest of the chain right over here but this double bond was broken this carbon lost an electron went to the hydrogen so this double this bond right here now forms between this carbon and that hydrogen so let me draw that bond so this bond actually draw it that you have this electron right there that electron will now be with the hydrogen's let me draw the electron and now we have that orange hydrogen I'll try to keep the colors consistent so that we know where things came from and then we have this bond now to the hydrogen now this carbon now only has three valence electrons one two three it has to sitting in its first shell so it actually has a total of five electrons has six protons so as a positive charge this carbon right here has a positive charge and another way to think about it is it was completely neutral and then it lost an electron so now we'll have a positive charge right over there so this is what we are left after that step of the reaction now as you can imagine oh and of course we can't forget bromine over here was neutral it had seven valence electrons and that's when bromine is neutral but now has eight so now this will have a negative charge this will have a negative charge this will have a negative charge because it gained an electron and in general your total charge over here our total charge is zero so our total charge should still be zero we have a negative and we have a positive they would cancel out so our total charge is still zero so what's likely to happen for the next step of our reaction well we have this positive thing here maybe you know bromine just bumped just the right way to into this to let go of this guy and steals electron but now you have this guy whose negative and this guy who's positive maybe they'll be attracted to each other maybe they'll just bump into each other at the exact right way and if they bump in the exact right way maybe this guy can swap swipe the electron from the from the bromide ion from this negative ion right here and you might say hey it isn't bromine more electronegative than carbon well it might be but this guy is electron rich it's not just a regular bromine atom this is this is a this is a bromine plus an extra electron so he's already hugged an electron so he's electron rich so in this situation he's negative he's positive he's he can give this guy an electron so they bump in just the right way this electron this electron can be swiped by this carbon right over here by that carbon right over here and this positive carbon just to give you terminology and we'll go go over it in more detail in future videos it's actually called a carbo-cation it's a positive ion of carbon that's where the word comes from but anyway if this electron gets swiped by this car but it'll then form a bond because remember this was the electron that was originally in a bond in a bond with this hydrogen it's still going to be you could imagine pair it up with this other purple or magenta electron right over there so if that happens then we're going to be left with so the next step is going to be so on this end of the molecule we have C carbon hydrogen hydrogen then we have this orange hydrogen that we stole from the hydrogen bromide then we have then we have this carbon right here it has a hydrogen they have the rest of the chain ch2 ch2 ch3 and now since this guy stole an electron it will bond will form with the bromine so now let me draw it let me draw it so he's going to steal this let me draw it this way so a bond will form he's stealing this electron so now this electron is with this carbon so I can do it as one end of the bond the other end of the bond will be that magenta electron right there on the bromine and now the bromine lost an electron so it's neutral it once again has 1 2 3 4 5 6 7 valence electrons carbon is now neutral because it gained an electron it once again has 1 2 3 4 electron so now everyone is happy again so we in this video we're able to get a mechanism and we can talk in the future about how likely it is to happen how quickly it might happen we were able to start with hydrogen bromide and 1-pentene or pent 1e and get to this thing and just by pushing around electrons and just thinking about what is likely to happen based on what's electronegative and what might be able to gain or lose an electron in just a bit of a review what is this thing right here this might be let's see this is 1 2 3 4 5 carbon steel so it's going to be a pent it's now it's now an alkane no double bond so it's pentane and it has one group on it so we'll start numbering close to the group so 1 2 it's 2 bromo pentane to bromo pentane so we were able to figure out a reaction mechanism to get us from hydrogen bromide and 1-pentene to to bromo pentane