Alkene reactions
Introduction to Reaction Mechanisms Introduction to Reaction Mechanisms . Addition reaction to an alkene
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- Let's think about how hydrogen bromide might react
- with this thing right here.
- Let's think about what this would be called.
- We have 1, 2, 3, 4, 5 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.
- the double bond starts at the number 1 carbon, pent-1
- and it's obviously an alkene.
- It has a double bond, so it's pent-1-ene.
- Sometimes this is called 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 solvent's not specified
- it's usually water or alcohol.
- If this was water
- then 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 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.
- If that's a completely foreign concept to you
- I'll do a little bit of a review.
- This is the Periodic Table.
- Electronegativity increases from the bottom left
- to the top right of the Periodic Table.
- 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 to hog electrons so much.
- So if we think about hydrogen bromide
- hydrogen's way up here
- on the left side of the Periodic Table
- bromine's 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
- and you'll have a partial positive charge
- on the hydrogen end.
- And, whenever you do these reactions
- it's actually useful to draw all the valence electrons.
- So bromine right here has an atomic number of 35
- 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 7.
- If you just count from here, 1, 2, 3, 4, 5, 6, 7
- or has 7 valence electrons. Let me draw that.
- Seven electrons in its outermost shell.
- Let me draw them right here.
- So 1, 2, 3, 4, 5, 6
- and then the 7th is in the bond with the hydrogen.
- Hydrogen obviously 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 it 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 take this electron away
- this green hydrogen electron.
- And maybe that happens. Let me just draw it out.
- So he takes that electron, that green electron, away.
- It's just getting closer and closer.
- If there was 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 about
- which one is more likely to give up the electron
- at this stage of the reaction.
- But just for simplicity, let's assume that
- this carbon right here gives up an electron.
- Where are its valence electrons?
- And just as a bit a review
- here's a carbon on the Periodic Table.
- it has 6 protons and 6 electrons in its stable state.
- Two are sitting in its first shell
- and then the other 4 are its valence electrons.
- And you can see that. It's in Group 4, right? 1, 2, 3, 4
- We can ignore these for now
- and usually you're not going to be dealing with
- well, we'll ignore those for now.
- It has four 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 of those right now.
- That's 4 valence electrons and you see it right here.
- It has 1, 2, 3, 4 valence electrons.
- It has 2 in its one shell, so it actually has 6
- but you only draw the 4 out there.
- So what we're saying is
- this green electron can go to the bromine
- as long as the hydrogen can take an electron
- maybe from this carbon right here.
- So let's draw that. So 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.
- And 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 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 to 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 bromine.
- So what's going to happen right after this step?
- So right after that happens
- what will everything look like?
- Well, 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 7 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 pent-1-ene
- 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
- then 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 bond right here now forms
- between this carbon and that hydrogen.
- So let me draw that bond.
- You have this electron right there.
- That electron will now be with the hydrogen.
- So 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 3 valence electrons.
- 1, 2, 3. It has two sitting in its first shell
- so it actually has a total of five electrons.
- It has 6 protons, so it has 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 it will have a positive charge right over there.
- So this's what we're left after that step of the reaction.
- 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 it has eight
- so now this will have a negative charge
- because it gained an electron.
- And in general, your total charge
- over here our total charge is 0
- so our total charge will still be 0.
- We have a negative and we have a positive.
- They would cancel out, so our total charge is still 0.
- So what's likely to happen
- for the next step of our reaction?
- Well, we have this positive thing here.
- Maybe bromine just bumped, just the right way
- to let go of this guy and steal his electron.
- But now you have this guy who's 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 swipe the electron
- from the bromide ion
- from this negative ion right here.
- And you might say, hey,
- isn't bromine more electronegative than carbon?
- Well, it might be, but this guy's electron rich.
- It's not just a regular bromine atom.
- This is a bromine plus an extra electron.
- So he's already hogged an electron
- so he's electron rich.
- So in this situation, he's negative. He's positive.
- He can give this guy an electron.
- So if they bump in just the right way, this electron
- can be swiped by this carbon right over here.
- And this positive carbon
- just so it gives you a little terminology
- and we'll go over it in more detail in future videos
- is actually called a carbocation.
- It's a positive ion of carbon.
- That's where the word comes from.
- But anyway
- if this electron gets swiped by this carbon
- it will then form a bond.
- Because remember, this was the electron
- that was originally in a bond with this hydrogen.
- It's still going to be, you could imagine, paired 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 this carbon right here.
- It has a hydrogen.
- You have the rest of the chain, CH2-CH2-CH3.
- And now, since this guy stole an electron
- a bond will form with the bromine.
- Let me draw it. So he's going to steal this
- 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.
- 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 electrons.
- So now everyone is happy again.
- In this video, we were able to get a mechanism.
- 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-1-ene, and get to this thing
- 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.
- And just a bit of a review
- what is this thing right here?
- This might be-- This is 1, 2, 3, 4, 5 carbons, still.
- So it's going to be a pent.
- It's now an alkane, no double bond. So it's pentane.
- And it has one group on it
- so we'll start numbering closer to the group.
- So 1, 2. It's 2-bromopentane.
- So we were able to figure out a reaction mechanism
- to get us from hydrogen bromide and 1-pentene
- to 2-bromopentane.
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At 5:31, how is the moon large enough to block the sun? Isn't the sun way larger?
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