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Course: Organic chemistry > Unit 5
Lesson 5: Sn1 and Sn2- Identifying nucleophilic and electrophilic centers
- Curly arrow conventions in organic chemistry
- Intro to organic mechanisms
- Alkyl halide nomenclature and classification
- Sn1 mechanism: kinetics and substrate
- Sn1 mechanism: stereochemistry
- Carbocation stability and rearrangement introduction
- Carbocation rearrangement practice
- Sn1 mechanism: carbocation rearrangement
- Sn1 carbocation rearrangement (advanced)
- Sn2 mechanism: kinetics and substrate
- Sn2 mechanism: stereospecificity
- Sn1 and Sn2: leaving group
- Sn1 vs Sn2: Solvent effects
- Sn1 vs Sn2: Summary
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Curly arrow conventions in organic chemistry
Conventions for drawing curved arrows that represent the movements of electrons. Created by Sal Khan.
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- In the hydroxide ion (OH) and methyl bromide (CH3Br) example, why doesn't he have the full arrow pointing from oxygen lone pair to the space between O and C? If you point the arrow at the space, I think you could imply that you are placing two electrons between O and C, thereby making a bond. Well, he did say it was his own convention.(4 votes)
- That is the usual convention. A double-barbed arrow shows the motion of a pair of electrons moving to another atom.
Single-barbed arrows show the movement of a single electron from each atom to form a bond between them.
His personal convention is to show the movement of a single electron of a pair to form a bond.(3 votes)
- This video helped so much... before this I was really confused on why he was moving single electrons with a full arrow. Thanks!(3 votes)
- So in a nutshell half arrow means transfer of single electron where as full arrow means transfer of pairs of electrons
Am I right ? please correct me if I am wrong.(2 votes)- Yes, half arrows (sometimes called fish hooks) correspond to the movement of a single electron, while full double headed arrows correspond to the movement of a pair of electrons.(2 votes)
- Does the movement of electron pair go towards positively charged species? For example, like the lone pair on O in OH goes towards the delta positive C. But then, if this is the case, why does the electrons in the covalent bond breaks off from the C and going towards the delta negative Br, if the rule is that movement of electron pair always go to positively charged species?(2 votes)
- No, electron pairs always go towards the more electronegative atom. Bromine, being more electronegative attracts the electron pair towards itself.(2 votes)
- At 5.52 he says that electron is moving by itself ,then won't electricity be generated during the formation of the compound..could someone guide me(2 votes)
- Electron pairs are driving the movement but they are still attached to their nucleophile, e.g. NH3 has a lone pair which remains attached to the nitrogen whilst bonding.(2 votes)
- what happens when you have two potential leaving groups? for example, when 4-bromo-1-pentanol reacts with NaH? how do you determine which R-group (either the bromine ion or the alcohol) will depart in the reaction?(1 vote)
- it depends upon the leaving group ability of the groups which generally is inversely proportional to the basic strength of the group. that is among the two compare the basic strength and then depart the one which has lesser strenght(2 votes)
- why steric hindrance is the most affect SN2 reaction?(1 vote)
- In the movement of electron as "part of pair" from Sal's example, part of the electron of the electron between C and Br is moving to the Br, rather than the entire pair is moving to the Br and hydroxide group brings two electrons, right?(1 vote)
- Yes, the OH⁻ uses two electrons to form the bond, and two electrons move to the Br as it leaves.(1 vote)
- How to convert benzene into benzyl(1 vote)
Video transcript
Sal: What I want to do in
this video is talk a little bit about the curly
arrow conventions used in organic chemistry and the
slight variations I use in many of the videos
here on Khan Academy. There's two types of
curly arrows you will see. You will see a curly full arrow like this, a curly full arrow like this. And I make sure to draw it curly, you will always see the curly like this. And you will see a curly half
arrow that looks like this, curly half arrow or fish hook arrow. The convention is a full
arrow or a typical arrow that you're used to
seeing, this is talking about the movement of
pairs, of electron pairs. Movement of pairs is the convention. I'll show you in a second
that I do a slight variation of that, and I do that
because it helps me account for electrons, and it
helps me at least visualize or conceptualize how
things are, or essentially how things are happening,
a little bit better. The general convention is
that this is movement of pairs and this is movement
of electron by itself. Electron, electron not part, electron by itself, maybe
I'll write it this way. By itself. The full arrow is what you're going to see through most of organic chemistry. This is the one that you're going to see most typically, the movement of pairs. The movement of electrons
by itself, this is going to show up more in free radical
reactions, which we do do, but this is later on, and
most of organic chemistry is going to be dealing
with the movement of pairs. What I've drawn over here is a curly arrow showing the same thing happening. I'm showing you the slight
variation that I do. I do it because it helps me, once again, account for the electrons, and it helps me conceptualize what is going on. The typical way that this type
of mechanism will be shown, we'll say you have this
electron pair on this oxygen, and this electron pair,
sometimes we will say, and you will learn about this
reaction in not too long, is going to the carbon,
or I guess you could say it's attacking the carbon right over here. The reason why this I find
a little bit less intuitive is that the whole pair is
not going to the carbon, that the oxygen is still
going to maintain half of this pair and it's
going to form a bond. Essentially one end of this
pair is going to end up at the carbon, one end of this
pair is going to end up at the oxygen, and they
are going to form a bond. The way I draw it, still
drawing the full arrow. Here I'm still talking
about pairs but I'm talking about the movement of an
electron as part of a pair. That's kind of the slight non-conventional thing that I do with the full arrow. Movement, movement of electron,
electron as part of pair. I'll often times draw the back
of the arrow from that electron, but It's important to recognize
that electron is not moving by itself, it's just ending
up on one side of a bond, it is moving as part of a pair. Another way to think of
it is this electron is going to be on the other side of the bond. I also want to be clear again. When I talk about electrons
on either side of bonds, I like to think about that because it helps me do it for accounting purposes. We know that these covalent
bonds, this one electron just doesn't sit on one side of a
bond and the other electron doesn't just sit on the
other side of the bond. In fact everything we do
in organic chemistry isn't anywhere near as clean as
the way we draw it, but I do this to remind myself that
there are two electrons here, and when you have a bond
there is some probability that one of the electrons
is closer to the hydrogen and there's some probability
that that electron is closer to the carbon, and
so you can kind of imagine that there are electrons on
either sides of the bond. The actual reality is that
there's a blur over them and depending on which molecule
is more electronegative the probability blur is a
little bit more weighted on one side or another, but
of course we like to clean things up with these
formalisms right over here. This is kind of the example
when you have this attacking pair, why I like to
think of the full arrow as the movement of an
electron as part of a pair. And this breaking bond over
here is another example. In the typical convention
you have this bond here. Remember a bond is made up
of two, this covalent bond right over here is made
up of two electrons. If they wanted to show this
bond breaking and both of these electrons going to
this bromine, the convention is to go from the middle
of the bond to the bromine. That I've never found that
intuitive because here, once again, bromine already
essentially had part of the bond, it was already
on one end of the bond. I like to visualize that it's
getting the other electron that it wasn't, it's now
getting both electrons. One part of the bond was
already closer to the bromine, now it's getting the other,
it's the other part of the bond. We're going to use full
arrows for these mechanisms, just as we would typically use
full arrows, but I'll often conceptualize it as the movement
of an electron as part of a pair, as opposed to the
entire pair, but the full arrows are still used the way it
would be conventionally used. Later on when we do free
radical reactions we're going to talk about an
electron moving by itself. Notice this electron right
over here, it's moving or it's doing something and
it's not part of a pair, it's by itself so we use
the fish hook arrows. Right over here we see a
bond breaking but instead of both electrons going
to one of the atoms or another one of the atoms,
as right over here. When both electrons went to one
of the atoms we use the full arrow, this already you can say
had one and now it's gaining another one so use the full
arrow, but here the bond is breaking and each electron
is going to a different atom. Once again the electron is moving, the electron is moving by itself. Maybe I'll put this
right, moving by itself, and here is a movement of the
electron as part of a pair. Hopefully that clarifies it a little bit. It's important to keep in
mind a lot of the notation I use is a departure from
the traditional organic chemistry notation, but I
think at least in my mind it's helped me build more of an
intuition of what's going on in the mechanisms and
account for the electrons. If you're in a course, and
especially depending on how it's graded, you might want to
stick to whatever the professor uses, which is probably going
to be a little bit closer to the using the full arrow as
the whole pair, and going from the middle of the bonds, the
middle of the pairs, as opposed from one of the electrons
moving as part of the pair.