Hey guys, professor Dave here. today I wanna talk to you about nucleophilic substitution
specifically the SN2 reaction
so for any substitution reaction one atom or group
is being replaced by another atom or group. so let's take a look at this
generalized SN2 here
and so that's exactly what we're seeing, we've got a hydroxide
ion replacing a chloro group on this molecule
for any SN2 reaction we want to identify three
key players. so the first is the nucleophile. the nucleophile is something that has
excess electron density so this could be
a formally negatively charged atom or
a polyatomic ion or it could be something with a lone pair
basically anything with excess electron density like hydroxide and that's got
a formal negative charge
then there's always going to be on electrophile, so this is just the opposite
this is something with electron deficiency, so here
this carbon atom is acting as the electrophile because of the polarity of
this bond
so we all know that
chlorine is more electronegative than carbon therefore this is a polar
covalent bond
meaning the electrons in that bond are being hogged a bit more by chlorine
leaving a partial positive charge on that carbon and so a formal
negative charge
is going to be attracted to a partial positive charge
and then lastly every SN2 is gonna have a leaving group
because the only way that this carbon can accept a bond
from an incoming nucleophile is if it then loses a bond
to some other atom or group, because carbon can only have four bonds
and so the chlorine is the leaving group here
and so when the hydroxide attacks, the chlorine
will leave and at the end we have methanol
and a chloride ion
now that we know what's going on in an SN2 reaction let's take a closer look at
the mechanism
so for one thing whenever we're drawing mechanisms for organic reactions we have
to be very careful with our
electron pushing arrows, these are arrows that depict the direction of electron flow
and they have to be drawn in a very specific way so
we're always gonna go from electron-rich to electron-poor
so this first arrow goes from the oxygen atom because that's the oxygen
rich atom that's the one with the negative charge so
the arrow starts there and it's going to go directly to
the carbon atom because that is the electron deficient site, that's
where there's the partial positive charge. this arrow
is gonna originate in the center of this bond
meaning that the two electrons in that covalent bond
are going to leave with the chlorine atom leaving the
chloride ion so the electron arows must be drawn in a very specific way
so watch that on your exams that's a that's an easy place to lose a lot of points
so let's take a look at some other aspects of the SN2 reaction
so this is a this is an sp3 hybridized
carbon with tetrahedral geometry. there's four electron domains
so these are 109.5 degree bond angles
now for the SN2 reaction there has to be
a backside attack for the nucleophile meaning for any reaction
the highest occupied molecular orbital
of what is doing the attacking is going to dump electrons into the lowest
unoccupied molecular orbital of whatever is acting as
the electrophile so here on this substrate the lowest
unoccupied molecular orbital is 180 degrees
from this carbon-chlorine bond so here's the leaving group, that means that
the nucleophile is going to have to attack
precisely 180 degrees from there so it is always gonna go this way
and then for any for any step in any reaction we can always look at something
called the transition state
so that's this, now as this approaches
so basically this is all happening in one step, the key with the SN2 reaction
is that it is a one-step reaction
for nucleophilic substitution so
it's a legitimate question to wonder why it's called SN2 even though it's one step
so the 2 is not referring to the number of steps, it refers to the fact that it is a
bimolecular transition state meaning that the substrate is coordinated
loosely both to the nucleophile and the leaving group simultaneously in the
transition state
so basically we can see a partial covalent bond
beginning to form between the oxygen and the carbon and a partial bond
here between the carbon and the chlorine as the chlorine begins to leave
so you can think of it almost as like a half bond each
in the transition state. now the interesting thing happening in terms of
geometry
is that in SN2 the substrate is gonna
invert like an umbrella so we can see here that we have one
orientation but then as this electron density from the nucleophile approaches
this is gonna start to invert like that
until we have this a trigonal bipyramidal-like
transition state here because of the five electron domains
and then as the bond materializes between the oxygen and carbon
the chlorine leaves, we return to sp3
tetrahedral geometry but we do have an inversion
at that center so if SN2 occurs
at a chiral center it will necessarily
invert the stereochemistry there so if it was R
the product will be S or vice versa
thanks for watching, guys. subscribe to my channel for more tutorials
and as always feel free to email me with questions