Hey everyone, professor Dave here. I wanna talk to you a little bit about
resonance structures and how to assign formal charge.
so a very important concept in organic chemistry
is resonance, and in order to understand resonance we're going to have to understand
pi electrons because pi electrons are the ones that participate in resonance
so a pi bond basically, if we remember
a double or a triple carbon-carbon bond consists
of one sigma bond and either one or two
pi bonds, so here is a carbon-carbon double bond
one of those bonds is a sigma bond and the other is a pi bond so let's take a
look at some
orbitals and see what's going on. so let's take this, a flat
alkene remembering that carbons that are participating in a double bond
are sp2 hybridized, they have hybridized their
atomic orbitals into sp2
molecular orbitals because they have 3
electron domains each, the other carbon and then
whatever else they're bound to, so let's take this
flat molecule and we're gonna flip it
like this and view it edge on
so these hydrogens are away from us, further
past the board, these hydrogens are towards us precisely in a plane
perpendicular to the
board and then what we're seeing is that
the carbon is bound to these other three atoms, the carbon and the two
hydrogen atoms
by its sp2 molecular orbitals
but that leaves an unhybridized
p orbital extending in perpendicular fashion
up and down in the opposite directions
and it is the overlap of these
p orbitals that is generating the pi bond
this carbon-carbon bond is the
sigma bond and then this is the pi bond
now the reason this is important is because it is the
electrons in pi bonds as well as electrons that are present as lone pairs
that are the ones that are going to participate in resonance.
so most resonance structures are going to contain atoms that bear
a formal charge, so before we investigate these structures I want to review really quickly
how we can tell whether an atom will bear a formal charge
so take a look at some of these polyatomic ions
for example in this structure why is it that that oxygen atom
has a formal negative charge localized on that oxygen atom?
well in order to tell if an atom's gonna have a formal charge we can
look in the periodic table to see how many valence electrons
that atom ought to have. now oxygen,
if we look at the table is in the sixth column so it should have six
valence electrons but here if we were to go ahead and draw in
all the lone pairs we could see that this oxygen atom
is actually showing 7 electrons in this
lewis dot structure because we have
3 lone pairs so that is six electrons and then
one of the electrons in this covalent bond belongs to that oxygen
the other belongs to this carbon so this oxygen is showing
7 electrons in this lewis dot structure that's one more than its typical valence of 6
one additional negatively charged particle
means that this oxygen has a formal negative charge
now for example let's look at the nitrogen atom in the nitrate ion
now nitrogen ought to have 5
valence electrons but here it is contributing
only four to this lewis dot structure because it is participating in
four covalent bonds, one here, one here
and then two, both the sigma and a pi bond in that double bond
so 1 per bond, 4 electrons
that this nitrogen is contributing to this lewis dot structure
4 is one less than 5
one fewer negatively charged particles means that this
nitrogen bears a formal positive charge
so that is the simplest way to tell if an atom has a formal charge
a lot of books and teachers will give you this very strange
way of using arithmetic and adding and subtracting all these things but
in my opinion this is the simplest way, just compare
the number of electrons that an atom is contributing to a lewis dot structure
to its typical valence and if they're not the same
that atom must bear a formal charge
so now that we're able to understand that lets take a look at some of these
resonance structures
now it is the case that pi electrons
are delocalizable within a structure
that means that if pi electrons can be rearranged to form some other stable
resonance structure
they will do so. so once again sigma
electrons cannot do this because they
are electrons that are involved in direct orbital overlap
so those are not delocalizable, but the pi electrons, the ones that are
the lateral overlap of p orbitals or
lone pairs, those are free to move around
so for example we've got a negative charge here which is
essentially just a lone pair so let's say a lone pair
goes ahead and forms a double bond there
the only way that this carbon can accept
another electron from some other atom is if it then
loses an electron, in other words it must lose
a bond. it can accept a bond from this electron
sorry, from this oxygen if it then loses a bond
and those electrons go somewhere else. well this oxygen
is just as capable of accommodating a negative charge
as this one is so that means that this is an equally valid resonance structure
as this one is, so we can have the negative charge on this oxygen
or go ahead and form the carbonyl there and bring the negative charge
up to that oxygen, so these are two
equally valid resonance structures of the same molecule
now the convention is to list them in brackets with this double-headed arrow
between them, that's just how we do it and then also
with regards to these electron pushing arrows those are something we're gonna
see a lot more
later with some of the mechanism stuff and these are very important to be drawn
in a very particular way
this must go from electron-rich to electron-poor, in other words from
the lone pair
to the bond itself. so we will get
much deeper into those arrows later. the important thing that we want to
understand about resonance structures
is that individual resonance structures
do not exist. we draw them that way
because they help us understand chemistry and how molecules behave
but in reality the only thing that exists
is a composite resonance structure that represents
the distribution of pi electron density around the molecule
so that would be this. so the reality of the fact is that there is
partial pi electron density distributed
all along that portion of the molecule. and that's what truly exists in nature
and there's a lot of ways that we can prove this from molecular shape
to just electron distribution, a lot of different things but
this is absolutely true that it is not the case
that these electrons are flipping back and forth really fast between the two
oxygen atoms
in fact the reality is this composite structure
that bears an overall delocalized
negative charge. so taking a look at the nitrate ion just as above
we can push these pi electrons to form a pi bond here
as long as we push the pi bond up to this other oxygen atom
that would give us this structure and then likewise the double bond
can also exist connecting to that third oxygen so
just as above these are three equivalent
resonance structures but once again it is important to understand that none of these
discrete resonance structures exist, it is actually only this composite resonance structure
that shows delocalized pi electron density
distributed about all the portions of the molecule
that have the delocalized electron density
in this case the whole thing so we have an overall
formal negative charge that is delocalized about the whole molecule
now in the prior examples we were looking at resonance structures that
were precisely
equally contributing but there could be other examples in which we have valid
resonance structures
that are not
equally contributing so let's take a look at why that might be
for here let's see that we have a formal positive charge on this carbon
these pi electrons are delocalizable so
we could lose this bond to oxygen and neutralize that carbon
if this pi bond moves down here oxygen will lose an electron
carbon will gain an electron therefore carbon will neutralize, but now oxygen
will have a formal positive charge
now this is a case where we have the formal charge on two different kinds
of atoms
depending on which resonance structure we're looking at. because this is a
positive charge
the atom of the two that is more electropositive
will have an easier time accommodating that formal
positive charge so we know about electronegativity and we know that
oxygen, because of its position in the periodic table is more electronegative than
carbon, therefore carbon is more electropositive. because carbon is more
electropositive it can better accommodate the positive charge
and therefore this resonance structure is more strongly contributing
so they are both valid resonance structures
however the composite resonance structure would
show a skewing of the partial pi electron density
closer to the carbon atom or actually
in this case more toward the oxygen because the carbon
can better accommodate the positive charge. now let's look at precisely the
opposite scenario, over here we have a formal
negative charge on that carbon so this is an enolate anion that we will see
quite a bit later and we can see that that can go ahead
and form a carbon-carbon double bond, kicking
this pi bond up onto that oxygen atom giving the other resonance structure
with the oxyanion and this is precisely the opposite case
we have two resonance structures each with a formal negative charge
but they are on two different kinds of atoms
so here because it's a formal negative charge it's the more
electronegative atom that is going to be able to accommodate the negative charge best
in this case it's the oxygen so this is the more
strongly contributing resonance structure, this is still a valid resonance structure
but the distribution of the electron density on the composite resonance structure is
going to more closely resemble
this resonance structure. and then lastly
if we look at an example like this where let's say this pi bond
goes and leaves that nitrogen and just
plops a negative charge right there on that carbon this is a valid
resonance structure
however resonance structures with all neutral atoms are necessarily more contributing
than other valid resonance structures that have formal positive or negative charges
so that's why this is going to be a much more strongly contributing resonance structure
overall. and lastly any resonance structure
must be a valid lewis dot structure so we can't have a resonance structure where
carbon has five bonds
or some other thing that violates the rules of Lewis dot structures.
so hopefully this clarifies a little bit about
resonance and how to draw proper resonance structures
thanks for watching, guys. subscribe to my channel for more tutorials
and as always, feel free to email me with questions