Leah here from leah4sci.com and in this video we're going to look at Lewis Structures specifically
for Radicals.
As a reminder, most of Organic Chemistry is going to feature an electron pair.
This means the bond between two atoms is a pair of electrons or a lone pair of electrons
is another pair for electrons.
Radicals which show up less frequently refer to a lone electron.
That means a single electron without a pair, without a partner making it unstable and very
reactive.
When it comes to Lewis Structure for Radicals we're going to use the same approach that
i teach for regular lewis structures which you can find linked below or see video 5 in
my Orgo Basics series at leah4sci.com/orgobasics.
As a reminder, the checklist includes atoms, octet, electrons and formal charge.
Let's start with an easier example, the superoxide ion or O2-.
We'll keep an eye on the checklist starting with atoms.
In this case because we have two oxygen atoms and nothing else, we don't start with a more
or a less electronegative atoms, we just have two oxygens so we place them next to each
other.
Atoms check!
The next step, even though we know we're not going to keep all the electrons is still to
fill the octet, again this is different from what you typically learn but it's a full proof
way to make sure that everything is accounted for and that you don't miss steps.
To complete the octet, we'll add three lone pairs to each oxygen atom so that with the
bond between them they each have 8 electrons.
Octet is complete.
Electrons is where we'll find the difference between a radical and a non-radical and figure
out what the heck is going on.
We start with how many electrons we should have on this radical ion.
Each oxygen has 6 electrons, times two gives us twelve plus one more electron for that
negative charge for a total of thirteen.
This right here, this is your clue that we're dealing with a radical because electron pairs
are pairs.
They're even and when you're dealing with an odd number you have a lone electron that
is not bound to anything that is not paired with anything.
We should have thirteen electrons, how many do we have on the current molecule?
2, 4, 6, 8, 10, 12, 14.
We have 14 electrons, we should have 13, that means all we have to do is knock off one and
we're good to go.
Because we're dealing with 2 oxygen atoms it doesn't matter which one I choose so randomly
I'll knock off this electron, that means we subtract one from the fourteen that we had
initially giving me a total of 13, I should have 13 I'm good to go but I'll only confirm
it only after completing the checklist which means I need to check formal charge.
We'll use the formal charge shortcut that I also use in the Orgo Basic Series linked
below, should, minus, has.
Oxygen should have 6 directly attached to this oxygen, we have 2,4,6,7.
Six minus seven equals negative one and this gives me the negative charge for this ion
and then we'll look at the oxygen on the right.
Oxygen on the right should have six, directly attached I have 1,2,3,4,5,6.
Remember this one is no longer here because we crossed it out.
And if I should have six and directly attached, I have six, that means the formal charge is
zero.
Let's redraw it because it looks a bit of a mess right now.
We have oxygen single bound to oxygen, the left oxygen has 3 lone pairs and a formal
charge of negative one.
The right oxygen has two lone pairs and one unpaired radical electron with no formal charge.
For the next example,we'll look at Nitrogen Monoxide or NO which is somewhat tricky but
comes up often enough to recognize.
The reason NO is tricky is that you're given 2 simple atoms, Nitrogen and Oxygen.
There's no charge, nothing looks off about it so you'll expect to have a regular drawing.
Let's use the checklist and start with atoms.
We have just two atoms so we place them next to each other with a single bond between them.
Atoms checked!
Next we'll look at Octet.
Since they have one bond between them, we'll add 3 lone pairs to each atom including the
bond, they each have a total of 8 electrons.
But how many electrons should this molecule have?
We have 5 valence electrons for Nitrogen and 6 valence electrons for Oxygen for a total
of 11.
Once again we have an odd number that means there's going to be one unpaired electron
giving us a radical.
Where is that radical?
let's find out.
Our current version of this molecule has a total of 14 electrons.
According to our calculation we need only 11.
That means we have to get rid of 3 electrons.
In the Lewis structure video we learned how to reduce the electron count by two.
Anytime you have two neighboring atoms that each have a lone pair.
By cancelling the lone pair on nitrogen and cancelling a lone pair in oxygen, each atom
lost 2 electrons.
But placing a bond between them gives them back to electrons while only adding in two
back into the system when we initially cut out 4.
By removing 2 electrons we bring the countdown to 12.
But we need to reduce it to one more to get 11.
Do we remove it from the oxygen or the nitrogen?
This isn't a very obvious answer, instead we jump ahead to formal charge and use that
to help us figure out what makes sense keeping in mind that charges are burden and if possible
the atoms do not want to have a charge.
I redrew the molecule twice so that we can try both options.
On your exam you can try one option in your head or make a quick mark, cross out and calculate
formal charge so that you don't have to redraw it.
We'll try one version where we cross out electron on nitrogen and another where we cross it
out on oxygen and determine which one looks happier.
We'll start with`the formal charge for Nitrogen where Nitrogen should have 5 and directly
attached we have 1,2,3,4,5.
Looks good, 5 minus 5 is zero.
Oxygen should have 6, directly attached we have 6 which gives us zero.
Hint, hint, this one looks good.
Let's try the structure on the ring.
Nitrogen which should have 5 has 6 directly attached.
5 minus 6 is negative 1.
Oxygen which should have 6 has 1,2,3,4,5 directly attached giving me a charge of plus 1.
The structure on the right has a separation of charge with negative one and positive one
that's not happy, that's not stable that is very likely not going to happen.
But the structure on the left has a formal charge of zero on both oxygen and nitrogen
making it as happy as it can be under the circumstances and therefore the correct answer.
Let's redraw this quickly so we see exactly what's going on..
We have Nitrogen double bound to oxygen where nitrogen has one radical and one lone electron
pair and oxygen has 2 electron pairs.
We have no formal charge on the molecule.
We want to verify that we have the correct number of electrons that's 2,4,6,8,10,11 and
going back to our checklist, electrons are accounted for, formal charges of zero are
accounted for, and we are good to go.
To review Lewis Structure and formal charge, watch the Orgo Basics series linked below
or visit my website leah4sci.com/orgobasics.
For more on radicals click the link below or visit leah4sci.com/radicals.