Reactive Intermediates
Ions
An ion is an atom or molecule that has a net charge- either positive or negative. An anion is a negatively charged ion. Some people remember this by using the following mnemonic.
Anion = A Negative ION
A cation is a positively charged ion. Some people remember this because the “t” in the middle of the word “cation” looks like a positive sign.
Cation = ca+ion
Reactive Intermediates
In an organic reaction, sometimes a species is made that is short-lived and reactive. They usually have an unusual number of bonds. These species are created on the way to making some product. They are intermediates in the reaction. We call these species reactive intermediates. There are four main types of reactive intermediates.
Reactive Intermediates
Carbocations
A carbocation is a carbon atom that is a cation (a positive ion). The simplest example of a carbocation is methyl carbocation.
Methyl carbocation
A methyl carbocation only has hydrogen atoms attached to the carbocation. A primary carbocation has one carbon atom attached to the carbocation. A secondary carbocation has two carbon atoms attached to the carbocation. A tertiary carbocation has three carbon atoms attached to the carbocation.
We use the terms primary, secondary, and tertiary often in organic chemistry to describe the generic forms of molecules. For example, below is how we name generic amines and generic alcohols. R = the Rest of the molecule. In this case, it means at least a carbon atom is there.
Primary vs. secondary vs. tertiary
Carbocation stability
A carbocation has three sigma bonds and an empty p-orbital on the carbon atom. The empty p-orbital has no negative electrons in it. This is what gives it its positive charge. Nature abhors a localized charge like this. So, in order to try to fix this positively charged problem, the positively charged, empty p-orbital, wants to have more negative electrons donated to it. If it could only get some negative electrons given to it, it would be relieved. It is like a dry mouth that wants to be quenched with water. Water needs to flow towards the dry mouth to help, not away from it.
Through Hyperconjugation
So, how can water flow towards the dry mouth? How can negative electrons flow towards the empty p-orbital to help stabilize it? One very common way is through a simple process that uses a ten-dollar word, hyperconjugation. Don’t let the big word discourage you. Hyperconjugation is simple to understand. To begin to understand it, let’s look at primary carbocations.
There are two electrons in each C-H bond. This bond overlaps with the empty p-orbital. Both the C-H bond and the empty p-orbital are in the plane of the paper. When they overlap, the electrons in the bond can donate “through space” to the empty p-orbital. This helps quench the thirst of the p-orbital. It helps stabilize the carbocation.
If we twist it to another angle, you can maybe better see that the C-H bond and the empty p-orbital are both pointing vertically and can overlap.
Because of the way we draw molecules on paper, it may seem strange that these negative electrons in the C-H bond are pushing or moving through such a long distance to help stabilize the positive charge. But, remember that bonds are not really thin lines like we draw. The C-H bond is an sp3 hybridized carbon orbital overlapping with a hydrogen 1s orbital.
In fact, both the C-H bond and the empty p-orbital are so fat they actually overlap and touch each other. The electrons in the C-H bond can “leak” into the positive p-orbital of the carbocation to help stabilize it. They don’t have to jump or travel very far at all.
Now, let’s look at the hyperconjugation that can occur in all four types of carbocations we have seen.
Hyperconjugation stability of carbocations
Notice that with no sp3 C-H bond neighbors the methyl carbocation cannot hyperconjugate at all. Its thirst is not quenched. The positive charge is all alone and not stabilized in any way. This is why methyl carbocations are very rare. Primary carbocations have one attached carbon group that can help the positive charge. Its thirst is quenched a little, but it is still quite thirsty. There is a little stabilization of the positive charge. Secondary carbocations have two carbon groups that can help. Its thirst is beginning to be satisfied. Tertiary carbocations have three carbon groups that can help through hyperconjugation. This positive charge is the most stabilized with three sets of negative electron bonds donating into the positive p-orbital.
You will find the order of stability of carbocations to be very useful when we predict reactions in the future. But, I warn you to not simply memorize this list. Think deeply about hyperconjugation until it simply makes sense that you want more carbon groups attached to the positively charged, empty p-orbital. If you can truly understand this concept, you will never again forget that tertiary carbocations are the most stable.
Through Resonance
Another way to stabilize carbocations is through resonance. If there is a pair of negative electrons in a pi bond one full step away from the carbocation, those electrons can be pushed towards the carbocation through resonance. This helps by spreading out the positive charge.
The secondary carbocation above is stabilized by two carbon groups through hyperconjugation. It is further stabilized through resonance. In the end, it has similar stability to a tertiary carbocation.
Another way resonance can help stabilize a carbocation is if a lone pair of negative electrons is nearby.
In this case, the lone pair of electrons can be pushed towards the carbocation to help stabilize it. Notice, this only works if the lone pair of electrons is on an atom one full step away from the carbocation. Below are several examples of heteroatoms that can help stabilize carbocations.
Be careful though, the lone pairs of electrons are not always drawn, so we need to be alert to when we draw them in ourselves to make resonance forms. You will often see them drawn without the lone pairs of electrons.
13. Draw in the appropriate number of lone pairs of electrons on each heteroatom. Then, draw an arrow to push electrons on each molecule to show how the carbocation is stabilized through resonance.
Through Induction
One final consideration for the stability of a carbocation is to look at inductive effects or induction. The inductive effect is an effect that takes place through the sigma bonds or the sigma bond framework. Notice that in hyperconjugation and resonance, the stability took place above or below the sigma bonds. Hyperconjugation can happen above or below the sigma bond and resonance can happen above or below the sigma bond. That depends on the orientation we happened to draw the molecules.
But, the inductive effect does not take place above or below the sigma bonds, but right through them! Usually, there is some atom or group attached that in some way affects the electron density. This group either sucks electrons towards itself (if it is electronegative or has a positive charge) or it donates electrons away from itself (if it is not very electronegative or has a negative charge). Below are four carbocations with a group (G) that either donates electron density away or attracts electron density towards itself.
Induction stability of carbocations
Below are two similar carbocations. The only difference between them is one atom, a different halogen.
One of these carbocations is more stable, and one is less stable. Let’s reason through this together.
14. If negative electrons are pushed towards the positive p-orbital of a carbocation, does this help or hurt the carbocation? ____________
15. If negative electrons are sucked away from the positive p-orbital of a carbocation, does this help or hurt the carbocation? _____________
You should be able to do the next problem in your head. If not, review the periodic table and periodic trends.
16. The strength of an atom to pull negative electrons towards itself in a bond is called electronegativity. Between F and Br, which is more electronegative? _________
17. Since F and Br are both fairly electronegative, do they help or hurt the carbocation they are on? _____________
18. Which hurts the carbocation more, F or Br? __________
19. Circle the structure below that is the most stable (or is hurt the least).
Sometimes the group that affects the electron density is close to the carbocation p-orbital, and sometimes it is farther away. Of course, when the group is close, it has a larger effect than if it is far away.
In the following two structures, having the F atom attached hurts one carbocation worse than the other.
20. Draw a circle around the molecule (above) that is hurt worse by the electronegative fluorine atom.
21. Now, think about which is the happiest and draw a square around the most stable carbocation of the two molecules above.
Free Radicals
Free radicals are much like carbocations. They are sp2 hybridized but the p-orbital has one electron in it. Free radicals are not as positively charged as carbocations, but they are slightly electron poor. Much like carbocations, free radicals want more electrons!
Stability of Free Radicals
Free radicals are stabilized the same way as carbocations. Tertiary free radicals are the most stable because alkyl groups would donate electrons towards the electron poor free radical carbon through hyperconjugation.
Order of stability of free radicals
Resonance forms can help stabilize free radicals if the lone electron can be shared over more than one atom.
Resonance stability of free radicals
Electronegative atoms attached to the free radical would hurt the electron deficient free radical by trying to pull the negative electrons away from it. Free radicals prefer groups, like alkyl groups, that donate electrons towards them.
Carbanions
A carbanion is an anionic carbon atom. It is a carbon atom with three bonds and a lone-pair of electrons. Carbanions are sp3 hybridized with the lone pair of electrons in one of the hybrid orbitals. Carbanions are nucleophilic because of their negative charge.
Carbanion examples
Carbanion stability
Nature abhors a localized charge like this. So, in order to try to fix this negatively charged problem, the negatively charged carbon wants to push the negative electrons away from it. If negative electrons try to come towards the carbanion, the carbanion is destabilized.
How about hyperconjucation?
What do alkyl groups do on a carbanion? Do they help or hurt the negative charge? We know that alkyl groups (R groups) donate electrons through hyperconjugation. If they donate electrons in towards the lone pair of electrons on our carbanion, this hurts the stability of the carbanion. This makes the negative charge even more negative! So, alkyl groups hurt carbanions and make them less stable. Therefore, the stability of carbanions is:
Carbanion stability
How about resonance?
Resonance can help spread out the negative charge. If the lone pair of electrons can be pushed away using resonance, it helps. Often, if the carbanion is next to a carbonyl, a nitrile, or a nitro group, it is stabilized through resonance.
Resonance stability of carbanions
How about induction?
Induction is through the sigma bond framework. Let’s look at a generic group, G, on a carbanion.
Induction stability of carbanions
So, what type of group do we want G to be to help stabilize the negative carbanion? Electronegative atoms help. Fluorine would be an excellent group to have on a carbanion.
22. Rank the following carbanion from most stable (1) to least stable (3).
23. Rank the following carbanions from the most stable (1) to the least stable (4).
Carbenes
Carbenes have a formal charge of zero. They are very reactive species. Carbenes are carbon atoms with two bonds and a lone pair of electrons. Carbenes are sp2 carbon atoms with a lone pair of electrons in an sp2 hybrid orbital and an empty p-orbital. Carbenes are both electrophilic and nucleophilic. They are electrophilic because of the empty p-orbital and nucleophilic because of the lone pair of electrons.
Carbenes
We will learn more about carbene chemistry when we study alkenes, but in general, they turn alkenes into cyclopropanes.
24. Carbenes are:
a) electrophilic
b) nucleophilic
c) both electrophilic and nucleophilic
Answers
13.
14. If negative electrons are pushed towards the positive p-orbital of a carbocation, does this help or hurt the carbocation? Helps
15. If negative electrons are sucked away from the positive p-orbital of a carbocation, does this help or hurt the carbocation? Hurts
16. The strength of an atom to pull negative electrons towards itself in a bond is called electronegativity. Between F and Br, which is more electronegative? F
17. Since F and Br are both fairly electronegative, do they help or hurt the carbocation they are on? Hurt
18. Which hurts the carbocation more, F or Br? F
19. Circle the structure below that is the most stable (or is hurt the least).
20. Draw a circle around the molecule that is hurt worse by the electronegative fluorine atom.
Distance matters. The fluorine being closer to the carbocations hurts worse than if it is farther away.
21. Now, think about which is the happiest, and draw a square around the most stable carbocation of the two.
22.
Electronegative chlorine atoms help carbanions by pulling electrons away through induction. The methyl group hurts the carbanion because it donates electron density towards the carbanion through hyperconjugation.
23.
The closer the electronegative chlorine atoms are to the carbanion, and the more electronegative chlorine atoms there are, it helps the carbanion.
24. c) both electrophilic and nucleophilic