Introduction
Although we have mentioned many types of chemical species in this blog, a class of species we have not broached on much is the class of charged species. For organic chemistry specifically, we will be focussing on carbocations and carbanions (Fig. 1), although much interesting reactivity exists as well for other types of chemical species.
Fig. 1: Carbocations and carbanions.
Both carbocations and carbanions involve ionic charges on the molecules. The presence of this ionic charge also indicates the lack or presence of electron density on the molecule, exposing it to attack by species such as nucleophiles and electrophiles.
The introduction, as we can see, is relatively short, because most of this article is devoted to specifically unpacking carbocations and carbanions and their differences. There is not much of an introduction to carbocations or carbanions as they are both relatively simple classes of molecules (which nevertheless have much interesting chemistry to be discussed).
So, without further ado, let us jump into looking at carbocations and carbanions.
Carbocations
There are very few classes of molecules which possess reactivity just because of electron deficiency (take note that electron deficiency and electron density deficiency are different things). Carbocations are examples of such a class of molecules. There is some confusion with carbocations because the name is interchangeably used to describe two distinctly different ions.
We discuss carbocations in terms of one carbon ions. Firstly, there is the CH3+ carbocation (Fig. 2), also known as the methenium ion. This is, relatively, much more commonly seen in organic chemistry, mostly because it is formed easily when a nucleofuge bonded to the carbon atom departs (for example, in the first step of the SN1 mechanism).
Fig. 2: Structure of a CH3+ carbocation.
The second type of carbocation is the CH5+ ion (Fig. 3), known as the methanium (or methanonium) ion. Unlike the CH3+ ion it is worth taking a look at the bonding in this ion, because it is much more unintuitive. It is easy to understand why CH3+ indeed exists as a viable ion, but why is CH5+ possible? It appears to involve a carbon forming five bonds (pentavalence), which should be impossible because carbon can only form a maximum of four bonds.
Fig. 3: Structure of a CH5+ carbocation.
The truth is that the chemical formula itself is deceptive. Instead of possessing five bonds in CH5+, the carbon in fact only has four bonds, as we would expect for typical carbon atoms. CH5+ has an example of a three-center, two-electron bond, which means that two electrons are shared between three atoms.
In the case of CH5+, the two electrons from carbon are shared between two hydrogens and the carbon atom. These hydrogens are not ‘lone’ atoms; rather, they constitute a molecule of hydrogen (H2). Thus, a better and more accurate chemical formula for CH5+ would be CH3(H2). Alternatively it is possible that the carbon alternates between either hydrogen, because this has a low energy barrier. This is known as pseudorotation.
The formation of CH3+ carbocations is also interesting, as there are several methods to do so. Obviously, the first and most direct method is by solvolysis, which results in ionization. The most commonly seen solvolysis is in the first step of the SN1 reaction, as we have mentioned earlier. Solvolysis is a reversible reaction.
A second type of ionization is also known, and this time it is by a leaving group, not the solvent. A good example of this would be the ionization of aryl diazonium salts, which are formed by reaction between primary amines and nitrous acid (HONO). Although this type of ionization is also reversible in some cases, for the above case it is not, because the resulting product is N2, which is highly stable.
Carbocations do not necessarily need to be formed by ionization; other methods of formation are also known, such as the breakage of a multiple bond between carbon and another atom. This is most commonly by addition of a proton to the atom bonded to carbon. Addition of a proton to a C=O, C=S, or C=N bond will always lead to the formation of a carbocation.
Carbocationic Reactivity
The reactivity of carbocations is interesting to discuss. First we will note the factors that make carbocations reactive, and second we will take a look at a few reactions of carbocations. For the factors, there is a large number of them. Take note that we are focussing on the CH3+ carbocations; the CH5+ carbocations are not well understood.
The factors include the type of carbocation, geometry, presence of steric hindrance, inductive effect, field effect, resonance and heteroatoms. Fortunately, most of these are easy to discuss, and each of the factors (with some of them grouped together because they are relatively similar) treated in one paragraph.
The type of carbocation refers to whether the carbocation is tertiary, secondary or primary. The tertiary carbocation is favored because of the presence of more electron-donating substituents (alkyls) to stabilize the carbocation (inductive effect). There is clear empirical evidence of this mostly from SN1 reactions; the reactants mostly involve tertiary carbons.
We will discuss the four effects together, steric hindrance, inductive effects and field effects. For steric hindrance, it is clear that the bulkier the groups near to the carbocation, the less reactive it will be, as it is less likely for reagents to approach the carbocation and attack it. Inductive and field effects are similar, and the difference between them is discussed here. They both refer to the presence of electron-donating or electron-withdrawing groups near the carbocation. Since a carbocation lacks electron density, the presence of electron-donating groups is stabilizing, and the presence of electron-withdrawing groups is destabilizing.
Resonance is treated separately from the previous three effects because it plays a far more notable role in two prominent cases. These cases involve the formation of a carbocation at the allylic or benzylic position (or, in many cases, directly on a benzene carbon). The resonance of a benzyl carbocation is more important, with resonance of the benzene ring being able to stabilize the positive charge. This plays a big part in the reactivity of benzenes.
There is also the presence of heteroatoms, which can influence the reactivity of carbocations by two effects: resonance and inductive effects. Usually, stabilizing heteroatoms (by resonance) will contain lone pairs, which create additional resonance structures with the carbocation, allowing for stabilization of the positive charge. A notable case is the acyl carbocation, which may form a triple bond. It is also possible for the heteroatom to be electron-donating or withdrawing, and thus allow for inductive effects to occur.
Finally, there is the geometry of the carbocation. This refers to the specific case of cyclopropane, which possesses bent bonding, where electron density lies outside of the cyclopropane ring. The cyclopropylmethyl cation (with the positive charge on the methyl carbon) is more stable than the benzyl cation, because these electrons can interact with the empty orbital of the carbocation and stabilize it.
Let us focus on the reactions of carbocations. Most of these reactions involve stabilization of the carbocation, of which a few ways are possible. Firstly, there is the combination of the carbocation with a negative charge (or electron pair). Alternatively, the carbocation may lose a proton or another positive ion to form a neutral (and most likely stable product. There are also other ways of stabilization, by adding to a double bond or rearranging, although these are less common.
Carbanions
The chemistry of carbanions are interesting and they are seen even more often than carbocations, because any acid-base reaction involving an acidic hydrogen on carbon (such as É‘-hydrogens) will lead to the formation of a carbanion in the conjugate base. In terms of the structure of carbanions, the negative charge stems from the presence of a lone pair of electrons on carbon. There is only one type of carbanion, RCH3-.
Speaking of acid-base reactions, it should also be prudent to touch on the Lewis definition of acids and bases. Given that a carbanion contains a lone pair, the definition of a Lewis base (electron pair donor) allows for carbanions to be considered bases. The more unstable a carbanion, the stronger it is as a Lewis base, because the more readily it will react with a proton to form a conjugate acid.
The chemistry we note above can be used to calculate the stability of carbanions. The stronger the Lewis base, the weaker the conjugate acid. The strength of the conjugate acid may thus be used to calculate how stable a carbanion is. However, this is not easy to do simply because carbanions are too unstable and reactive. The simplest carbanion, CH3-, is particularly reactive as evidenced by how weak the conjugate acid is (pKa = 56).
Despite this, however, the relative stabilities of two different carbanions can be compared if we react an ionic salt of the carbanion and a polar covalent compound of the carbanion together. The reaction should be reversible. The ionic carbanion product that is formed in the highest amounts will be the most stable one. For example, the reaction between Li+C2H3- and CH3I forming Li+CH3- and C2H3I is reversible (Fig. 8). If the equilibrium lies toward the right, the formation of CH3- is favored and thus it has to be the more stable one.
Fig. 4: Reaction between Li+C2H3- and CH3I forming Li+CH3- and C2H3I.
As for the actual results calculated by the above method, we will leave that to be discussed next. Without further ado, let us get into it.
Carbanionic Reactivity
By the above method that we have discussed, it has been worked out that the approximate order of stabilities for carbanions is phenyl > cyclopropyl > ethyl > n-propyl > isopropyl > neopentyl. It is interesting to look at the list and observe the patterns. For example, we see that primary carbanions are usually more stable than secondary and tertiary carbanions. This is because primary carbanions have less alkyl groups bonded to them. Alkyl groups are electron-donating groups, so their presence on the carbanion is destabilizing. In essence, the effect is opposite to that of carbocations.
Why phenyl is at the top is very obvious, because it possesses several resonance structures that delocalize the negative charge throughout the ring, allowing for extensive stabilization to occur. In fact, conjugation of this electron pair with a double bond, which is the most common, will also stabilize the carbanion quite well. This is the reason why allylic carbanions are also quite stable.
The cyclopropyl group is more interesting, because it seems that a cyclopropyl carbanion is a secondary carbon and thus should be quite unstable, more than that of the ethyl carbanion. However, recall that cyclopropane possesses bent bonding. The electron density in the ring (including the lone pair electron density) lies outside the ring, and this gives the cyclopropyl carbanion more stability.
Finally, there is resonance, which stabilizes carbanions just as it stabilizes carbocations. The difference is that if resonance involves a C=O or a C=N bond with the carbanion (which are relatively common), in one of the resonance forms the negative charge will be placed on the oxygen or nitrogen atom, which is desirable because these atoms are electronegative and bear the negative charge well. For example, the enolate resonance form (Fig. 9) contributes more to a resonance hybrid (suggesting that it is more stable) than the carbanion resonance form.
P.S. Sorry for there being no post on Dec 24, because of Christmas!