The following discussion has been contributed by Saurja DasGupta
A covalent bond is formed when electrons are shared between two atoms in the classical sense. A single bond (sigma bond) is thus made up of two electrons. Now a chemical reaction takes place when old bonds are broken and new ones are created. So how can one break a single bond—there are plainly two ways to go about breaking a bond as shown below.
Homolytic fission is where each atom of the bond keeps an electron each resulting in species called free radicals. Radicals are important intermediates in organic chemistry and we will talk about them later. As the bond breaks to give two similar species each keeping an electron this form of bond breaking is called Homolytic Fission.
In this case we can see that one of the atoms carry a negative charge after bond cleavage indicating that it has both the electrons of the bond and the other has no electrons at all. Hence it is electron deficient thus positively charged. As the electrons are not divided equally after bond cleavage this is called Heterolytic Fission. In a case the C atom carries a positive charge it is called a carbocation and in the case it carries both the electrons of the broken bond and is negatively charged, it is quite intuitively called a Carbanion. Carbocation and Carbanions are the most important carbon intermediates in organic chemistry and hence warrant further discussion.
Please note that both types of fissions are applicable to both homoatomic and heteroatomic bonds (bonds between two different atoms say C-N or C-O). Now let us discuss the three intermediates we talked about in some detail.
These are neutral intermediates, formed due to homolytic cleavage of a single bond. Some common bonds which cleave to give free radicals in organic chemistry are shown: C-O, C-Cl, C-Br, C-I, C-C, C-H. Carbon free radicals are mainly generated by:
- Photolysis (action of light) like acetone alpha cleavage
- Other radical initiator like allylic bromination by N-Bromosuccinimide (NBS)
There has been a certain degree of debate as to what the shape and geometry of a free radical is like. Revisiting the theory of hybridization, there can be two basic shapes of these radicals.
If the centre carbon atom of the radical is sp3 hybridized (remember the one which was made of one s and three orbitals as in CH4), the geometry will be tetrahedral.1 But in the case of a radical there are only three groups attached to the sp3 hybridized carbon atom so they we will have a shape of what resembles a pyramid—it’s a tetrahedron with its head cut off. So sp3 hybridized radicals are pyramidal in shape. The single electron of the radical would then be housed in a sp3 orbital. The other option is sp2 hybridization. In that case the C atom is sp2 hybridized, so as discussed previously the shape would be planar with the single electron in the unhybridized p-orbital with the three substituents having sp2 hybridized bonds.
So to summarize free radicals:
- Formed under activation by light or use of additional compounds called Radical Initiators.
- They are very reactive, because they have an unpaired electron which wants to get paired up.
- They are either pyramidal or planar with the lone electron in their sp3 or p orbitals respectively.
- Because of their high reactivity, they tend to be less selective. In simple terms it means that it sometimes difficult to predict what products are formed in reactions which involve free radicals and we actually get several products from a single reaction.
This reaction shows the formation of two products with the Br atom attached to different carbons.
Carbocations are formed from the heterolytic cleavage of a carbon-heteroatom (meaning a non carbon atom in general) bond where the other atom is more electronegative than carbon like a C-O, C-N, C-X (X can be Cl, Br, I, etc) bond. This is quite logical as after the cleavage if a carbocation is to be formed the two electrons of the bond must go to the other atom. And this is favoured if that other atom is electronegative. Carbocations can be made in difficult conditions by using so-called superacids, developed by George Olah (Nobel Prize, 1994), which helps stabilize these intermediates substantially to be analyzed. Formation of carbocations can be assisted by using cations like Ag+, with alkyl halides as substrates.
The precipitating out of the silver salt forces the equilibrium to shift towards the forwards reaction.2
The positively charged carbon atom in carbocations is sp2 hybridized, which means it’s planar as we know by now. The three substituents of the carbocation lie in a plane leaving the unhybridized empty p orbital perpendicular to them.
These intermediates react with species which are electron rich (quite obvious) and being charged are stabilized in polar solvents. (Just as Na+ is soluble and stable in polar water). Carbocations are important intermediates in most mechanisms along with carbanions as we shall see later.
To summarize carbocations:
- Formed due to heterolysis of a C-X bond (where X is more electronegative) and thus has a positive charge.
- Planar in shape (sp2 hybridized carbon), with empty p orbital perpendicular to the plane of the molecule.
- Reactive towards electron rich species.
These are intermediates also formed as a result of heterolysis, but here the electron pair from the bond is kept by the carbon atom. From what we saw earlier the more electronegative atom keeps the electrons, so in this case carbon must the more electronegative of the two atoms making up the bond. Now there are only a few atoms (non-metals; metals are not usually part of organic chemistry) which are less electronegative, so the most common bond cleavage which yields carbanions is the C-H bond. The ease of breaking this bond and creating a carbanion is also a measure of the compound’s acidity, because a H+ is also generated with the carbanion, which makes the molecule an acid in the Bronsted sense.
Carbanions have three groups attached to each other and a lone pair of electrons which gives it its negative charge (similar to the ammonia molecule where the central N has 3 Hs and a lone pair of electrons). So its geometry is pyramidal (tetrahedral but since there is no fourth group again it’s like a tetrahedral with head cut off) and the carbon atom is sp3 hybridized.
Carbanions are also stable in polar solution (electrostatic stabilization).
To summarize carbanions:
- Formed due to heterolysis of a C-X bond (where X is less electronegative) and thus has a negative charge.
- Pyramidal is shape (sp3 hybridized) with the excess electrons placed in one sp3 hybrid orbital.
- Reactive towards positively charged (electron deficient species).
Stability of intermediates
Most organic reactions take place via formation of intermediates. So the study of different intermediates would help us predict the course of the reaction and the main aspect to look at would be their stability. No organic mechanism has been conclusively ‘PROVEN’, all the mechanism we see are the most plausible ones derived from many experiments, a major component of which is isolating and studying the intermediates. It is difficult to say that a certain mechanism is absolutely correct, but it is quite simple to point out an incorrect mechanism. One of the ways a chemist would confirm an incorrect mechanism is if it involves a very unstable intermediate. The good thing about this is that with a few empirical rules and principles in mind, it is quite simple to assign relative stability of intermediates like radicals, carbocations and carbanions. And what is even better is that we have already discussed these principles.
For carbocations and free radicals (both electron poor species), any group which donates electron density to the carbon centre would stabilize it and inversely electron withdrawing groups would increase electron deficiency on the carbon centre leading to destabilization. (Remember charge is not desirable, the most stable species are usually neutral). So following the same logic the effect should just be opposite in the case of carbanions as they are electron rich (negatively charged) instead of being electron deficient like the above two. So groups which pull away electrons from the charged carbon atom would have a stabilizing effect whereas electron donation would destabilize the intermediate as it loads more negative charge on an already negatively charged atom.