Carbon atoms deviate from this configuration in one way or another, we get an unstable, and therefore highly reactive species. First we learned about carbocations. These occur when a tetravalent carbon atom loses a bond and therefore makes only three bonds, for a total of six electrons in its outer shell. Contributing only three electrons to the Lewis structure instead of four, the carbon must bear a formal positive charge. Carbocations are therefore electrophilic, and seek to coordinate to some electron density, so as to fill their outermost shell and neutralize the formal charge. If this doesn’t happen, elimination is likely to occur, in order to stabilize via alkene formation, giving that carbon a fourth bond. Carbanions, on the other hand, have a full outermost shell of eight electrons, as they have four electron domains, but one of them is a lone pair.
Contributing five electrons to the Lewis structure instead of four, the carbon must bear a formal negative charge. Carbanions are therefore nucleophilic, and will seek to coordinate to some electron deficiency, so as to share the lone pair with some other atom and neutralize itself. Then we learned about carbon radicals. These have seven electrons in their outermost shell, as one of them is unpaired. Contributing four electrons to the Lewis structure, the carbon atom is of neutral charge, but because it doesn’t have a full octet, it is still a form of electron deficiency. Carbon radicals will seek to gain a single electron from something else, so as to complete its octet, usually through propagation steps that produce other, more stable radicals.
Now it is time to learn about one more unstable carbon species, and that is the carbene. Carbenes, like carbocations, have only six electrons in their outermost shell, but in the form of two covalent bonds and two additional electrons. Because four of these electrons belong to the carbon atom itself, the carbon is of neutral charge, though it does not possess a full octet. Carbenes tend to react with neutral partners to form two new bonds, in order to achieve a full octet and stabilize. Much like carbon radicals, carbenes are very reactive, and they come in two forms. There are triplet carbenes and singlet carbenes. Here we can see a triplet carbene. The carbon atom is sp2 hybridized, just like a carbocation or a carbon radical. This results in a trigonal planar geometry, with the remaining unhybridized p orbital extending in perpendicular fashion with respect to the plane of the molecule.
With the triplet carbene, one of the nonbonding electrons sits unpaired in the third sp2 orbital, and the other nonbonding electron sits unpaired in the unhybridized p orbital. With the singlet carbene, however, both of the nonbonding electrons are in the sp2 orbital, paired and with opposite spin. The difference in stability for these forms is very small, as little as one kcal per mole when highly substituted, and up to eight kcal per mole for simpler structures, and either form may be the more stable form, depending on the specific carbene in question. So now that we know what carbenes are, how do we make them? Carbenes were first reported in 1903 by German chemist Eduard Buchner, who perhaps surprisingly is not the guy who invented the Buchner funnel.
He observed that the decomposition of some diazo compounds in the presence of alkenes produced cyclopropanes, and correctly identified the intervening intermediate as a carbene. This is a reaction that we will elucidate in the next tutorial. Formation of carbenes from diazo compounds remains a common technique to this day. Take for example diazomethane, an explosive toxic yellow gas with a boiling point of -24 degrees Celsius, shown here in its two resonance structures. If we focus on the structure on the right, we can see that this readily allows for the release of molecular nitrogen, leaving behind a methylene carbene, and this decomposition will proceed upon heating, driven by the stability of diatomic nitrogen. Given that these electrons are already paired, this reaction will produce a singlet carbene.
We can also form halogenated carbenes, where the carbon bearing the nonbonding electrons is bound to halogens instead of carbons or hydrogens. For example, observe the following reaction of chloroform. If chloroform is deprotonated with strong base, such as hydroxide, it produces the trichloromethanide anion. To neutralize the formal charge, one of the chlorines can leave in the form of a chloride ion, taking both of the electrons in the carbon-chlorine bond with it. This leaves a halogenated carbene, which again, will be in the singlet form. As we may begin to gather, most reactions that produce carbenes do produce singlet carbenes.
It is a bit tricky to generate triplet carbenes, but it is possible that the singlet form can produce the triplet form through photochemistry, meaning interaction with light of a particular wavelength. This is a large topic unto itself, so for now we will just understand that carbene formation usually generates singlet carbenes, which is preferable anyway, since singlet carbenes are more useful in organic synthesis, as we will see in a moment. So let’s move forward and see exactly what we can do with these things.