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Introduction
Addition is one of the most important transformations in organic chemistry; we could argue that all of them are important, but addition reactions are one of the more well-researched classes of reactions in all of organic chemistry. In this introduction, we will give a brief explanation of what addition reactions are, and how they form addition compounds. It is often useful to first understand the definition of the addition reaction. The IUPAC Gold Book, one of the most important sources of information for nomenclature, gives the following definition: “A chemical reaction of two or more reacting molecular entities, resulting in a single reaction product containing all atoms of all components, with formation of two chemical bonds and a net reduction in bond multiplicity in at least one of the reactants.” What this basically means is that the addition reaction involves two reactants combining two form a new, single product, that differs from the reactants chemically.
In particular, we would like to focus on two ideas of addition reactions: firstly, there is a net reduction in bond multiplicity, which means that a multiple bond is involved in the reaction, and that the reaction has to ‘turn’ a triple bond into a double bond, or a double bond into a single bond. The second key thing to note is that two chemical bonds are formed. When a multiple bond is broken, the two atoms forming the bond both ‘need another sigma (single) bond’ to be formed to them, in order to ensure that they have a noble gas electronic configuration. This is why two bonds are formed. The definition given of addition reactions means that reactions like hydrogenation or nucleophilic and electrophilic addition can be considered under a single category, addition reactions.
Now that we know the basics of addition reactions, we can begin to consider addition compounds (which are basically products of addition reactions). Of course, we will not be stopping to consider the ‘easiest’ of the addition compounds; such compounds are wide-ranging and are not ‘chemically interesting’ anyways.
EDA Complexes
Addition compounds can generally be split into four types; the first type is of electron donor-acceptor complexes (or also commonly termed charge-transfer complexes). In such cases, the reactants of the addition reaction are known as donors and acceptors. The donor donates a lone pair of electrons to the acceptor. In these cases, we could either regard the complex as being composed of an electron donor and acceptor that form a noncovalent bond, or it could be that no bond is formed at all, but rather electrostatic attractions are involved between the donor and the acceptor, holding the molecules together. In the former case, it would be a tad similar to ionic bonds, but it is very unlikely that the transfer of the lone pair would be ‘complete’; in cases where the transfer is complete, the EDA complex would be relatively stable, but in the majority of cases, the transfer will be incomplete, making the EDA complex so unstable that even the presence of a polar solvent can destroy the donor-acceptor interaction.
There are several different types of EDA complexes; we discuss two of them. In the first type, the acceptor is a metal ion while the donor can be an alkene, or an aromatic ring. A common case of the above is ferrocene (Fig. 1), which involves an iron atom as the acceptor and two cyclopentadienyls as donors, although this is debatable. It may sound familiar to some, because it is also an example of a coordination complex. In fact, many coordination complexes can generally be considered EDA complexes, as they involve an acceptor and a donor. In coordination chemistry jargon, the donor molecules would be known as ligands. Note that there is some grey area between these two types of complexes, because the ligand (donor) does not always have to be an alkene or aromatic ring in coordination complexes, but for EDA complexes, the donor must always be an alkene or an aromatic ring. In ferrocene, there is pentahapto coordination; this means that all five of the atoms in cyclopentadienyl are donor atoms.
Fig. 1: Structure of ferrocene.
The second type of EDA complex has an organic molecule (meaning a majority of carbons is involved) as the acceptor. Many organic molecules can act as acceptors, such as picric acid and 1,3,5-trinitrobenzene. Other types of organic molecules may also act as acceptors, but particularly molecules with electron-withdrawing groups, the simplest of which involve halides. Some of these organic acceptors, interestingly, are observed to participate in EDA complexes despite lacking empty orbitals; yet, the bonding observed cannot simply be attributed to electrostatic effects such as induced dipole attractions, since these attractions are too weak to explain relatively stronger bonding that is seen. We will list an example of a EDA complex formed by an organic molecule as an acceptor; the acceptor in this case is picric acid (Fig. 2), and the complexes it forms are known as picrates (they should not be confused with the salts of picric acids).
Fig. 2: Structure of picric acid.
Conclusion
In this article, we discussed an introduction to addition compounds, as well as EDA complexes. There were two types of EDA complexes we explained: complexes with metal ion acceptors and ligand donors, as well as complexes with organic molecules as acceptors. In the next articles, we will hopefully be discussing more types of addition compounds, such as cyclodextrins, inclusion compounds, crown ethers and cryptands.
Part 2 of this article is here.