Adding Up: Addition

Abstract

Abstract Adding up, or more formally ‘addition’, is just what it says: it is the attachment of atoms to a sensitive spot on a molecule. I need to stand back for a moment and explain what I mean by a ‘sensitive spot’. You already know (from Reaction 13) that some atoms are held to each other by an electron cloud (a ‘single bond’) and others by a doubly dense cloud (a ‘double bond’). There is a third type in which the atoms are held together by an even denser, triply dense cloud of electrons, forming a ‘triple bond’. Here I am concerned with the latter two types of bond, the so-called ‘multiple bonds’. These are the sensitive spots of organic molecules for it is quite easy to attack a multiple bond, rearrange the clouds, and attach other groups. For simplicity, I shall deal only with the more common type of multiply bonded molecule, one with a double bond. A double bond is a region rich in electrons, so you should suspect that any missile that will attack it will be an electrophile (a seeker-out of negative charge, of electron richness, Reaction 16). I shall consider a very simple case: the addition of bromine to cyclohexene, 1. As we have seen in Reaction 15, the presence of a bromine atom, Br, in a molecule is often the starting point for building on other groups of atoms, so this is an important reaction in a chain that might be used to construct something useful, such as a pharmaceutical. Bromine is a liquid composed of Br2 molecules. Cyclohexene is a liquid composed of hexagonal benzene-like molecules but with only one double bond in each molecule. Why I have chosen this slightly elaborate molecule rather than something simpler, such as ethylene (ethene, Reaction 13), will soon become clear. Let’s shrink together down to our normal molecular size and watch what happens as the bromine is poured into the cyclohexene. We already know from Reaction 16 that a bromine molecule has a nose for negative charge, so you should not be surprised to see one homing in on the electron-rich double bond of a nearby cyclohexene molecule.