General Chemistry Review
By James Ashenhurst
From Gen Chem to Org Chem, Pt. 9 – Acids and Bases
Last updated: March 21st, 2019
First of all, back to the shepherds:
Oxygius, having six sheep to take care of, found he could make a bargain with two of the Hydrus shepherd boys, whom each brought a sheep of their own. In so doing, Oxygius fulfilled his commitments under the law of Emperor Octavius, and each of the Hydrus shared the tending of a pair of sheep with Oxygius.
One day one of the Hydrus shepherd boys told Oxygius he wanted to leave the arrangement. He wanted to take his lone sheep and explore some of the lands on distant hillsides, as he used to do before he met Oxygius.
Oxygius was furious. He said a deal was a deal, and the Hydrus couldn’t just pack up and leave with his sheep. They had agreed to share the sheep, and the Hydrus must honor the agreement. : “You can’t just leave. I forbid it”, he said.
It was then that the young Hydrus realized that sometimes it is easier to get into arrangements than get out of them. Although he was nominally sharing his sheep with Oxygius, for all intents and purposes, it was under Oxygius’ control. He had entered into an unequal partnership and was not strong enough to win a wrestling match with the more powerful shepherd.
So the Hydrus decided to leave for the distant fields anyway, but had to leave his sheep behind with Oxygius. However, before leaving, Oxygius made a concession. He gave him a necklace with a (+) on it, to signify he was owed a sheep, and Oxygius wore a different necklace with a (-) to signify that he had an extra head in his flock.
Acids and bases.
Last time we talked about how many bonds are not an equal sharing of electrons, but instead the more electronegative partner tends to take the majority of the electron density from the less electronegative partner. So it is with water, where the oxygen, being more electronegative, contains a partial negative charge and hydrogen, being electron deficient, contains a partial positive charge. [This is the basis for hydrogen bonding, by the way – more about that later].
When the water bond breaks, there are two ways in which it could do so. It could break homolytically, whereby each partner obtains one electron. The other way the bond could break is heterolytically, where one partner takes both electrons of the bond and the other partner takes none, and is left with a positive charge. There are two ways the bond will break heterolytically. After heterolytic cleavage, the more electronegative partner will have the negative charge, and the less electronegative partner will have the positive charge.
Since the electronegativity of H is 2.2, and most of the other elements are on the right hand side are higher (F,O,N,Cl,Br, C, S, I) hydrogen is often in the position of being partially positive. When bonds containing H break, they tend to break heterolytically, and the H leaves as H+.
There’s a word for this behavior. It’s called acidity. Brønsted acidity. in fact.
A Brønsted acid is a chemical species which can donate H+. Conversely, a Brønsted base is any chemical species that can accept H+.
Examples of strong Bronsted acids include HCl, HBr, HI, H2SO4, and HClO4. When an acid loses a proton, it forms what is called the conjugate base. In general, strong Brønsted acids contain a species that is very stable as an anion. When a Brønsted acid reacts with a Brønsted base to generate a protonated species, the protonated species is called the conjugate acid. In general, acid base reactions proceed such that the acid and base react to give a weaker acid and a weaker base. The measure of acidity is pKa, which I’ve alluded to before, but is a measure of how easily a species parts with H+ .
There’s another way to look at this reaction. Since you could make the analogy that electron flows are to chemistry what currency flows are to economics, it’s helpful to look at this purely from a basis of examining the flow of electrons. If you look closely at what’s going on in the example above, you’ll see that the oxygen of H2O donates a lone pair to the proton of HCl, and the proton accepts the lone pair from the oxygen. This is a useful way to look at reactions because similar types of reactions (that don’t involve H+ per se) are extremely common in chemistry, like the reaction of BF3 with NH3:
So Gilbert Lewis generalized this concept: in the Lewis definition, a Lewis acid is a chemical species that accepts a lone pair, and a Lewis base is a species that donates a lone pair. This is a much broader definition of acidity/basicity and one we will encounter again and again.
Metaphorically, we can even extend it further. What’s the Lewis base and the Lewis acid in this picture? (Yes, that’s a plug on the left – things are a little different over here).
There’s other relationships you could extend the metaphor to as well – hand-glove, sword-scabbard, foot-shoe – the list could go on. As we’ll see, the reactions of bases with acids are examples of nucleophiles attacking electrophiles, which is one of the key concepts to master in order to understand the vast array of reactions you’ll be presented with in organic chem.
Take home message: understand the definitions of Brønsted and Lewis acidity, and understand the differences between them.