Chemists use the term ‘delocalization of charge’ to describe this situation. What this means, you may recall, is that the negative charge on the acetate ion is not located on one oxygen or the other: rather it is shared between the two. The two resonance forms for the conjugate base are equal in energy, according to our ‘rules of resonance’. For acetic acid, however, there is a key difference: a resonance contributor can be drawn in which the negative charge is localized on the second oxygen of the group. In both species, the negative charge on the conjugate base is held by an oxygen, so periodic trends cannot be invoked. What makes a carboxylic acid so much more acidic than an alcohol? As before, we begin by considering the conjugate bases. The first model pair we will consider is ethanol and acetic acid, but the conclusions we reach will be equally valid for all alcohol and carboxylic acid groups.ĭespite the fact that they are both oxygen acids, the pK a values of ethanol and acetic acid are very different. Now, it is time to think about how the structure of different organic groups contributes to their relative acidity or basicity, even when we are talking about the same element acting as the proton donor/acceptor. In the previous section we focused our attention on periodic trends - the differences in acidity and basicity between groups where the exchangeable proton was bound to different elements.
This illustrates a fundamental concept in organic chemistry that is important enough to put in red: The atomic radius of iodine is approximately twice that of fluorine, so in an iodine ion, the negative charge is spread out over a significantly larger volume: But in fact, it is the least stable, and the most basic! It turns out that when moving vertically in the periodic table, the size of the atom trumps its electronegativity with regard to basicity.
Because fluorine is the most electronegative halogen element, we might expect fluoride to also be the least basic halogen ion. In order to make sense of this trend, we will once again consider the stability of the conjugate bases. This is best illustrated with the halides: basicity, like electronegativity, increases as we move up the column.Ĭonversely, acidity in the haloacids increases as we move down the column. When moving vertically within a given column of the periodic table, we again observe a clear periodic trend in acidity. Once again, a more reactive (stronger) conjugate base means a less reactive (weaker) conjugate acid. The nitrogen lone pair, therefore, is more likely to break away and form a new bond to a proton - it is, in other words, more basic. Oxygen, as the more electronegative element, holds more tightly to its lone pair than the nitrogen. We can use the same set of ideas to explain the difference in basicity between water and ammonia.īy looking at the pK avalues for the appropriate conjugate acids, we know that ammonia is more basic than water. Thus, the ethoxide anion is the most stable (lowest energy, least basic) of the three conjugate bases, and the ethyl anion is the least stable (highest energy, most basic). The more electronegative an atom, the better it is able to bear a negative charge. Remember the periodic trend in electronegativity: it also increases as we move from left to right along a row, meaning that oxygen is the most electronegative of the three, and carbon the least. In the ethyl anion, the negative charge is borne by carbon, while in the methylamine anion and ethoxide anion the charges are located on a nitrogen and an oxygen, respectively. Look at where the negative charge ends up in each conjugate base. The key to understanding this trend is to consider the hypothetical conjugate base in each case : the more stable (weaker) the conjugate base, the stronger the acid. We can see a clear trend in acidity as we move from left to right along the second row of the periodic table from carbon to nitrogen to oxygen. We’ll use as our first models the simple organic compounds ethane, methylamine, and ethanol, but the concepts apply equally to more complex biomolecules, such as the side chains of alanine, lysine, and serine. \( \newcommand\)įirst, we will focus on individual atoms, and think about trends associated with the position of an element on the periodic table.