Two very important bases in organic chemistry are pyrrole and pyridine. These are heterocyclic bases. Firstly, what is a heterocycle? A heterocycle is a cyclic compound with a heteratom in the ring. A heteroatom in this context is any atom other than carbon. Pyrrole is a 5 membered ring containing a nitrogen atom: Pyrrole has some signs of aromaticity, as we … Read more
By way of introduction, let us first consider the simplest aromatic base, aniline: As you can see, aniline has resonance forms which lower the energy of the normal form of aniline. This resonance is allowed because the nitrogen lone pair is in an sp2 hybridised orbital, and can therefore overlap with the pi orbital system of the benzene ring. However upon protonation, no resonance stabilisation is available: The net … Read more
If we attach electron withdrawing groups to an amine, we can significantly reduce its basicity by removing the tendency for the electron pair of the nitrogen to attack a proton. e.g.: The species shown above is not very basic at all due to the electron withdrawing groups. The analogous species with three CF3 groups is practically not basic at … Read more
The trends of bases of the type NR3, where R is an alkyl group or hydrogen, can be explained quite well, and quite simply. The first point to note is that the strength of a base of this type is related to the availability of the electron pair on the nitrogen atom (as it is this pair that will attack … Read more
Just as with acid strength, base strength can be determined by considering an equilibrium: The equilibrium constant for a base is Kb. As before, the term for water does not appear in the expression because it is in vast excess, and can be assumed to be constant. It is therefore incorporated into the term for Kb. However, in … Read more
We know from previous discussions that Ka is not an absolute value for a particular substance, because it depends intimately on the surroundings; such as solvent. Most values are quoted for water at 25ºC. It is necessary to quote a temperature with a pKa or Ka, because just as with solvent, Ka varies with temperature. This is, of course, obvious … Read more
A carboxyl group is inductively electron withdrawing. As we have already shown, the presence of electron withdrawing groups can be expected to increase the acidity of compounds. Hence we might expect that compounds with two acid groups would be stronger acids, and this is indeed the case as illustrated by the following values: Carboxylic Acid pKa Dicarboxylic derivative … Read more
The effect of substituents on phenols is not as straightforward as with aliphatic acids. Phenol itself is weakly acidic, with a pKa of 9.95. Above is shown phenol (left), and the phenolate ion (right). The strength of phenol as an acid can be increased by the addition of electron-withdrawing groups. For example, we can add a nitro group (a powerful electron withdrawer). This has both inductive (through σ-bonds) … Read more
This discussion will focus on factors that are internal to the acid which affect its strength. Consider the following acids: Acid Name pKa A Chloroacetic 2.85 B Dichloroacetic 1.25 C Trichloroacetic 0.66 The strength of acid increases with the addition of extra chlorine atoms. This simply illustrates that as X– becomes more electronegative, the anion becomes a more stable conjugate base as it … Read more
Water is the most common solvent with respect to acids and bases. Often, the critical factor in determining the strength of an acid is played by the solvent, especially when that solvent is water. Unfortunately, given that water is an ionising solvent, many organic acids do not dissolve sufficiently in it to begin with. However, if that … Read more