Core ConceptsThis article will describe organic acids, organic bases, and how to differentiate their strength.Organic AcidsOrganic acids are molecules that contain carbon and can donate a proton. The general formula is as follows: HA (aq) + H2O (l) –> A– (aq) + H3O+ (aq). The acid, HA, donated a hydrogen atom or an acidic proton to the water molecule.Organic BasesOrganic bases are molecules that contain a carbon and can accept a proton. The general formula is as follows: B (aq) + H2O (l) –> BH(aq) + HO– (aq). The base, B, accepted hydrogen from the water molecule.Strength of Organic AcidsSome acids (HA) are considered strong while others are considered weak. The A– formed when HA loses hydrogen is called a conjugate base. The more stable the conjugate base, the stronger the acid.Strong acids contain more electronegative molecules. When the hydrogen is lost on HA and the molecule becomes A–, the electronegative molecules are stable while taking the negative charge. Looking at the periodic table period below, the molecules closer to the right are more electronegative.Strong acids have large conjugate bases. The larger size creates a more stable conjugate base. Looking at the periodic table below, the molecules at the bottom of the column are larger.The table below lists the six strong acids.Aside from the convention general chemistry acids, there are many others used in organic chemistry. Strong acids that dissociate into H+ are more soluble in basic solutions. The OH– in basic solutions reacts with the hydrogen ion to form water. Some examples are citric acid and oxalic acid.Strength of Organic BasesSome bases (B) are considered strong while others are considered weak. The HB formed when B accepts a hydrogen is called the conjugate acid. The more stable the conjugate acid, the stronger the base.Strong bases contain less electronegative molecules. Looking at the periodic table period below, the molecules closer to the left are less electronegative.Strong base conjugate acids are large. The larger sizes create a more stable conjugate acid. Looking at the periodic table below, the molecules at the bottom of the column are larger.The table below lists the six strong bases.Aside from the convention general chemistry bases, there are many others used in organic chemistry. Strong bases that dissociate into OH– are more soluble in acidic solutions. The H+ in acidic solutions reacts with the hydroxide to form water.Super BasesIn organic chemistry, much stronger bases are needed than the ones discusses above. Super bases are molecules that have a high affinity for hydrogens so they remove protons from molecules. They can create carbanions and often dissolve in organic solvents. Some examples include sodium hydride, butyl lithium, and lithium diethylamide. We will study lithium diisopropylamide, a commonly used base in organic chemistry, in depth.Lithium diisopropylamide or LDA is a super base that is often used to form enol anions. The negative nitrogen takes a hydrogen from a carbon which begins the arrow pushing as shown below.Dissociation and pKaStrong acids and bases completely dissociate in water.An example of a strong acid dissociation would be: HCl (aq) –> Cl– (aq) + H+ (aq)An example of a base dissociating would be: NaOH (aq) –> Na+ (aq) + OH– (aq)pKa is a numerical measurement of how easily an acid donates its proton. The dissociation of an acid, as shown above, is measured by the dissociation constant “Ka.” Studying pKa mathematically, students can use the formula -log10(Ka ) = pKa. The lower the value of the pka, the more acid the molecule because the molecule is able to easily donate its proton.Inductive EffectInducive effect must be studied when discussing acids, as it affects pka. A strong electron withdrawing group will decrease the pKa, creating a more acidic molecule. This is because an electron withdrawing groups makes the molecule more able to stabilize negative charge, creating a stable conjugate base.Carboxylic acids have a low pKa, however adding addition electron withdrawing group such as alcohols or halogens further decreases the pKa. Electron withdrawing groups bring the negative charge towards themselves, which stabilizes the conjugate base. This is illustrated in the example section below.Inductive Effect ExampleUsing the principles of inductive effect, we can compare the three molecules below and rank them in order of the most acidic to least acidic.To determine which is the most acidic, we must look at their conjugate bases and determine which is the most stable. Their conjugate bases are shown below.Since Fluorene is the most electronegative, compared to chlorine and hydrogen, we know that is best pulls the electrons toward itself. This makes the negative oxygen less negative and therefore more stable. So, the acid with the electron withdrawing group of Fluorene is the most acidic and has the lowest pKa value.Using this logic, we know that Chlorine is the second most electronegative. It will draw in less of the negative charge than Fluorene and more than hydrogen. Therefore it will stabilize the conjugate base less than Fluorene and more than Hydrogen. It will have the middle pKa value.Hydrogen is not very electronegative and does not draw the negative charge well. This does not stabilize the negative charge on the conjugate base. The pKa value would be the highest and therefore the least acidic.ConclusionWhen comparing acids or bases, we can use the periodic table to identify which acids/bases are strongest and weakest. Organic chemistry uses super bases, which are stronger than the typical base molecules. Using the inductive effect we can better understand how electron withdrawing groups relate to pKa values and conjugate bases.
