Factors Affecting the Strengths of Acids and Bases:
The strengths of acids and bases based upon many factors. It was stated before, that apart from the occurrence of functional groups; structural deviations in molecules as well effect their acidic or basic properties. We will now center our attention on some effects which occur due to structural transforms in the molecule. A amend in molecular structure can affect the reactivity of the molecule via changing the electron distribution of the system; in that case it is termed an electronic effect. An additional possibility is that two or more groups or atoms might come close sufficient in space so that the London interactions between them become important. The effects arising from these interactions are termed steric effects.
We will start our conversation through the study of an electronic effect, identified as inductive effect.
We are previously familiar through the reality that when 2 diverse atoms form a covalent bond the shared pair of electrons is pulled more via the more electronegative atom. These asymmetrical electron distribution consequences in partial separation of accuse and we get a dipole in that one atom has a partial positive charge and another atom (the more electronegative one) has a partial negative charge. These polarizations of a bond can be felt via adjacent groups as well. This phenomenon of the diffusion of charge through a chain of atoms connected via ∂ ∂ bonds is termed inductive effect
As we know that the inductive effect is a permanent effect.
Let us now analyse how inductive effect reasons a transform in the acidity or basicity of a molecule. Let us take the example of ethanoic acid whose structure is shown below:
If we substitute one of the hydrogen atoms on the C - 2 carbon atoms through a substituent X, then, the nature of the substituent group might affect the electron density of the O - H bond resulting in a change in the acidity of the molecule.
Depending upon whether the substituent X is electron-withdrawing of electron donating, the electron density will reduce or amplify, correspondingly. If the electron density between the bonds formed via O and H atoms reduces, then, the loss of H as H+ ion is facilitated resulting in the amplified acidity of the molecule. On the other hand, an enhance in the election density at the bond between O and H atoms will build the proton liberate hard, thereby, decreasing the acidity.
(i) When the substitution X is electron withdrawing, it reduces the electron density at H as given away below:
(ii) While the substituent X is electron donating, it enhances the electron density at H as symbolized below:
Table: Inductive effect of various functional groups
The consequence of several of such substituents on the acidity of the substituted acids in expressions of their pKa values is given away in Table.
Table: pKa values for some substituted acids determine in water at 298 K.
Table illustrates the reduced acidity for propanoic acid (larger pKa value) as compared to the ethanoic acid. As we know that the propanoic acid has a methyl group in lace of H in ethanoic acid. The methyl group is electron-donating in nature and hence, has a + I result that consequences in the reduce in the acidity. But the acidity enhances when the electron-withdrawing substituents these as F, Cl, Br and l are here. As we familiar that the enhance in acidity is in accordance through the electro negativity of such elements.
The inductive effect of such substituents is additional increased through the enhance in the number of such substituents. This is represented in Table.
Table: Effect of increase in the number of chiorine substituents on acidity of ethanoic acid.
In monochloroethanoic acid, one of the 3 hydrogen atoms in ethanoic acid has been changed via an electron withdrawing chlorine atom. Therefore, the electron pair constituting the C - Cl bond is drawn closer to the chlorine atom. This consequence is transmitted during other atoms forming δ bonds to the OH bond of the
-C- O - H group. These consequences in a shift of the electrons constituting the O - H bond towards oxygen as shown below:
These electron withdrawal through chlorine atom, therefore, facilities the department of the proton and thus, enhances the acidic character of monochloroethanoic acid as compared to ethanoic acids, the existence of 2nd and 3rd chlorine.
In the di - and trichloroethanoic acids, the presence of 2nd and 3rd chlorine
Atoms consequences in more electron withdrawal away from hydrogen of the O - H bond and would, hence, further raises the acidity of such compounds as compared to ethanoic acid or chloroethanoic acid. Hence, we can organize such acids in the increasing order of their acidities as ethanoic acid < chloroethanoic acid < dichloroethanoic acid < trichloroethanoic acid.
The position of electron-withdrawing substituents in a molecule also influences its acidic character. This is shown by the pka values of isomeric monochlorobutanoic acids given in Table.
Table: Consequence of position of substituent on acidity
It can be seen that even though in each of such acids a chlorine atom has swapped a hydrogen atom but they illustrate diverse acidities. As we know that as the distance of the electron withdrawing chlorine atom from the reaction site (that is O - H of the COOH group) enhances, the acid strength reduces. Therefore, the influence of the inductive consequence on acid strength is greatest when the electron withdrawing chlorine atom is present on the carbon next to the carboxylic group and it reduces rapidly through enhance in the distance. This result is roughly negligible after the 4th carbon atom in the chain.
Comparable electron withdrawals take places when a completely charged group is present in a molecule. A positive centre these as (CH3)3 N+- (trimethyl ammonium) or +NH3 (ammonium), eases the departure of proton by withdrawing electrons and hence, increases the acid character of the molecule. This is illustrated in the instance given below:
As we know that here as well through enhance in the distance between the positively charged group and the carboxyl group, the inductive effect reduces.
If the presence of a positively charged group raises the acidity of a molecule, then a negatively charged group should decrease the acidity. Consider the dissociation of propanedioic acid, as given below:
Where Ka1 is the dissociation steady.
Here, a proton is lost from one of the 2 carboxyl groups of the molecule. The dissociation constant for this dissociation is termed the 1st dissociation constant and is symbolized via Ka1. Additional dissociation of the anion attained in the above dissociation is hard since it involves the elimination of the proton from a negatively charged species. Therefore, this step has a pKa value equal to 5.69. This is termed pKa2 because Ka2 symbolizes the 2nd dissociation constant.
Always remember that Ka1 is larger than Ka2 for a dicarboxylic acid. Hence, for such acids pKa1 is lower than pKa2. From the above discussion, we can state that the substituents having - 1 effect enhance the acidity whereas the substituents having + effect diminish the acidity. On this basis, let us now examine the stability of carbocations that are reactive intermediates structured during the chemical reactions. Look at the subsequent instances of carbocations:
The carbocations are classified through the degree of alkyl substitution at the positively charged carbon atom as primary, secondary or tertiary carbocations, as follow away below:
Where R is the alkyl group since the alkyl groups are electron donating in nature, the + I effect raises through enhance in the number of alkyl groups. Therefore, the raise in the number of alkyl groups in a carbocation assists in the dispersal of its positive charge.
Hence, a tertiary carbocation is more stable than a secondary carbocation ion that is, in turn, more stable than a primary carbocation. Therefore, we can arrange the above carbocations in the subsequent order of their stabilities;
Since the substitution having + 1, effect reduce the acidity, their presence should as well enhance the basicity. This is what is actually examined when the hydrogen atoms of ammonia are consecutively swapped via methyl groups to provide methylamine and dimethylamine whose basicities enhance through the increase in the number of methyl groups, as given away below via the pKa values of their conjugate acids.
Till now, we have been learning the inductive effect of different substituents on the acidities and basicities of molecules. In reality, the inductive effect influences the electron density of the H-A bond. An additional factor that influences liberate of protons from the acid HA is the stability of the anion, A-, structured through the loss of proton from the acid HA.
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