Resonance effect and Hyperconjugation:

One of the features that stabilize the A^- anion through respect to the acid HA, is resonance effect. Let us 1^st revise the basic thoughts about resonance.

Resonance:

We are previously familiar through the reality that several covalent molecules or ions can't be symbolized satisfactorily via a single Lewis structure. Hence, for these species, more than one Lewis structure is possible. Such Lewis structures are termed resonance structures or resonance contributors and the definite molecule or ion is said to be a resonance hybrid of such resonance structures. Because we will be contracting through the resonance structures of different molecules in explaining their reactivity, we should be able to mark all the feasible resonance structures of a molecule. For this reason, definite rules are to be followed. Such rules are as listed below:

1. Only nonbonding electrons and electrons constituting the multiple bonds change locations from one resonance contributor to another. The electrons in single covalent bonds aren't involved. This is given away in the instances below:

Not that a double headed arrow (→ ←) is utilized to represent the resonance contributors. It should be obvious to us that it doesn't mean that the resonance contributors are in speedy equilibrium but it implies that the genuine molecule has 1 structure that has the contribution from a variety of resonance contributors.

2. The nuclei of several atoms indifferent resonance contributors are in similar position. Therefore, the structures that as given away below isn't resonance structures since the location of the chlorine atom is different in them.        

3. All reasonable contributors must have similar number of paired and unpaired electrons. This is demonstrated below:

It is vital to comprehend that the individual resonance structures contain no actuality and the actual compound isn't a mixture of the diverse resonance contributors, but it is a weighted average of such structures. When we utilize the words weighted average, it is implied that several resonance structures are more significant than the additional and hence, contribute more to the hybrid formation. But, how to identify that structure is more significant than the others and hence, contribute more to the hybrid structure. But how to know that structure is more significant than the others to estimate the relative significance of different resonance structures, their stabilities are compared through considering each structure as a divide entity or species. In other terms, we suppose each resonance formation to be real. Therefore the most stable structure is the most significant ones. Specified below are several guidelines to enable us to assess the relative significance of resonance structures.

Table lists several groups that specify or withdraw electrons due to resonance. Groups that donate electrons via resonance are termed + R groups. Several instances of the +R groups being the hydroxyl (OH), amino (-NH₂), alkoxy (-OR), halogens (-X) and alkylamino (-NHR and, - NR₂) groups. On the other hand, the groups that withdraw electrons through resonance are termed - R groups. The instances of - R groups are nitro (-NO₂), cyano (-C=N), carbonyl (>C=O), and suphonics (-SO₃H) groups.

Table: Resonance effects of various groups

Let us now study how resonance affects the acidity and basicity of various molecules. Consider the pKa values for ethanoic acid and ethanol as given below:

        O

        ||

CH₃C - OH                CH₃CH₂OH

ethanoic                     ethanol

pK_a 4.76                    pK_a 17

Consider the dissociation of these compounds as shown below:

       O                                                          O

        ||                                                         ||

CH₃C - OH + H₂O                         CH₃ - C - O - + H₃O⁺

                                                          ethanoate ion

CH₃CH₂OH + H₂O                       CH₃CH₂O ^- + H₃O⁺

                                                           ethoxide ion

We find that the anion of ethanoic acid can be represented as a resonance hybrid of the following two resonance structures.

Because such 2 structures are equivalent, they contribute similarly to the actual structure that can be symbolized as shown below:

Therefore, we can pronounce that in the ethanoate anion the charge isn't localized on any one of the oxygen atoms but is allocated uniformly, or is delocalized, over both the oxygen atoms. This dispersal of charge resulting from the delocalization stabilities this anion. But, the delocalization of accuse diminishes the availability of electrons, thus resulting in reduce in the basicity of the anion. Therefore, the symmetry lies in the further direction resulting in the dissociation of the acid

Analogous resonance stabilization isn't possible for the ethoxide ion since these as stabilization is feasible only if the system has Π electrons. Since of the absence of resonance stabilization of the ethoxide anion, ethanol is less acidic as compared to ethanic acid. Resonance formations discussed in this division grip Π electrons and in several cases nonbonded electrons. In the next section, we will learn hyper conjugation that involves Π and δ electrons.

Alike to acidity, the basicity of compounds is as well affected via the resonance. For instance, in case benzenamine (aniline), additionally to the electron withdrawing nature (-1 effect) of the aryl group, the subsequent resonance structure are possible.

Such resonance structures obviously illustrate that the nonbonding electrons of the nitrogen atom are delocalized over the aromatic ring. Therefore, the electron density at the nitrogen atom decreases that result in the lower basicity of aniline as compared to ammonia.

Hyperconjugation:

Hyperconjugation entails the conjugation of sigma-electrons through adjacent pi electrons, as given away below:

This is as well identified as δ - π   conjugation. This kind of delocalization leads to a situation where there is no bond between the hydrogen and the carbon atom of the molecule. Thus, it is as well recognized as hydrogen no-bond resonance. Keep in mind that the proton doesn't go away its position and because the nuclei or the atoms don't transform their positions, thus, the hyperconjugation becomes similar to resonance. Hyperconjugation also results in the delocalization of charge, as we will now learn in case of carbocations. Hyperconjugation involving hydrogens is the most general. The stability of carbocations has been previous illustrated on the basis of inductive consequence of the alkyl groups. Let us think once more a primary carbocation, these as the one given below in Fig.

Fig: the hyperconjugation in a carbocation

Build a model of this carbonation and induce our self about the overlap as given here.

It is clear from the above formation that the electrons forming α - C - H bond can overlap, into the blank p orbital of the carbon atom taking the positive charge. The C - H bond adjacent to the >C = C< or carbonation is referred here as α-C-H bond. The resulting hyperconjugation can be symbolized as demonstrated below:

As we know that hyperconjugation creates several further bonding between the electron-deficient carbon and the adjacent carbon atom. Therefore, hyperconjugation results in the stabilization of carbocation via delocalizing the positive charge. Evidently, the more the number α -C - H bonds which can participate in hyperconjugation, the more stable will be the carbocation. We can see that in case of the primary carbocation given above, there are 3 such α - C - H bonds. Let us now observe the secondary and the tertiary carbocations.

For hyperconjugation to take place, the substituent next to the completely charged carbon must have a filled ∂ orbital accessible to overlap through vacant p orbital of the carbon atom carrying the positive charge.

The secondary carbocation has 6 α -C - H bonds that can participate in hyperconjugation while the tertiary carbocation has 9 α - C - H bonds. Indeed, more delocalization of charge is possible in case of a tertiary carbocation than in a secondary carbocation that is in turn more than the feasible in a primary carbocation. Hence, the tertiary carbocation is more stable than the secondary carbocation that is more stable-than the primary carbocation.

Hyperconjugation has as well been utilized to illustrate the comparative stabilities of substituted alkenes. Consider the subsequent order of stability of several alkenes.

We can see that in an alkene, the more the number of α-C-H bonds that can contribute in hyperconjugation, the higher is its stability.

I spite of the reality that hyperconjugation can be utilized to explain many otherwise unattached phenomena, it is controversial as it entails the formation of a weaker pi bond at the expense of a strong sigma bond. Additionally to the resonance, an additional factor that donates to the stability of the anion A, hydrogen bonding which we will now learn.

Hydrogen bonding

We are previously familiar through the idea of hydrogen bonding from our previous study. If we analyse the pK_a values of benzenecarboxylic acid and 2-hydroxy-benzenecarboxylic acid, as given below, then we will terminate that 2-hydroxy benzenecarboxylic acid is much more acidic than benzenecarboxylic acid. This is since the anion structured from 2-hydroxy benzenecarboxylic acid is stabilized through hydrogen bonding, as given below:

Benzenecarboxylic acid                  2- hydroxy benzenecarboxylic acid

pK_a 4.2                                                           pK_a   2.98

Hydrogen bonding stabilizes the anion through delocalizing the charge. No alike stailisation is possible for the benzenecarboxylate anion; hence benzenecarboxylic acid is less acidie than 2-hydroxy benzenecarboxylic acid. In the next section, we will learn the steric result on molecular reactivity.

Steric effect

The effect arising from the spatial interactions between the groups is termed the steric effect. We have previously studied the effect of these interactions on the constancy of geometrical isomers (where we studied that the trans-isomer is more constant than the cis-isomers) and conformational isomers (where we studied that the staggered conformation is more stable than the eclipsed conformation). As the acid-base behaviour or the molecular reactivity is associated to the accessibility of the electrons, steric factors might as well influence the molecular reactivity. For instance, they can inhibit the delocalization of charge, as examined in case of N, N-dimethyl-o-toluidine. The delocalization of the nonbonded electron pair on nitrogen, as given away in the formation of N, N-dimethylaniline in Fig (a).

Fig: (a) Delocalization of nonbonded electrons on nitrogen into arom ring in N,N-dimethylaniline (b) Such a delocalization in not possible in N, N-denethy-o-toluidine requires that the p-orbital of nitrogen and those of the aromatic ring should be coplanar. Such co planarity is inhibited in the case N, N-dimethyl-o-toluidine due to the presence of the ortho methyl group, as shown in Fig (b). Hence, in  this molecule the electron pair is not delocalized but is available for bonding with  the proton which makes this molecule more basic than N,N-dimethylaniline. This kind of steric consequence is identified as steric, inhibition of resonance.

The most ordinary steric effect is, though, the steric impediment where the presence of the bulky collections creates the approach of the reagent to the reaction site hard. These steric hindrances can account for the lower basicity of tertiary amines as compared to secondary amines. The 3 alkyl groups connected to the nitrogen atome of the tertiary amine give increase to steric hindrance and interfere throughout the solvation (see next subsection) of its conjugate acid. Therefore, as given in Fig, the trimethylammonium cation, that is the conjugate acid of trimethylamin, is sterically the most obstructed.

Consider that the steric hindrance influences the molecular reactivity not by increasing or decreasing the electron availability but due to spatial congestion. Therefore, it is different from electronic effects.

Activity:

Make models of primary, secondary and tertiary amines and compare the steric hindrance observed in these molecules.

Fig: A comparison of the solvation of trimethylammonium and methylammonium ions

It is therefore least stabilized via solvation, leading to the lower basicity of trimethylamine in water as compared to dimethylamine and methylamine. Though, in the gas phase or nonaqueous media, the electron-donating inductive consequence of a methyl group makes trimethylamine the most basic among the methylamines. Let us now learn what are solvation and the role of solvent on the reactivity of the molecules.

The occurrence of a solvent in acid-base reactions guides to the solvation of the ionized species that are the conjugate acid and the conjugate base whenever we are dealing through Bronsted acids and bases. Solvation refers to the interaction of the melted species and solvent molecules wherein numerous solvent molecules contain the dissolved species via shaping a solvent shell or solvent cage around it, as given below:

The greater the solvation, the greater is the delocalization of the charge on the species. Therefore, raised solvation amplifies the dissociation of an acid or a base via increasing the stability of the ions. Such interfaces are particularly significant when water is utilized as a solvent where the hydrogen bonding participates a significant role in solvating the anions. The high dielectric steady of water as well assists in the dissociation of the acids. Therefore, the ionization and the acidity of a material rise through amplify in the dielectric constant of the solvent. This is demonstrated in Table.

Table: Effect of solvent on pKa of ethanoic acid at 298 K Solvent pKa Benzene

Therefore, as the percentage of water in the solvent system rises, pK_a value of the acid diminishes. Water is peculiar solvent as it can act both as an acid too as a base. But its utilize has a limitation in the logic that several organic compounds aren't soluble in it.

Having conversed the various features of acids and bases, let us now focus our consideration on an internal acid-base procedure termed tautomerism.

Tautomerism:

The expression tautomerism designates a quick and reversible interconversion of isomers that are related to each other through the actual movement of electrons as well as of one or more atoms. These isomers are termed tautomers. Therefore, tautomerism is a chemical reaction and is to be differentiated from resonance in that the nuclei don't move. It is, thus, symbolized via the equilibrium sign (↔) between the tautomers. Tautomers that fluctuate from each other only in the location of a hydrogen atom and a double bond are termed proton tautomers. Table shows some examples of proton tautomers.

In contrast to resonance formation, tautomers are actual compounds and are capable of independent subsistence.

A particular instance of tautomerism involving the ketones as carbonyl compounds is termed keto-enol tautomerism and is symbolized below:

The keto-enol tautorism is enormous significance as we will learn later in this course and as well in the Organic Reactions Mechanism lessons. In keto-enol tautomers, the keto structure is generally the more stable shape and, hence, it predominates at equilibrium.

The mechanism of enolisation includes solvent mediated proton shift steps rather than a direct intramolecular jump of the proton from carbon to oxygen.

Proton tautomerism in some cases leads to the formation of a ring in one of the tautomers. Such a tautomerism is termed as ring-chain tautomerism and is demonstrated below:

Another kind of tautomerism, known as valence tautomerism involves a shift in interatomic distance within a molecule, with no the division of any atom from the rest of the molecule, as an intermediate stage. This kind of tautomerism occurs as a result of movement of valence electrons of the molecule. An example of valence tautomerism is shown below:

Cyclooctatetraene

The valence tautomerism might appear alike to resonance but remember that the 2 are dissimilar. The difference is that the valence tautomerism includes making and breaking of δ and π electrons or the nonbonding electrons shift and the δ framework of the molecule isn't disturbed.

Several other dissimilarities between tautomerism and resonance are as follows:

i) Tautomerism might include a change in the hybridization of atoms that might consequence in a change in the shape of the molecule. Whilst in resonance there is no these transform in the hybridization and geometry of the molecule.

ii) The tautomers have a physical actuality as the resonance structures are imaginary.

iii) Tautomerism includes symmetry between 2 or more tautomers. On the other hand, the resonance implies that the actual structure of the molecule in the weighted standard of various resonance contributors and not a mixture on them.

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