How does aromaticity affect basicity

This is consistent with the increasing trend of electronegativity along the period from left to right. The connection between electronegativity and acidity can be explained as the atom with a higher electronegativity being able to better accommodate the negative charge of the conjugate base, therefore stabilizing the conjugate base in a better way.

Therefore, the more stable conjugate base, the weaker the conjugate base is, and the stronger the acid is. For the discussions in this section, the trend in the stability or basicity of the conjugate bases often helps to explain the trend of the acidity. When moving vertically within a given group on the periodic table, the trend is that acidity increases from top to bottom. This can be illustrated with the haloacids HX and halides as shown below: the acidity of HX increases from top to bottom, and the basicity of the conjugate bases X — decreases from top to bottom.

The acidity of the H in thiol SH group is also stronger than the corresponding alcohol OH group, following the same trend. In order to make sense of this trend, we will once again consider the stability of the conjugate bases. When moving vertically in the same group of the periodic table, the size of the atom overrides its electronegativity with regards to basicity.

The atomic radius of iodine is approximately twice that of fluorine, so in an iodide ion, the negative charge is spread out over a significantly larger volume, so I — is more stable and less basic, making HI more acidic. For elements in the same group, the larger the size of the atom, the stronger the acid is; the acidity increases from top to bottom along the group. The resonance effect accounts for the acidity difference between ethanol and acetic acid.

For both ethanol and acetic acid, the hydrogen is bonded with the oxygen atom, so there is no element effect that matters. However, the p K a values and the acidity of ethanol and acetic acid are very different. What makes a carboxylic acid so much more acidic than an alcohol? As stated before, we begin by considering the stability of the conjugate bases, remembering that a more stable weaker conjugate base corresponds to a stronger acid.

For acetate, the conjugate base of acetic acid, two resonance contributors can be drawn and therefore the negative charge can be delocalized shared over two oxygen atoms. However, no other resonance contributor is available in the ethoxide ion, the conjugate base of ethanol, so the negative charge is localized on the oxygen atom.

As we have learned in section 1. Once the information from reliable sources is obtained, the next step is to Organize the information to provide a framework of understanding or knowledge. More graphic organizers are necessary in undergraduate chemistry to provide successful learning strategies. Filling in a two-dimensional grid provides spatial separation of concepts and is better than a list to memorize. Finally, after organizing the Information, one of the best ways to show knowledge is the Prediction of the solution to a new problem.

The SOP system can be used in any class flipped or lecture , is student centered active learning and, once mastered, may be used to evaluate progress during the semester or on written evaluations, such as a final exam. Are all these details necessary to show students how to use structures to predict relative acidity or basicity?

The use of only electronic effects on the substrate as a guide to acidity or basicity is a gross simplification that still works for teaching undergraduates today. It is important to indicate that, whereas the textbook may deal with these exceptions, we do not test students on data outside the standard cases. There also seems to arise a lore in organic chemistry circles that to predict relative acidity among a series of acids one must analyze the stability of their conjugate bases, and then use the reciprocal strength relationship between acids and bases.

This generalization does not hold in all cases. After all, values of p K a measured for the acidic substrate provide evidence of the relative experimental acidity Rossi, In this article, we will only focus on the inductive and resonance effects in the acid and show how it can be extrapolated to compare the relative base strength.

There are seven electronic effects that affect the reactivity of substituted acids Stock, These effects are listed as:. Whereas most of these effects are shown in Introductory Chemistry, effects 5 to 7 are usually not seen by undergraduate students until the chapters on aromaticity in the Organic Chemistry textbooks.

This delay in the presentation of fundamental principles of reactivity also delays the ability to apply these effects and limits the knowledge of the students. K a X is the ionization constant of the substituted benzoic acid and K a H that of benzoic acid itself. Inductive F and resonance effects R are quantified and collected in databases according to the substrate. Instead of K a values indicated in the Hammett equation, most tabulated data provide p K a values for substituted benzoic acids.

The p K a of unsubstituted benzoic acid of 4. As every other substituted benzoic acid will be measured according to this standard, we place this molecule in the center of the 3 x 3 grid Figure 2 grid and will organize the other substituents in relation to this standard.

If students are provided with a list of substituted benzoic acids, one ability expected of them is to Search for the corresponding p K a values using the name or Lewis structure of the compound. They should be able to prepare a chart such as Table 1 structure vs.

Figure 3. Grid position for substituted benzoic acids. The first realization from this organizational grid is that the acidity relates to the position in the grid Figure 4. Figure 4. It might not be clear why the-meta-nitro compound was placed Figure 3 in the square of [Neutral by resonance and Withdrawal by induction] if it is known that the nitro group is a group [Withdrawing by resonance and induction] in the electrophilic aromatic substitution grid.

Indeed, for the para -nitrobenzoic acid, both withdrawing effects are present. However, in the situation of meta -substitution, the relative ability of the nitro substituent to extract by resonance is limited because there is no direct resonance interaction between the nitro and carboxyl groups, usually rationalized by showing the extended Lewis structures of the resonance forms Figure 5 , yet the inductive effect continues through the sigma framework.

Thus, a general rule can be formed: if the substituent is in the meta -position, the relative resonance effect is neutral, without need to draw the complete resonance structures for each substituent. This supposition is supported by the quantitative data for the nitro substituent Hansch et al.

The electron-density withdrawal from the para -position is only slightly more than in the meta -position, thus the values of p K a for the para - and meta -compounds are similar.

Figure 5. Resonance structures for m -nitrobenzoic acid. Now let us apply the Prediction phase of the process. If a new molecule is provided, with a substituent less common or with an unknown p K a, can we use organized data Information to create Knowledge?

Note it is not necessary to provide an absolute numerical prediction but rather a relative prediction of p K a.

One test case is m -methoxybenzoic acid, Figure 6. Figure 6. Structure of m -methoxybenzoic acid and prediction question. Figure 7. Predicted placement of m -methoxybenzoic acid in grid relative to benzoic acid. Using our organization in a grid, we can easily apply the general rules and place the molecule Figure 7 in the right-hand side of the chart [Neutral by resonance and Withdrawal by induction].

Furthermore, the position directly relates to a prediction of a p K a less than benzoic acid. Resonance does not always win. It is possible to write a complete set of Lewis resonance structures Figure 8 that show no delocalized contact between the two functional groups, yet it would be advantageous to predict the acidity without resorting to this long process. A complete search of the databases shows that m -methoxybenzoic acid has a p K a of 4. Figure 8. Resonance structures for m -methoxybenzoic acid.

Continuing the Prediction process, let us apply the grid system to another test case, o -methoxybenzoic acid Figure 9. Figure 9. Structure of o -methoxybenzoic acid and prediction question. This prediction would lead one to assign the o -isomer with a p K a greater than benzoic acid. Figure Incorrect Predicted placement of o -methoxybenzoic acid in grid relative to benzoic acid.

However, this prediction of acidity would be wrong, even if the electronic effects are correctly predicted. The experimental p K a, 4. When simplistic predictions and experimental results disagree, this discrepancy shows a problem with our models. The first factor is that an electron-withdrawing oxygen is present, which can remove some of the electron density from nitrogen.

Look at that resonance form on the right. Therefore, the basicity of a nitrogen is decreased when attached to a pi-acceptor. What are pi-acceptors, again? Examining the resonance forms of DMAP is illuminating.

In the key resonance form, the nitrogen in the ring bears a negative charge. The NMe 2 is made less basic by being a pi-donor see above but the pyridine nitrogen is made more basic because it is the pi-acceptor here. Another example of how basicity of nitrogen can be increased by attachment to pi-donors is found in guanidines.

This is borne out by pKaH values. Not resonance, by the way! T he lone pair in pyridine is in the plane of the ring, and thus not in conjugation with the p-orbitals. It turns out that the nitrogen in pyrrole is unusually non-basic. In fact, even when subjected to acid, pyrrole reacts at carbon C-2 , and not on the nitrogen.

The conjugate acid is not aromatic. Removal of the lone pair on nitrogen through protonation would destroy the conjugation of the lone pair with the other p orbitals of the ring and render the molecule non aromatic. This might ring a distant bell. You might remember that cyclopentadiene is an unusually strong acid for a hydrocarbon.

The bottom line here is to be on the lookout for situations where forming a new N-H bond might disrupt aromaticity. OK, I said there were 5 factors which affected acidity, but I covered six factors here. If you understand the factors that stabilize negative charge and therefore make an atom less basic , by definition you also understand the factors that destabilize negative charge and therefore make an atom more basic.

How do we handle situations where multiple factors come into play? We must resort to experimental measurement pK a H. Note 1. Recall, however, that across a row of the periodic table, basicity is inversely correlated with electronegativity. The less electronegative the element, the less stable the lone pair will be and therefore the higher will be its basicity.

Another useful trend is that basicity decreases as you go down a column of the periodic table. This is because the valence orbitals increase in size as one descends a column of the periodic table. Note 2. Note how a significant resonance form of the conjugate acid is a substituted version of the aromatic cyclopropenium cation. This helps to drive the equilibrium towards the conjugate acid. The pKaH here is about 27! Thank you for granting access to the informative and educational technical paper.

My question is: if I want to reverse the Lyotropic selectivity of sulfate first and chloride second in a Type 1 strong base anion exchange resin tertiary amine or quaternary amine to chloride first and sulfate second, what high basicity amines are best suited in the amination step post chloromethylation? Thank you.

In inversion of amines an sp3 hybridized nitrogen passes through a planar sp2 intermediate en route to the inverted sp3 nitrogen. In theory one would expect that an sp3 hybridized nitrogen would be more basic than an sp2 hybridized nitrogen than an sp hybridized nitrogen.

Amines are more basic than nitriles after all. Therefore one could look at nitrogens in systems where inversion is impossible and compare their basicities to nitrogens where inversion happens. One good comparison would be, say, triethylamine with quinuclidine.