Sunday, October 27, 2019

Bruice Organic Chemistry Note #5: Acids and Bases

Once I dove in chemical synthesis, I realize how important it is to understand the concept of acids and bases. I realize how acids and bases are central of organic chemistry. So here in this post, I am going to discuss Chapter 2 in Bruice Organic Chemistry Book: Acids and Bases: Central to Understanding Organic Chemistry. 

Based on Bronsted and Lowry, acid is a species that loses a proton, while base is a species that receive the proton. In acids and bases reaction, there are 2 terms that we have to be familiar with, they are pKa and pH.

pKa is the degree of acidity possessed by a compound, a tendency of a compound to lose its proton. In a reaction of acids and bases, the degree of acid to dissociates is called the acid dissociation constant (Ka). The pKa value depends on Ka value. The larger the acid dissociation constant, the stronger the acids. Below is the equation:

The degree of the acidity of a compound based on its pKa value can be seen from the below table:

How about pH, pH is not a value possed by a compound, the value is obtained from the solution. So, pH is defined as the concentration of proton in a solution and the equation is shown below.

There are 4 compounds in organic chemistry that as an organic chemist we have to be familiar with because these 4 compounds are always popping up in an organic reaction mostly: (1) carboxylic acids, (2) alcohols, (3) amines, and (4) protonated compounds.

Carboxylic acids, for example, acetic acids and formic acid. 

Alcohols, for example, methyl alcohol and ethyl alcohol.

Amines, for example, methylamine and ammonia.

Protonated compounds, in order to understand this, there is an important state you have to know:
"The stronger the acid, the weaker its conjugate base". Look at the pKa value of these protonated compounds.

Protonated methylamine has pKa 10.7 compared with protonated ethylamine that pKa is 11.0, protonated methylamine is a stronger acid, means that from the states above, protonated methylamine has a weaker conjugate base than ethylamine.

Another example, protonated methyl alcohol, pKa -2.5, protonated ethyl alcohol, pKa -2.4, while protonated acetic acid, pKa -6.1 means that protonated acetic acid has a weaker conjugate base than ethyl alcohol, ethyl alcohol has a weaker conjugate base than methyl alcohol. 

By the way, as those compounds are very important to be remembered. Below is the table of the approximate pKa value of those compounds, so from the table, it can be easily recited. 

Some of the compounds can act as acid or base, depend on the situation of the reaction. So, how to predict it? Based on the pKa of the compounds involved in the reaction. 

For example, a reaction involving HCl (pKa -7) and H2O (pKa 15.7), we can see that HCl is stronger as an acid compared to H2O, so in this reaction HCl acts as the acid, while H2O as the base. 

Meanwhile, in different reaction involving NH3 (pKa 36) and H2O (pKa 15.7). H2O is a stronger acid than NH3, so H2O acts as the acid, while NH3 acts as the base. 

Now, after knowing which compound acts as the acid or the base, it's time to know, which part of the reaction is favored (reactant or product formation). As we know already from several reaction in equilibrium, the arrow is shown in 2 direction. However, there are some cases when one of the arrow is drawn longer than the other to show the part which is favored more. 

For example is the reaction below:

So, to know which one is favored, we can see from the pKa, once again, is all about the pKa value. The reaction is favored after comparing the pKa of the acid in both part of the reaction. The longer arrow will direct to the weaker acid formation. So from the reaction above, it's clear that the reaction favored to the product formation. Compared to the reaction below, the reaction is favored to the reactant side where the weaker acid is formed. 

I know it's always hard to recite numbers. It's not impossible, but it's too troublesome to recite all the pKa value of  all compounds. So, no need to recite all. Nevertheless, at least, we have to know the approximate pKa value of the compound. There are at least, 3 factors that can affect the pKa value: (1) electronegativity, (2) hybridization, and (3) size. 

Talking about electronegativity, it can be applied when the compounds being compared have the same size, I mean in the same bar of Periodic Table, such as C, N, O, and F. From that case, we can see the difference of the electronegativity that F is more electronegative than O > N > C. So, based on the electronegativity, the more electronegative the compound is, the stronger the acid. 

Hybridization is about sp3, sp2, or sp. Those differ in single, double, or triple bond. We also knew already that sp is more electronegative than sp2, and sp2 is more electronegative than sp3. So sp is a stronger acid than sp2, sp2 > sp3. 

Back to the statement I mentioned earlier, the stronger the acid, the weaker the conjugate base. A weaker conjugate base also means the more stable it is as a base. The stability of a base also depends on the size. The larger the size, the more stable it is. So in periodic table, in a column going down, between HF, HCl, HBr, and HI, the most stable base is HI, means the strongest acid. Even though it is the lowest in electronegativity, but the effect of electronegativity is put aside by the size. Size does matter more than the electronegativity. That's why, when we are talking about the effect of electronegativity, the size of the compounds compared must be in the same size. 

These there factors affect the pKa value directly. There is another factor that can indirectly affect the pKa value which is the substituent. See the example below.

When hydrogen is substituted with Br, Cl, and F, the pKa value is changing, and the compound becomes a stronger acid when substituted with the more electronegative atom. That's why the pKa becomes smaller when substitute with Fluorine atom. But how come substituent can affect the pKa? This is due to the inductive electron withdrawal. The electron in oxygen atom is pulled toward the halogen, so the electron density is reduced and stabilize it as it is being stabilized, the stronger the acid is. That's why the acidity increases. 

How about the position of the substituent, will it also affect the pKa value? The answer is yes. The closer the substituent, the stronger the acid. 

Now, let's flashback to a table that showed the pKa value of carboxylic acid and methanol. pKa value of carboxylic acid approximately around 5, while methanol around 15. How come carboxylic acid is stronger than methanol? This is also due to inductive electron withdrawal from the oxygen. In addition, delocalization also occurs there between both oxygen. As it is delocalized, it becomes more stable. While in methanol, there is only localization that happens. As the carboxylic acid is more stable, the stronger the acid than methanol. 

Here, we come to the end of acids and bases discussion. The effect of pH to determine the structure of the organic compound. When there is a compound in a solution with a certain pH, the structure is determined by the pH of the solution. A compound can be in acidic or basic form.
a. If pH of the solution is the same with the pKa of the compound, the compound will be in 50% acidic form and 50% basic form.
b. If pH is less than pKa, the compound will be in its acidic form.
c, If pH is more than pKa, the compound will exist in its basic form.

That's all from me. I apologize if there are some mistakes regarding my explanation. Thank you for visiting. See you in my next post!

Bruice, P. Y. 2017. Organic Chemistry Eighth Edition. England: Pearson Education Limited.


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