Acid-Base Catalysis

The world around us is full of different chemicals. Every day several chemical reactions take place in our environment. These reactions are crucial for several important processes.

Catalysis is an important process that aids in many chemical reactions. This process is widely used in different industries to turn raw materials into useful products. It can be safely said that catalysis is the backbone of several industrial processes.

Simply put, Catalysis is the process of starting a chemical reaction at a faster rate and a much lower energy requirement than without a catalyst.  So catalysts are agents that carry out the entire process of catalysis. This is a very important chemical process that students of chemistry must be well acquainted with.

 In this article, we will understand the process of acid-base catalysis. Acid and bae act as catalysts for many industrial chemical reactions. Moving on, we will try to understand why acid and bases act as a catalyst and what is the mechanism behind such reactions.

With the help of certain examples, the concept of acid-base catalysis should be easily grasped. You will also be informed of the research behind the acid-base catalysis reaction. Lastly, we will take a look at how catalysis is crucial to our lives.

What is Acid-Base Catalysis?

Acid and bases are the most widely used reagents in chemical laboratories. In various reactions, it performs the role of catalyst. Let us discuss what acid-base catalysis reaction is? Acid-base catalysis is the addition of an acid or a base to a chemical reaction to speed up the reaction without consuming the acid or base. Acid catalysis, as in the decomposition of sucrose into glucose and fructose in sulfuric acid, or base catalysis, as in the addition of hydrogen cyanide to aldehydes and ketones in the presence of sodium hydroxide, are two examples of catalytic reactions. Acids and bases both catalyze a variety of reactions.

General Acid-Base Catalysis Mechanism

The Bronsted-Lowry principle of acids and bases describes the mechanism of acid- and base-catalyzed reactions as an initial transfer of protons from an acidic catalyst to the reactant or from the reactant to a basic catalyst. The reaction involves the exchange of an electron pair donated by a base catalyst or embraced by an acid catalyst, according to the Lewis theory of acids and bases.

Acid catalysis is used in many industrial processes, including the conversion of petroleum hydrocarbons to gasoline and other products. High-molecular-weight hydrocarbons are decomposed (cracked) using alumina-silica catalysts (Bronsted-Lowry acids), unsaturated hydrocarbons are polymerized using sulfuric acid or hydrogen fluoride (Bronsted-Lowry acids), and aliphatic hydrocarbons are isomerized using aluminium chloride (Bronsted-Lowry acids) (a Lewis acid). There are two types of acid catalysis: basic acid catalysis and general acid catalysis.

Specific Catalysis

A protonated solvent is the catalyst in basic acid catalysis. The rate of reaction is proportional to the amount of protonated solvent molecules SH+ present. The acid catalyst (AH) only helps to accelerate the rate by changing the chemical equilibrium between the solvent S and the AH in favour of the SH+ species. Strong acids in polar solvents, such as water, often undergo this form of catalysis.

\[S + AH \rightarrow  SH^{+} + A^{-} \]

In an aqueous buffer solution, for example, the rate of reaction for reactants R is affected by the pH of the system but not by the concentrations of different acids.

\[Rate = -\frac{dR1R2}{dt} = K [SH^{+}] R1R1 \; R2R2 \]

When the reactant R1 is in a quick equilibrium with its conjugate acid R1H+, which then continues to react slowly with R2 to produce the reaction product, this form of chemical kinetics is observed; for example, in the acid catalysed aldol reaction.

General Catalysis

In general, all species capable of contributing protons contribute to the acceleration of reaction rates in acid catalysis. The most powerful acids are the most effective. General acid catalysis is seen in reactions where proton transfer determines the rate of the reaction, such as diazonium coupling reactions.

\[Rate = -d \frac{R1R1}{dt} = K1 [SH^{+}] R1R1\; R2R2 + K2\; A1H \;A1H \;R1R1\; R2R2 + K3 A2H\; A2H \;R1R1 \;R2R2 +.. \]

A change in the rate indicates general acid catalysis when the pH is kept constant but the buffer concentration is changed. The presence of a constant rate indicates the presence of a particular acid catalyst. This type of catalysis is significant when reactions are carried out in nonpolar media because the acid is often not ionized.

General acid catalysis and/or general base catalysis can be essential mechanisms for specificity and rate enhancement in any reaction involving proton transfer. Acid-base catalysis can be divided into two types: general catalysis and specific catalysis. Specific acid or specific base catalysis occurs when a hydronium \[H_{3}O^{+}\] or hydroxide \[HO\] ion catalyses a reaction that is determined solely by the pH, rather than the buffer concentration. Consider the hydrolysis of ethyl acetate as an example of how complex acid-base catalysis functions. Since both the nucleophile \[H_{2}O\] and the electrophile (the carbonyl of ethyl acetate) are unreactive at neutral pH, this reaction is extremely slow. 

However, if the reactivity of either the nucleophile or the electrophile could be increased, the reaction rate could be accelerated. The concentration of the hydroxide ion, which is a much stronger nucleophile than water, increases as the pH rises, and the rate of hydrolysis rises as well. Similarly, lowering the pH raises the concentration of the hydronium ion, which can protonate the ester carbonyl, raising its electrophilicity and speeding up the hydrolysis process. If that's the case, the hydrolysis rate should be doubled when the base and acid are combined, right? Not. As acid is added to a base in solution, it is neutralised, and any catalytic effect is lost.

It's important to understand that the pKa values of amino acid side chain groups inside enzyme active sites aren't quite the same as those determined in the solution. In addition, in hydrophobic conditions, pKa values can change dramatically. As a result, the removal of higher pKa protons from substrates by active-site bases may not be as absurd as it seems if only solution chemistry is considered.

When General Acid Catalysis Occurs?

When acids other than the hydronium ion accelerate the reaction rate, this is known as general acid catalysis. When bases other than the hydroxide ion accelerate the pace, this is known as general base catalysis. As the reaction rate increases with increasing buffer concentration at a constant pH and ionic strength in solution, general acid-base catalysis is demonstrated, with a greater increase with a buffer that contains a more concentrated acid or base portion. Since the concentration of hydronium or hydroxide ions does not change (the pH remains constant), the reaction must be catalysed by the buffer. General acid-base catalysis occurs when an acidic or simple residue at the active site is used to promote proton transfers in a reaction mediated by an enzyme.

Consider the enzyme -chymotrypsin as an example of general acid-base (and covalent) catalysis. The enzyme's commonly accepted mechanism of action, which involves a serine hydroxyl group attacking the carbonyl group of a peptide bond with a nucleophilic attack. Although the serine hydroxyl group is not generally thought to be a strong nucleophile, Blow and colleagues have linked aspartic acid and histidine residues nearby to the conversion of serine to an alkoxide through a mechanism known as the charge relay method. 

Researches on Acid-Base Catalysis Mechanism

Some researchers discovered the Asp-102, His-57, and Ser-195 hydrogen-bonding networks. The aspartate carboxylate removes a proton from the histidine imidazole (pKa of the acid in solution is 6.1), which then removes a proton from the serine hydroxyl group (pKa 14 in solution). The suggestion would be hydrogen-bonding to any reputable organic chemist; how can a base like an aspartate, whose conjugate acid is two pKa units lower than the histidine imidazole, efficiently remove the imidazole proton, and then how can this imidazole remove the proton from the hydroxyl group of serine? Which of the following is eight pKa units higher than histidine's (protonated) imidazole? The equilibrium favours the first proton transfer by just 1%, while the second proton transfer equilibrium favours the back direction by a factor of 108. 

One theory may be that some of these acids and bases have different pKa values at the active site than in the solution. Furthermore, because these groups are kept close together at the active site, as the proton is withdrawn from the serine hydroxyl group, the charge density continues to the next level (attack of the oxygen electron density at the peptide carbonyl), driving the equilibrium forward. This is the beauty of enzyme-catalyzed reactions: the proximity of the groups and the fluidity of the active-site residues combine to allow reactions that would be virtually impossible in solution to occur.

Role of Acid-Base Catalysis in Hydrolysis Reaction

Hydrolysis is a reaction involving a water molecule that splits large molecules into smaller ones. It is catalysed by proton or hydroxide (and occasionally inorganic ions such as phosphate ions) that are present in the aquatic environment and are involved in general acid-base catalysis. The rate of hydrolysis is affected by pH and temperature, However, various components in the real aquatic environment, such as dissolved organic matter (humic substances, metal ions, and so on), can alter the hydrolytic profile of pollutants.

Redox Reactions Involving Acid Catalysis Mechanism

Humic acids (HA) are redox-active compounds that are found in abundance in natural aquatic and soil environments. The oxidation or reduction of ions and molecules, including several organic pollutants, may be catalysed by redox-active functional groups found in humic substances and mineral surfaces. Mineral form, solution chemistry, and microbial activity all influence the pathways and rates of these processes. Humic substances could be involved in extensive redox reaction networks, particularly in organic-rich environments like soils.

Importance of Catalysis

As can be understood by now, catalysis is an important process and can help save a lot of time and energy in many chemical reactions in different industries. You will be surprised to know that many daily things that you use have been prepared using the process of catalysis including notebooks, detergents, cheese, beer etc. However, catalysis is not limited to industrial uses. It also plays a crucial role in other aspects. Some of these are as follows:

• The human body also uses many catalysts to carry out crucial life processes. Enzymes for example are nothing but proteins that act as catalysts for many processes

• Using the right catalyst can reduce the toxic waste in many industrial chemical reactions. Therefore the process of using the right catalyst can have a big positive effect on our environment.

• Catalysis can help by breaking down cellulose found in biomass to help produce fuel.

• The main function of a catalyst is to save energy in any chemical reaction and make the entire process more quick and efficient.

• Scientists are searching for new catalysts every day that can help build more energy-efficient products and reduce toxic chemical waste.

Did You Know?

• Enzymes use general-acid and general-base catalysis to catalyse reactions.

• Do you know many reactions are catalyzed by both acids and bases?