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Introduction
For those who were expecting an article on molecular orbital theory (as I had promised in the previous article), I apologize, because I have to finish a piece of coursework this week. A more detailed explanation of this is given at the end of the article.
The addition of a catalyst (Fig. 1) can affect many aspects of a reaction. These aspects include not just the rate of the reaction, but also the stereochemistry, and the feasibility of the reaction itself. In this article, we will learn how the catalyst can affect these three aspects of a reaction, and more.
Fig. 1: Visualization of a catalyst.
Specifically, there are five aspects this article will go into: the type of catalyst, how it speeds up a reaction (mechanism of action) and how it affects the stereochemistry of products. We will also look at enzymes and an example of a catalyzed reaction, catalytic hydrogenation.
But before we can get into that, we must know what exactly a catalyst is. The definition of a catalyst, according to the IUPAC Gold Book, is “a substance that increases the rate of a reaction without modifying the overall standard Gibbs energy change in the reaction”.
For this Gibbs energy to remain unmodified by the catalyst, it must be chemically unaffected throughout the entire reaction. This does not mean it has to be chemically inert, and only unaffected by the reaction conditions.
This criterion is very important when considering what is or is not a catalyst. It is usually not possible for a reactant to speed up a reaction and be used up in the process, except if a different mechanism is followed when this new reactant is added.
Enzymes (Fig. 2) are examples of catalysts biologically, because they are able to speed up the process of breakdown of some molecules without being chemically changed themselves. However, they are considered separate from the field of chemistry and are discussed at the end.
Fig. 2: Visualization of an enzyme.
Based on what we have discussed in the criterion, is it possible for a physical change, such as an increase in temperature or pressure, to be termed a ‘catalyst’? Technically, a catalyst has to be a chemical substance, which means that such physical changes cannot be termed catalysts, especially in the scientific field.
Furthermore, as we will learn later, catalysts speed up reactions differently as compared to heat or other physical changes. This will be explained in detail next.
How does a catalyst speed up a reaction?
In terms of chemistry, there are a few ways to explain how the catalyst speeds up the reaction. We can either analyse the reaction rate (physical chemistry) or look at the mechanistic pathway (organic chemistry). Although I’m more of an organic person, we will still look at both aspects.
We start with the physical chemistry aspect first. Simply stated, a catalyst lowers the activation energy of a reaction (Fig. 3). How does this increase the rate of a reaction?
Fig. 3: Activation energy graph.
Firstly, lowering the activation energy allows for more successful collisions between reactant molecules. By the kinetic particle theory, reactant particles collide to react, but only if they collide with a sufficient amount of energy (the activation energy).
Thus, by lowering the activation energy, there are more successful collisions in a given amount of time, which increases the rate of reaction. That is the physical aspect of catalysis.
However, I always feel that it is superficial: why does a catalyst lower the activation energy of a reaction? To answer this question, we have to look at the organic aspect of it. Catalysts are able to increase the rate of the reaction by two ways: changing the mechanistic pathway or increasing the stability of the intermediate.
For example, a reaction, A + B, occurs at a slow rate. By using C, the catalyst, an alternative reaction mechanism occurs, which is much faster and yet does not chemically change C. A real-life instance of C would be transition metal catalysts like palladium or platinum.
Increasing the stability of an intermediate will also help with the rate of the reaction, because it makes the forward reaction more energetically favorable. If we were to go into detail on this, it would broach a bit more into the physical and not the organic part of chemistry.
What types of catalysts are there?
What are the types of catalysts? There are, in general, two types of catalysts, which is heterogeneous and homogeneous catalysts. Heterogeneous catalysts possess a different state from the reaction medium. For example, a solid catalyst and a liquid solvent.
Homogeneous catalysts, in contrast, possess the same state as the reaction medium. For instance, a liquid catalyst and a liquid solvent. There are many examples of both heterogeneous and homogeneous catalysts, but they are featured most prominently for hydrogenation reactions.
In a hydrogenation reaction of alkenes, a solid catalyst, Raney nickel, is used, while the reaction medium is usually a liquid. As such, Raney nickel is termed a heterogeneous catalyst. There is a reason why it is a heterogeneous catalyst, but this will be discussed later in ‘what is catalytic hydrogenation’?
How does a catalyst affect stereochemistry?
The catalyst will affect the stereochemistry of a product if either the reactant and the catalyst are chiral. More information about chirality, enantiomers and stereochemistry are found in some articles I have previously written.
For two different enantiomers, they may also react at different rates with chiral compounds, which includes chiral catalysts. The rates can vary greatly up till deciding whether a reaction is feasible or not.
If neither the catalyst nor the reactants are optically active, then the product will also not be optically active. Furthermore, if the starting material were racemic, the product of the reaction will also be racemic.
What are enzymes and how do they speed up reactions?
Enzymes are termed ‘biological catalysts’ for an important reason, because they are able to speed up the breakdown of biological reactants, and are not used up in the process. Thus we must also consider this when we are discussing catalysts.
Enzymes work only by one simple mechanism, formulated as the lock-and-key theory. It suggests that the enzyme is the lock and the substrate (which is being broken down) is the key. As such, for every substrate there is a specific type of enzyme.
An enzyme is able to speed up a reaction by facilitating the breakdown of the substrate (Fig. 4). By the lock-and-key theory, the substrate enters an active site of the enzyme (a ‘hole’). It is subsequently broken down into individual parts by the enzyme.
Fig. 4: Mechanism of an enzyme.
We will not, however, go into the specifics of this, because this is a chemistry blog, not a biology one!
What is catalytic hydrogenation?
Finally, we will be learning about catalytic hydrogenation. In catalytic hydrogenation, the catalyst is Raney nickel, a heterogeneous one. How is it able to speed up the reaction? We have long learnt that reactants of different states of matter cannot react.
We mentioned before that from an organic perspective, there are two ways a catalyst can work: by changing the mechanistic pathway or stabilizing the intermediate. Catalytic hydrogenation does the former, and a different mechanism operates.
Typically catalytic hydrogenation will take place at relatively moderate temperatures, while uncatalyzed hydrogenation has to take place at extremely high temperatures. That is the power of the heterogeneous catalyst.
Now, let us learn how Raney nickel changes the mechanistic pathway. Instead, something special, known as adsorption, occurs (Fig. 5). This means that a multiple bond ‘sticks’ to the surface of the metal, but without the formation of a chemical bond. This adsorption is affected by steric hindrance.
Fig. 5: Adsorption of multiple bonds.
Anyway, once the multiple bond has adsorbed, a molecule of hydrogen (H2) is also adsorbed. By adsorption, it breaks into two individual hydrogen atoms, which are radicals. In the next step, the hydrogen atom attaches to an alkene carbon, breaking the multiple bond. The same process is done for the other alkene carbon, resulting in the alkane.
P.S. For those who are wondering why this article is not about molecular orbital theory, here is the answer. To cover molecular orbital theory, I could either cover just the most basic definition and aspects of it, or I could delve deeper and give a more detailed article.
Of course, I wanted to do the latter, but my professor assigned a piece of coursework that is highly tedious (I won’t go into the details). So here is a simple article on catalysis instead. I may go back to the molecular orbital theory article when I have time, hopefully in late February or March.