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You probably followed a recipe. Recipes can be very precise - combine 100g of this with two teaspoons of that, stir three times, and bake for exactly 22 minutes. They walk you through the process of baking, one step at a time. It's not just as simple as throwing everything into a bowl and hoping for the best!
Recipes are to baking as reaction mechanisms are to chemistry. All organic reactions have various steps that the overall equation simply doesn't show. However, with reaction mechanisms, we can peel apart a reaction to see its inner workings and go through it step-by-step.
- This article is about reaction mechanisms in organic chemistry.
- We'll start by defining reaction mechanism.
- We'll then look at different types of reaction mechanisms, including substitution, addition, and elimination reactions.
- We'll also explore nucleophilic and electrophilic reactions.
- Finally, we'll consider the importance of reaction mechanisms.
What are reaction mechanisms?
At the start of this article, we introduced you to the idea of reaction mechanisms.
A reaction mechanism is a step-by-step description of the changes involved in a chemical reaction.
You can think of reaction mechanisms as instructions for building a new chemical molecule. It might involve dismantling an old one and putting the pieces back together, or perhaps combining lots of smaller molecules into one larger one, or maybe just simply swapping one functional group for another. However, these aren't instantaneous processes. They all involve lots of smaller steps that aren't normally visible to the eye. Reaction mechanisms break down the process and show you each step along the way.
Reaction mechanisms usually involve diagrams. These show a few things.
- The reactants.
- The products.
- The intermediates.
- The movement of electrons.
- The breaking and formation of bonds.
Intermediates are highly reactive, short-lived compounds that exist for a fraction of a second in a chemical reaction. Once formed, they quickly react and turn into more stable compounds.
If we go back to our baking analogy, the reactants are like the basic ingredients of the cake: flour, butter, and sugar. The products are, of course, the finished cake. Each step in the reaction is akin to another instruction in the recipe - weigh the flour out, beat the butter and sugar together. At the end of each instruction, you've created something new, something that's different from your starting ingredients, but will change again before it becomes your finished cake. This represents your intermediates. Intermediates are simply species made in the process of the reaction which then change into something else.
Drawing reaction mechanisms
We now know what a reaction mechanism is. But how exactly do you show one?
There are a few key ideas that you need to know when it comes to drawing reaction mechanisms. We explore these ideas in much more detail in Drawing of Reaction Mechanisms, but we'll summarise the main points now:
- Organic molecules are typically shown using displayed formulae, which show every bond and atom in the molecule. However, large or complicated molecules use a type of modified displayed formula, making the species simpler to understand.
- We show all charges using the positive and negative signs, + and -. Partial charges are represented by the delta symbol, δ.
- A pair of dots represents a lone pair of electrons, whilst a single dot represents an unpaired electron. These are typically found in free radicals.
- Curly arrows show the movement of electrons. Full-headed arrows are used to show the movement of an electron pair, whilst half-headed arrows show the movement of a single electron.
Reaction mechanism types
Reaction mechanisms can seem a little tricky. You might wonder how you will ever remember all the different movements of electrons, but in actual fact, reaction mechanisms can be grouped into a few different categories. Once you know the basic mechanism, it is easy to apply it to a specific reaction. These kinds of reactions include:
- Substitution reactions.
- Addition reactions.
- Elimination reactions.
We'll walk you through an example of each. However, you can always check out Nucleophilic Substitution Reactions, Reactions of Alkenes, and Elimination Reactions respectively, for more detailed explanations of these terms and concepts. We'll then consider the difference between nucleophilic and electrophilic reactions.
Substitution reaction mechanism
Substitution reactions are reactions in which an atom or functional group in a molecule is replaced by a different atom or functional group.
In substitution reactions, a molecule is attacked by a particular species. This species replaces a different atom or functional group on the original molecule. An example of a substitution reaction is the reaction between bromoethane (CH3CH2Br) and a hydroxide ion (OH-). In this case, the hydroxide ion replaces the bromine atom, resulting in a bromide ion and an organic compound with a hydroxyl group. Here's the mechanism.
Fig. 1 - Nucelophilic substitution reaction mechanism
The hydroxide ion has a lone pair of electrons. This lone pair of electrons attacks bromoethane’s partially positive carbon atom. The electrons are transferred from the hydroxide ion, and used to make a new covalent bond between the carbon atom and the hydroxide ion. Their movement is shown using a curly arrow. At the same time, the C-Br bond breaks and the electron pair is transferred to the bromine atom, forming a bromide anion (Br-). Once again, electron movement is shown using a curly arrow.
Note that the bromide ion also has a lone pair of electrons, represented by two dots.
Addition reaction mechanism
Addition reactions are reactions in which two molecules combine to form one larger molecule, with no other products. They involve breaking a double or triple bond.
In addition reactions, a double or triple bond breaks and the electron pair is used to form a single covalent bond with another species. An example is reacting an alkene such as ethene (CH2CH2) with hydrogen bromide (HBr). Here's the mechanism.
Fig. 2 - Electrophilic addition reaction mechanism
In the first step, one of the electron pairs involved in ethene's C=C double bond attacks the partially charged hydrogen atom in hydrogen bromide. This forms a C-H single bond and leaves behind both an organic molecule with a positive carbon ion, called a carbocation, and a bromide ion (Br-). In the second step, the bromide ion adds to the carbocation, using its lone pair of electrons to form a single covalent bond. This forms bromoethane (CH3CH2Br).
Elimination reaction mechanism
Elimination reactions are reactions in which two substituents are removed from a larger molecule. The substituents come together to form a smaller molecule.
In elimination reactions, two smaller species are removed from a larger molecule. These species generally react together to form a new product, and a double bond forms in the initial larger molecule. They are the reverse of addition reactions.
We already looked at the reaction between bromoethane and the hydroxide ion as a substitution reaction, but under different conditions, it can actually be an elimination reaction. This produces ethene, water, and a bromide ion. Take a look at the reaction mechanism:
Fig. 3 - Elimination reaction mechanism
One of ethene's hydrogen atoms is first attacked by the hydroxide ion. The hydroxide ion uses its lone pair of electrons to form a bond with hydrogen, producing water; the repsective C-H bond breaks and its electrons are used to turn an adjacent C-C single bond into a C=C double bond. This causes the C-Br bond to break heterolytically. The pair of electrons from this bond is transferred to the bromine atom, which is released as a bromide ion.
Other types of reaction mechanism
In organic chemistry, you might also come across these further types of reaction.
- In hydrolysis reactions, a molecule is broken down by water.
- In oxidation reactions, a species loses electrons. Oxidation can also be used to describe the addition of oxygen or the removal of hydrogen.
- In reduction reactions, a species gains electrons. Chemically, reduction is the reverse of oxidation and can also be used to describe the removal of oxygen or the addition of hydrogen.
Head over to Redox to learn more about oxidation and reduction reactions.
Nucleophilic and electrophilic reactions
You might have noticed in some of our examples above that we used terms such as nucleophilic and electrophilic. For example, the substitution of bromoethane was a nucleophilic reaction, whilst the addition of ethene was an electrophilic reaction. What do these words mean?
Well, they refer to the type of species responsible for the reaction. In nucleophilic reactions, the targeted organic molecule is attacked by a nucleophile, whilst in electrophilic reactions, the organic molecule is attacked by an electrophile.
Nucleophiles are electron pair donors. On the other hand, electrophiles are electron pair acceptors.
Importance of reaction mechanisms
You should now feel confident at drawing and interpreting reaction mechanisms for a variety of different reactions. But why are reaction mechanisms important?
As we explored earlier, reaction mechanisms are step-by-step guides to a chemical reaction. They offer the following benefits:
- They break a reaction down into separate chunks, each of which can be analysed separately.
- They show the movement of electrons.
- They show us any intermediates formed in the reaction.
- They let us see the effect of catalysts on a reaction.
- They show us the transition state of a reaction, helping us work out the reaction's activation energy.
- They help us determine the kinetics of a chemical reaction. In other words, they give us information about the rate of reaction.
- Knowing how a reaction proceeds can help us choose favourable conditions.
Reaction mechanism and rate of reaction
Are you ready to learn more? For those of you wanting to stretch your understanding, we're now going to take a deep dive into how reaction mechanisms relate to the rate of reaction, and the order of a reaction.
Reaction mechanisms show the individual steps of a chemical reaction. Each step is called an elementary process, or elementary step, and represents a geometric change in the molecules involved in the reaction. You can think of an overall chemical reaction as a sequence of multiple elementary processes.
Elementary processes can be uni-, bi- or termolecular, depending on how many molecules they involve.
- Unimolecular elementary processes involve just one molecule.
- Bimolecular elementary processes involve two molecules. These could be from the same species or from different species.
- Termolecular elementary processes involve three molecules. Once again, these could all be from the same species, or from different species.
Termolecular elementary processes are relatively rare. For a reaction to occur, molecules need to collide at just the right time, with enough energy, and just the right orientation. It's quite uncommon for two molecules to do this, let alone three!
So, how do elementary processes relate to rate equations?
In Rate Equations, we explored what a rate equation is: an equation showing how the rate of a chemical reaction depends on the concentration of certain species. Reactions all have a rate-determining step. In other words, they have a rate-determining elementary process. This is the slowest part of a reaction, and all the species involved in elementary processes up to and including this step feature in the rate equation. Rate laws can be determined for each elementary process, showing how the rate of each step depends on a particular species.
The combined rate laws of all of the steps up to and including the rate-determining elementary process make up the rate equation. If we are given information about a rate equation and a reaction mechanism, we can work out the rate-determining step of a reaction, and vice versa.
Here's a handy table showing how elementary processes and rate laws are linked for three imaginary species. Let's call them A, B, and C, and we'll name the product D.
| Type of elementary process | Equation | Rate law |
| Unimolecular | k = [A] | |
| Bimolecular | k = [A]2 | |
| k = [A] [B] | ||
| Termolecular | k = [A]3 | |
| k = [A]2 [B] | ||
| k = [A] [B] [C] |
Here's the rate equation for the reaction:
k = [H2] [ICl]
You'll notice that the rate equation for this reaction doesn't involve all of the molecules present in the overall equation. In fact, it only features one molecule of hydrogen (H2) and one molecule of iodine monochloride (ICl). This means that the only species that feature in the steps up to and including the rate-determining elementary process are one molecule of hydrogen, and one molecule of iodine monochloride. We can therefore predict that the overall reaction mechanism has two distinct steps.
In the first step, one molecule of hydrogen and one molecule of iodine monochloride react to form an intermediate and hydrogen chloride (HCl):
In the second step, the intermediate reacts with another molecule of iodine monochloride to form hydrogen chloride and iodine (I2):
Another example is the reaction between nitrogen dioxide (NO2), and carbon monoxide (CO). It has the following equation and rate equation:
k = [NO2]2
The rate equation features two molecules of nitrogen dioxide, but no molecules of carbon monoxide. We can therefore predict that the reaction again takes place in two distinct steps. The first step involves two molecules of nitrogen dioxide reacting to form nitrogen monoxide (NO) and an intermediate. This must be the rate-determining step, as these two nitrogen dioxide molecules are the only molecules involved in the rate equation. In the second step, the intermediate reacts with carbon monoxide to form nitrogen dioxide and carbon dioxide. You can see this below:
If we combine the two equations, one of the nitrogen dioxide molecules and the intermediate molecule appear on both the left- and the right-hand side, and so don't feature in the reaction's overall equation:
Reaction Mechanism - Key takeaways
- A reaction mechanism is a step-by-step description of the changes involved in a chemical reaction.
- Reaction mechanisms show the reactants, products, and intermediates in a chemical reaction. They also show the movement of electrons and the making and breaking of bonds.
- Reaction mechanisms are generally drawn using displayed formulae. We show lone pairs of electrons using two small dots, free radicals using one dot, and the movement of electrons using curly arrows.
- Types of reaction mechanism include substitution, addition, and elimination reactions.
- Reaction mechanisms are useful because they break a reaction down into smaller parts, each of which can be analysed separately.
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Frequently Asked Questions about Reaction Mechanism
What are reaction mechanisms?
Reaction mechanisms are step-by-step descriptions of the changes involved in a chemical reaction.
How do you draw reaction mechanisms?
You draw reaction mechanisms using displayed formulae, and curly arrows to show the movement of electrons. Make sure to include partial charges, ions, free radicals, and lone pairs of electrons on your diagram.
How are reaction mechanisms determined?
It can be quite hard to determine reaction mechanisms experimentally because they happen extremely quickly, on a microscopic level. However, techniques include measuring the enthalpy change of the reaction to determine activation energy, measuring the effect of ionic strength on rate of reaction, and detecting the stereochemistry of reactants, products, and intermediates at different stages of the reaction.
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