Are you a LEGO fan? To build a LEGO set, you follow a series of step-by-step diagrams showing you how to combine individual bricks to make a complex set. The diagrams are pretty handy. By relying solely on pictures and avoiding any written language, the instructions can be understood by anyone around the world, regardless of their native language. Plus, the diagrams show you exactly where to put each piece and the correct way to orientate it, so there is no confusion whatsoever. When it comes to LEGO sets, the old saying is right - a picture really is worth a thousand words!
In chemistry, we use a similar principle to show reactions. Instead of trying to describe how molecules react using words, we show the process using a clear, visual reaction mechanism: a step-by-step description of the changes involved in a chemical reaction. In this article, we'll learn all about the steps and process of drawing reaction mechanisms.
- This article is about drawing reaction mechanisms in organic chemistry.
- We'll define reaction mechanisms before looking at why they are important.
- We'll then learn the rules of drawing reaction mechanisms.
- After that, we'll discuss the steps involved in drawing reaction mechanisms.
- You'll be able to practice your skills with our worked examples.
- To finish, we'll briefly consider the four arrow pushing patterns in reaction mechanisms.
Reaction mechanism: definition
In the introduction, we gave you the definition of a reaction mechanism.
A reaction mechanism is a step-by-step description of the changes involved in a chemical reaction.
You should know that we can use chemical equations to represent a reaction. However, these only show the reaction's starting reactants and final products. Reaction mechanisms, on the other hand, show all the individual changes within the reaction, such as which old bonds are broken and which new bonds are formed. They might also show the action of catalysts. In addition, they show the formation of intermediates: species made in the process of the reaction which then change into something else. All in all, the additional information that reaction mechanisms give us makes them much more useful for finding out about the finer details of a reaction than a simple chemical equation.
Reaction mechanisms typically show the following:
- Organic species involved in the reaction, such as the reactants, products, and intermediates.
- Charges and partial charges.
- Lone pairs of electrons and unpaired electrons.
- The movement of electrons, both homolytically and heterolytically.
- A change in functional group.
Like with LEGO instructions, we draw reaction mechanisms using clear, simple diagrams. Sure, you could try and describe a reaction using words, but this can be confusing and ambiguous (as you'll see later on in the article). Written text also relies on a good grasp of the language. Instead, by using diagrams that follow set-in-stone rules, we can break down a chemical reaction into a series of visual steps that anyone can understand.
Drawing reaction mechanisms: rules
Reaction mechanisms only make sense if you know how to interpret them. They follow specific rules to make them universally understandable. Fortunately, the rules are fairly straightforward. Let's explore them now.
Organic species
Firstly, we need to find out how we show organic species in reaction mechanisms:
- We typically represent organic species using displayed formulae. These show every bond and atom in the molecule. We represent single bonds with single straight lines and double bonds with a pair of straight lines.
- However, if we have a particularly large or complicated molecule, we might use a modified displayed formula. In this formula, we group some of the carbons and hydrogens into alkyl groups. Modified displayed formulae make the mechanism clearer, less cluttered, and easier to understand.
- We could also replace part of the molecule that isn't involved in the reaction with an R group.
- Charges are written slightly above and to the right of the relevant atom or group. Partial charges are shown using the delta symbol (δ).
Here are a few different ways of representing the intermediate carbocation CH3CH2+ formed during the addition reaction between ethene (CH2CH2) and hydrogen bromide (HBr):
Fig. 1: Ways of displaying organic species in reaction mechanisms. Here, we show a positive carbocation.Vaia Originals
Head over to Organic Compounds for a guide to the different types of formulae we use to represent molecules in organic chemistry. You can also find out more about the particular addition reaction between ethene and hydrogen bromide in Reactions of Alkenes.
Electrons
Many species are particularly reactive because of their electron configuration. For example, they might have a lone pair of electrons which they try to donate, making them a nucleophile. Other species could be unstable free radicals with an unpaired electron! Because both unpaired electrons and lone pairs of electrons are so important to chemical reactions, we show them in our reaction mechanisms.
- Lone pairs of electrons are shown using two dots.
- Unpaired electrons in free radicals are shown using one dot.
Here is a negative hydroxide ion (OH-) and a chlorine radical (Cl). We've included the negative charge of the hydroxide ion.
Fig. 2: Representing electrons in reaction mechanisms. We use two dots to show lone pairs of electrons and a single dot to represent an unpaired electron.Vaia Originals
Electron movement
We learned earlier on that reaction mechanisms break down a reaction into smaller steps. Each step involves one particular thing: the movement of electrons. We show this movement using curly arrows. Curly arrows tell you how electrons are transferred from one bond or atom to another, and thus represent the breaking and making of bonds.
- Curly arrows start at the lone pair of electrons or covalent bond that the electrons are coming from.
- They finish at the atom or bond where the electrons end up.
- Full-headed curly arrows show the movement of a pair of electrons, such as in heterolytic fission. In this type of electron movement, two electrons are transferred to the same atom or bond.
- Half-headed curly arrows show the movement of a single electron, such as in homolytic fission. In this type of electron movement, just one electron is transferred to an atom or bond.
Heterolytic fission is a type of bond breaking where both electrons are transferred to a single species. On the other hand, homolytic fission is a type of bond breaking where the bonded electron pair splits up. Here, one electron goes to each of the two species involved.
We could form the hydroxide ion we saw above by splitting an O-H bond in water using heterolytic fission. This involves the movement of a pair of electrons and so uses a full-headed arrow. Conversely, chlorine free radicals are formed in homolytic fission. This involves the movement of single electrons and so uses half-headed curly arrows.
Fig. 3: Heterolytic and homolytic fission in organic reaction mechanisms. Note the different types of curly arrows used.Vaia Originals
You don't have to use a different colour for your curly arrows - we've simply drawn them in this way to help you spot them in the mechanism a little more easily. However, you should make sure that your curly arrows always start from either a covalent bond or a lone pair of electrons, and end at an atom or another bond.
Drawing reaction mechanisms: steps
Now that we know the basic rules for drawing reaction mechanisms, we can look at how we use them to represent the inner workings of a chemical reaction. Here are some handy steps you should follow when drawing reaction mechanisms.
- First, draw the reactants out using displayed formulae.
- Show how these reactants react using curly arrows to represent the movement of electrons as bonds break and form.
- Chemical reactions are a step-by-step process - some bonds must break before others can form (and vice versa) whilst some break or form at the same time. You should draw all arrows showing simultaneous electron movement in the same step.
- The electron movement changes the structure of the reactants to form new intermediates. Draw the intermediates formed in a new step. Once again, add curly arrows to represent the electron movement in this second step of the reaction to show how the intermediates react further.
- Continue adding steps, complete with displayed formulae and curly arrows, until you arrive at your final products. Each step should show the intermediate products formed in the step directly before it, and how these intermediates react in the next stage of the reaction.
- Finally, link the steps using straight arrows to show the progress of the reaction.
Make sure to include the end products of your reaction in your reaction mechanism using one final diagram, once again linked to the steps before it with a straight arrow!
Let's see an example.
Read the following text describing the addition reaction between ethene (CH2CH2) and hydrogen bromide (HBr), which we introduced you to earlier on, and compare it to the reaction mechanism we show below. This particular reaction has two steps. However, we also show the final products of the reaction in a final diagram, so you should end up with three parts to your mechanism.
Here's what happens:
- In the first step, HBr is attracted to the high electron density of ethene's C=C double bond. The C=C bond breaks heterolitically and both electrons are used to form a bond with H from HBr. At the same time, the H-Br bond in HBr breaks heterolitically, and both electrons are transferred over to Br.
- Overall, the first step creates a positive carbocation, as well as a negative bromide ion with a lone pair of electrons. These products carry on over to the second step. Here, the lone pair of electrons from the negative bromide ion are used to form a bond with the positive carbocation. Both electrons end up at the same place, and so we continue using full-headed arrows.
- The end product is a halogenoalkane.
Fig. 4: The reaction mechanism for the addition reaction between ethene and hydrogen bromide.Vaia Originals
Don't worry if you don't understand the ins and outs of this reaction - you'll learn all about it in Reactions of Alkenes. For now, concentrate on relating the movement of electrons that we describe in the text to the arrows shown in the reaction mechanism diagram. You should hopefully see how using simple arrows and displayed formulae make the individual steps within a chemical reaction much easier to understand, compared to trying to explain the process in written text!
Drawing reaction mechanisms: examples
With any luck, you now feel confident at drawing reaction mechanisms, and you understand their benefits when it comes to examining chemical reactions. Have a go at applying your knowledge to the following example problems. You'll be able to practice drawing more reaction mechanisms, as well as predicting the products of a reaction from its mechanism.
Practice interpreting the following written text and turning it into a reaction mechanism diagram:
A hydroxide ion (OH-) with a negative charge and a lone pair of electrons attacks the polar molecule bromoethane (CH3CH2Br). The lone pair of electrons is used to form a C-OH bond. At the same time, the C-Br bond in bromoethane breaks heterolytically; both electrons are transferred to Br. The reaction produces ethanol (CH3CH2OH) and a negative bromide ion (Br-).
Well, we start by drawing the structures of the two species mentioned (bromoethane and the hydroxide ion), making sure to include their charges and partial charges. We then consider what happens to the molecules as they react and how we show this in a mechanism.
Firstly, the text tells us that one new C-OH bond is formed, using the hydroxide ion's lone pair of electrons. Therefore, we draw a full-headed arrow from the hydroxide ion's lone pair of electrons to one of the carbon atoms in bromoethane to show the movement of electrons.
Secondly, the text also informs us that an old C-Br bond is broken. This happens at the same time as the C-OH bond is formed, and so occurs in the same step. In particular, the bond breaks heterolytically, and both bonded electrons move to Br. Therefore, we draw a full-headed arrow from the C-Br bond to the bromine atom.
To finish, we draw our final products adjacent to the first part of the mechanism, linking the two diagrams with a straight arrow.
Here's what you should end up with:
Fig. 5: The reaction mechanism for the nucleophilic substitution reaction between bromoethane and the hydroxide ion.Vaia Originals
Note that all the electron movement involved a pair of electrons, and so we used exclusively full-headed arrows in this mechanism. Also notice that we showed both products: the alcohol and the negative bromide ion. Make sure to include the bromide ion's negative charge.
The following mechanism shows the first step of a chemical reaction. Use your knowledge to draw the structure of the intermediate formed.
Fig. 6: An unknown reaction mechanism. Can you draw the intermediate formed? Vaia Originals
To tackle this question, we take the organic species shown and consider how its structure changes as it reacts, using what the mechanism diagram tells us about the electron movement within this step of the reaction.
We can see that there are two species present: an aldehyde (RCHO) and a cyanide ion (CN-). The cyanide ion has a negative charge and a lone pair of electrons. The full-headed arrow that comes from the cyanide ion's lone pair of electrons and goes to the aldehyde's C=O carbon atom shows us that the lone pair of electrons is used to form a C-CN bond. This means that the cyanide ion joins onto the aldehyde and loses its negative charge. The second full-headed arrow, coming from the C=O bond and ending at the C=O oxygen atom, tells us that one of the C=O covalent bonds breaks, and both of its electrons move to O. This means that O ends up with a lone pair of electrons and a negative charge.
Here is the final intermediate formed:
Fig. 7: The intermediate formed in the unknown reaction mechanism shown above.Vaia Originals
The first of these two example questions showed nucleophilic substitution, which you'll discover further in Nucleophilic Substitution Reactions and Nucleophilic Substitution Mechanism. The second introduced you to the first step of the reduction of an aldehyde, another reaction you need to learn about for your A-level exam. You'll explore its workings in Reactions of Aldehydes and Ketones.
Drawing reaction mechanisms: patterns
The reaction mechanisms that we’ve looked at today all show the movement of electrons using curly arrows. This technique is known as electron pushing (or arrow pushing) and was first developed by Sir Robert Robinson in the 20th century. Although electron pushing may seem complicated from afar, you’ll hopefully have realised today that it isn’t too tricky to understand. In fact, all electron pushing organic reaction mechanisms are based on just four specific types of electron movement, which we call patterns. These four are:
- Nucleophilic attack.
- Loss of a leaving group.
- Proton transfer.
- Rearrangement.
In the first of the two examples above, we drew a mechanism for the nucleophilic substitution reaction between bromoethane and the hydroxide ion. This reaction features two different electron pushing patterns:
- A C-OH bond is formed, which is an example of nucleophilic attack.
- At the same time, the C-Br bond is broken, which is an example of the loss of a leaving group.
Electron pushing patterns rely on knowledge of nucleophiles and electrophiles, which you can learn about in Types of Reaction Mechanism. You'll discover more reactions involving these species as you progress further through organic chemistry, and they are an important part of your A-level studies. However, knowledge of electron pushing patterns will not be tested in your exams!
Drawing Reaction Mechanisms - Key takeaways
- Reaction mechanisms are step-by-step descriptions of the changes involved in a chemical reaction.
- When drawing reaction mechanisms, you should obey the following rules:
- Represent species using displayed or simplified displayed formulae.
- Show lone pairs of electrons using two dots and unpaired electrons using single dots.
- Show electron movement using curly arrows.
- Draw the reaction in multiple steps, grouping together simultaneous electron movement. Each step should show how the reactant or intermediate produced in the step beforehand changes.
- Link the steps of the reaction using straight arrows.
Explanations 
Textbooks 
Exams 
Magazine 
