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Have you ever baked a cake?
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.
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.
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 like another instruction in the recipe - weigh this out, beat that 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.
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'll walk you through them, using a couple of examples along the way.
We first need to learn how you represent different species in reaction mechanisms. We'll start with organic molecules.
Organic molecules are generally shown in reaction mechanisms using displayed formulae. These show every bond and atom in the molecule. You represent single bonds with single straight lines and double bonds with a pair of straight lines. Here's an example showing bromoethane (CH3CH2Br).
However, if you have a particularly large or complicated molecule, you might want to use a modified displayed formula. In this type of formula, you group some of the carbons and hydrogens together into methyl groups. It makes the mechanism clearer and easier to understand.
Head over to Organic Compounds for a guide to the different types of formulae we use to represent molecules.
You might know that bromoethane is a polar molecule. The carbon atom in the C-Br bond is partially positively charged, while the bromine atom is partially negatively charged. To represent partial charges, we use the delta symbol, δ. Draw it slightly above and to the side of the appropriate atom, as we've shown below:
In reaction mechanisms, organic molecules are often attacked by ions. We tend to show ions using letters with any charges in the upper corner, as you would write them in a chemical equation. However, we also show any lone pairs of electrons. We do this using two small dots. For example, here's a hydroxide ion (OH-):
Whilst ions often have pairs of electrons, sometimes you come across species with one single unpaired electron. These are known as free radicals.
A free radical is an atom, ion, or molecule with an unpaired outer shell electron. They are extremely reactive and typically short-lived.
To represent the unpaired electron in a free radical, we use a single dot. Here's an example of a chlorine radical:
We know that reaction mechanisms break down a reaction into smaller steps. Each step involves one particular thing: the movement of electrons. We use curly arrows to show electron movement. They show how electrons are transferred from one bond or atom to another, thus representing the breaking and making of bonds.
Curly arrows start at the atom or bond that the electrons are coming from, and end at the atom or bond where you want your electrons to end up. However, depending on the number of electrons moving, you might need either a full-headed curly arrow, or a half-headed curly arrow.
Full-headed curly arrows show the movement of a pair of electrons. This is the most common type of electron movement in reaction mechanisms. One example of this is heterolytic fission.
Heterolytic fission is a type of bond breaking where both electrons are transferred to a single species.
In this step of the reaction mechanism shown below, a C-Cl bond is breaking. Both electrons move to the chlorine atom, forming a chlorine ion. This is an example of heterolytic fission, so we use a full-headed curly arrow.
Sometimes, we want to show the movement of just one electron. This is where half-headed arrows come in. They represent the movement of a single electron. You might use half-headed arrows when working with free radicals or homolytic fission.
Homolytic fission is a type of bond breaking where the bonded electron pair splits up, one electron going to each of the two species involved.
Here’s an example of homolytic bond fission. Here, the C-Cl bond breaks apart, and one electron goes to the carbon atom whilst the other electron goes to the chlorine atom.
Your curly arrows should always start from a covalent bond or a lone pair of electrons.
Now that we’ve covered the basics, we can have a go at drawing a reaction mechanism. We’re going to use the example of the nucleophilic substitution of bromoethane using a hydroxide ion, which produces ethanol and a bromide ion.
This reaction is explained in more depth in Substitution Reactions.
When drawing reaction mechanisms, use the following steps.
Let’s apply this to our reaction. This is what the mechanism should look like:
Let’s break it down a little.
In the first step, we start with bromoethane (CH3CH2Br) and a hydroxide ion (OH-). Bromoethane has partial charges, and the hydroxide ion has a negative charge and 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. This 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, this is shown using a curly arrow.
This mechanism only really has one step, because both pairs of electrons move at the same time. However, it is helpful to draw out the products of the reaction as an additional step. In this second step, we can see that we have produced ethanol and a bromide ion. Note the following:
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:
We'll walk you through an example of each.
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. We've already seen an example of a substitution reaction: the reaction between bromoethane and a hydroxide ion. 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 again.
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.
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 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 reactions are reactions in which two atoms or groups of atoms are removed from a larger 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:
A hydrogen atom 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 C-H bond breaks and the 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.
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:
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.
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 mechanisms are step-by-step descriptions of the changes involved in a chemical reaction.
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.
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.
What is a reaction mechanism?
A step-by-step description of the changes involved in a chemical reaction.
What do you show in a reaction mechanism?
The movement of electrons
What is each step in a reaction mechanism also known as?
An elementary process.
True or false? Reaction mechanisms show intermediates.
What are intermediates in a chemical reaction?
Highly reactive, short lived compounds that exist for a fraction of a second in a chemical reaction.
Which type of formula do you typically use in reaction mechanisms?
What symbol do we use to represent partial charges in reaction mechanisms?
What do we use to show the movement of electrons in reaction mechanisms?
Curly arrows. A full-headed curly arrow shows the movement of a pair of electrons, whilst a half-headed curly arrow shows the movement of a single electron.
True or false? Curly arrows always start at a covalent bond.
What is heterolytic fission?
A type of bond breaking where both electrons are transferred to a single species.
What is homolytic fission?
A type of bond breaking where the bonded electron pair splits up and one electron goes to each of the two species involved.
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