Reaction Mechanism
The Basic Components of a Mechanism
Before we dive into the steps, let's meet the key players in any reaction mechanism.
| Component | Description | Simple Analogy |
|---|---|---|
| Reactants | The starting substances in a chemical reaction. | Flour and eggs for a cake. |
| Products | The substances that are formed by the reaction. | The finished cake. |
| Reaction Intermediate | A temporary, unstable species formed during the reaction steps but not present in the final products. | The mixed batter before it goes into the oven. |
| Activation Energy | The minimum energy required for a reaction to start. It's like a hill the reactants must climb. | The effort needed to push a boulder over a hill before it rolls down by itself. |
| Transition State | The highest-energy point in a reaction step, where bonds are partially broken and partially formed. It cannot be isolated. | The precise moment at the top of the hill when the boulder is perfectly balanced before falling. |
Following the Electrons: Curved Arrow Formalism
Curved arrows are the language of reaction mechanisms. They are like a map that guides us through the reaction. The most important rule is: arrows always start from a source of electrons (a bond or a lone pair) and point to where those electrons are going (an atom or a bond).
For example, when a hydroxide ion (OH$^{-}$) reacts with hydrochloric acid (HCl), the mechanism is simple. The hydroxide ion has a lone pair of electrons. The hydrogen in HCl has a partial positive charge because chlorine is more electronegative. The arrow starts from the oxygen's lone pair and points to the hydrogen atom, forming a new O-H bond. Simultaneously, the H-Cl bond must break, so a second arrow is drawn from the H-Cl bond to the chlorine atom, giving it a lone pair and forming a chloride ion (Cl$^{-}$). The products are water (H_2O) and chloride ion.
Common Types of Elementary Steps
A reaction mechanism is made up of one or more "elementary steps"—the simplest single events in a reaction. Most organic and biochemical reactions involve a few common types of steps.
| Step Type | Description | Example |
|---|---|---|
| Heterolytic Bond Cleavage | A bond breaks unevenly, with both electrons going to one atom, forming a cation and an anion. | A-B → A$^{+}$ + B$^{-}$ |
| Homolytic Bond Cleavage | A bond breaks evenly, with one electron going to each atom, forming two radicals. | A-B → A• + B• |
| Coordination | A species with a lone pair (a Lewis base) donates both electrons to form a new bond to an electron-deficient atom (a Lewis acid). | N: + B → N-B (where N: is the lone pair) |
| Proton Transfer | A specific and very common type of coordination where a base accepts a proton (H$^{+}$). | B$^{-}$ + H-A → B-H + A$^{-}$ |
A Step-by-Step Example: The SN2 Reaction
Let's look at a classic example: the SN2 reaction (Substitution Nucleophilic Bimolecular). This is how a nucleophile replaces a leaving group in a molecule. A common example is the reaction of bromomethane (CH_3Br) with a hydroxide ion (OH$^{-}$) to form methanol (CH_3OH).
Step 1: The Approach. The hydroxide ion, with its lone pair of electrons (the nucleophile), approaches the carbon atom from the side opposite the bromine atom (the leaving group). This "backside attack" is crucial.
Step 2: The Transition State. As the C-OH bond begins to form, the C-Br bond begins to break. In the transition state, the carbon is simultaneously bonded to both oxygen and bromine. The three hydrogen atoms are pushed into a flat, planar arrangement.
Step 3: The Departure. The C-OH bond fully forms, and the C-Br bond fully breaks, releasing a bromide ion (Br$^{-}$). The final product, methanol, has its OH group where the Br was, but the 3D structure is inverted, like an umbrella turning inside out in the wind.
Mechanisms in Action: Combustion and Digestion
Reaction mechanisms are not just for test tubes; they explain everyday phenomena.
Combustion: The burning of natural gas (methane, CH_4) is a radical chain reaction. It starts when heat breaks an O=O bond in oxygen, creating highly reactive oxygen radicals. These radicals then attack methane molecules, stripping a hydrogen atom and creating a methyl radical (•CH_3). This sets off a chain of rapid steps that ultimately produces carbon dioxide and water, releasing a lot of heat and light energy.
Digestion: In your body, enzymes act as biological catalysts to speed up the breakdown of food. For instance, the enzyme amylase in your saliva breaks down starch into sugars. The mechanism involves the enzyme providing an alternative pathway with a lower activation energy. It holds the starch molecule in a specific way that makes it easier for a water molecule to break the chemical bonds between sugar units, a process called hydrolysis.
Important Questions
Why can't we see a reaction mechanism?
What is the difference between a reaction intermediate and a transition state?
How does a catalyst affect a reaction mechanism?
Footnote
1 SN2: Stands for Substitution Nucleophilic Bimolecular. It describes a reaction where a nucleophile substitutes a leaving group in a single, concerted step that involves two molecules in the rate-determining step.
2 Nucleophile: A "nucleus-loving" species that donates a pair of electrons to form a new chemical bond. It is a Lewis base.
3 Leaving Group: An atom or group of atoms that is displaced in a substitution reaction, taking a pair of electrons with it when it departs.
4 Radical: A highly reactive atom or molecule that has an unpaired electron.
5 Activation Energy (Ea): The minimum energy that reacting particles must possess for a successful collision to result in a chemical reaction.