Introduction to Reaction Mechanisms Revision Notes for OCR A-Level Chemistry A

Introduction to reaction mechanisms

Understanding how chemical reactions occur is essential in organic chemistry. While a chemical equation tells you what reactants are used and what products are formed, it doesn't reveal the step-by-step process of how the reaction actually happens. This is where reaction mechanisms come in - they show us the pathway a reaction takes and how electrons move during the process.

Types of bond fission

Before we can understand reaction mechanisms, we need to know how covalent bonds break. A covalent bond consists of a shared pair of electrons between two atoms. When a reaction occurs, these bonds must break, and they can do so in two different ways: homolytic fission or heterolytic fission.

infoNote

The way a bond breaks determines what species are formed and significantly affects the reaction pathway. Understanding these two breaking patterns is fundamental to predicting and explaining organic reaction mechanisms.

Homolytic fission

In homolytic fission, a covalent bond breaks in such a way that each atom receives one electron from the bonded pair. This creates an equal split of the electrons.

When this happens:

Each atom ends up with a single unpaired electron
Species with unpaired electrons are called radicals
Radicals are highly reactive due to their unpaired electron

lightbulbExample

Homolytic Fission of Ethane

When ethane undergoes homolytic fission:

$\text{H}_3\text{C-CH}_3 \rightarrow \text{H}_3\text{C}^{\bullet} + {}^{\bullet}\text{CH}_3$

The dot notation ( $\bullet$ ) represents the unpaired electron on each radical. Note that radicals are not charged - they're neutral species with an odd number of electrons.

Heterolytic fission

In heterolytic fission, the covalent bond breaks unevenly - one atom takes both electrons from the bonded pair, while the other atom receives none.

When this occurs:

The atom that takes both electrons becomes a negative ion (anion)
The atom that loses both electrons becomes a positive ion (cation or carbocation if it's a carbon atom)
This type of fission produces charged species rather than radicals

lightbulbExample

Heterolytic Fission of Chloromethane

When chloromethane undergoes heterolytic fission:

$\text{H}_3\text{C-Cl} \rightarrow \text{H}_3\text{C}^+ + \text{Cl}^-$

The carbon atom forms a positively charged carbocation, while the chlorine atom becomes a chloride ion. Heterolytic fission typically occurs in polar bonds where one atom is more electronegative than the other.

In this diagram, you can see the partial charges ( $\delta^+$ and $\delta^-$ ) on the carbon and chlorine before the bond breaks. The chlorine is more electronegative, so it attracts the electron pair more strongly. When the bond breaks heterolytically, both electrons move to the chlorine, as shown by the curly arrow.

Reaction mechanisms and curly arrows

A reaction mechanism shows the detailed step-by-step pathway of how a reaction proceeds, including the movement of electrons. This is crucial information that a simple chemical equation cannot provide.

Curly arrows

To show electron movement in mechanisms, chemists use curly arrows. These arrows are essential tools for understanding organic reactions:

A full curly arrow (with a full arrowhead) represents the movement of an electron pair
The arrow starts from where the electrons are initially located
The arrow points to where the electrons are moving to
Curly arrows always show electron movement, never atom movement

In heterolytic fission, curly arrows show how the electron pair moves from the bond to one of the atoms. For example, in the breaking of a carbon-chlorine bond, a curly arrow would start at the C-Cl bond and point toward the chlorine atom, showing that both electrons move to chlorine.

chatImportant

Common Arrow-Drawing Mistakes

Always draw curly arrows carefully - they should start from a bond or a lone pair of electrons, not from an atom itself. Common mistakes include starting arrows from atomic symbols or drawing arrows pointing the wrong direction.

Fish-hook arrows

When dealing with homolytic fission and radicals, a different type of arrow is used:

A fish-hook arrow (with half an arrowhead) represents the movement of a single electron
These are used in radical mechanisms
Each breaking bond requires two fish-hook arrows (one electron going to each atom)

lightbulbExample

Homolytic Fission Using Fish-Hook Arrows

When bromine undergoes homolytic fission:

$\text{Br-Br} \rightarrow \text{Br}^{\bullet} + \text{Br}^{\bullet}$

Two fish-hook arrows would show one electron moving to each bromine atom.

Three main types of organic reactions

Organic reactions can be classified into three main categories based on what happens during the reaction: addition, substitution, and elimination. Understanding these reaction types helps you predict products and understand mechanisms.

Addition reactions

In an addition reaction, two molecules combine to form a single product. This type of reaction is characteristic of compounds containing double bonds, such as alkenes.

Key features:

Two reactant molecules join together
The double bond in the alkene breaks
One product molecule is formed
Nothing is lost from the reactants

lightbulbExample

Addition Reaction: Hydration of But-2-ene

When but-2-ene reacts with water, the water molecule adds across the double bond to form butan-2-ol (a secondary alcohol):

In this reaction:

The $\text{C=C}$ double bond in but-2-ene breaks
Water adds across this bond
The product is a saturated alcohol
No atoms are removed from either reactant

infoNote

Addition reactions are common in alkene chemistry and are important in the synthesis of alcohols and other organic compounds. The double bond provides the reactive site where molecules can add across.

Substitution reactions

In a substitution reaction, one atom or group of atoms in a molecule is replaced by a different atom or group of atoms.

Key features:

One functional group is exchanged for another
An atom or group leaves the molecule (leaving group)
A different atom or group takes its place (entering group)
The carbon skeleton usually remains unchanged

lightbulbExample

Nucleophilic Substitution of 1-Bromopropane

When 1-bromopropane reacts with hydroxide ions, the bromine atom is replaced by a hydroxyl group to form propan-1-ol:

In this reaction:

The bromine atom (leaving group) is replaced
The hydroxide ion (nucleophile) takes its place
The carbon chain structure remains intact
A bromide ion is produced as a by-product

Substitution reactions are particularly important in haloalkane chemistry and in converting one functional group to another.

chatImportant

Identifying Substitution Components

In substitution reactions, always identify what is leaving and what is substituting. The hydroxide ion is a nucleophile (electron-pair donor), so this is specifically a nucleophilic substitution reaction.

Elimination reactions

In an elimination reaction, a small molecule is removed from a larger molecule. This typically results in the formation of a double bond.

Key features:

A small molecule (often water or hydrogen halide) is removed
The parent molecule loses atoms
A double bond usually forms
Two products are formed: the main organic product and the small eliminated molecule

lightbulbExample

Elimination Reaction: Dehydration of Propan-1-ol

When propan-1-ol is heated with an acid catalyst (such as concentrated sulfuric acid), a water molecule is eliminated to form propene:

In this reaction:

A water molecule ( $\text{H}_2\text{O}$ ) is eliminated
The alcohol loses an H from one carbon and OH from an adjacent carbon
A double bond forms between these two carbon atoms
The product is an alkene (propene)

Elimination reactions are important for converting alcohols to alkenes and are often the reverse of addition reactions.

chatImportant

Don't Confuse Elimination with Substitution

Students sometimes confuse elimination with substitution. Remember: elimination removes a small molecule and creates a double bond, while substitution exchanges one group for another without forming a double bond.

Practical applications

These three reaction types form the foundation of organic synthesis:

Addition reactions are used to convert alkenes (unsaturated) into alcohols and other functional groups
Substitution reactions allow chemists to transform one functional group into another while maintaining the carbon skeleton
Elimination reactions enable the creation of double bonds, converting saturated compounds to unsaturated ones

Understanding which conditions favour each reaction type is crucial for synthetic planning and will be developed further as you study specific functional groups.

bookmarkSummary

Key Points to Remember:

Homolytic fission splits a bond evenly, giving each atom one electron and forming radicals (species with unpaired electrons)
Heterolytic fission splits a bond unevenly, with one atom taking both electrons, forming a positive ion (carbocation) and a negative ion
Curly arrows show the movement of electron pairs in reaction mechanisms - they should start from bonds or lone pairs, not from atoms
Addition reactions involve two molecules joining to form one product (common in alkenes)
Substitution reactions involve one atom or group being replaced by another (common in haloalkanes)
Elimination reactions involve removing a small molecule from a larger one, usually forming a double bond (converting alcohols to alkenes)

Introduction to Reaction Mechanisms (OCR A-Level Chemistry A): Revision Notes