What is the fundamental difference between homolytic and heterolytic fission?

Homolytic fission involves a bond breaking where each atom takes one electron, forming two radicals. Heterolytic fission involves one atom taking both electrons, resulting in the formation of a positive and a negative ion.

How does a nucleophile differ from an electrophile in terms of electron behavior?

A nucleophile is an electron-pair donor that is attracted to positive centers. An electrophile is an electron-pair acceptor that is attracted to electron-rich areas.

What is the difference between an addition reaction and a substitution reaction?

In an addition reaction, two molecules combine to form a single product with no atoms left over. In a substitution reaction, one atom or group is swapped for another, resulting in two products (the main organic product and a leaving group/byproduct).

What is a common error when drawing the start of a curly arrow in a mechanism?

A common error is starting the arrow from the chemical symbol of an atom. It must always start from a specific source of electrons, such as a lone pair or the center of a covalent bond.

Why is it a mistake to draw a curly arrow pointing toward a nucleophile?

Curly arrows represent the movement of electrons. Since nucleophiles are electron-rich and donate electrons, the arrow must point away from the nucleophile toward an electron-deficient electrophile.

What happens to the charge of an atom if a curly arrow points away from its lone pair?

The atom will become more positive (or less negative) because it is losing 'possession' of an electron pair to form a new bond. Forgetting to update the formal charge is a frequent mistake.

Define a 'Free Radical'.

A free radical is a highly reactive chemical species that contains at least one unpaired electron, typically formed through homolytic fission.

What is 'Hydrolysis' in the context of organic mechanisms?

Hydrolysis is a reaction where a chemical bond is broken by the addition of water, often catalyzed by acids or bases to split a large molecule into two smaller ones.

What does the symbol [O] represent in an organic equation?

The symbol [O] represents an oxygen atom supplied by an oxidizing agent, used to simplify equations for oxidation reactions where the specific reagent is not the focus.

What is a 'Carbocation'?

A carbocation is an organic ion in which a carbon atom has a positive charge and only three bonds, making it a common electron-deficient intermediate in heterolytic reactions.

Fundamentals of Reaction Mechanisms | AQA A-Level Chemistry

Fundamentals of Reaction Mechanisms

Summary

Reaction mechanisms provide a step-by-step description of how chemical bonds break and form during a transformation. By tracking the movement of electron pairs and identifying the nature of reactive intermediates, chemists can predict the outcome, speed, and selectivity of organic reactions.

1. Bond Fission: Homolytic and Heterolytic

Homolytic fission occurs when a covalent bond breaks and the shared pair of electrons is divided equally between the two atoms. This process results in the formation of free radicals, which are highly reactive species possessing an unpaired electron.
Heterolytic fission involves the unequal breaking of a covalent bond where the more electronegative atom retains both electrons from the shared pair. This leads to the creation of ions: a negatively charged anion (which took the electrons) and a positively charged cation (which lost them).
The choice between these pathways often depends on the environment; for example, non-polar solvents and UV light favor homolysis, while polar solvents often promote heterolysis.

2. Reactive Species: Nucleophiles and Electrophiles

A nucleophile is an electron-rich species, often carrying a lone pair or a negative charge, that seeks out electron-deficient centers to donate its electron pair. The term literally means 'nucleus-loving,' reflecting its attraction to the positive charge of an atomic nucleus.
An electrophile is an electron-deficient species, such as a cation or a molecule with a partial positive charge, that acts as an electron pair acceptor. These 'electron-loving' species are typically attacked by nucleophiles to form new covalent bonds.
Understanding the relative strength of these species is vital; for instance, a full negative charge generally makes a species a more potent nucleophile than a neutral molecule with only a partial negative dipole.

Diagram showing a curly arrow originating from a lone pair on a nucleophile and pointing towards an electrophilic center, representing the formation of a new bond.

3. The Grammar of Mechanisms: Curly Arrows

Curly arrows are the standard notation used to visualize the movement of electron pairs during a reaction. A full-headed arrow represents the movement of an electron pair, while a half-headed (fishhook) arrow represents the movement of a single electron.
The tail of the arrow must always start at a source of electrons, such as a lone pair or the center of a covalent bond. The head of the arrow points exactly to the atom or bond where the electrons are moving to form a new connection or charge.
Correct placement is critical for accuracy; an arrow starting from an atom symbol rather than a specific bond or lone pair is considered a conceptual error in chemical notation.

4. Classification of Organic Reactions

5. Key Distinctions in Reaction Pathways

6. Exam Strategy & Tips

Fundamentals of Reaction Mechanisms

Summary

1. Bond Fission: Homolytic and Heterolytic

Homolytic fission occurs when a covalent bond breaks and the shared pair of electrons is divided equally between the two atoms. This process results in the formation of free radicals, which are highly reactive species possessing an unpaired electron.
Heterolytic fission involves the unequal breaking of a covalent bond where the more electronegative atom retains both electrons from the shared pair. This leads to the creation of ions: a negatively charged anion (which took the electrons) and a positively charged cation (which lost them).
The choice between these pathways often depends on the environment; for example, non-polar solvents and UV light favor homolysis, while polar solvents often promote heterolysis.

2. Reactive Species: Nucleophiles and Electrophiles

A nucleophile is an electron-rich species, often carrying a lone pair or a negative charge, that seeks out electron-deficient centers to donate its electron pair. The term literally means 'nucleus-loving,' reflecting its attraction to the positive charge of an atomic nucleus.
An electrophile is an electron-deficient species, such as a cation or a molecule with a partial positive charge, that acts as an electron pair acceptor. These 'electron-loving' species are typically attacked by nucleophiles to form new covalent bonds.
Understanding the relative strength of these species is vital; for instance, a full negative charge generally makes a species a more potent nucleophile than a neutral molecule with only a partial negative dipole.

Diagram showing a curly arrow originating from a lone pair on a nucleophile and pointing towards an electrophilic center, representing the formation of a new bond.

3. The Grammar of Mechanisms: Curly Arrows

Curly arrows are the standard notation used to visualize the movement of electron pairs during a reaction. A full-headed arrow represents the movement of an electron pair, while a half-headed (fishhook) arrow represents the movement of a single electron.
The tail of the arrow must always start at a source of electrons, such as a lone pair or the center of a covalent bond. The head of the arrow points exactly to the atom or bond where the electrons are moving to form a new connection or charge.
Correct placement is critical for accuracy; an arrow starting from an atom symbol rather than a specific bond or lone pair is considered a conceptual error in chemical notation.

4. Classification of Organic Reactions

Addition reactions involve two or more molecules combining to form a single product, typically occurring across double or triple bonds where the pi-bond is broken to accommodate new substituents.
Substitution reactions occur when an atom or functional group in a molecule is replaced by a different atom or group. This is common in saturated compounds like halogenoalkanes.
Elimination reactions are the reverse of addition, where a small molecule (like $H_2O$ or $HCl$ ) is removed from a larger molecule, usually resulting in the formation of a double bond.
Redox reactions in organic chemistry are often identified by the gain or loss of oxygen or hydrogen. Oxidation is frequently characterized by the addition of oxygen or removal of hydrogen, while reduction involves the addition of hydrogen or removal of oxygen.

5. Key Distinctions in Reaction Pathways

Feature	Nucleophilic Substitution	Electrophilic Addition
Target	Saturated Carbon (e.g., C-X)	Unsaturated Carbon (C=C)
Reagent	Nucleophile (e.g., $OH^-$ )	Electrophile (e.g., $H^+$ )
Outcome	One group replaces another	Two groups add across a bond
Intermediate	Often involves a transition state	Often involves a carbocation

Distinguishing between these pathways requires looking at the functional group of the reactant. Alkenes, being electron-rich due to their pi-clouds, naturally attract electrophiles, whereas polar bonds in alkanes attract nucleophiles to the partial positive carbon.

6. Exam Strategy & Tips

Always draw dipoles: Explicitly marking $\delta^+$ and $\delta^-$ on polar bonds helps justify why a nucleophile or electrophile is attacking a specific site. This is often a requirement for full marks in mechanism questions.
Check the arrow origin: Ensure your curly arrow starts exactly on a lone pair or the middle of a bond. If it starts in 'empty space' or on a nucleus, it does not correctly represent electron movement.
Conserve Charge: The total charge of the reactants must equal the total charge of the products and any intermediates. If you start with a neutral molecule and a negative ion, your intermediate or final product set must have a net negative charge.
Intermediate Stability: When multiple products are possible, identify the most stable intermediate. For carbocations, the stability order is Tertiary > Secondary > Primary due to the inductive effect of alkyl groups.

Feature

Nucleophilic Substitution

Electrophilic Addition

Target

Saturated Carbon (e.g., C-X)

Unsaturated Carbon (C=C)

Reagent

Nucleophile (e.g.,

OH^-

)

Electrophile (e.g.,

H^+

)

Outcome

One group replaces another

Two groups add across a bond

Intermediate

Often involves a transition state

Often involves a carbocation