Reactivity of some classes of compounds and their characteristic organic reaction mechanisms

Welcome to Organic Mechanisms: The "How" of Chemistry!

Ever wondered why some organic molecules react instantly while others sit there doing nothing? Or why certain molecules prefer to swap atoms while others like to add them on? Welcome to Section 11.3: Organic Reactions and Mechanisms. Think of this chapter as the "rulebook" for how molecules interact. Instead of just memorizing reactions, we are going to look under the hood to see how electrons move. Don't worry if it seems like a lot of moving parts at first—once you see the patterns, it’s like solving a puzzle!

1. The Cast of Characters: Key Terminology

Before we look at the reactions, we need to know who the players are. Organic chemistry is essentially the study of electron movement.

Electrophiles vs. Nucleophiles

Most reactions involve an electron-rich species attacking an electron-poor species.

Nucleophile (Lewis Base): These are "nucleus lovers." They are electron-rich species (often having a lone pair or a negative charge) that are looking for a positive center to attack.
Think of them as "The Givers."

Electrophile (Lewis Acid): These are "electron lovers." They are electron-deficient species (often having a positive charge or a partial positive \(\delta+\) charge) that want to accept electrons.
Think of them as "The Takers."

Free Radicals and Carbocations

Sometimes, bonds break in different ways:

1. Homolytic Fission: The bond breaks evenly. Each atom takes one electron from the shared pair, creating Free Radicals (highly reactive species with an unpaired electron).
\(X—Y \rightarrow X\cdot + Y\cdot\)

2. Heterolytic Fission: The bond breaks unevenly. One atom takes both electrons, usually forming a Carbocation (a carbon atom with a positive charge) and an anion.
\(C—X \rightarrow C^+ + :X^-\)

Quick Review: Degrees of Substitution

You will often hear terms like Primary (1°), Secondary (2°), and Tertiary (3°). This simply refers to how many carbon groups are attached to the carbon atom we are looking at.
• Primary: 1 carbon attached.
• Secondary: 2 carbons attached.
• Tertiary: 3 carbons attached.
• Quaternary: 4 carbons attached.

Key Takeaway: Electrons flow from electron-rich sites (Nucleophiles) to electron-poor sites (Electrophiles).

2. Why Do They React? Understanding Reactivity

The syllabus requires us to understand why different classes of compounds behave the way they do based on their structure.

The "Lazy" Alkanes

Alkanes are generally unreactive. Why?
• They only have C—C and C—H bonds, which are strong and non-polar.
• There are no electron-rich or electron-poor sites for nucleophiles or electrophiles to attack.

The "Electron-Magnet" Alkenes

Alkenes are much more reactive than alkanes because of the C=C double bond. This double bond contains a \(\pi\) (pi) bond, which is an area of high electron density sitting above and below the plane of the molecule. This makes alkenes a prime target for electrophiles.

Benzene: The Stable Ring

Even though benzene looks like it has three double bonds, it doesn't react like an alkene. Because of the delocalisation of \(\pi\) electrons in the ring, benzene is extra stable. It prefers Substitution (swapping an atom) over Addition (breaking the ring) because addition would destroy that special stability.

Electronic and Steric Effects

• Electronic Effects: Some groups "push" electrons toward a center (electron-donating), while others "pull" them away (electron-withdrawing). This affects how stable an intermediate (like a carbocation) is.
• Steric Effect (Steric Hindrance): This is just a fancy way of saying "it's too crowded!" Large groups can physically block a reagent from reaching a carbon atom.

Key Takeaway: Reactivity depends on electron density (Electronic) and how much space there is (Steric).

3. The Mechanisms: Step-by-Step

A mechanism uses curly arrows to show where electrons are going. A full arrow represents the movement of a pair of electrons, while a half-arrow (fishhook) represents a single electron.

Mechanism 1: Free-Radical Substitution (Alkanes)

Example: Ethane + Chlorine (\(UV\) light)
1. Initiation: \(UV\) light breaks the \(Cl—Cl\) bond homolytically to form chlorine radicals.
2. Propagation: Radicals react with molecules to form new radicals. This keeps the reaction going like a chain.
3. Termination: Two radicals meet and form a stable molecule, ending the chain.

Mechanism 2: Electrophilic Addition (Alkenes)

Example: Ethene + Bromine (\(Br_2\))
The electron-rich \(\pi\) bond of the alkene attacks the \(Br—Br\) molecule. This creates a carbocation intermediate. The remaining \(Br^-\) then attacks the positive carbon.

Did you know? If the alkene is unsymmetrical, we use Markovnikov’s Rule: the hydrogen (from \(HX\)) attaches to the carbon that already has more hydrogens. "The rich get richer!"

Mechanism 3: Electrophilic Substitution (Arenes)

Example: Nitration of Benzene
Because the benzene ring is so stable, it needs a very strong electrophile (like \(NO_2^+\)). The electrophile attacks the ring, temporarily disrupting the delocalisation, but then a hydrogen atom leaves as \(H^+\) to restore the stable ring.

Mechanism 4: Nucleophilic Substitution (\(S_N1\) and \(S_N2\))

This is how halogenoalkanes react. There are two "paths":

• \(S_N1\) (Substitution Nucleophilic Unimolecular): Happens in two steps. The halogen leaves first, forming a carbocation. This is preferred by tertiary halogenoalkanes because 3° carbocations are stable.
Outcome: Racemisation (a 50/50 mix of optical isomers).

• \(S_N2\) (Substitution Nucleophilic Bimolecular): Happens in one smooth step. The nucleophile attacks from the back as the halogen leaves. This is preferred by primary halogenoalkanes because there is less steric hindrance.
Outcome: Inversion of configuration (like an umbrella turning inside out).

Mechanism 5: Nucleophilic Addition (Carbonyls)

Example: Aldehydes/Ketones + \(HCN\)
The \(C=O\) bond is very polar (\(C^{\delta+}—O^{\delta-}\)). The nucleophile (\(CN^-\)) attacks the \(\delta+\) carbon. This is a crucial way to add a carbon atom to a chain!

Key Takeaway: Mechanisms are categorized by who attacks (Nucleophile/Electrophile) and what happens (Addition/Substitution).

4. Comparing Reactivities: Common Pitfalls

Students often struggle with comparing different molecules. Here are two big ones for the H2 syllabus:

Halogenoalkanes vs. Halogenoarenes (e.g., Chlorobenzene)

Why is chlorobenzene so unreactive compared to chloroethane?
1. Delocalisation: The lone pair on the chlorine atom overlaps with the benzene \(\pi\) system, making the \(C—Cl\) bond stronger and harder to break.
2. Steric Hindrance: The benzene ring physically blocks nucleophiles from attacking the carbon from the back.
3. Electron Repulsion: The electron-rich benzene ring repels incoming nucleophiles.

Alkenes vs. Benzene

Alkenes undergo addition easily because their \(\pi\) electrons are localized. Benzene undergoes substitution because its \(\pi\) electrons are delocalised and very stable; addition would require too much energy to break that stability.

Quick Review Box:
• Alkanes: Free-radical substitution.
• Alkenes: Electrophilic addition.
• Benzene: Electrophilic substitution.
• Halogenoalkanes: Nucleophilic substitution.
• Carbonyls: Nucleophilic addition.

Final Encouragement

Don't worry if these mechanisms feel tricky at first! The secret to mastering Organic Chemistry is practice. Try drawing the curly arrows for the \(S_N2\) and \(S_N1\) mechanisms side-by-side. Once you understand the "flow" of electrons from negative to positive, you'll be able to predict reactions you've never even seen before! You've got this!