Chemistry | Electron sharing reactions

Welcome to Reactivity 3.3: Electron Sharing Reactions!

Hello future Chemists! You’ve already learned how atoms share electrons to form covalent bonds (Structure 2.2). Now, we’re moving into the exciting world of mechanisms—understanding the step-by-step "dance" that molecules perform when these shared bonds break and form during a reaction.

This chapter, "Electron sharing reactions," is absolutely crucial because it lays the foundation for understanding almost all organic chemistry. We are shifting focus from *what* reacts (Reactants and Products) to *how* it reacts (the mechanism).

Don't worry if this seems tricky at first. Mechanisms are like reading a language. Once you learn the alphabet (the reactive species) and the grammar (the curly arrows), you can understand any chemical conversation!

1. The Language of Mechanisms: Following the Electrons

1.1 What is a Reaction Mechanism?

A reaction mechanism is the detailed sequence of steps showing the movement of electrons, bond breaking, and bond forming that occurs during a chemical change.

Chemical reactions rarely involve all bonds breaking and forming simultaneously. Instead, they occur in a series of smaller, high-energy steps involving highly reactive intermediates.

1.2 Visualizing Electron Movement: Curly Arrows

In organic mechanisms, we use special arrows, called curly arrows (or curved arrows), to show the path of electrons. This is the single most important tool in this topic!

• Full-Headed Arrow (\(\curvearrowright\)): Shows the movement of a pair of electrons (two electrons). This is used in most ionic or polar reactions (heterolytic processes).
• Half-Headed Arrow (Fish-Hook) (\(\curvearrowright\)): Shows the movement of a single electron. This is used in radical reactions (homolytic processes).

Rule to remember: Curly arrows always start at an electron source (a lone pair, a bond, or a negative charge) and point towards an electron sink (an atom, or a place with a partial positive charge).

1.3 Breaking the Covalent Bond (Fission)

Before new bonds can form, old bonds must break. This process, called fission, can happen in two main ways, depending on how the shared electron pair is split:

Homolytic Fission (The Fair Split)

• Definition: The shared bond breaks evenly, with one electron going to each atom.
• Products: This generates highly reactive, neutral species called free radicals. A radical has an unpaired electron (represented by a single dot, e.g., \(Cl\bullet\)).
• Notation: Uses half-headed (fish-hook) arrows.
• Example: Chlorine gas reacting under UV light.

  \(Cl-Cl \longrightarrow Cl\bullet + Cl\bullet\)

Heterolytic Fission (The Unfair Split)

• Definition: The shared bond breaks unevenly, with both electrons going to the more electronegative atom.
• Products: This generates charged ions—a carbocation (positive carbon) and an anion (negative ion).
• Notation: Uses full-headed curly arrows.
• Example:

  \(R_3C-Cl \longrightarrow R_3C^+ + Cl^-\)

2. The Key Players: Nucleophiles, Electrophiles, and Radicals

In electron sharing reactions, species are classified by whether they are looking for electrons or looking for a place to donate electrons.

2.1 Nucleophiles (\(\text{Nu}^-\)): The Electron Donors

• Meaning: "Nucleus-loving" or positive-charge loving.
• Characteristics: They are electron rich. They either have a negative charge (\(OH^-\), \(CN^-\)) or possess accessible lone pairs of electrons (\(H_2O\), \(NH_3\)).
• Role in Reaction: They attack areas of low electron density (positive centers or partially positive atoms, like the carbon atom bonded to a halogen).
• Analogy: The nucleophile is the generous person who has extra money (electrons) and is looking for someone poor to help out (the electrophile).

2.2 Electrophiles (\(\text{E}^+\)): The Electron Acceptors

• Meaning: "Electron-loving" or negative-charge loving.
• Characteristics: They are electron poor. They often have a positive charge (\(H^+\), \(NO_2^+\)) or have an incomplete octet, making them desperate for electrons (like \(BF_3\) or the carbon atom in a carbocation).
• Role in Reaction: They accept a pair of electrons from a nucleophile or an electron-rich bond (like the double bond in an alkene).
• Did you know? The double bond in an alkene is considered electron-rich because of the cloud of \(\pi\) (pi) electrons, making alkenes highly susceptible to attack by electrophiles.

2.3 Free Radicals (\(\text{R}\bullet\)): The Highly Reactive Singles

• Characteristics: Free radicals have a single, unpaired electron. They are electrically neutral but incredibly reactive because they are unstable and want a partner electron immediately.
• Role in Reaction: They participate in chain reactions (like the radical substitution of alkanes with halogens), often involving a cycle of initiation, propagation, and termination steps.

3. Main Types of Electron Sharing Reactions

When we look at chemical change at the atomic level, we categorize the reactions based on what primary action is occurring: swapping, adding, or removing groups. All these actions rely on the movement of shared electrons.

3.1 Substitution Reactions

Substitution means replacing one atom or group with another. The total number of bonds to the central atom remains the same.

Nucleophilic Substitution

• Reactants: Typically occurs with saturated compounds, like haloalkanes (R-X).
• Mechanism Core: A nucleophile (\(\text{Nu}^-\)) uses its electron pair to attack the partially positive carbon atom attached to the leaving group (X, usually a halogen). The bond between C and X breaks heterolytically.
• Example: The reaction of a haloalkane with hydroxide ion (\(OH^-\)) to form an alcohol. The \(OH^-\) substitutes the halogen atom.

  \(R-X + OH^- \longrightarrow R-OH + X^-\)

Radical Substitution

• Reactants: Typically occurs with saturated compounds, like alkanes.
• Mechanism Core: Driven by free radicals (homolytic fission). This reaction is usually hard to control because the radicals react non-selectively, leading to mixtures of products.

3.2 Addition Reactions

Addition reactions occur when two molecules combine to form a single, larger molecule. This usually happens when a multiple bond (\(\pi\) bond, like those in alkenes or alkynes) breaks and new single bonds are formed.

Electrophilic Addition

• Reactants: The defining reaction of alkenes and alkynes.
• Mechanism Core: The electron-rich double bond acts as a nucleophile and attacks an electrophile (like \(H^+\) from \(HBr\) or \(Br_2\)). The \(\pi\) bond breaks, electrons move, and two new \(\sigma\) bonds form.
• Key Rule: When adding an unsymmetrical reagent to an unsymmetrical alkene, the reaction often follows Markovnikov’s Rule: The hydrogen atom adds to the carbon atom in the double bond that already has more hydrogen atoms. (This happens because the reaction stabilizes the more substituted carbocation intermediate).

3.3 Elimination Reactions

Elimination reactions are the opposite of addition. A small molecule (like \(H_2O\) or \(HX\)) is removed from a larger molecule, often resulting in the formation of a multiple bond (alkene).

• Mechanism Core: A base often removes a proton (\(H\)) from a carbon adjacent to the carbon attached to the leaving group. The electrons from the C-H bond move to form the new double bond, and the leaving group leaves (heterolytic fission).
• Context: Elimination often competes with Nucleophilic Substitution, especially when using strong bases/nucleophiles (like concentrated \(NaOH\)). Changing the solvent and temperature can favor one mechanism over the other.

Key Takeaway Summary

Understanding electron sharing reactions means mastering the movement of electrons.

• The Tool: Curly arrows show electron flow.
• The Splitting: Homolytic fission leads to radicals; heterolytic fission leads to ions (nucleophiles/electrophiles).
• The Actors: Nucleophiles donate electrons (attack positive centers); Electrophiles accept electrons (attack negative/electron-rich centers).
• The Actions:
  • Substitution: Swapping groups (keeps saturation).
  • Addition: Adding across a multiple bond (loses saturation).
  • Elimination: Removing a group to form a multiple bond (loses saturation).