Welcome to Reactivity 3.3 & 3.4: Electron Sharing Reactions!

Hi future chemists! In the previous chapters of Reactivity, we looked at how protons move (acids and bases) and how electrons transfer completely (redox). Now, we are diving into the heart of organic chemistry mechanisms: reactions where electrons or pairs of electrons are shared to break and form covalent bonds. This is often called Mechanisms of Organic Change.

Understanding these mechanisms (the step-by-step path a reaction takes) is like being a detective. It helps you predict why a reaction happens, what controls its speed, and what products are formed. Don't worry if this seems tricky at first; we will break down the movement of electrons using simple concepts and curved arrows!

Part 1: The Fundamentals of Bond Breaking

Before any electron sharing reaction can happen, a covalent bond usually has to break. There are two crucial ways this can occur, depending on how the electrons are distributed:

1. Homolytic Fission (The Fair Split)

The prefix Homo- means "same." In homolytic fission, the shared pair of electrons breaks evenly. Each atom gets one electron from the original bond.

  • Mechanism: We use a fishhook arrow (a half-headed curved arrow, \(\curvearrowright\)) to show the movement of a single electron.
  • Result: This creates highly reactive species called free radicals.
  • Example: \(A:B \rightarrow A\cdot + B\cdot\)
  • Key Feature: These reactions are non-polar and often initiated by energy (like UV light or heat). They fall under Reactivity 3.3: Electron sharing reactions.

2. Heterolytic Fission (The Uneven Split)

The prefix Hetero- means "different." In heterolytic fission, one atom hogs both electrons from the bond.

  • Mechanism: We use a full curved arrow (\(\curvearrowright\)) to show the movement of an electron pair.
  • Result: This creates two charged ions: a positive ion (which lost the electrons) and a negative ion (which gained the pair). These charged ions are known as intermediates (e.g., carbocations or carbanions).
  • Example: \(A:B \rightarrow A^+ + :B^-\)
  • Key Feature: These are polar reactions and are driven by differences in electronegativity. They are the focus of Reactivity 3.4: Electron-pair sharing reactions.
Quick Review: Fission Types
  • Homolytic (Radical): Single electron moves, forms neutral radicals (\(A\cdot\)).
  • Heterolytic (Polar): Electron pair moves, forms ions (\(A^+\) and \(B^-\)).

Part 2: The Key Players in Polar Reactions (R3.4)

In electron-pair sharing reactions (polar mechanisms), we need two main actors: the one looking for electrons, and the one providing them.

1. Electrophiles (\(E^+\))

Electrophiles are "electron lovers" (Electro = electron, phile = loving). They are species that are electron deficient and seek to accept an electron pair to form a new bond.

  • Characteristics: They are usually positively charged or contain an atom with a strong partial positive charge ($\delta^+$).
  • Analogy: Think of an electrophile as someone who is hungry for energy (electrons).
  • Examples: Carbocations (\(R_3C^+\)), \(H^+\) (protons), or atoms in polar molecules like the carbon atom in C=O.
  • Memory Aid: Electrophiles Explore for Electrons.

2. Nucleophiles (\(Nu^-\))

Nucleophiles are "nucleus lovers" (Nucleo = nucleus/positive charge, phile = loving). They are species that are electron rich and seek to donate an electron pair to form a new bond.

  • Characteristics: They have lone pairs of electrons and/or a negative charge.
  • Analogy: Think of a nucleophile as a generous person ready to donate a pair of spare electrons.
  • Examples: Hydroxide ion (\(OH^-\)), halide ions (\(Cl^-\)), water (\(H_2O\), due to lone pairs on oxygen), or the double bond in an alkene.
  • Memory Aid: Nucleophiles Nuclear attack (they attack the positive nucleus).

Key Takeaway: All electron-pair sharing reactions involve a nucleophile attacking an electrophile.

Part 3: Reaction Mechanisms: Radical Substitution (R3.3)

Radical reactions are typically seen when substituting atoms on alkanes, which are generally unreactive due to their strong, non-polar C–H bonds. We need extreme conditions (like UV light) to initiate homolytic fission.

Example: Chlorination of Methane (\(CH_4 + Cl_2 \rightarrow CH_3Cl + HCl\))

This mechanism occurs in three distinct stages:

1. Initiation

UV light provides the energy to break the weakest bond (the Cl–Cl bond) homolytically, creating two highly reactive chlorine radicals.

$$ Cl-Cl \xrightarrow{UV \ light} 2 Cl\cdot $$

2. Propagation (The Chain Reaction)

This stage involves steps where one radical reacts to form a new radical, allowing the reaction to continue in a chain. These two steps repeat rapidly.

Step 1: The chlorine radical attacks methane, grabbing a hydrogen atom and forming a methyl radical. $$ CH_4 + Cl\cdot \rightarrow CH_3\cdot + HCl $$

Step 2: The methyl radical attacks a chlorine molecule, forming the product (chloromethane) and regenerating the highly important chlorine radical. $$ CH_3\cdot + Cl_2 \rightarrow CH_3Cl + Cl\cdot $$

3. Termination

The reaction stops when two radicals collide and bond together, removing the radical species necessary for propagation.

$$ Cl\cdot + Cl\cdot \rightarrow Cl_2 $$ $$ CH_3\cdot + CH_3\cdot \rightarrow C_2H_6 $$ $$ Cl\cdot + CH_3\cdot \rightarrow CH_3Cl $$

Common Mistake to Avoid: The initiation step requires a specific trigger (UV or heat); the reaction will not proceed without it.

Part 4: Reaction Mechanisms: Polar Reactions (R3.4)

These reactions involve the movement of electron pairs (nucleophiles attacking electrophiles) and are critical for understanding how functional groups react.

1. Nucleophilic Substitution Reactions (S$_{\text{N}}$)

In this mechanism, a nucleophile (\(Nu^-\)) replaces a "leaving group" (L) attached to a carbon atom. This is common in halogenoalkanes. We classify these based on kinetics (the rate-determining step):

a) S$_{\text{N}}$2 Mechanism (Substitution, Nucleophilic, Bimolecular)

The 2 means the rate depends on the concentration of two species: the nucleophile and the substrate (e.g., halogenoalkane).

  • Process: This is a one-step concerted mechanism. The nucleophile attacks the carbon atom at the same time the leaving group departs.
  • Intermediate: None, but a transition state is formed where the carbon is bonded partially to both the nucleophile and the leaving group.
  • Stereochemistry: The attack must happen from the back side (opposite the leaving group), resulting in an inversion of configuration (like an umbrella turning inside out).
  • Substrate Preference: S$_{\text{N}}$2 favors primary halogenoalkanes (RCH$_2$X) because the carbon is less crowded (less steric hindrance), allowing the nucleophile easy access.
  • Analogy: It's a quick, efficient handshake. As the new partner (Nu) approaches, the old partner (L) leaves immediately.
b) S$_{\text{N}}$1 Mechanism (Substitution, Nucleophilic, Unimolecular) (HL Focus)

The 1 means the rate depends on the concentration of one species: only the substrate.

  • Process: This is a two-step mechanism.
    1. Step 1 (Slow/Rate-Determining): Heterolytic fission occurs, and the leaving group departs first, forming a stable carbocation intermediate (\(R_3C^+\)).
    2. Step 2 (Fast): The nucleophile quickly attacks the planar carbocation.
  • Stereochemistry: Because the carbocation is planar (flat), the nucleophile can attack from either side, often resulting in a racemic mixture (a mix of both possible stereoisomers).
  • Substrate Preference: S$_{\text{N}}$1 favors tertiary halogenoalkanes (\(R_3CX\)) because the tertiary carbocation formed is more stable (due to electron-donating alkyl groups).
  • Analogy: It's a waiting game. The substrate breaks up first (forms the carbocation), and then the nucleophile attacks the lonely ion.

Did you know? Carbocation stability increases as you add more alkyl groups: Primary (least stable) < Secondary < Tertiary (most stable). This difference dictates whether a reaction goes S$_{\text{N}}$1 or S$_{\text{N}}$2!

2. Electrophilic Addition Reactions (R3.4)

Electrophilic addition is the characteristic reaction of compounds containing carbon-carbon double bonds (alkenes). The double bond is rich in electrons, making it the perfect target for an electrophile.

Example: Addition of HBr to Propene (\(CH_3CH=CH_2\))

The mechanism follows these steps:

  1. Step 1: Attack by Electrophile (Slow): The \(\pi\)-electrons of the C=C double bond act as a nucleophile and attack the partially positive hydrogen atom (\(H^{\delta+}\)) in H–Br. The H–Br bond breaks heterolytically, forming a carbocation intermediate and a bromide ion (\(Br^-\)).
  2. Step 2: Attack by Nucleophile (Fast): The bromide ion (\(Br^-\)), which is a nucleophile, attacks the positively charged carbocation, completing the addition product.

Regioselectivity (Markovnikov’s Rule): When adding an unsymmetrical reagent (like H–Br) to an unsymmetrical alkene (like propene), two products are possible. However, the major product is formed via the more stable carbocation intermediate (tertiary > secondary > primary).

  • In propene: Adding H to the end carbon produces a secondary carbocation (\(CH_3CH^+CH_3\)). Adding H to the middle carbon produces a primary carbocation (\(CH_3CH_2CH_2^+\)).
  • Result: Since the secondary carbocation is more stable, the major product is 2-bromopropane. This is famously summarized as: "The rich get richer" (the H atom goes to the carbon already having more H atoms).

Comprehensive Summary: Mechanisms of Chemical Change

You have now explored the three major classes of reaction mechanisms:

Mechanism Type Reactivity Section What Moves? Key Intermediates
Proton Transfer R3.1 \(H^+\) (Proton) Conjugate Pairs
Electron Transfer R3.2 Full Electron (e$^-$) Ions (Oxidation States Change)
Electron Sharing (Radical) R3.3 Single Electrons Free Radicals (\(A\cdot\))
Electron-Pair Sharing (Polar) R3.4 Electron Pairs Carbocations (\(R_3C^+\)), Carbanions

Great job! Understanding these fundamental ways that electrons and electron pairs move is the key to unlocking the entire world of chemical synthesis.