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Organic Mechanisms: Curly Arrows, Electrophiles, Nucleophiles

Learn and apply Organic Mechanisms: Curly Arrows, Electrophiles, Nucleophiles in the published Chemistry course sequence.

  • GCE A-Level H2 Chemistry 9476-2027
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Organic Mechanisms: Orientation

Mechanism questions reward a chemically consistent electron story. This lesson connects the prescribed reaction language, bond fission, reactive species and electronic effects so that curly arrows become consequences of structure rather than decorative marks.

Use it beside Representations and Nomenclature and return to the Organic Chemistry hub to apply the toolkit to each functional-group family.

Definitions (Must Know)

A. Core reaction language

  • Addition: two species combine and no atom group is replaced.
  • Substitution: one atom or group is replaced by another.
  • Elimination: atoms or groups are removed to form a multiple bond.
  • Condensation: two molecules join with loss of a small molecule.
  • Hydrolysis: a bond is broken by reaction with water.
  • Oxidation / reduction: in organic chemistry, track oxygen gain/hydrogen loss or oxygen loss/hydrogen gain, while using oxidation states when needed.

B. Bond fission and intermediates

  • Homolytic fission: each atom takes one bonding electron, producing radicals.
  • Heterolytic fission: one atom takes both bonding electrons, producing ions.
  • Free radical: a species with an unpaired electron.
  • Carbocation: an organic ion with a positively charged carbon centre.

C. Reagent roles

  • Electrophile: an electron-pair acceptor; a Lewis acid.
  • Nucleophile: an electron-pair donor; a Lewis base.

Primary, secondary and tertiary describe how many carbon groups are attached to the carbon centre being classified.

Detailed Explanations

A. Electronic and steric effects

  • The inductive effect transmits electron donation or withdrawal through sigma bonds and changes charge stability.
  • The mesomeric effect arises when a lone pair, charge or pi bond is delocalised across adjacent p orbitals.
  • A steric effect occurs when bulky groups impede approach to a reaction centre.

Use these effects to explain evidence; do not cite them as unexplained labels. For example, a more substituted carbocation is stabilised by electron-releasing alkyl groups, whereas crowding around a carbon centre hinders SN2 attack.

B. Mechanism-selection workflow

  1. Identify the functional group and the bond that changes.
  2. Assign the reagent’s role: radical source, electrophile, nucleophile, acid, base, oxidant or reductant.
  3. Use the solvent, temperature, catalyst and kinetic information.
  4. Choose the reaction family and likely intermediate.
  5. Draw every required electron movement.
  6. audit atoms, charge, lone pairs and catalyst regeneration.

C. One toolkit, several mechanisms

  • Free-radical substitution: single-headed arrows; initiation, propagation and termination.
  • Electrophilic addition: the alkene pi bond attacks an electrophile.
  • Electrophilic substitution: the arene attacks an electrophile, then restores aromaticity.
  • Nucleophilic substitution: a nucleophile attacks an electron-poor carbon as the leaving group departs, in one step or through a carbocation.
  • Nucleophilic addition: a nucleophile attacks the carbonyl carbon and the pi pair moves to oxygen.

The arrow does not mean “this species attacks”; it identifies exactly which electrons move and where.

Worked Examples

Modelled example 1

Classify Ammonia

Core

Problem

Is NH₃ a nucleophile or an electrophile? Explain in one line.
Study the worked solution

  1. Locate an available electron pair

    Method

    Identify the lone pair on nitrogen.

    Reason

    This pair is available to form a new covalent bond.

    Working

    :NH₃ has an electron-pair donor site.

  2. Apply the definition

    Method

    Classify ammonia as a nucleophile.

    Reason

    A nucleophile donates an electron pair to an electron-deficient centre.

    Working

    NH₃ is a nucleophile because nitrogen can donate its lone pair.

Guided practice 2

Classify the Proton

About 4 min

Problem

Is H⁺ a nucleophile or an electrophile? Explain in one line.

Classify by electron-pair role

Role
Electron-pair action

Hints

Hint 1: electron state
H⁺ is electron deficient.
Hint 2: definition
The species receiving an electron pair is the electrophile.
View solution step by step

  1. Identify electron deficiency

    Method

    Recognise that H⁺ has no electron pair to donate.

    Reason

    It can form a bond by receiving an electron pair from another species.

    Working

    H⁺: electron-pair acceptor.

  2. State the classification

    Method

    Call H⁺ an electrophile.

    Reason

    Electrophiles accept electron pairs.

    Working

    H⁺ is an electrophile because it accepts an electron pair.

Common misconception 3

Attack Site in Bromoethane

Find and correct the mistake

Learner claim

A learner says a nucleophile attacks bromine in CH₃CH₂Br because bromine is the more electronegative atom. Correct the attack site using partial charges.

Track bond polarity and attraction

Carbon charge
Attack site

View solution step by step

  1. Assign the bond polarity

    Method

    Label carbon δ + and bromine δ-.

    Reason

    Bromine draws bonding electron density toward itself.

    Working

    C\delta⁺-Br\delta⁻.

  2. Choose the attack site

    Method

    Attack the δ + carbon with the nucleophile.

    Reason

    The nucleophile donates an electron pair to the electron-deficient centre.

    Working

    Nucleophile → carbon bonded to Br.

Challenge 4

Fission of Chlorine

Minimal support

Bond-fission transfer

Classify Cl₂ → 2Cl. as homolytic or heterolytic fission, explain the electron split and state the usual initiating condition.

Read products and infer electron movement

Fission
Electrons received per Cl
Condition

Hints

Hint 1: product symbol
The dot on each chlorine product represents an unpaired electron.
Hint 2: split
Two neutral radicals form only if the bonding pair divides evenly.
View solution step by step

  1. Infer the electron split

    Method

    Give one bonding electron to each chlorine atom.

    Reason

    Equal sharing produces two neutral radicals, each with one unpaired electron.

    Working

    Cl-Cl → Cl. + .Cl.

  2. Name fission and condition

    Method

    State homolytic fission initiated by UV light.

    Reason

    Photochemical energy breaks the halogen bond and starts a radical chain.

    Working

    Cl₂ → [UV] 2Cl.: homolytic fission.

Mind Stretchers

Mind stretcher 1: Auditing an impossible arrowExtension

A proposed carbonyl mechanism draws an arrow from the δ + carbonyl carbon to CN⁻ and leaves the C = O bond unchanged. Diagnose both errors and give the correct electron-flow sequence.

Show Hint

Identify the electron-rich species first, then ask where the carbonyl pi pair must move when carbon gains a new bond.

Show Answer

The electron pair cannot start at the electron-poor carbon; it starts at the carbon lone pair/negative charge of CN⁻ and points to the carbonyl carbon. As the new carbon–carbon bond forms, a second full-headed arrow moves the C = O pi pair to oxygen. The resulting alkoxide is then protonated.

Mind stretcher 2: Using rate evidence to reject a mechanismExtension

Hydrolysis of a halogenoalkane has rate equation rate = k[RX] and is unchanged when hydroxide concentration doubles. Explain what this evidence says about the slow step and why a concerted bimolecular proposal is inconsistent.

Show Hint

A reactant present in the rate-determining elementary step normally appears in the rate equation.

Show Answer

Only the halogenoalkane concentration affects rate, so the slow step involves RX but not hydroxide. This is consistent with slow heterolytic C-X cleavage to a carbocation followed by fast nucleophilic attack. A concerted bimolecular step would involve both RX and OH⁻ and would normally predict hydroxide dependence.