Introduction to intermediate and reaction involving Substitution, addition, elimination, oxidation-reduction.

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Introduction to Intermediates & Reactions — Substitution, Addition, Elimination, Redox

Introduction to Intermediates & Reactions

Comprehensive concise notes on reaction types: Substitution, Addition, Elimination, and Oxidation–Reduction. Notation uses HTML <sub> and <sup> for chemical formulas and entities like &rarr; for arrows.

1. Overview

Chemical reactions can be classified by how bonds are broken and formed. Key categories covered here:

  • Substitution — one atom/group replaces another.
  • Addition — two fragments add to a multiple bond.
  • Elimination — removal of atoms/groups to form a multiple bond.
  • Oxidation–Reduction (Redox) — transfer of electrons; changes in oxidation state.

2. Common Reaction Intermediates

  • Carbocations — positively charged carbon: R–C+ (planar, sp2, 120°). Stabilized by resonance and alkyl groups (hyperconjugation).
  • Carbanions — negatively charged carbon: R–C- (pyramidal or sp2/sp depending on stabilization).
  • Radicals — species with an unpaired electron: R· (usually planar or slightly pyramidal). Reactivity influenced by stabilizing substituents.
  • Carbenes & Nitrenes — neutral species with divalent carbon/nitrogen; highly reactive (singlet or triplet states).
  • Oxonium/Nitronium/Acyl cations — heteroatom-centred reactive intermediates in many polar mechanisms.

3. Substitution Reactions

Substitution is the replacement of one group (leaving group) by another (nucleophile or electrophile). Major pathways in organic chemistry are SN1 and SN2.

SN2 (Bimolecular Nucleophilic Substitution)

Mechanism: single concerted step. Nucleophile attacks from the back side as leaving group departs. Rate = k[substrate][nucleophile].

  • Stereochemistry: inversion of configuration (Walden inversion).
  • Favoured: primary & secondary substrates, strong nucleophiles, polar aprotic solvents.
  • Typical example:
    CH3CH2Br + OH- → CH3CH2OH + Br-

SN1 (Unimolecular Nucleophilic Substitution)

Mechanism: two-step via carbocation intermediate. Rate = k[substrate].

  • Stereochemistry: racemization (if chiral carbocation), because planar carbocation is attacked from either side.
  • Favoured: tertiary substrates, weak nucleophiles, polar protic solvents that stabilise ions.
  • Typical example:
    (CH3)3C–Cl → (CH3)3C+ + Cl-; then (CH3)3C+ + H2O → (CH3)3COH + H+

Notes: Competing elimination (E2/E1) may occur with strong base or heat. Leaving group ability follows: I- > Br- > Cl- > F- (in protic solvents).

4. Addition Reactions

Addition reactions add atoms or groups across multiple bonds (C=C, C?C, C=O). Common types:

  • Electrophilic addition — typical for alkenes. Sequence: electrophile adds to give carbocation (or cyclic intermediate), then nucleophile attacks. Example: HBr addition to alkene (Markovnikov addition):
    RCH=CH2 + HBr → RCH(Br)CH3
  • Nucleophilic addition — common to carbonyls (C=O). Nucleophile attacks carbonyl carbon; tetrahedral intermediate forms. Example: addition of CN- to aldehyde:
    RCHO + CN- → RCH(OH)CN
  • Radical addition — initiated by radicals, e.g., HBr addition with peroxides (anti-Markovnikov via radical chain).
  • Hydrogenation — catalytic addition of H2 across double bonds:
    RCH=CH2 + H2 (Pd/C) → RCH2CH3

5. Elimination Reactions

Elimination removes atoms/groups to form a multiple bond. Two principal mechanisms: E2 (concerted) and E1 (via carbocation).

E2 (Bimolecular Elimination)

Concerted removal of a proton and leaving group. Rate = k[substrate][base]. Requires anti-periplanar geometry for best overlap.

Example:
CH3CHBrCH3 + OH- → CH3CH=CH2 + H2O + Br-

E1 (Unimolecular Elimination)

Two-step: leaving group departs ? carbocation; then base removes a proton to form alkene. Rate = k[substrate]. Often competes with SN1.

Zaitsev's rule: the more substituted (stable) alkene is usually formed preferentially unless bulky base or special conditions favour Hofmann product.

Note: Stereochemistry and regiochemistry depend on substrate geometry and base strength. E2 requires accessible ?-H in anti-periplanar arrangement.

6. Oxidation–Reduction (Redox)

Redox involves electron transfer. In organic chemistry, oxidation often means loss of hydrogen or gain of oxygen (formal), while reduction is gain of hydrogen or loss of oxygen. Use oxidation numbers for precise bookkeeping.

Key concepts

  • Oxidation: increase in oxidation state. Example: primary alcohol to aldehyde to carboxylic acid.
    RCH2OH → RCH=O → RCOOH
  • Reduction: decrease in oxidation state. Example: ketone to alcohol by NaBH4.
    RCOR → RCHOH R' (by NaBH4)
  • Redox agents: oxidising agents (KMnO4, CrO3, PCC) and reducing agents (LiAlH4, NaBH4, H2/Pd).

Balancing redox (inorganic example)

Half-reaction method (acidic solution): split into oxidation and reduction half-reactions, balance atoms and charge with H2O and H+, then combine so electrons cancel.

7. Mechanistic & Problem-Solving Tips

  1. Identify electrophiles and nucleophiles; determine leaving-group ability.
  2. Check substrate: primary/secondary/tertiary affects SN/E pathways.
  3. Consider solvent: polar protic stabilises ions (SN1/E1), polar aprotic favours SN2.
  4. Strong bases favour elimination (E2) and can compete with SN2.
  5. Look for resonance and neighbouring group participation that stabilises intermediates.
  6. Use stereochemical consequences (inversion, racemization, syn/anti elimination) to deduce mechanism.

8. Quick Comparison Table

Reaction Mechanism Rate law Favoured conditions
SN2 Concerted nucleophilic attack/back-side k[substrate][nucleophile] Primary substrate, strong nucleophile, polar aprotic
SN1 Carbocation intermediate k[substrate] Tertiary substrate, polar protic solvent
E2 Concerted base-induced elimination k[substrate][base] Strong base, anti-periplanar ?-H
E1 Carbocation then deprotonation k[substrate] Weak base, heat, tertiary substrates

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