Mechanism of Organic Reactions
The mechanism of an organic reaction describes the step-by-step sequence of elementary reactions by which overall chemical change occurs. It provides insight into the bond breaking and bond forming processes, the intermediates formed, and the transition states involved. Understanding reaction mechanisms is crucial for predicting reaction outcomes and designing new synthetic routes.
1. Fundamental Concepts
1.1. Types of Bond Fission (Bond Breaking)
Homolytic Fission (Homolysis):
- A covalent bond breaks in such a way that each atom retains one of the shared electrons.
- Results in the formation of free radicals (species with unpaired electrons).
- Indicated by half-headed arrows (⌢).
- Favoured by: High temperatures, UV light, or presence of peroxides.
- Example: $Cl-Cl \xrightarrow{UV\ light} Cl\cdot + Cl\cdot$
Heterolytic Fission (Heterolysis):
- A covalent bond breaks in such a way that one atom retains both of the shared electrons, and the other atom gets none.
- Results in the formation of ions (carbocations, carbanions, or other charged species).
- Indicated by full-headed curved arrows (↷).
- Favoured by: Polar solvents and presence of polar reagents.
- Example: $CH_3-Cl \xrightarrow{H_2O} CH_3^+ + Cl^-$
1.2. Electron Displacement Effects
These effects influence the distribution of electron density within a molecule, affecting its reactivity.
Inductive Effect (+I and -I):
- The permanent displacement of electron density along a saturated carbon chain towards a more electronegative atom or group.
- -I Effect: Electron-withdrawing groups (e.g., $−NO_2$, $−CN$, $−COOH$, $−X$ (halogens), $−OH$, $−OR$). They pull electron density away.
- +I Effect: Electron-donating groups (e.g., alkyl groups $−CH_3$, $−C_2H_5$). They push electron density.
- Effect decreases rapidly with distance.
Resonance Effect (Mesomeric Effect, +M and -M):
- The delocalization of $\pi$-electrons within a conjugated system.
- +M Effect: Electron-donating groups through resonance (e.g., $−OH$, $−OR$, $−NH_2$, $−NR_2$, $−X$). They push lone pairs/$\pi$-electrons into the conjugated system.
- -M Effect: Electron-withdrawing groups through resonance (e.g., $−CHO$, $−COR$, $−CN$, $−NO_2$, $−COOH$). They pull $\pi$-electrons from the conjugated system.
- Permanent effect. Stronger than inductive effect in conjugated systems.
Hyperconjugation (No bond resonance):
- The delocalization of $\sigma$-electrons (from C-H bonds) of an alkyl group interacting with an adjacent empty p-orbital or a $\pi$-bond.
- Stabilizes carbocations, free radicals, and alkenes.
- More $\alpha$-hydrogens lead to greater hyperconjugation and stability.
- Stability of Carbocations/Free Radicals: $3^\circ > 2^\circ > 1^\circ > CH_3^+$
1.3. Types of Reagents
Electrophiles (Electron-loving species):
- Electron-deficient species that seek electrons.
- Usually positively charged ions or neutral molecules with an empty orbital (Lewis acids).
- Examples: $H^+$, $NO_2^+$, $R_3C^+$, $BF_3$, $AlCl_3$.
Nucleophiles (Nucleus-loving species):
- Electron-rich species that donate electrons.
- Usually negatively charged ions or neutral molecules with at least one lone pair of electrons (Lewis bases).
- Examples: $OH^-$, $CN^-$, $RO^-$, $RNH_2$, $H_2O$.
2. Reaction Intermediates
Transient species formed during a reaction that are not reactants or products but are formed and consumed within the reaction pathway.
Carbocations (Carbonium Ions):
- Carbon atom carrying a positive charge.
- $sp^2$ hybridized, trigonal planar geometry.
- Electron deficient (sextet of electrons).
- Stability: $3^\circ > 2^\circ > 1^\circ > CH_3^+$
Carbanions:
- Carbon atom carrying a negative charge.
- $sp^3$ hybridized, pyramidal geometry.
- Electron rich (lone pair + octet).
- Stability: $CH_3^- > 1^\circ > 2^\circ > 3^\circ$
Free Radicals:
- Species with an unpaired electron on a carbon atom.
- $sp^2$ hybridized (planar) or $sp^3$ hybridized (pyramidal), often planar or near-planar.
- Electron deficient (septet of electrons).
- Stability: $3^\circ > 2^\circ > 1^\circ > CH_3^\cdot$
Carbenes & Nitrenes:
- Carbenes: Neutral carbon species with two unshared electrons and two bonds. Highly reactive.
- Nitrenes: Nitrogen analog of carbenes, with two unshared electrons and one bond. Highly reactive.
3. Types of Organic Reactions
3.1. Substitution Reactions
An atom or group is replaced by another atom or group.
Nucleophilic Substitution ($S_N$):
- $S_N1$: Two-step, carbocation intermediate. Rate depends only on substrate. Favoured by $3^\circ$ alkyl halides, weak nucleophiles, polar protic solvents. Racemization occurs.
- $S_N2$: One-step (concerted), no intermediate. Rate depends on both substrate and nucleophile. Favoured by $1^\circ$ alkyl halides, strong nucleophiles, polar aprotic solvents. Inversion of configuration (Walden inversion) occurs.
Electrophilic Substitution ($S_E$):
- Common for aromatic compounds. An electrophile replaces a hydrogen atom on the aromatic ring.
Free Radical Substitution ($S_R$):
- Common for alkanes. Mechanism involves initiation, propagation, and termination steps.
3.2. Addition Reactions
A molecule adds across a multiple bond (double or triple bond), breaking the $\pi$-bond(s) and forming new $\sigma$-bonds.
Electrophilic Addition ($A_E$):
- Common for alkenes and alkynes. Follows Markovnikov’s rule.
Nucleophilic Addition ($A_N$):
- Common for aldehydes and ketones. Nucleophile attacks the electrophilic carbonyl carbon.
3.3. Elimination Reactions
Atoms or groups are removed from adjacent carbon atoms, forming a multiple bond.
E1 and E2 Mechanisms:
- E1: Two-step, carbocation intermediate. Competes with $S_N1$.
- E2: One-step (concerted). Competes with $S_N2$. Requires anti-periplanar arrangement.
- Saytzeff’s Rule: Major product is the more substituted alkene.
3.4. Rearrangement Reactions
An atom or group migrates within the same molecule, often to form a more stable intermediate.
4. Important Considerations
- Solvent Effects: Polar protic solvents favour $S_N1$/E1. Polar aprotic solvents favour $S_N2$/E2.
- Leaving Group Ability: Good leaving groups are weak bases (e.g., $I^- > Br^- > Cl^- > F^-$).
- Nucleophilicity/Basicity: Strong nucleophiles and strong bases favour $S_N2$ and E2.
- Steric Hindrance: Increased steric hindrance favours $S_N1$/E1 over $S_N2$.
5. Important MCQs with Explanations
Here are 40 important multiple-choice questions to test your understanding of the concepts.
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1. Which of the following bond fissions results in the formation of ions?
(a) Homolytic fission (b) Heterolytic fission (c) Both (d) Neither
Explanation: The correct answer is (b). Heterolytic fission involves the unequal breaking of a covalent bond, producing a cation and an anion. Homolytic fission produces free radicals.
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2. A species with an unpaired electron is called a(n):
(a) Carbocation (b) Carbanion (c) Free radical (d) Electrophile
Explanation: The correct answer is (c). Free radicals possess an unpaired electron. Carbocations are positively charged, and carbanions are negatively charged. Electrophiles are electron-deficient.
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3. Which of the following effects involves the delocalization of $\sigma$-electrons?
(a) Inductive effect (b) Resonance effect (c) Hyperconjugation (d) Mesomeric effect
Explanation: The correct answer is (c). Hyperconjugation involves the delocalization of $\sigma$-electrons from C-H bonds to an adjacent empty p-orbital or $\pi$-system.
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4. An electrophile is a species that is:
(a) Electron-rich (b) Electron-deficient (c) Negatively charged (d) A proton donor
Explanation: The correct answer is (b). Electrophiles are “electron-loving” and seek electrons, making them electron-deficient. They are typically Lewis acids.
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5. Which of the following is an example of a nucleophile?
(a) $H_2O$ (b) $BF_3$ (c) $AlCl_3$ (d) $NO_2^+$
Explanation: The correct answer is (a). Water ($H_2O$) has lone pairs on oxygen and can donate them, acting as a nucleophile. The other options are electrophiles.
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6. The stability order of carbocations is:
(a) $1^\circ > 2^\circ > 3^\circ$ (b) $3^\circ > 2^\circ > 1^\circ$ (c) $CH_3^+ > 1^\circ > 2^\circ$ (d) All are equally stable
Explanation: The correct answer is (b). The stability of carbocations increases with alkyl substitution due to the electron-donating (+I) effect and hyperconjugation. Thus, tertiary ($3^\circ$) is the most stable.
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7. Which of the following organic reactions proceeds via a carbocation intermediate?
(a) $S_N2$ reaction (b) $S_N1$ reaction (c) E2 reaction (d) Free radical addition
Explanation: The correct answer is (b). $S_N1$ reactions are two-step processes where a carbocation intermediate is formed in the rate-determining step.
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8. Walden inversion is associated with which reaction mechanism?
(a) $S_N1$ (b) $S_N2$ (c) E1 (d) E2
Explanation: The correct answer is (b). Walden inversion, an inversion of configuration at a chiral center, is a characteristic feature of $S_N2$ reactions due to the backside attack of the nucleophile.
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9. Which type of solvent favours $S_N2$ reactions?
(a) Polar protic solvents (b) non-polar solvents (c) Polar aprotic solvents (d) Any solvent
Explanation: The correct answer is (c). Polar aprotic solvents (e.g., DMSO, acetone) do not solvate nucleophiles, keeping them reactive and favouring the $S_N2$ mechanism.
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10. Markovnikov’s rule is primarily applicable to:
(a) Nucleophilic substitution (b) Elimination (c) Electrophilic addition (d) Free radical substitution
Explanation: The correct answer is (c). Markovnikov’s rule governs the regioselectivity of electrophilic addition reactions to unsymmetrical alkenes.
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11. Which of the following is the best leaving group?
(a) $OH^-$ (b) $F^-$ (c) $Cl^-$ (d) $I^-$
Explanation: The correct answer is (d). Good leaving groups are weak bases. Basicity decreases down the halogen group, making $I^-$ the best leaving group.
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12. Saytzeff’s rule (Zaitsev’s rule) predicts the major product in:
(a) Addition reactions (b) Substitution reactions (c) Elimination reactions (d) Rearrangement reactions
Explanation: The correct answer is (c). Saytzeff’s rule predicts the more substituted alkene as the major product in elimination reactions.
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13. The geometry of a carbocation is:
(a) Tetrahedral (b) Pyramidal (c) Trigonal planar (d) Linear
Explanation: The correct answer is (c). A carbocation’s central carbon is $sp^2$ hybridized, resulting in a trigonal planar geometry.
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14. Which factor favours $S_N1$ reactions over $S_N2$ reactions?
(a) Strong nucleophile (b) Primary alkyl halide (c) Polar protic solvent (d) Low temperature
Explanation: The correct answer is (c). Polar protic solvents stabilize the carbocation intermediate, which is key to $S_N1$ reactions.
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15. The carbon atom in a carbanion is typically:
(a) $sp^2$ hybridized (b) $sp$ hybridized (c) $sp^3$ hybridized (d) $dsp^2$ hybridized
Explanation: The correct answer is (c). A carbanion has a lone pair and three bonds, leading to $sp^3$ hybridization and a pyramidal geometry.
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16. Which reaction is characteristic of alkanes?
(a) Electrophilic addition (b) Nucleophilic substitution (c) Free radical substitution (d) Nucleophilic addition
Explanation: The correct answer is (c). Alkanes primarily undergo free radical substitution reactions, such as halogenation in UV light.
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17. The intermediate formed during the nitration of benzene is a(n):
(a) Carbanion (b) Carbocation (sigma complex) (c) Free radical (d) Nitrene
Explanation: The correct answer is (b). The nitration of benzene is an electrophilic aromatic substitution, which proceeds through a resonance-stabilized carbocation intermediate called a sigma complex.
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18. Which of the following is a two-step reaction mechanism?
(a) $S_N2$ (b) E2 (c) $S_N1$ (d) All of the above
Explanation: The correct answer is (c). $S_N1$ and E1 reactions are two-step processes, while $S_N2$ and E2 are one-step (concerted) mechanisms.
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19. In an $S_N2$ reaction, the rate depends on:
(a) Concentration of substrate only (b) Concentration of nucleophile only (c) Concentrations of both substrate and nucleophile (d) Concentration of leaving group only
Explanation: The correct answer is (c). The $S_N2$ mechanism is bimolecular, so its rate depends on the concentration of both the substrate and the nucleophile.
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20. Which effect explains the ortho/para-directing nature of electron-donating groups?
(a) Inductive effect (b) Hyperconjugation (c) Resonance effect (d) Steric effect
Explanation: The correct answer is (c). Electron-donating groups activate the ortho and para positions of an aromatic ring by increasing electron density through the resonance effect (+M effect).
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21. A molecule with an incomplete octet on a carbon atom and a positive charge is a:
(a) Carbanion (b) Free radical (c) Carbocation (d) Electrophile (but specifically an ion)
Explanation: The correct answer is (c). A carbocation has a carbon atom with only six valence electrons and a positive charge.
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22. Which of the following reactions involves the formation of a free radical intermediate?
(a) Friedel-Crafts alkylation (b) Halogenation of methane under UV light (c) Nitration of benzene (d) Dehydration of alcohol
Explanation: The correct answer is (b). The halogenation of alkanes proceeds via a free radical chain mechanism initiated by UV light.
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23. The relative stability of free radicals is:
(a) $1^\cdot > 2^\cdot > 3^\cdot$ (b) $3^\cdot > 2^\cdot > 1^\cdot$ (c) $CH_3^\cdot > 1^\cdot > 2^\cdot$ (d) All are equally stable
Explanation: The correct answer is (b). The stability of free radicals increases with increasing alkyl substitution due to hyperconjugation.
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24. Which type of reaction mechanism is dominant for primary alkyl halides with strong nucleophiles/bases?
(a) $S_N1$ (b) $S_N2$ (c) E1 (d) $A_E$
Explanation: The correct answer is (b). Primary alkyl halides undergo $S_N2$ reactions effectively due to minimal steric hindrance.
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25. A reaction that involves the removal of atoms or groups from adjacent carbon atoms to form a multiple bond is called:
(a) Substitution (b) Addition (c) Elimination (d) Rearrangement
Explanation: The correct answer is (c). Elimination reactions are defined by the removal of groups to form a double or triple bond.
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26. The rate-determining step in an $S_N1$ reaction is:
(a) Attack of the nucleophile (b) Formation of the carbocation (c) Deprotonation (d) Formation of the product
Explanation: The correct answer is (b). The slowest, rate-determining step in an $S_N1$ reaction is the formation of the carbocation intermediate.
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27. Which of the following is an electron-withdrawing group by inductive effect (-I effect)?
(a) $-CH_3$ (b) $-OH$ (c) $-NH_2$ (d) $-OCH_3$
Explanation: The correct answer is (b). The oxygen in the hydroxyl group is more electronegative than carbon and pulls electron density away through the inductive effect.
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28. What happens to the configuration of a chiral centre in an $S_N1$ reaction?
(a) Complete inversion (b) Complete retention (c) Racemization (partial inversion and retention) (d) No change
Explanation: The correct answer is (c). The planar carbocation intermediate allows attack from either side, leading to a racemic mixture of products.
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29. Nucleophilic addition reactions are characteristic of:
(a) Alkanes (b) Alkenes (c) Aldehydes and ketones (d) Aromatic compounds
Explanation: The correct answer is (c). Aldehydes and ketones contain an electrophilic carbonyl carbon, which is readily attacked by nucleophiles.
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30. Which effect contributes to the extra stability of benzene?
(a) Inductive effect (b) Hyperconjugation (c) Resonance (aromaticity) (d) Steric hindrance
Explanation: The correct answer is (c). The stability of benzene is due to the delocalization of $\pi$-electrons, a phenomenon known as resonance or aromaticity.
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31. The intermediate formed in Hofmann bromamide degradation is a:
(a) Carbene (b) Nitrene (c) Free radical (d) Carbanion
Explanation: The correct answer is (b). The Hofmann degradation proceeds through the formation of a nitrene intermediate.
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32. An E2 reaction typically occurs with:
(a) Weak base in protic solvent (b) Strong base in aprotic solvent (c) Weak acid in protic solvent (d) Strong acid in aprotic solvent
Explanation: The correct answer is (b). E2 reactions are favoured by strong bases and polar aprotic solvents.
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33. What is the stability order of carbanions?
(a) $1^\circ > 2^\circ > 3^\circ$ (b) $3^\circ > 2^\circ > 1^\circ$ (c) $CH_3^- > 1^\circ > 2^\circ > 3^\circ$ (d) All are equally stable
Explanation: The correct answer is (c). The stability of carbanions decreases with increasing alkyl substitution because electron-donating alkyl groups destabilize the negative charge.
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34. The rearrangement of carbocations from less stable to more stable is often achieved by:
(a) Addition of nucleophile (b) Hydride or alkyl shifts (c) Oxidation (d) Reduction
Explanation: The correct answer is (b). Carbocation rearrangements occur via 1,2-hydride or 1,2-alkyl shifts to form a more stable carbocation.
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35. Electrophilic aromatic substitution is a characteristic reaction of:
(a) Alkanes (b) Alkenes (c) Arenes (aromatic compounds) (d) Alkyl halides
Explanation: The correct answer is (c). Aromatic compounds undergo electrophilic substitution while preserving their aromaticity.
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36. Which of the following functional groups is a strong electron-withdrawing group by resonance (-M effect)?
(a) $-OH$ (b) $-CH_3$ (c) $-NO_2$ (d) $-NH_2$
Explanation: The correct answer is (c). The nitro group ($-NO_2$) is a powerful electron-withdrawing group through the resonance effect, pulling electron density from the conjugated system.
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37. The anti-Markovnikov’s addition of HBr to an alkene occurs in the presence of:
(a) Acid catalyst (b) Peroxides (c) UV light (d) Lewis acid
Explanation: The correct answer is (b). The addition of HBr to an alkene follows the anti-Markovnikov’s rule in the presence of peroxides via a free radical mechanism.
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38. Which type of reaction involves the replacement of an atom or group by another atom or group?
(a) Addition (b) Elimination (c) Substitution (d) Rearrangement
Explanation: The correct answer is (c). Substitution reactions are characterized by the replacement of one group with another.
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39. The most reactive alkyl halide towards $S_N2$ reactions is:
(a) Tertiary alkyl halide (b) Secondary alkyl halide (c) Primary alkyl halide (d) Methyl halide
Explanation: The correct answer is (d). Methyl halides have the least steric hindrance around the carbon atom, making them the most reactive in $S_N2$ reactions.
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40. A nucleophile can also be termed as a:
(a) Lewis acid (b) Brønsted-Lowry acid (c) Lewis base (d) Electrophile
Explanation: The correct answer is (c). Nucleophiles are electron-rich species that donate an electron pair, which is the definition of a Lewis base.