Fiche de révision : Nucleophilic Substitution Mechanisms

📋 Course Outline

1. Nucleophilic Substitution
2. SN2 Mechanism
3. SN1 Mechanism
4. Halogenoalkane Reactions
5. Substitution of NH2
6. Cyanide Substitution
7. Oxidation of Alcohols
8. Alcohol Dehydration
9. Esterification

📖 1. Nucleophilic Substitution

🔑 Key Concepts & Definitions

• Nucleophile: A species that donates a pair of electrons to form a chemical bond (source content). It acts as an electron pair donor during substitution reactions.
• Nucleophilic substitution: A reaction mechanism where a nucleophile replaces a leaving group in a molecule, typically involving halogenoalkanes (source content).
• General substitution reactions involving halogenoalkanes and nucleophiles: Reactions where halogenoalkanes undergo replacement of halogen atoms with nucleophiles such as OH-, NH3, or CN-, resulting in alcohols, amines, or nitriles.
• Difference between nucleophilic substitution and elimination reactions: Substitution involves replacing a leaving group with a nucleophile, whereas elimination results in the removal of a group to form an alkene, often under similar conditions but with different mechanistic pathways.
• Role of heat under reflux in substitution reactions: Reflux involves heating a reaction mixture to allow continuous boiling and condensation, ensuring the reaction proceeds at an elevated temperature without loss of volatile components, thus increasing reaction efficiency (source content).

📝 Essential Points

• Nucleophilic substitution mechanisms include SN2 (bimolecular) and SN1 (unimolecular), with SN2 occurring mainly in primary halogenoalkanes and SN1 in tertiary ones (source content). The SN2 mechanism involves a backside attack by the nucleophile and a transition state, while SN1 involves carbocation formation.
• In reactions with halogenoalkanes, nucleophiles such as OH- can replace halogen atoms, producing alcohols, with heat under reflux often used to facilitate the process (source content).
• The stability of carbocations influences the mechanism: tertiary carbocations are stabilized by inductive effects, favoring SN1, while primary carbocations are less stable, favoring SN2.
• Reactions with nucleophiles like NH3 and CN- produce primary amines and nitriles, respectively, with subsequent hydrolysis possible to acids (source content).
• Heat under reflux allows reactions to proceed efficiently by maintaining a constant temperature and preventing the loss of volatile reactants or products.

💡 Key Takeaway

Nucleophilic substitution involves a nucleophile replacing a leaving group in a molecule, with the reaction pathway (SN1 or SN2) influenced by the structure of the halogenoalkane and reaction conditions such as heat under reflux, which enhances reaction rate and efficiency.

📖 2. SN2 Mechanism

🔑 Key Concepts & Definitions

• SN2 mechanism: A bimolecular nucleophilic substitution process that occurs predominantly in primary halogenoalkanes, involving a single concerted step where the nucleophile attacks the carbon atom from the opposite side of the leaving group, leading to inversion of configuration (see source page 5).

• Mechanism of SN2: The nucleophile uses its lone pair to attack the δ+ carbon atom from the side opposite the leaving group, forming a transition state where bonds to both the nucleophile and leaving group are partially formed and broken simultaneously (see source page 5).

• Energy profile of SN2 reactions: The reaction involves a single energy peak corresponding to the transition state, with the energy barrier determined by the strength of the bonds being broken and formed during the nucleophilic attack (see source page 5).

• Steric hindrance preventing SN2 in tertiary halogenoalkanes: The large alkyl groups in tertiary halogenoalkanes create steric hindrance that blocks the nucleophile's approach, preventing the SN2 mechanism and favoring other pathways such as SN1 (see source page 5).

• Primary halogenoalkanes do not undergo SN1: Due to the lack of carbocation stability in primary halogenoalkanes, they cannot favor the formation of a carbocation intermediate required for the SN1 mechanism, thus favoring SN2 instead (see source page 5).

📝 Essential Points

• The SN2 mechanism is characterized by a one-step process where the nucleophile attacks from the opposite side of the leaving group, leading to an inversion of stereochemistry at the carbon center.

• The energy profile of SN2 reactions features a single transition state with an energy maximum; the height of this barrier depends on the nature of the halogenoalkane and the nucleophile.

• Steric hindrance is a critical factor: in tertiary halogenoalkanes, bulky groups prevent the nucleophile from accessing the carbon atom, thus SN2 is effectively blocked, and SN1 becomes the dominant pathway.

• Primary halogenoalkanes do not undergo SN1 reactions because they cannot stabilize the carbocation intermediate, making SN2 the preferred mechanism.

💡 Key Takeaway

The SN2 mechanism involves a direct, concerted attack by the nucleophile on primary halogenoalkanes, with a characteristic energy profile and stereochemical inversion, while steric hindrance in tertiary halogenoalkanes prevents this pathway, favoring alternative mechanisms.

📖 3. SN1 Mechanism

🔑 Key Concepts & Definitions

• SN1 mechanism: A unimolecular nucleophilic substitution process that occurs predominantly in tertiary halogenoalkanes, involving a two-step reaction where carbocation formation is the rate-determining step. (source: Page 6)
• Formation and stabilization of tertiary carbocation intermediate: The process whereby a tertiary carbocation is generated during SN1, stabilized by the positive inductive effect of alkyl groups, making the reaction more favorable in tertiary halogenoalkanes. (source: Page 6)
• Rate-determining step is carbocation formation: The slowest step in SN1, where the halogen leaves, forming a carbocation; this step controls the overall reaction rate. (source: Page 6)
• Steric hindrance preventing SN2 in tertiary halogenoalkanes: The large alkyl groups surrounding the tertiary carbon hinder nucleophilic attack from the opposite side, thus favoring SN1 over SN2. (source: Page 6)
• Order of reactivity: tertiary > secondary > primary halogenoalkanes in SN1: Due to carbocation stability, tertiary halogenoalkanes react fastest via SN1, followed by secondary, with primary halogenoalkanes being least reactive. (source: Page 6)

📝 Essential Points

• SN1 occurs mainly in tertiary halogenoalkanes because the carbocation formed is stabilized by the positive inductive effect of alkyl groups, which is crucial for the reaction's feasibility.
• The mechanism involves a two-step process: first, the leaving group departs, forming a tertiary carbocation; second, the nucleophile attacks the carbocation, resulting in substitution.
• The rate of SN1 depends solely on the concentration of the halogenoalkane, as carbocation formation is the rate-determining step.
• Steric hindrance caused by bulky alkyl groups prevents SN2 in tertiary halogenoalkanes, making SN1 the predominant pathway.
• The reactivity trend (tertiary > secondary > primary) is directly related to the stability of the carbocation intermediate.

💡 Key Takeaway

SN1 mechanism is characterized by carbocation formation as the rate-determining step, with tertiary halogenoalkanes reacting fastest due to carbocation stabilization and steric hindrance preventing SN2 pathways.

📖 4. Halogenoalkane Reactions

🔑 Key Concepts & Definitions

• Reactions of halogenoalkanes include substitution and elimination processes, often facilitated by reagents such as aqueous NaOH or KOH, and ethanol with NaOH or KOH for elimination. These reactions typically occur under reflux conditions to ensure complete reaction (see pages 5-7).

• Testing halogen in halogenoalkanes involves adding AgNO3 solution to a halogenoalkane solution. The formation of a precipitate indicates the presence of a halogen, with precipitate colors: AgCl (white), AgBr (cream), AgI (yellow). The solubility of these precipitates in NH3 confirms the halogen type (see pages 8-10).

• Precipitate colors for silver halides:

  • AgCl: White
  • AgBr: Cream
  • AgI: Yellow

• Trend in rate of halide ion release from halogenoalkanes during testing: tertiary > secondary > primary. Tertiary halogenoalkanes release halide ions faster because they form more stable carbocations, which facilitates halide ion release until stabilized by the positive inductive effect of alkyl groups (see pages 8-10).

• Use of reflux apparatus in halogenoalkane reactions involves heating the reaction mixture while condensing vapors back into the reaction vessel. This ensures continuous heating without loss of volatile reactants or products, crucial for reactions like substitution and elimination (see pages 5-7).

📝 Essential Points

• Elimination reactions of halogenoalkanes with ethanolic NaOH or KOH produce alkenes, such as propene from 2-bromopropane, via an E2 mechanism. These reactions require heating under reflux, and the removal of the halogen and hydrogen occurs from adjacent carbons (pages 6-7).

• Testing for halogens involves adding dilute nitric acid to remove impurities, then AgNO3 solution. The formation and solubility of the precipitate in NH3 distinguish the halogen:

  • AgCl dissolves in dilute NH3
  • AgBr dissolves in concentrated NH3
  • AgI remains insoluble, confirming the halogen (pages 8-10).

• Substitution of NH2 and CN groups occurs under specific conditions:

  • Ethanolic NH3 in a sealed tube produces primary amines and ammonium salts (page 8).
  • NaCN or KCN in ethanol under reflux forms nitriles, which can be hydrolyzed to carboxylic acids, extending the carbon chain (pages 8-10).

• Reflux conditions are essential for reactions like halogen substitution with HCl, SOCl2, PCl3, PBr3, and PI3, ensuring complete conversion and preventing loss of volatile products (pages 10-11).

💡 Key Takeaway

Reactions of halogenoalkanes involve substitution and elimination mechanisms that are influenced by the structure of the halogenoalkane, reaction conditions, and reagents used. Testing for halogens via silver halide precipitates and understanding the rate trends in halide ion release are crucial for analyzing halogenoalkane reactivity. Reflux apparatus plays a vital role in facilitating these reactions efficiently.

📖 5. Substitution of NH2

🔑 Key Concepts & Definitions

• Substitution of NH2: The process where an amino group (-NH2) replaces a halogen atom in a halogenoalkane through a nucleophilic substitution reaction, typically using ethanolic NH3 under pressure and heat in a sealed tube.

• Ethanolic NH3 in sealed tube: A reaction setup where ammonia dissolved in ethanol is used under pressure and heat within a sealed container to prevent OH- ions from substituting instead of NH2, ensuring the formation of primary amines.

• Formation of primary amines and ammonium salts: When halogenoalkanes react with ethanolic NH3, primary amines (e.g., CH3CH2NH2) are formed along with ammonium salts (e.g., NH4Br), depending on reaction conditions.

• Importance of using ethanolic NH3: Ethanolic solution of NH3 is crucial because it minimizes competing substitution by OH- ions, which would otherwise lead to unwanted alcohol formation, and the sealed tube prevents the escape of NH3 due to its lower density than air.

• Properties of NH3 affecting setup: Ammonia's properties, such as being less dense than air, influence the reaction setup by causing it to rise and escape if not contained properly, which is why a sealed tube is essential for controlled reaction conditions.

📖 6. Cyanide Substitution

🔑 Key Concepts & Definitions

• Substitution of CN group using NaCN or KCN in ethanol under reflux: A reaction where a halogenoalkane reacts with sodium cyanide (NaCN) or potassium cyanide (KCN) dissolved in ethanol, heated under reflux, resulting in the replacement of a halogen atom with a cyanide group (-C≡N).

• Formation of nitriles from halogenoalkanes: The process by which halogenoalkanes undergo nucleophilic substitution with cyanide ions to produce nitriles, which are organic compounds containing the -C≡N functional group.

• Ionic mechanism of cyanide substitution: The reaction proceeds via an ionic pathway where the cyanide ion (CN-) acts as a nucleophile, attacking the electrophilic carbon atom of the halogenoalkane, leading to the displacement of the halogen atom and formation of a nitrile.

• Importance of cyanide substitution in carbon chain lengthening: This reaction is significant because it allows for the extension of the carbon chain in organic synthesis; the nitrile product can be hydrolyzed to form a carboxylic acid, effectively increasing the chain length.

• Hydrolysis of nitriles to carboxylic acids using acid or alkali under reflux: The conversion of nitriles into carboxylic acids by treatment with dilute acids (e.g., HCl or H2SO4) or alkalis (e.g., NaOH) under reflux conditions, breaking the -C≡N bond and forming a -COOH group.

📖 7. Oxidation of Alcohols

🔑 Key Concepts & Definitions

• Oxidation of primary alcohols (see source content): The process where primary alcohols are oxidized using acidified potassium dichromate (K₂Cr₂O₇) to form aldehydes initially, and further oxidation produces carboxylic acids.
• Color change during oxidation: The observable transition from orange to green signifies the reduction of dichromate ions (Cr₂O₇²⁻) to chromium ions (Cr³⁺), indicating oxidation progress (source content).
• Conditions for partial oxidation (distillation): When primary alcohols are oxidized to aldehydes, the aldehyde is distilled off immediately to prevent further oxidation to carboxylic acids. This involves keeping the alcohol in excess and using distillation.
• Conditions for complete oxidation (reflux): To fully oxidize primary alcohols to carboxylic acids, the reaction mixture is heated under reflux with excess oxidizing agent, ensuring continuous oxidation until the acid forms.
• Oxidation of secondary alcohols: Secondary alcohols are oxidized to ketones under reflux with acidified potassium dichromate, with no further oxidation to acids (source content).
• Tertiary alcohols: These do not oxidize because they lack hydrogen on the carbon bearing the hydroxyl group (source content).

📝 Essential Points

• The oxidation process uses acidified potassium dichromate (K₂Cr₂O₇ + H₂SO₄), with the color change from orange to green indicating the reduction of Cr₂O₇²⁻ to Cr³⁺ (source content).
• For primary alcohols, distillation is used to isolate aldehydes, which are volatile and can be separated early, preventing further oxidation to carboxylic acids (source content).
• Complete oxidation of primary alcohols to carboxylic acids requires reflux with excess oxidizing agent, heating the mixture continuously until the reaction is complete (source content).
• Secondary alcohols are oxidized to ketones by reflux, with no further oxidation due to the stability of the ketone structure (source content).
• Tertiary alcohols do not undergo oxidation because they lack hydrogen on the carbon with the hydroxyl group, preventing oxidation (source content).

💡 Key Takeaway

Oxidation of alcohols using acidified potassium dichromate involves a color change from orange to green, with primary alcohols oxidized to aldehydes (via distillation) or carboxylic acids (via reflux), secondary alcohols to ketones, and tertiary alcohols remaining unoxidized.

📖 8. Alcohol Dehydration

🔑 Key Concepts & Definitions

• Dehydration of alcohols to alkenes using Al2O3 catalyst or concentrated H2SO4: A chemical reaction where alcohols are heated with a catalyst such as Al2O3 or concentrated sulfuric acid, resulting in the removal of a water molecule (OH and H from neighboring carbons) to form an alkene.

• Removal of OH group and hydrogen from neighboring carbon: The essential step in dehydration, where the hydroxyl group (OH) and a hydrogen atom from an adjacent carbon are eliminated, facilitating alkene formation.

• Dehydration of complex alcohols producing mixtures of alkenes: When complex alcohols (with multiple possible elimination sites) undergo dehydration, they form a mixture of different alkenes, including cis and trans isomers, due to multiple possible elimination pathways.

📝 Essential Points

• The dehydration process involves heating alcohols in the presence of Al2O3 or concentrated H2SO4, which acts as a dehydrating agent (see "Dehydration of alcohols to alkenes using Al2O3 catalyst or concentrated H2SO4").

• The removal of the OH group and a hydrogen atom from a neighboring carbon is critical for alkene formation, following an elimination mechanism.

• When dehydration occurs with complex alcohols such as butan-2-ol, it produces a mixture of alkenes: 50% but-1-ene, 25% cis-but-2-ene, and 25% trans-but-2-ene, reflecting multiple possible elimination pathways.

• Heating is a necessary condition for dehydration reactions, with specific temperature requirements depending on the alcohol's complexity and the catalyst used.

• Safety note: When dismantling apparatus after dehydration, always remove the delivery tube first before stopping the heating to prevent "suck back," which can cause water to rush back rapidly and potentially explode.

💡 Key Takeaway

Dehydration of alcohols to alkenes involves heating with Al2O3 or concentrated H2SO4, removing the OH group and hydrogen from neighboring carbons, and often produces mixtures of alkenes, especially with complex alcohols. Proper safety procedures, including careful dismantling of apparatus, are essential to prevent accidents.

📖 9. Esterification

🔑 Key Concepts & Definitions

• Esterification: A chemical reaction where alcohols react with carboxylic acids in the presence of concentrated sulfuric acid (H2SO4) and heat to form esters and water.
• Functional group of esters: The characteristic group in esters is (R)-C(=O)-O-(R'), where R and R' are alkyl groups.
• Reversible nature of esterification: The reaction can proceed in both directions, meaning esters can hydrolyze back into alcohols and acids under suitable conditions.
• Naming esters: Esters are named based on the alcohol (alkyl group) and the acid (acidic group) components, e.g., ethyl ethanoate from ethanol and ethanoic acid.
• Separation of esters: Due to differences in solubility, esters can be separated from reaction mixtures by adding water; esters are generally insoluble, while alcohols and acids are soluble.
• Uses of esters: Esters are widely used in perfumes and glues because of their pleasant odors and adhesive properties.

📝 Essential Points

• Esterification involves reacting alcohols with carboxylic acids in the presence of concentrated H2SO4 and heat, producing esters and water.
• The functional group (R)-C(=O)-O-(R') is key to identifying esters.
• The reaction is reversible, meaning esters can hydrolyze back into alcohols and acids, especially under acidic or basic conditions.
• Esters are named by combining the name of the alkyl group from the alcohol with the name of the acid, replacing the "-ic acid" suffix with "-ate" for esters.
• To separate esters from the reaction mixture, water is added; esters are insoluble and can be separated by decantation or extraction.
• Common applications include their use in perfumes for their fragrant properties and in glues for their adhesive qualities.

💡 Key Takeaway

Esterification is a reversible reaction that produces esters with distinctive functional groups, which are easily identified and widely utilized in perfumes and adhesives due to their pleasant odors and solubility differences from reactants.

📊 Synthesis Tables

⚠️ Common Pitfalls & Confusions

1. Confusing SN1 and SN2 mechanisms; remember SN1 is two-step with carbocation, SN2 is one-step concerted.
2. Assuming primary halogenoalkanes favor SN1; they predominantly undergo SN2.
3. Overlooking steric hindrance effects; bulky groups prevent SN2 but favor SN1.
4. Misunderstanding stereochemical outcomes; SN2 causes inversion, SN1 causes racemization.
5. Forgetting that carbocation stability influences SN1 reactivity trend.
6. Assuming SN1 occurs in primary halogenoalkanes; it generally does not.
7. Confusing the role of heat under reflux; it accelerates substitution reactions.

✅ Exam Checklist

• Know the definition of a nucleophile and how it participates in nucleophilic substitution reactions.
• Understand the difference between nucleophilic substitution and elimination, including conditions favoring each.
• Describe the SN2 mechanism: one-step, backside attack, inversion of stereochemistry, and its energy profile.
• Explain why primary halogenoalkanes favor SN2 and tertiary halogenoalkanes favor SN1, referencing carbocation stability.
• Know the mechanism of SN1: carbocation formation as rate-determining step, two-step process, and stereochemical outcomes.
• Recognize the effect of steric hindrance on SN2 and SN1 pathways.
• Recall the reactivity trend of halogenoalkanes in SN1 and SN2 reactions: tertiary > secondary > primary.
• Understand the role of heat under reflux in substitution reactions.
• Know the reactions of halogenoalkanes with nucleophiles such as OH-, NH3, and CN-, and the products formed.
• Be familiar with the tests for halogen presence using AgNO3 and the precipitate colors.
• Know SMITH's definition of the invisible hand in economic context.
• Understand the mechanisms and conditions for halogenoalkane reactions, including hydrolysis, elimination, and substitution.
• Recognize the importance of carbocation stability in substitution mechanisms.
• Know the key authors and their concepts: Markovnikov, Hammond, Walden inversion.
• Be able to compare SN1 and SN2 mechanisms in terms of rate, stereochemistry, and substrate preference.