Substitution Reactions Parts A and B
Page 1: Substitution Reactions
Definition of substitution reactions where one atom or group in a molecule is replaced by another atom or group.
Page 2: Introduction to Alkyl Halides
Alkyl Halide:
General structure: R-C-X (where R is an alkyl group and X is a halogen).
Classification of Alkyl Halides:
Based on the number of R groups attached to the carbon bearing the halide:
1° (Primary): One R group (e.g., methyl halide).
2° (Secondary): Two R groups.
3° (Tertiary): Three R groups.
Halogen Variants: X can be F, Cl, Br, or I.
Page 3: Alkyl Halides and Nucleophilic Substitution
Alkyl halides react via substitution with nucleophiles.
Nucleophilic Substitution Reaction:
General equation: R-X + :Nu → R-Nu + X:
Alkyl halides also undergo elimination reactions with bases, resulting in alkenes.
Page 4: General Features of Nucleophilic Substitution
General Reaction: R-X + :Nu → R-Nu + X:
Nucleophile attacks the sp³ hybridized carbon atom.
Examples: Include various substrates and their products:
Example 1: CH3-CI + :OH → CH3-ÖH + CI-
Example 2: CH3CH2CH2-I + :SH → CH3CH2CH2-SH + I-
Example 3: CH3CH2-Br + :ÖCH3 → CH3CH2-CH3 + Br
Page 5: The Leaving Group
Leaving Groups:
An effective leaving group can stabilize the negative charge it gains after departure.
Example: H2O is better than HO¯ due to its weaker basicity.
Page 6: Periodic Trends in Leaving Group Ability
Right to left in periods, basicity decreases while leaving group ability increases.
Row 2 Elements:
Better leaving group with increasing basicity: NH3 < H2O.
Down group trends show the same: I- is a better leaving group than F-.
Page 7: Nucleophilicity versus Basicity
Nucleophilicity trends:
Stronger base = stronger nucleophile (same atom)
Negatively charged nucleophile stronger than conjugate acid.
Right-to-left across a period, nucleophilicity increases with basicity.
Page 8: Nucleophilicity for Second-Row Elements
For elements with the same charge:
Example order: CH3-NH₂ > ROH.
Page 9: Evaluating Leaving Groups
EP2: Identify which is a better leaving group between: CI-, I- and NH3, NH
Assess the effectiveness of leaving groups based on basic strength.
Page 10: Identifying Stronger Nucleophiles
EP3: Compare pairings:
NH3 vs NH2-
CH3- vs OH-
CH3NH2 vs CH3OH
CH3COO- vs CH3CH2O-
Page 11: Impact of Steric Hindrance
Steric hindrance does not affect basicity but does impact nucleophilicity; larger groups can hinder attack on electrophiles.
Page 12: Solvent Effects - Polar Protic Solvents
Polar protic solvents solvate nucleophiles by ion-dipole and hydrogen bonding.
Common examples include water and alcohols.
Page 13: Nucleophilicity in Polar Protic Solvents
Nucleophilicity decreases across the periodic table due to solvating effects of the solvent shell around smaller anions.
Page 14: Aprotic Solvents Definition
Polar aprotic solvents do not hydrogen bond with themselves or nucleophiles, thus enhancing nucleophile reactivity.
Examples include DMSO, DMF, acetone.
Page 15: Solvated Anions in Aprotic Solvents
Examples of interactions that occur can affect nucleophile behavior.
Page 16: Nucleophilicity in Polar Aprotic Solvents
Nucleophilicity parallels basicity in polar aprotic solvents; larger anions become more reactive.
Page 17: Evaluating Nucleophiles in Various Solvents
EP4: Determine stronger nucleophiles under various conditions, specifically in both polar protic and aprotic environments.
Page 18: Alkyl Halides and Nucleophilic Substitution - Part B
Further examination of mechanistic processes for alkyl halides.
Page 19: Reaction Kinetics
Use of the method of initial rates to establish the rate equation for the reaction.
Page 20: Mechanism for SN2 Reaction
SN2 involves a backside attack by the nucleophile leading to configuration inversion in the substrate.
Page 21: Stereochemistry of SN2 Reactions
Nucleophilic backside attack results in inversion; cyclic systems can transform from cis to trans.
Page 22: Inversion of Configuration EP1
Tasks requiring identification of products with consideration of stereoselectivity and inversion from substrate to product.
Page 23: Reaction Mechanism Using Chair Conformations
Consider chair conformations in nucleophilic substitution reactions, particularly with bulky groups.
Page 24: R Group Impact on SN2 Reaction
As R group steric hindrance increases, nucleophile accessibility and reaction rates decrease.
Page 25: Faster SN2 Reaction Determination
EP3: Compare pairs of compounds based on R group structure and deduce SN2 reactivity involving sterics.
Page 26: Kinetics and Mechanism Summary
Characteristics of the SN2 mechanism reviewed with emphasis on kinetics and stereochemistry.
Page 27: tert-Butyl Chloride and SN1 Reaction
tert-Butyl chloride reaction dependence on formation of a carbocation, indicating SN1 behavior.
Page 28: SN1 Mechanism
Describes two-step mechanism; carbocation formation as rate-determining step.
Page 29: Rate-Determining Step in Multistep Reactions
In multistep reactions, the slowest step governs overall reaction kinetics.
Page 30: Product Identification in SN1 Mechanisms
Practical exercise to illustrate drawing mechanisms.
Page 31: Stereochemistry in SN1 Reactions
Carbocation components and effects on product formation included with possible stereochemical outcomes.
Page 32: Racemization in SN1 Reactions
Racemic mixtures formed due to loss of chirality during carbocation formation.
Page 33: Understanding Attack Mechanism on Carbocation
Mechanisms of front and back-side attack yielding racemic mixtures detailed.
Page 34: SN1 Reaction Products and Stereochemistry
Drawing products of specified reactions along with stereochemical notations.
Page 35: Carbocation Stability
Stability influenced by R group substituents; stability leads to increased rates.
Page 36: Inductive Effects on Carbocation Stability
Explanation of how electron-donating R groups stabilize positive charge in carbocations.
Page 37: Stability Comparison of Carbocations
EP6: Assessing stability among given 1° carbocations.
Page 38: Summary of Characteristics of SN1 Mechanism
Comprehensive recap of SN1 characteristics including kinetics and stereochemistry.
Page 39: Factors Determining SN1 vs. SN2 Mechanism
Various factors evaluated that categorize reactions into SN1 or SN2 pathways.
Page 40: Influence of the Substrate Nature
Predictive criteria for SN1 and SN2 based on substrate structure.
Page 41: Impact of Nucleophile Nature on Mechanisms
Overview of how nucleophile strength affects mechanism choice.
Page 42: Assessing Mechanisms Based on Nucleophilic Character
EP7: Determine likely mechanisms given the characteristics of reactions.
Page 43: Nature of Leaving Group
Analysis of how leaving group quality influences overall reaction speed and mechanism format.
Page 44: Rate Comparisons of Nucleophilic Substitutions
EP8: Comparative analysis of reaction rates based on nucleophilicity and leaving group efficacy.
Page 45: Solvent Effects on SN2 Reactions
Distinguishing polar protic and aprotic solvent impacts on nucleophilic substitution pathways.
Page 46: Solvent Effects on SN1 Reactions
Role of polar protic solvents in stabilizing carbocation intermediates in SN1 reactions.
Page 47: determining Mechanisms Based on Solvent Effects
EP9: Evaluate mechanism choice by correlating alkyl halides with nucleophiles and solvents.
Page 48: Summary of Factors Influencing Mechanisms
Comprehensive conditions that determine mechanism pathways for reactions provided in tabulated format for clarity.