Sevada Chamras, Ph.D. Glendale Community College Chemistry 105 Exam. 3 Lecture Notes Chapters 6 & 7 Description: Examples: 3 Major Types of Organic Halides: 1. Alkyl Halides: Chapter 6 (Part 1/2) : Alkyl Halides Examples: 2. Vinyl Halides: Examples: 3. Aryl Halides: Examples: 1
Nomenclature for Alkyl Halides: Example: a) Functional Class: Two separate words. The alkyl group is named on the basis of its longest continuous chain starting at the attachment point of the halogen. Cl 1-methylbutyl chloride Example: b) Substitutive: One word with halogen as a substituent. Chain numbering: halogen with the lowest number. Halogen has priority over alkyl substituents. Cl Types of Alkyl Halides: 2-chloropentane Based on the degree of substitution of the carbon bonded to halogen: 1. Methyl: Example: 2. Primary: Example: 3. Secondary: Example: 4. Tertiary: Example: 2
Types of Dihalides: 1. Vicinal: Example: 2. Geminal: Example: Exercise: Name the following compounds according to both the functional class and substitutive naming conventions: Br Br F F Cl Cl 3
Some Physical Properties & Trends: a) Boiling Point: BP SIZE CH 3 F CH 3 CH 2 F CH 3 CH 2 CH 2 F CH 3 CH 2 CH 2 CH 2 F BP: -78 o C -32 o C -3 o C 65 o C EXAMPLE: BP HALIDE POLARIZABLILTY CH 3 F CH 3 Cl CH 3 Br CH 3 I BP: -78 o C -24 o C 3 o C 42 o C b) Density: HALIDE SIZE ALKYL HALIDE DENSITY C 8 H 17 F D: 0.80 g.ml -1 C 8 H 17 Cl C 8 H 17 Br C 8 H 17 I 0.89 g.ml -1 1.12 g.ml -1 1.34 g.ml -1 4
Synthesis of Alkyl Halides: a) Free-Radical Halogenation of Alkanes: (Covered in Chapter 4) General Equation: b) Allylic Bromination: Especially successful with Bromination. Free-Radical mechanism. Allylic Position: Allylic Vinylic During allylic bromination, an allylic hydrogen is substituted with a bromine. Locate the allylic position(s): *Why are allylic radicals very stable? 5
Reaction Mechanism: Source of bromine radical: Molecular Bromine (Br 2 ): Used primarily for free-radical halogenation of alkanes (Chp. 4). Disadvantage of the direct use of molecular bromine for allylic halogenation: Preferred Mix of Reagents: NBS, hν *How does the new mix of reagents solve the problem? NBS: O N Br O 6
Example 1: Predict the products for the following reactions. If more than one product is possible, indicate the major product: NBS, h! NBS, h! NBS, h! Example 2: Predict the reactant that gives rise to the formation of the following allylic halides: Br NBS, h! NBS, h! Br 7
Reactions of Alkyl Halides: 1. Nucleophilic Substitution 2. Elimination General Equation: Nucleophilic Substitution Reactions of Alkyl Halides R LG + Nuc: R Nuc + LG Example: H 2 O CH 3 Cl + OH CH 3 OH + Cl Components of S N Reactions: 1. Substrate: ( R LG) The alkyl group bonded to the leaving group. The leaving group is usually attached to an sp 3 carbon. 2. Leaving Group: A substituent on the substrate. 3. Nucleophile: Is substituted in place of the leaving group. Nucelophiles are Lewis bases, and usually (not necessarily) anions. 4. Solvent: Provides the medium for the reaction to occur efficiently. 8
Types of S N Reactions: Two types will be covered. These two types proceed via different mechanisms. 1. S N 1 (Substitution, Nucleophilic Unimolecular) 2. S N 2 (Substitution Nucleophilic Bimolecular) ******************************************************************* S N 2 Reactions EXAMPLE: CH 3 Cl + I CH 3 I + Cl Reaction Rate Depends on Number of Reaction (Mechanistic) Steps Number of Intermediates Number of Suggested Transition States Ideal Solvent Ideal Nucelophile Ideal Substrate Energetics of S N 2 Reaction: Energy Reaction Coordinate 9
EXAMPLE: CH 3 Cl + I CH 3 I + Cl SM* Starting Materials: TS* Suggested Transition State Structure: P* Products: Complete Mechanism (Arrow Notation) for S N 2 Reaction: 10
Kinetics of S N 2 Reaction: [CH 3 Cl] (M) [I ] (M) Relative Rate 0.1 0.2 1 0.2 0.2 2 0.3 0.4 6 0.1 0.4 2 Stereochemistry of S N 2 Reaction: If the carbon holding the leaving group is chiral, then the absolute configuration at that carbon will be reversed as a result of the S N 2 reaction (an R stereocenter will turn into S, and vice versa). This phenomenon is called Inversion of Configuration. EXAMPLE: 11
Important Details about S N 2 Reactions: 1. The nature of the Nucleophile: The presence of a strong nucleophile results in a higher S N 2 rate (faster): *What makes a nucleophile strong? a) Presence of outstanding negative charge. Example: b) Lower electronegativity (Left to right). Periodic Table Example: c) Larger size (and therefore polarizability). *What is Polarizability? Example: 12
*List of Common Nucleophiles & Their Relative Nucleophilic Abilities: a) Strong: (C 2 H 5 ) 3 P, SH, I, (C 2 H 5 ) 2 NH, CN, (C 2 H 5 ) 3 N, OH, CH 3 O b) Moderate: Br, NH 3, CH 3 SCH 3, Cl, CH 3 COO c) Weak: F, H 2 O, CH 3 OH 2. Nucleophilicity Vs. Basicity (Kinetics vs. Thermodynamics) The role of a base: The role of a nucleophile: Kinetically-Driven Vs. Thermodynamically-Driven Processes S N 2 Acid-Base Reaction Energy Energy Reaction Coordinate 13 Reaction Coordinate
3. Nucleophilicity and Steric Effects (Bulk): O O Basicity? Nucleophilicity? Nucleophile performs a Back-Side Attack on the substrate 4. Ideal Substrate for S N 2: a) The overall structure of a substrate: The Substrate is subjected to a Back-Side Attack by the nucleophile 14
b) The nature of the leaving group on the substrate: The ideal leaving group should: 1. Be remarkably more electronegative than the carbon of the substrate bonded to it. Let us consider the transition state geometry for S N 2 an reaction: 2. Be relatively stable after leaving: An unstable (reactive) leaving group will affect the energy of the transition state geometry and slow down the reaction. (Also: Polarizability as part of stability) *List of Common Leaving Groups: Ions: Cl, Br, I, RSO 3, RSO 4, RPO 4 Neutral molecules: H 2 O, ROH, NR 3, PR 3 5. Ideal Solvent for S N 2: Solvent Types: a) Polar Protic: Examples: b) Polar Aprotic: Examples: *Why should the solvents of choice for S N 2 reactions be polar-aprotic? 15
EXAMPLE: S N 1 Reactions Br + CH 3 OH O + HBr Reaction Rate Depends on Number of Reaction (Mechanistic) Steps Number of Intermediates Number of Suggested Transition States Ideal Solvent Ideal Nucelophile Ideal Substrate Energetics of S N 1 Reaction: Energy Reaction Coordinate 16
EXAMPLE: Br + CH 3 OH O + HBr SM* Starting Materials: Carbocation: CC* Intermediate: Stabilities of Carbocations: P* Products: 17
Complete Mechanism (Arrow Notation) of S N 1 Reaction: 18
Kinetics of S N 1 Reaction: Substrate (M) Nucleophile Relative Rate (M) 0.1 0.2 1 0.2 0.2 2 0.3 0.4 3 0.1 0.4 1 Rate-Determining Step (RDS): Ideal Solvent for S N 1: 19
Ideal Nucleophile for S N 1: Ideal Substrate for S N 1: Stereochemistry of S N 1 Reaction: If the carbon holding the leaving group is chiral, then the absolute configuration at that carbon will be partly reversed and partly maintained as a result of the S N 1 reaction. When this happens to a 50:50 extent, it is called Racemization. EXAMPLE: 20
Solvolysis: A Special Case of S N 1: EXAMPLE: Rearrangement of Carbocations in S N 1 Reactions: EXAMPLE: 21