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Sevada Chamras, Ph.D.

Glendale Community College Chemistry 105

Exam. 3 Lecture Notes Chapters 6 & 7

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Chapter 6 (Part 1/2) : Alkyl Halides Description:

Examples: 3 Major Types of Organic Halides: 1. Alkyl Halides: Examples: 2. Vinyl Halides: Examples: 3. Aryl Halides: Examples:

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Cl

1-methylbutyl chloride

Cl

2-chloropentane

Nomenclature for Alkyl Halides:

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.

Example:

b) Substitutive: One word with halogen as a substituent.

Chain numbering: halogen with the lowest number.

Halogen has priority over alkyl substituents.

Example:

Types of Alkyl Halides:

Based on the degree of substitution of the carbon bonded to halogen:

1. Methyl: Example:

2. Primary: Example:

3. Secondary: Example:

4. Tertiary: Example:

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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

Cl

Br

F F

Cl

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CH3F CH3CH2F CH3CH2CH2F CH3CH2CH2CH2F

BP: -78oC -32

oC -3

oC 65

oC

BP SIZE

CH3F CH3Cl CH3Br CH3I

BP: -78oC -24

oC 3

oC 42

oC

BP HALIDE

POLARIZABLILTY

C8H17F

D: 0.80 g.mL-1

HALIDE SIZE

ALKYL HALIDE DENSITY

C8H17Cl C8H17Br C8H17I

0.89 g.mL-1

1.12 g.mL-1 1.34 g.mL

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Some Physical Properties & Trends:

a) Boiling Point:

EXAMPLE:

b) Density:

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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:

During allylic bromination, an allylic hydrogen is substituted with a bromine. Locate the allylic position(s): *Why are allylic radicals very stable?

Allylic

Vinylic

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Reaction Mechanism: Source of bromine radical: Molecular Bromine (Br2): 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:

N

O

O

Br

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Example 1: Predict the products for the following reactions. If more than one product is possible, indicate the major product: Example 2: Predict the reactant that gives rise to the formation of the following allylic halides:

NBS, h!

NBS, h!

NBS, h!

Br

NBS, h!

Br

NBS, h!

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Reactions of Alkyl Halides:

1. Nucleophilic Substitution 2. Elimination

Nucleophilic Substitution Reactions of Alkyl Halides

General Equation: R––LG + Nuc: R––Nuc + LG Example: H2O CH3Cl + OH– CH3OH + Cl–

Components of SN Reactions:

1. Substrate: ( R____LG) The alkyl group bonded to the leaving group. The leaving group is usually attached to an sp3 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.

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Types of SN Reactions: Two types will be covered. These two types proceed via different mechanisms.

1. SN1 (Substitution, Nucleophilic Unimolecular) 2. SN2 (Substitution Nucleophilic Bimolecular) *******************************************************************

SN2 Reactions

EXAMPLE:

CH3Cl + I– CH3I + 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 SN2 Reaction:

En

ergy

Reaction Coordinate

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EXAMPLE:

CH3Cl + I– CH3I + Cl–

SM* Starting Materials:

TS* Suggested Transition State Structure:

P* Products: Complete Mechanism (Arrow Notation) for SN2 Reaction:

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Kinetics of SN2 Reaction:

[CH3Cl] (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 SN2 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 SN2 reaction (an R stereocenter will turn into S, and vice versa). This phenomenon is called Inversion of Configuration. EXAMPLE:

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Important Details about SN2 Reactions:

1. The nature of the Nucleophile:

The presence of a strong nucleophile results in a higher SN2 rate (faster): *What makes a nucleophile strong?

a) Presence of outstanding negative charge. Example: b) Lower electronegativity (Left to right). Example:

c) Larger size (and therefore polarizability). *What is Polarizability? Example:

Periodic Table

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*List of Common Nucleophiles & Their Relative Nucleophilic Abilities:

a) Strong: (C2H5)3P, SH–, I–, (C2H5)2NH, CN–, (C2H5)3N, OH–, CH3O– b) Moderate: Br–, NH3, CH3SCH3, Cl–, CH3COO–

c) Weak: F–, H2O, CH3OH

2. Nucleophilicity Vs. Basicity (Kinetics vs. Thermodynamics)

The role of a base: The role of a nucleophile:

Kinetically-Driven Vs. Thermodynamically-Driven Processes

SN2

En

ergy

Reaction Coordinate

Acid-Base Reaction

En

ergy

Reaction Coordinate

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3. Nucleophilicity and Steric Effects (Bulk): Basicity? Nucleophilicity?

Nucleophile performs a “Back-Side Attack” on the substrate

4. Ideal Substrate for SN2:

a) The overall structure of a substrate:

The Substrate is subjected to a “Back-Side Attack” by the nucleophile

OO

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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 SN2 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–, RSO3–, RSO4

–, RPO4–

Neutral molecules: H2O, ROH, NR3, PR3

5. Ideal Solvent for SN2:

Solvent Types: a) Polar Protic:

Examples:

b) Polar Aprotic:

Examples:

*Why should the solvents of choice for SN2 reactions be polar-aprotic?

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SN1 Reactions

EXAMPLE:

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 SN1 Reaction:

En

ergy

Reaction Coordinate

Br CH3OH O + HBr+

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EXAMPLE:

SM* Starting Materials:

CC* Intermediate: Carbocation: Stabilities of Carbocations:

P* Products:

Br CH3OH O + HBr+

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Complete Mechanism (Arrow Notation) of SN1 Reaction:

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Kinetics of SN1 Reaction:

Substrate (M) Nucleophile (M)

Relative Rate

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 SN1:

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Ideal Nucleophile for SN1: Ideal Substrate for SN1: Stereochemistry of SN1 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 SN1 reaction. When this happens to a 50:50 extent, it is called Racemization. EXAMPLE:

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Solvolysis: A Special Case of SN1: EXAMPLE: Rearrangement of Carbocations in SN1 Reactions: EXAMPLE: