Chapter 7.1 Alkyl Halides and Nucleophilic Substitution Chemistry 2 Universiti Teknologi Petronas
Glendale Community College Chemistry 105 Exam. 3 Lecture ... · Nucleophilic Substitution Reactions...
Transcript of Glendale Community College Chemistry 105 Exam. 3 Lecture ... · Nucleophilic Substitution Reactions...
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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
-1
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: