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Organic Chemistry , 6 h Edition L. G. Wade, Jr. Chapter 6 Alkyl Halides: Nucleophilic Substitution and Elimination Jo Blackburn Richland College, Dallas, TX Dallas County Community College District ã 2006, Prentice Hall => Classes of Halides

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Chapter 6 Alkyl Halides: Nucleophilic Substitution and Elimination

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Chapter 6 alkyl halides nucleophilic substitution and elimination l.jpg

Organic Chemistry, 6h EditionL. G. Wade, Jr.

Chapter 6Alkyl Halides: Nucleophilic Substitution and Elimination

Jo Blackburn

Richland College, Dallas, TX

Dallas County Community College District

ã 2006,Prentice Hall


Classes of halides l.jpg

=>

Classes of Halides

  • Alkyl: Halogen, X, is directly bonded to sp3 carbon.

  • Vinyl: X is bonded to sp2 carbon of alkene.

  • Aryl: X is bonded to sp2 carbon on benzene ring. Examples:

Chapter 6


Polarity and reactivity l.jpg

Polarity and Reactivity

  • Halogens are more electronegative than C.

  • Carbon-halogen bond is polar, so carbon has partial positive charge.

  • Carbon can be attacked by a nucleophile.

  • Halogen can leave with the electron pair. =>

Chapter 6


Iupac nomenclature l.jpg

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IUPAC Nomenclature

  • Name as haloalkane.

  • Choose the longest carbon chain, even if the halogen is not bonded to any of those C’s.

  • Use lowest possible numbers for position.

Chapter 6


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Systematic Common Names

  • Name as alkyl halide.

  • Useful only for small alkyl groups.

  • Name these:

Chapter 6


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“Trivial” Names

  • CH2X2 called methylene halide.

  • CHX3 is a haloform.

  • CX4 is carbon tetrahalide.

  • Examples:

    • CH2Cl2 is methylene chloride.

    • CHCl3 is chloroform.

    • CCl4 is carbon tetrachloride. =>

Chapter 6


Classes of alkyl halides l.jpg

Classes of Alkyl Halides

  • Methyl halides: only one C, CH3X

  • Primary: C to which X is bonded has only one C-C bond.

  • Secondary: C to which X is bonded has two C-C bonds.

  • Tertiary: C to which X is bonded has three C-C bonds. =>

Chapter 6


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Classify These:

Chapter 6


Dihalides l.jpg

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Dihalides

  • Geminal dihalide: two halogen atoms are bonded to the same carbon.

  • Vicinal dihalide: two halogen atoms are bonded to adjacent carbons.

Chapter 6


Uses of alkyl halides l.jpg

Uses of Alkyl Halides

  • Solvents - degreasers and dry cleaning fluid.

  • Reagents for synthesis of other compounds.

  • Anesthetics: Halothane is CF3CHClBr

    • CHCl3 used originally (toxic and carcinogenic).

  • Freons, chlorofluorocarbons or CFC’s

    • Freon 12, CF2Cl2, now replaced with Freon 22, CF2CHCl, not as harmful to ozone layer.

  • Pesticides - DDT banned in U.S. =>

Chapter 6


Dipole moments l.jpg

Dipole Moments

  • m= 4.8 x d x d, where d is the charge (proportional to DEN) and d is the distance (bond length) in Angstroms.

  • Electronegativities: F > Cl > Br > I

  • Bond lengths: C-F < C-Cl < C-Br < C-I

  • Bond dipoles: C-Cl > C-F > C-Br > C-I1.56 D 1.51 D 1.48 D 1.29 D

  • Molecular dipoles depend on shape, too! =>

Chapter 6


Boiling points l.jpg

Boiling Points

  • Greater intermolecular forces, higher b.p.

    • Dipole-dipole attractions not significantly different for different halides

    • London forces greater for larger atoms

  • Greater mass, higher b.p.

  • Spherical shape decreases b.p. (CH3)3CBr CH3(CH2)3Br 73C 102C =>

Chapter 6


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Densities

  • Alkyl fluorides and chlorides less dense than water.

  • Alkyl dichlorides, bromides, and iodides more dense than water. =>

Chapter 6


Preparation of rx l.jpg

Preparation of RX

  • Free radical halogenation (Chapter 4)

    • produces mixtures, not good lab synthesis

    • unless: all H’s are equivalent, or

    • halogenation is highly selective.

  • Free radical allylic halogenation

    • produces alkyl halide with double bond on the neighboring carbon. =>

Chapter 6


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Halogenation of Alkanes

  • All H’s equivalent. Restrict amount of halogen to prevent di- or trihalide formation

  • Highly selective: bromination of 3 C

Chapter 6


Allylic halogenation l.jpg

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Allylic Halogenation

  • Allylic radical is resonance stabilized.

  • Bromination occurs with good yield at the allylic position (sp3 C next to C=C).

  • Avoid a large excess of Br2 by using N-bromosuccinimide (NBS) to generate Br2 as product HBr is formed.

Chapter 6


Reaction mechanism l.jpg

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Reaction Mechanism

Free radical chain reaction

  • initiation, propagation, termination.

Chapter 6


Substitution reactions l.jpg

Substitution Reactions

  • The halogen atom on the alkyl halide is replaced with another group.

  • Since the halogen is more electronegative than carbon, the C-X bond breaks heterolytically and X- leaves.

  • The group replacing X- is a nucleophile. =>

Chapter 6


Elimination reactions l.jpg

Elimination Reactions

  • The alkyl halide loses halogen as a halide ion, and also loses H+ on the adjacent carbon to a base.

  • A pi bond is formed. Product is alkene.

  • Also called dehydrohalogenation (-HX). =>

Chapter 6


S n 2 mechanism l.jpg

SN2 Mechanism

  • Bimolecular nucleophilic substitution.

  • Concerted reaction: new bond forming and old bond breaking at same time.

  • Rate is first order in each reactant.

  • Walden inversion. =>

Chapter 6


S n 2 energy diagram l.jpg

SN2 Energy Diagram

  • One-step reaction.

  • Transition state is highest in energy. =>

Chapter 6


Uses for s n 2 reactions l.jpg

Uses for SN2 Reactions

  • Synthesis of other classes of compounds.

  • Halogen exchange reaction.

=>

Chapter 6


S n 2 nucleophilic strength l.jpg

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SN2: Nucleophilic Strength

  • Stronger nucleophiles react faster.

  • Strong bases are strong nucleophiles, but not all strong nucleophiles are basic.

Chapter 6


Trends in nucleophilic strength l.jpg

Trends in Nucleophilic Strength

  • Of a conjugate acid-base pair, the base is stronger: OH- > H2O, NH2- > NH3

  • Decreases left to right on Periodic Table. More electronegative atoms less likely to form new bond: OH- > F-, NH3 > H2O

  • Increases down Periodic Table, as size and polarizability increase: I- > Br- > Cl- =>

Chapter 6


Polarizability effect l.jpg

Polarizability Effect

=>

Chapter 6


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Bulky Nucleophiles

Sterically hindered for attack on carbon, so weaker nucleophiles.

Chapter 6


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Solvent Effects (1)

Polar protic solvents (O-H or N-H) reduce the strength of the nucleophile. Hydrogen bonds must be broken before nucleophile can attack the carbon.

Chapter 6


Solvent effects 2 l.jpg

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Solvent Effects (2)

  • Polar aprotic solvents (no O-H or N-H) do not form hydrogen bonds with nucleophile.

  • Examples:

Chapter 6


Crown ethers l.jpg

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Crown Ethers

  • Solvate the cation, so nucleophilic strength of the anion increases.

  • Fluoride becomes a good nucleophile.

Chapter 6


Leaving group ability l.jpg

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Leaving Group Ability

  • Electron-withdrawing

  • Stable once it has left (not a strong base)

  • Polarizable to stabilize the transition state.

Chapter 6


Structure of substrate l.jpg

Structure of Substrate

Structure of Substrate

  • Relative rates for SN2: CH3X > 1° > 2° >> 3°

  • Tertiary halides do not react via the SN2 mechanism, due to steric hindrance. =>

Chapter 6


Steric hindrance l.jpg

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Steric Hindrance

  • Nucleophile approaches from the back side.

  • It must overlap the back lobe of the C-X sp3 orbital.

Chapter 6


Stereochemistry of s n 2 l.jpg

Stereochemistry of SN2

Walden inversion

=>

Chapter 6


S n 1 reaction l.jpg

SN1 Reaction

  • Unimolecular nucleophilic substitution.

  • Two step reaction with carbocation intermediate.

  • Rate is first order in the alkyl halide, zero order in the nucleophile.

  • Racemization occurs. =>

Chapter 6


S n 1 mechanism 1 l.jpg

=>

SN1 Mechanism (1)

Formation of carbocation (slow)

Chapter 6


S n 1 mechanism 2 l.jpg

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SN1 Mechanism (2)

  • Nucleophilic attack

  • Loss of H+ (if needed)

Chapter 6


S n 1 energy diagram l.jpg

SN1 Energy Diagram

  • Forming the carbocation is endothermic

  • Carbocation intermediate is in an energy well. =>

Chapter 6


Rates of s n 1 reactions l.jpg

Rates of SN1 Reactions

  • 3° > 2° > 1° >> CH3X

    • Order follows stability of carbocations (opposite to SN2)

    • More stable ion requires less energy to form

  • Better leaving group, faster reaction (like SN2)

  • Polar protic solvent best: It solvates ions strongly with hydrogen bonding. =>

Chapter 6


Stereochemistry of s n 1 l.jpg

Stereochemistry of SN1

Racemization: inversion and retention

=>

Chapter 6


Rearrangements l.jpg

Rearrangements

  • Carbocations can rearrange to form a more stable carbocation.

  • Hydride shift: H- on adjacent carbon bonds with C+.

  • Methyl shift: CH3- moves from adjacent carbon if no H’s are available. =>

Chapter 6


Hydride shift l.jpg

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Hydride Shift

Chapter 6


Methyl shift l.jpg

=>

Methyl Shift

Chapter 6


S n 2 or s n 1 l.jpg

Primary or methyl

Strong nucleophile

Polar aprotic solvent

Rate = k[halide][Nuc]

Inversion at chiral carbon

No rearrangements

Tertiary

Weak nucleophile (may also be solvent)

Polar protic solvent, silver salts

Rate = k[halide]

Racemization of optically active compound

Rearranged products =>

SN2 or SN1?

Chapter 6


E1 reaction l.jpg

E1 Reaction

  • Unimolecular elimination

  • Two groups lost (usually X- and H+)

  • Nucleophile acts as base

  • Also have SN1 products (mixture) =>

Chapter 6


E1 mechanism l.jpg

E1 Mechanism

  • Halide ion leaves, forming carbocation.

  • Base removes H+ from adjacent carbon.

  • Pi bond forms. =>

Chapter 6


A closer look l.jpg

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A Closer Look

Chapter 6


E1 energy diagram l.jpg

E1 Energy Diagram

Note: first step is same as SN1

=>

Chapter 6


Zaitsev s rule l.jpg

major

minor

=>

Zaitsev’s Rule

  • If more than one elimination product is possible, the most-substituted alkene is the major product (most stable).

  • R2C=CR2>R2C=CHR>RHC=CHR>H2C=CHR tetra > tri > di > mono

Chapter 6


E2 reaction l.jpg

E2 Reaction

  • Bimolecular elimination.

  • Requires a strong base.

  • Halide leaving and proton abstraction happens simultaneously - no intermediate. =>

Chapter 6


E2 mechanism l.jpg

E2 Mechanism

  • Order of reactivity: 3° > 2 ° > 1°

  • Mixture may form, but Zaitsev productpredominates.

=>

Chapter 6


E2 stereochemistry l.jpg

E2 Stereochemistry

=>

Chapter 6


E1 or e2 l.jpg

Tertiary > Secondary

Weak base

Good ionizing solvent

Rate = k[halide]

Zaitsev product

No required geometry

Rearranged products

Tertiary > Secondary

Strong base required

Solvent polarity not important

Rate = k[halide][base]

Zaitsev product

Coplanar leaving groups (usually anti)

No rearrangements =>

E1 or E2?

Chapter 6


Substitution or elimination l.jpg

Substitution or Elimination?

  • Strength of the nucleophile determines order: Strong nucleophile, bimolecular, SN2 or E2.

  • Primary halide usually SN2.

  • Tertiary halide mixture of SN1, E1 or E2

  • High temperature favors elimination.

  • Bulky bases favor elimination.

  • Good nucleophiles, but weak bases, favor substitution. =>

Chapter 6


Secondary halides l.jpg

Secondary Halides?

Mixtures of products are common.

=>

Chapter 6


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End of Chapter 6

Chapter 6


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