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Chapter 6 Alkyl Halides: Nucleophilic Substitution and Elimination

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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

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

=>

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
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

=>

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

systematic common names

=>

Systematic Common Names
  • Name as alkyl halide.
  • Useful only for small alkyl groups.
  • Name these:

Chapter 6

trivial names
“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
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

dihalides

=>

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
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
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
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

densities
Densities
  • Alkyl fluorides and chlorides less dense than water.
  • Alkyl dichlorides, bromides, and iodides more dense than water. =>

Chapter 6

preparation of rx
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

halogenation of alkanes

=>

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

=>

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

=>

Reaction Mechanism

Free radical chain reaction

  • initiation, propagation, termination.

Chapter 6

substitution reactions
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
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
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
SN2 Energy Diagram
  • One-step reaction.
  • Transition state is highest in energy. =>

Chapter 6

uses for s n 2 reactions
Uses for SN2 Reactions
  • Synthesis of other classes of compounds.
  • Halogen exchange reaction.

=>

Chapter 6

s n 2 nucleophilic strength

=>

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
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

bulky nucleophiles

=>

Bulky Nucleophiles

Sterically hindered for attack on carbon, so weaker nucleophiles.

Chapter 6

solvent effects 1

=>

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

=>

Solvent Effects (2)
  • Polar aprotic solvents (no O-H or N-H) do not form hydrogen bonds with nucleophile.
  • Examples:

Chapter 6

crown ethers

=>

Crown Ethers
  • Solvate the cation, so nucleophilic strength of the anion increases.
  • Fluoride becomes a good nucleophile.

Chapter 6

leaving group ability

=>

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
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

=>

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
Stereochemistry of SN2

Walden inversion

=>

Chapter 6

s n 1 reaction
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

=>

SN1 Mechanism (1)

Formation of carbocation (slow)

Chapter 6

s n 1 mechanism 2

=>

SN1 Mechanism (2)
  • Nucleophilic attack
  • Loss of H+ (if needed)

Chapter 6

s n 1 energy diagram
SN1 Energy Diagram
  • Forming the carbocation is endothermic
  • Carbocation intermediate is in an energy well. =>

Chapter 6

rates of s n 1 reactions
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
Stereochemistry of SN1

Racemization: inversion and retention

=>

Chapter 6

rearrangements
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

s n 2 or s n 1
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
E1 Reaction
  • Unimolecular elimination
  • Two groups lost (usually X- and H+)
  • Nucleophile acts as base
  • Also have SN1 products (mixture) =>

Chapter 6

e1 mechanism
E1 Mechanism
  • Halide ion leaves, forming carbocation.
  • Base removes H+ from adjacent carbon.
  • Pi bond forms. =>

Chapter 6

e1 energy diagram
E1 Energy Diagram

Note: first step is same as SN1

=>

Chapter 6

zaitsev s rule

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
E2 Reaction
  • Bimolecular elimination.
  • Requires a strong base.
  • Halide leaving and proton abstraction happens simultaneously - no intermediate. =>

Chapter 6

e2 mechanism
E2 Mechanism
  • Order of reactivity: 3° > 2 ° > 1°
  • Mixture may form, but Zaitsev productpredominates.

=>

Chapter 6

e2 stereochemistry
E2 Stereochemistry

=>

Chapter 6

e1 or e2
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
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
Secondary Halides?

Mixtures of products are common.

=>

Chapter 6

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