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Alkyl Halides. Nucleophilic Substitution and Elimination. Nomenclature of Alkyl Halides. Name halogen as substituent on alkane or cylcoalkane. Learn common names for some of the simple structures. e.g. chloroform, methylene chloride.

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Alkyl Halides


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alkyl halides

Alkyl Halides

Nucleophilic Substitution and Elimination

slide2

Nomenclature of Alkyl Halides

  • Name halogen as substituent on alkane or cylcoalkane.
  • Learn common names for some of the simple structures. e.g. chloroform, methylene chloride.
  • Note degree of substitution - name as type of C it is bonded to (i.e. 10, 20, 30).
  • Geminal (gem-) dihalide has two halogen atoms bonded to the same carbon.
  • Vicinal (vic-) dihalide has two halogens bonded to adjacent carbons.

Do problem 6-1, 6-2 and 6-3 of the text.

slide4

Uses and General Chracteristics

  • BOND DIPOLE (): + at C, - at X
    • all reactions based on this.
    • The bond dipole moments increase in the order: C—I < C—Br < C—F < C—Cl
  • Physical properties
    • generally, trends are similar to those seen in alkanes.
    • bp affected by London forces and dipole-dipole attractions.
  • Common uses: solvents, anesthetics, freons (refrigerants), pesticides.
slide5

Preparation of alkyl & allylic halides

  • Free radical halogenation of alkanes (Chpt 4)
  • You are expected to know the mechanism by which this transformation takes place.
slide6

Preparation of Alkyl Halides

  • Free radical halogenation of alkenes at allylic position
  • need to know resonance structures for intermediate & predict major/minor product

See pages 235-236 of the text. Do problems 6-8 and 6-9.

slide7

Nucleophilic Substitution (SN)

R—LG + Nuc:  R—Nuc + LG:

  • Substrate
  • Reagent/Nucleophile (Nuc)
  • Leaving Group (LG)
  • Solvent/Reaction Conditions
slide8

1. Identify electrophilic carbon in substrate

2. Identify nucleophilic electrons in nucleophile

3. Identify leaving group in substrate

slide9

Then draw product(s)

4. Draw substrate without LG but with bond

5. Add Nuc to bond where LG used to be

slide10

Factors influencing what products are formed

  • Substrate/steric effects
  • Strength of nucleophile vs. basicity of nucleophile
  • Stability of leaving group
  • Reaction conditions
    • Polarity of solvent
    • acidic/neutral/basic
slide11

Substitution Mechanisms

  • Continuum of possible mechanisms
  • Mechanism determined primarily by substrate steric effects

SN2 - methyl, 1º & unhindered 2º

SN1 - 3º, hindered 2º

slide12

Bimolecular (SN2) Nucleophilic Substitution

  • concerted reaction; Nuc attacks, LG leaves
  • pentacoordinate carbon in transition state
  • rate depends on conc. of both reactants
  • Me = methyl group; Et = ethyl group
slide13

Reaction is “stereospecific”

  • 100 % inversion of configuration

You should know how to represent this

mechanism in an energy diagram!!

factors that affect s n 2 reaction rates
Factors that Affect SN2 Reaction Rates
  • Strength of Nucleophile: species with negative charge is a stronger nuc than an analogous neutral species (e.g. -OH > H2O; -NH2 > NH3).
  • Nucleophilicity increases from left to right across the periodic chart (e.g. -OH > -F).
  • Nucleophilicity increased down the periodic table (I- > Br- > Cl- > F-) or (-SeH > -SH > -OH).
  • Solvent: Polar protic solvents (e.g. ethanol, ammonia decrease nucleophilicity. Polar aprotic solvents e.g. (acetonitrile, DMSO, acetone) increase nucleophilicity.
slide15

Factors that affect SN2 reaction rates

  • Steric Effects: When bulky groups interfere with a rxn. because of their size, this is called steric hindrance. Steric hindrance affects nucleophilicity, not basicity. (e.g. ethoxide ion is a stronger base than t-butoxide ion). Also, alkyl halide reactivity decreases from methyl to 10 to 20 to 30. In fact, 30 alkyl halides do not react by SN2.
  • Leaving group: The substrate should have a good leaving group. A good leaving group should be electron withdrawing, relatively stable, and polarizable. They are weak bases. Examples are Cl-, Br-, I-, RSO3-, RSO4-, RPO4-, and neutral molecules such as water, alcohols and amines. Strong bases (OH-, RO-, H2N-) are not good leaving groups!
slide16

Unimolecular (SN1) Nucleophilic Substitution

  • Two-step reaction
    • LG leaves, then Nuc: attacks
  • Tricoordinate carbocation intermediate
  • Solvolysis (when solvent is also the nucleophile = SN1 reaction
  • Rate depends on substrate conc. only
slide17

Mechanism of SN1 reaction

You must be able to represent this on an energy diagram!

slide18

Reaction Not StereoselectiveUnless Steric Factors Apply

  • Racemization - not always exactly 50/50. Carbocation can be attacked from the top or bottom face giving both enantiomers.
  • Steric hindrance gives attack at one side preferentially
  • Longer-lived carbocations give more racemization, shorter-lived give more inversion
slide19

Factors Influencing SN1 Reaction Rates

  • Stability of the carbocation*

Allylic 3° >> 3°  allylic 2° > 2°  allylic 1° >> 1° > Me Carbocations are stabilized by alkyl groups (through hyperconjugation and the inductive effect) and by resonance.

  • Leaving group stability: the better the leaving group, the faster the reaction.
  • Solvent polarity: the reaction is favored in polar protic solvents.

*must have neutral to acidic conditions to form carbocation

slide20

Rearrangement of Carbocations

  • Large difference in energy (stability) of 3° vs. 2° C+
  • H- (hydride) or R- will shift (migrate) to adjacent position to form more stable carbocation. E.g. when neopentyl bromide is boiled in methanol, only rearranged product is formed.
elimination reactions
Elimination Reactions
  • May proceed by a unimolecular (E1) or bimolecular (E2) mechanism.
  • In an alkyl halide, when a halide ion leaves with another atom or ion, the reaction is an elimination.
  • If the halide ion leaves with H+, the reaction is called a dehydrohalogenation.
slide22

Elimination Mechanisms

  • Mechanism determined primarily by substrate steric effects
slide24

E1 & SN1Competition

  • Always
    • by definition a nucleophile is Lewis Base
slide25

Carbocations generally always give both products

  • Relative amounts not easily predictable
  • Always assume formed in approximately equal amounts
slide26

E2 is Stereospecific

  • anti-coplanar elimination of H and LG
slide27

Product Distribution in E2

  • Seytzeff Product, most substituted
    • major with small base, i.e., ethoxide, small LG

+

minor

major

R2C=CR2 > R2C=CHR > RHC=CHR > R2C=CH2 > RHC=CH2

Decreasing alkene stability

slide28

E2 Mechanism

  • Concerted, anti-coplanar,  Stereospecific
  • strong base & good LG
slide30

Comparison of SN1 and SN2

E1

E2

  • Strong bases required
  • Substrates: reactivity order is 3o > 2o > 1o
  • Solvent polarity is not so important
  • Rate: depends on conc. of substrate and base.
  • Stereochemistry: coplanar arrangement required in transition state; Saytzeff rule followed
  • Rearrangements: not possible
  • Base strength unimportant
  • Substrates: reactivity order is 3o > 2o > 1o
  • Solvent: good ionizing solvent required
  • Rate: depends on substrate conc. only
  • Stereochemistry: no particular geometry required for slow step; Saytzeff rule followed
  • Rearrangements: very common