Alkyl Halides

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Alkyl Halides Chapter 9
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Outline 1. Preparation of alkyl halides by
halogenation of hydrocarbons 2. Introduction to
preparation of alkyl halides from alcohols 3.
Reaction of nucleophilic substitution SN1 4.
Reaction of nucleophilic substitution SN2 5.
Nucleophilicity. Competition between SN1 and SN2
reactions 6. Preparation of alkyl halides from
alcohols 7. Reactions of alkyl halides.
Substitution 8. Reactions of alkyl halides.
Elimination E2 9. Reactions of alkyl halides.
Elimination E1 10. Reaction of organic halides
with metals 11. Use of organic halides
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Required
background Carbocations Curved arrow
notation Surfaces of potential energy Stereochemis
try Reaction kinetics (from general chemistry)
Essential
for 1. Chemistry of alcohols and ethers 2.
Multistep reaction mechanisms 3. Planning organic
synthesis 4. Nucleophilic substitution in
aromatic compounds and at the carbonyl group 5.
Chemistry of Grignard reagents
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1. Preparation of alkyl halides by halogenation
of hydrocarbons 2. Introduction to preparation of
alkyl halides from alcohols 3. Reaction of
nucleophilic substitution SN1 4. Reaction of
nucleophilic substitution SN2 5. Nucleophilicity.
Competition between SN1 and SN2 reactions 6.
Preparation of alkyl halides from alcohols 7.
Reactions of alkyl halides. Substitution 8.
Reactions of alkyl halides. Elimination E2 9.
Reactions of alkyl halides. Elimination E1 10.
Reaction of organic halides with metals 11. Use
of organic halides
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1. Addition of Hal2 or HHal to alkenes (see unit
Alkenes II) 2. Radical halogenation of
alkanes a. Chain initiation Cl2 2Cl.
(requires irradiation by light or high
temperatures) b. Chain propagation RH Cl.
R. HCl R. Cl2 RCl Cl. c. Chain
termination 2Cl. Cl2 2R. R2 R.
Cl. RCl Br2 reacts like Cl2, but it is less
reactive and more selective. I2 does not react
this way, because I. is too stable to split the
C-H bond. F2 is so reactive that it breaks both
C-H and C-C bonds 7F2 C2H6 2CF4 6HF
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Allylic halogenation
Note 1. There is a competing reaction of
addition CH3-CHCH2 Br2 CH3-CHBr-CH2Br 2.
To eliminate this shortcoming, Br2 is usually
replaced with NBS
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1. Preparation of alkyl halides by halogenation
of hydrocarbons 2. Introduction to preparation of
alkyl halides from alcohols 3. Reaction of
nucleophilic substitution SN1 4. Reaction of
nucleophilic substitution SN2 5. Nucleophilicity.
Competition between SN1 and SN2 reactions 6.
Preparation of alkyl halides from alcohols 7.
Reactions of alkyl halides. Substitution 8.
Reactions of alkyl halides. Elimination E2 9.
Reactions of alkyl halides. Elimination E1 10.
Reaction of organic halides with metals 11. Use
of organic halides
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R-OH HHal R-Hal
H2O Reactivity of alcohols tertiary gt secondary
gt primary
Why do different types of alcohols react
differently?
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1. Preparation of alkyl halides by halogenation
of hydrocarbons 2. Introduction to preparation of
alkyl halides from alcohols 3. Reaction of
nucleophilic substitution SN1 4. Reaction of
nucleophilic substitution SN2 5. Nucleophilicity.
Competition between SN1 and SN2 reactions 6.
Preparation of alkyl halides from alcohols 7.
Reactions of alkyl halides. Substitution 8.
Reactions of alkyl halides. Elimination E2 9.
Reactions of alkyl halides. Elimination E1 10.
Reaction of organic halides with metals 11. Use
of organic halides
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SN1 animation
The SN1 mechanism takes place 1. For tertiary
substrates always 2. For secondary substrates
sometimes 3. For primary substrates - never
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1. Preparation of alkyl halides by halogenation
of hydrocarbons 2. Introduction to preparation of
alkyl halides from alcohols 3. Reaction of
nucleophilic substitution SN1 4. Reaction of
nucleophilic substitution SN2 5. Nucleophilicity.
Competition between SN1 and SN2 reactions 6.
Preparation of alkyl halides from alcohols 7.
Reactions of alkyl halides. Substitution 8.
Reactions of alkyl halides. Elimination E2 9.
Reactions of alkyl halides. Elimination E1 10.
Reaction of organic halides with metals 11. Use
of organic halides
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V ksubstrateNu-
SN2 animation
The SN2 mechanism takes place 1. For tertiary
substrates never 2. For secondary substrates
sometimes 3. For primary substrates - always
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1. Preparation of alkyl halides by halogenation
of hydrocarbons 2. Introduction to preparation of
alkyl halides from alcohols 3. Reaction of
nucleophilic substitution SN1 4. Reaction of
nucleophilic substitution SN2 5. Nucleophilicity.
Competition between SN1 and SN2 reactions 6.
Preparation of alkyl halides from alcohols 7.
Reactions of alkyl halides. Substitution 8.
Reactions of alkyl halides. Elimination E2 9.
Reactions of alkyl halides. Elimination E1 10.
Reaction of organic halides with metals 11. Use
of organic halides
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Factor SN1 SN2
Stability of carbocation No effect
Steric hindrance No effect -
Nucleophilicity No effect
Good leaving group
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Nucleophilicity is the ability to donate a pair
of electrons at the moment of nucleophilic
substitution. Nucleophilicity depends on the
ability of the nucleophile to donate a pair of
electrons to H (basicity) and on the influence
on such ability from the substrate (usually
polarizability). Polarizability is
characteristic for large atoms and greatly
increases nucleophilicity.
Weaker bases make better leaving groups.
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1. Preparation of alkyl halides by halogenation
of hydrocarbons 2. Introduction to preparation of
alkyl halides from alcohols 3. Reaction of
nucleophilic substitution SN1 4. Reaction of
nucleophilic substitution SN2 5. Nucleophilicity.
Competition between SN1 and SN2 reactions 6.
Preparation of alkyl halides from alcohols 7.
Reactions of alkyl halides. Substitution 8.
Reactions of alkyl halides. Elimination E2 9.
Reactions of alkyl halides. Elimination E1 10.
Reaction of organic halides with metals 11. Use
of organic halides
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How to improve the leaving group? Make it
neutral (decrease its basicity)
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1. Preparation of alkyl halides by halogenation
of hydrocarbons 2. Introduction to preparation of
alkyl halides from alcohols 3. Reaction of
nucleophilic substitution SN1 4. Reaction of
nucleophilic substitution SN2 5. Nucleophilicity.
Competition between SN1 and SN2 reactions 6.
Preparation of alkyl halides from alcohols 7.
Reactions of alkyl halides. Substitution 8.
Reactions of alkyl halides. Elimination E2 9.
Reactions of alkyl halides. Elimination E1 10.
Reaction of organic halides with metals 11. Use
of organic halides
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In ethanol (solvent) the reaction completely
changes CH3-CH2-Br NaOH gt CH2CH2 Competing
elimination takes over. Now we will consider
elimination in more detail
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1. Preparation of alkyl halides by halogenation
of hydrocarbons 2. Introduction to preparation of
alkyl halides from alcohols 3. Reaction of
nucleophilic substitution SN1 4. Reaction of
nucleophilic substitution SN2 5. Nucleophilicity.
Competition between SN1 and SN2 reactions 6.
Preparation of alkyl halides from alcohols 7.
Reactions of alkyl halides. Substitution 8.
Reactions of alkyl halides. Elimination E2 9.
Reactions of alkyl halides. Elimination E1 10.
Reaction of organic halides with metals 11. Use
of organic halides
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E2 animation
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Regioselectivity of E2
Reminder Alkoxides are stronger bases, than OH-
due to weaker solvation
This is another example of the Zaitsevs rule.
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Stereoselectivity of E2
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1. Preparation of alkyl halides by halogenation
of hydrocarbons 2. Introduction to preparation of
alkyl halides from alcohols 3. Reaction of
nucleophilic substitution SN1 4. Reaction of
nucleophilic substitution SN2 5. Nucleophilicity.
Competition between SN1 and SN2 reactions 6.
Preparation of alkyl halides from alcohols 7.
Reactions of alkyl halides. Substitution 8.
Reactions of alkyl halides. Elimination E2 9.
Reactions of alkyl halides. Elimination E1 10.
Reaction of organic halides with metals 11. Use
of organic halides
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Nucleophilicity and basicity of Y- do not affect
the reactions E1 and SN1, because Y- is not
involved in the rate limiting step.
E1 animation
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1. Preparation of alkyl halides by halogenation
of hydrocarbons 2. Introduction to preparation of
alkyl halides from alcohols 3. Reaction of
nucleophilic substitution SN1 4. Reaction of
nucleophilic substitution SN2 5. Nucleophilicity.
Competition between SN1 and SN2 reactions 6.
Preparation of alkyl halides from alcohols 7.
Reactions of alkyl halides. Substitution 8.
Reactions of alkyl halides. Elimination E2 9.
Reactions of alkyl halides. Elimination E1 10.
Reaction of organic halides with metals 11. Use
of organic halides
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Organolithium and organomagnesium compounds are
very useful. Some of their reactions will be
covered in the Spring semester.
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1. Preparation of alkyl halides by halogenation
of hydrocarbons 2. Introduction to preparation of
alkyl halides from alcohols 3. Reaction of
nucleophilic substitution SN1 4. Reaction of
nucleophilic substitution SN2 5. Nucleophilicity.
Competition between SN1 and SN2 reactions 6.
Preparation of alkyl halides from alcohols 7.
Reactions of alkyl halides. Substitution 8.
Reactions of alkyl halides. Elimination E2 9.
Reactions of alkyl halides. Elimination E1 10.
Reaction of organic halides with metals 11. Use
of organic halides
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Chloroform (CHCl3) and dichloromethane (CH2Cl2)
are non-flammable organic solvents
Tetrachloroethylene, trichloroethylene are
dry-cleaning solvents
Herbicides and pesticides
Refrigerants (chlorofluorohydrocarbons)
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