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More About the Families in Group II
The families in Group II all have an electronegative atom or group
that is attached to an sp3 carbon.
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Strongly Basic Leaving Groups
Cannot Be Displaced
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Acid Converts the Poor Leaving Group
into a Good Leaving Group
Alcohols have to be “activated” before they can react.
Only weakly basic nucleophiles can be used.
Strongly basic nucleophiles would react with the proton.
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Converting Alcohols to Alkyl Halides
Primary and Secondary alcohols require heat; tertiary alcohols do not.
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The Reactions of Secondary and Tertiary Alcohols
with Hydrogen Halides are SN1 Reactions
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An SN1 Reaction Creates a Carbocation Intermediate,
So Watch Out for Carbocation Rearrangements
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Ring Expansion
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The Reactions of Primary Alcohols
with Hydrogen Halides are SN2 Reactions
Recall that Br– is a good nucleophile in a protic solvent.
Alcohols undergo SN1 reactions unless they would have to form a primary carbocation.
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Cl– is the Poorest Nucleophile of the Halide Ions
ZnCl2 increases the rate of the reaction
by creating a better leaving group than water.
‫רשות‬
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Why is It Important to Be Able
to Convert Alcohols to Alkyl Halides?
Alcohols are readily available but unreactive.
Alkyl halides are less available but reactive and
can be used to synthesized a wide variety of compounds.
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Other Methods to Convert
Alcohols into Alkyl Halides
phosphorus tribromide, phosphorus trichloride, thionyl chloride
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The Mechanism for the Reaction
with PBr3 or PCl3
‫רשות‬
A bromophosphite (chlorophosphite) group
is a better leaving group than a halide ion.
Pyridine is the solvent and acts as a base.
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The Mechanism for the Reaction with SOCl2
‫רשות‬
A chlorosulfite group is a better leaving group than a halide ion.
Pyridine is the solvent and acts as a base.
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Converting Alcohols to Alkyl Halides
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Primary and Secondary Alcohols Can Be
Activated by Being Converted to Sulfonate Esters
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A Sulfonic Acid is a Strong Acid
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Common Sulfonyl Chlorides
tosyl chloride = TsCl
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The Mechanism
‫רשות‬
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A Sulfonate Ester Reacts with Nucleophiles
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Dehydration of an Alcohol
Dehydration of an alcohol is an elimination reaction.
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Dehydration is a Reversible Reaction
To prevent the alkene from adding water and reforming the alcohol,
the water is removed as it is formed.
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Dehydration of Secondary and Tertiary Alcohols
are E1 Reactions
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Dehydration
The major product is the more stable alkene.
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The More Stable Alkene Has the More Stable Transition State
Leading to Its Formation
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Tertiary Alcohols are
the Easiest to Dehydrate
The rate of dehydration reflects the ease of carbocation formation.
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An E1 Reaction Creates a Carbocation Intermediate,
So Watch Out for Carbocation Rearrangements
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Dehydration of a Primary Alcohol
is an E2 Reaction
Both E2 and SN2 products are obtained.
Alcohols undergo E1 reactions unless they would have to form a primary carbocation.
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Dehydration
The major product is the stereoisomer with the
largest groups on opposite sides of the double bond.
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Oxidation of Secondary Alcohols
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Oxidation of Primary Alcohols
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Tertiary Alcohols Cannot Be Oxidized
to a Carbonyl Compound
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Oxidation by Hypochlorous Acid (HOCl)
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The Mechanism
‫רשות‬
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Alcohols and Ethers Have Similar Leaving Groups
Alcohols and ethers have to be “activated” before the compounds can react.
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The Leaving Group of an Ether Can Be
Activated by Protonation
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The Mechanism
If a relatively stable carbocation is formed when ROH leaves,
it will be an SN1 reaction.
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The Mechanism
If a relatively stable carbocation would not be formed when ROH leaves,
it will be an SN2 reaction.
Ethers undergo SN1 reactions unless they would have to form a primary carbocation.
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Nomenclature of Epoxides
‫אלקנים‬-6
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Epoxides are More Reactive Than Ethers
Because of Ring Strain
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The Acid-Catalyzed Mechanism
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If the Epoxide is Not Symmetrical,
Which Carbon Does the Nucleophile Attack?
The nucleophile attacks the more substituted ring carbon.
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Under Acidic Conditions the Nucleophile Attacks
the More Substituted Ring Carbon
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Under Neutral or Basic Conditions the
Nucleophile Attacks the Less Substituted Carbon
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The Conditions Determine the Site of
Nucleophilic Attack
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Using Epoxides in Synthesis
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‫ריאגנט גריניאר‬
Grignard Reagents: Alkylmagnesium halide, RMgX, produced from reaction of
alkyl halides, RX, with magnesium metal in ether or tetrahydrofuran (THF) solvent
The carbon atom is both nucleophilic and basic
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‫ריאגנט גריניאר‬
They react with weak acids such as H2O, ROH, RCO2H, and RNH2 to
abstract a proton and yield hydrocarbons
Since hydrocarbons are weak acids, with pKa’s in the range of 44 to 60,
carbon anions are very strong bases
Because even trace amounts of moisture can destroy an organometallic
compound, it is important that all reagents be dry when organometallic
compounds are being synthesized and when they react with other reagents
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