Chapter 10.5: Conversion of Alcohols to Haloalkanes and Sulfonates

Question 1

What reagents are involved in converting an alcohol to a haloalkane with a mixed stereochemistry?

Answer 1

HCl, HBr, or HI

Question 2

How do you form alkylsulfonates?

Answer 2

o Alcohols react with compounds called sulfonyl chlorides to form alkylsulfonates

Question 3

What reagents are involved in converting an alcohol to a haloalkane with an inverted stereochemistry?

Answer 3

PBr3

Question 4

What is the stereochemistry of the haloalkane made from an alcohol and PBr3?

Answer 4

Stereochemistry: Inversion

Question 5

Describe the mechanism of a Reaction of a 1° Alcohol with HBr—An Sn2 Reaction

Answer 5

 Step 1: Add a Proton
• Rapid and reversible proton transfer from the hydronium ion to the hydroxyl group of the alcohol gives an oxonium ion, which converts –OH, a poor leaving group, into –OH2, a better leaving group
 Step 2: Make a New Bond between a Nucleophile and an Electrophile and Simultaneously Break a Bond to Gove Stable Molecules or Ions
• Nucleophilic displacement of H2O by Br- gives the bromoalkene

Question 6

Desscibe the mechanism behind the reaction of a Primary Alcohol with PBr3

Answer 6

 Step 1: Make a New Bond between a Nucleophile and an Electrophile and Simultaneously Break a Bond to Give Stable Molecules or Ions
• Nucleophilic displacement on phosphorus by the oxygen atom of the alcohol gives a protonated dibromophosphite group, which –OH, a poor leaving group, into a good leaving group
 Step 2: Make a New Bond between a Nucleophile and an Electrophile and Simultaneously Break a Bond to Give Stable Molecules or Ions
• Nucleophilic displacement of the protonated dibromophosphite group by bromide ion gives the bromoalkene. The other two bromine atoms on phosphorus are replaced in similar reactions, giving three moles of RBr and one mole of phosphorous acid H3PO3

Question 7

Describe the mechanism of Rearrangement upon Treatment of Neopentyl Alcohol with HCl

Answer 7

 Step 1: Add a Proton
• Rapid and reversible proton transfer from the hydronium ion to the hydroxyl group of the alcohol gives an oxonium ion, which converts –OH, a poor leaving group, into –OH2, a better leaving group
 1,2 Shift and Simultaneously Break a Bond to Give Stable Molecules of Ions
• Two changes take place simultaneously in this step; the C—O bond breaks and a methyl group with its pair of bonding electrons migrates to the site occupied by the departing H2O group. The result of these changes is loss of H2O and the formation of a 3° carbocation.
 Step 3: Make a New Bond between a Nucleophile and an Electrophile
• Reaction of the 3° carbocation (an electrophile) with chloride ion (a nucleophile) gives the 3° haloalkane

Question 8

o Alcohols react with compounds called sulfonyl chlorides to form ?

Answer 8

alkylsulfonates

Question 9

What is the function of the amine in an alcohol reaction with SOCl2 or SOBr2?

Answer 9

 The function of the amine (a weak base) is:
• Catalyze the reaction by forming a small amount of the alkoxide in equilibrium. The alkoxide is more reactive than the alcohol as a nucleophile.
• The amine neutralizes the HCl or HBr generated during the reaction

Question 10

What is the stereochemistry of the haloalkane made from an alcohol and HBr, HCl, or HI?

Answer 10

o Stereochemistry: scrambled if starting with chiral alcohol

Question 11

Describe the mechanism of a reaction of a 3° Alcohol with HBr—An Sn1 Reaction

Answer 11

 Step 1: Add a Proton
• Rapid and reversible proton transfer from the hydronium ion to the hydroxyl group of the alcohol gives an oxonium ion, which converts –OH, a poor leaving group, into –OH2, a better leaving group
 Step 2: Break a Bond to Give Stable Molecules of Ions
• Loss of water gives a 3° carbocation intermediate
 Step 3: Make a New Bond between a Nucleophile and an Electrophile
• Reaction of the 3° carbocation (an electrophile) with bromide ion (a nucleophile) gives the haloalkane

Question 12

What is the stereochemistry of the product from a reaction of an alcohol with SOCl2 or SOBr2?

Answer 12

o Stereochemistry: inversion of configuration

Question 13

What is a sulfonate anion a good leaving group?

Answer 13

o A sulfonate anion is a very weak base and stable anion; therefore, it is a very good leaving group in nucleophilic substitution reactions

Question 14

Describe the reaction of an alcohol with thionyl chloride or thionyl bromide

Answer 14

o SOCl2 and SOBr2
o Reactions are most commonly carried out in the presence of pyrimidine or a tertiary amine
o Formation of an alkyl chlorosulfite, which converts –OH-, a poor leaving group, into a chlorosulfite that now contains a good leaving group
o Nucleophilic displacement of this leaving group by chloride ion gives the product