UB2 OS ÜB 7 Part 2: Alcohol Reactions

• This worksheet is part 2 of UB2 OS ÜB 7 and covers SN reactions and O & N-Nucleophiles.

Elimination Reactions

• Alkenes are formed under acidic conditions and high temperatures.
• Strong and bulky bases at high temperatures are used if the OH group was previously converted into a good leaving group.

Nucleophilic Reactions

• Act as Nucleophiles, especially after deprotonation
• Can react as Bases

Electrophilic Reactions

• Alcohols can act as electrophiles in SN reactions
• The OH group must be converted into a good leaving group first.

Oxidation Reactions

• Primary alcohols oxidize to aldehydes and further to carboxylic acids.
• Secondary alcohols oxidize to ketones.

Reactions of Alcohols

• UB2 OS ÜB7 Part 1 covered the acid-base properties of the OH group in alcohols.
• The OH group is acidic because it can act as a proton donor.
• The acidity of a group is influenced by substituents.
• Protonation of the OH group increases the chemical reactivity of the alcohol because it converts the OH group to a better leaving group, H2O.

Elimination Reactions

• The reactions of alcohols to alkenes
• Favor elimination when no good nucleophiles are present and at higher temperatures, which removes the resulting alkene from equilibrium.
• Acid-catalyzed elimination of primary alcohols occurs via an E2 mechanism, while secondary and tertiary alcohols eliminate via an E1 mechanism.
• Carbenium ions can rearrange before being deprotonated to form alkenes or adding a nucleophile.

Alcohol Reactions in SN Reactions

• Halogenated alkanes can be produced by reacting alcohols with acids like HCl, HI, or HBr in an equilibrium reaction.
• Secondary and tertiary alcohols react via an SN1 mechanism, while primary alcohols react via an SN2 mechanism.
• Phenols do NOT react with hydrogen halides to form halogen-aryl compounds because the phenolic OH group cannot be substituted by either SN1 or SN2 mechanisms under these conditions.

Task 1

• General reaction equation for the reaction of alcohols with hydrogen halides to form halogenated alkanes: R-OH + H-X -> R-X + H2O

Reactivity Series

• Primary alcohols react only via SN2 because 1° carbenium ions are too high in energy to exist as intermediates.
• 2° & 3° Alcohols react with SN1 because their carbenium ions.
• S№2 reaction is faster with higher concentrations of the electrophile (protonated alcohol) and the nucleophile.
• The protolysis equilibrium of alcohols in aqueous media favors the reactants, even at high HX concentrations
• Steric hindrance at the trigonal-bipyramidal coordinated C-atom in the transition state of the SN2-reaction increases the activation energy.
• Protonated alcohols can undergo elimination of H2O to form carbenium ions, which then quickly add a nucleophile because of their high electrophilicity.
• The activation barrier for forming the carbenium ion is lower than the activation barrier for the SN1-reaction

SN2 Reaction

• S№2 reaction with protonated alcohols proceeds slowest because the concentration of protonated alcohol is low, and the energy of the transition state.

Task 2

• React alcohols with H-Br to produce the substitution product.

Propan–1–ol

• Reaction with H-Br produces 1–Brompropan

Butan–2–ol

• Reaction with H-Br produces 2–Brombutan

Reaction of Alcohols with Acids, Acid Halides, and Anhydrides

• Esters are the condensation and/or substitution products of alcohols reacting with acids, acid halides, and acid anhydrides.

Alcohol Reaction with Acids

• Acid + Alcohol -> Ester + Water
• The OH group of the acid and its binding electron pair are substituted by the oxygen atom of the alcohol's OH group.
• The alcohol attacks the acid as a nucleophile.

Esters

• Consist of an acid residue and an alcohol residue, with the acid residue bound to the ester via the oxygen atom of the alcohol residue.
• When the inorganic acid is a hydrogen halide acid, the halide ion nucleophilically attacks the protonated alcohol, substituting the alcohol's OH group which follows the schema on page 2.

Alcohol Reaction with Acid Halides

• Acid halide + Alcohol -> Ester + Hydrogen halide
• The halogen atom in the acid halide is substituted by the alcohol.

Alcohol Reaction with Acid Anhydrides

• Acid anhydride (with oxo bridge) + Alcohol -> Ester + Acid
• Alcohol attacks the anhydride nucleophilically and substitutes an acid residue under simultaneous formation of the ester.

Acid Halide and Acid Anhydride Definitions

• These names derive from the fact that these substances transform into acids when reacted with water.

Oxoacids General Structure

• General Form: EOx(OH)y
• E = Nonmetal or Semi-metal atom

Task 3

• Read the general procedure for the esterification of alcohols with HBr to answer the following questions.

General Procedure for Esterification of Alcohols with Hydrogen Bromide

• 1 mol primary alcohol is cooled and mixed with 0.5 mol concentrated sulfuric acid, and then with 1.5 mol hydrogen bromide.
• The mixture is heated to boiling.
• Secondary and tertiary alcohols are esterified without adding H2SO4 to reduce the formation of olefins. Variant A: Easily volatile alkyl bromides are distilled directly from the reaction mixture
• Variant B: Heavier alkyl bromides are boiled under reflux for 6 hours, then distilled with steam, and, finally, the alkyl bromide is separated using a separatory funnel.
• The crude product is shaken carefully with 1/5 volume of cold conc. sulfuric acid, or the same volume of conc. hydrochloric acid in a separatory funnel to remove the ether byproduct. -The crude bromide is washed with water and boiling alkyl bromides are washed twice with 75 ml each of 40% aqueous methanol and then neutralized
• The product is dried over calcium chloride and distilled using a 20cm Vigreux column.

Task 4

• Molar structure of a 48% HBr solution

Solution

• Base 100g HBr w(HBr) = 48%
• 48g HBr => nHBr = 48 g / 80.908 g/mol = 0.5933 mol
• 52g H2O => nH2O = 52 g / 18.016 g/mol = 2.886 mol
• XH₂O = 2.886 mol / (2.886 mol +0.5933 mol) = 0.83 & XH₂O = 0.5933 mol / (2.886 mol +0.5933 mol) = 0.17
• In a 48% HBr solution, 83% of all molecules are water molecules.

Task 5

• Apply key principles to understand the esterification of alcohols with HBr.

Propyl Bromide Description and IUPAC Name

• Skeletal formula and IUPAC name: 1-Bromopropane

Alcohol Formed

• Formed with HBr to propyl bromide: Propan-1-ol

Overall Reaction

• Reaction equation for forming the alcohol in b: Propan-1-ol + H-Br -> 1-Brompropan + H2O

Task 6

• Apply given information to analyze esterification reactions of alcohols with HBr.

Isopropyl Bromide

• Skeletal formula and IUPAC name: 2-Bromopropane

Starting Alchohol

• With HBr to Isopropyl bromide is esterified: propan-2-ol

Reaction Equation for the Protonation of the Alcohol in b

• Propan-2-ol + H+ <-> H+ - Propan-2-ol

Ester Formation

• The formation of esters involves a general pattern
• Protonation of the of the OH group.
• The fate of the protonated group then depends on the nature of the substrate

SN2 Substrate (1°)

• Protonated OH group is substituted by the nucleophile The elimination of a proton often follows because of ether formation

SN1 Substrate (2°, 3°, Benzylic & Allylic)

• Includes the elimination of H2O
• Add the nucleophile to the carbenium ion

Task 7

• Compare the syntheses of Propyl bromide and Isopropyl bromide.

The Velocity Equation

• Considers only the velocity-limiting Step

Velocity Equation for SN1

• v=k₁[Protonated Alcohol]

Velocity Equatio for SN2

• v=k₂[Protonated Alcohol]·[Br-]

Why is sulfuric acid added in the production of propyl bromide?

• The addition of sulfuric acid increases the concentration of protonated alcohol

Task 8

• General procedure calls for extraction with sodium hydrogen carbonate solution in SN Reactions.

Chemical Formulas

• Sodium hydrogen carbonate: Na(HCO3)
• Sodium carbonate: Na2(CO3)
• Carbonic acid: H2CO3 or CO(OH)2.

Forms for Hydrogencarbonation and Carbonation

• Hydrogencarbonation
• Carbonation
• Carbonic acid Formulations

Removing a Water Molecule

• CO2 is the acid anhydride of carbonic acid which results in the following reaction Säureanhydrid + H2O = Säure

General Formula with Nonmetallic structure

• EOx(OH)y nonmetal based

Task 9

• General reaction involves ether products.

The reaction Formulations

• Involves the formation of Dioprpylether, the formation of Diisopropylether.

Reaction Mechanisms

• Reaction Mechanisms is the same as the analogous compounds
• Replace the incoming nucleophile with the analogous alcohol

Task 10

• In the experiment it is important to understand what can push the reaction to the end result.

Halogenated Alkane Use

• Used is to have the compounds unmixed with the reaction medium

Pentylbromide

• The use of the pentylbromide product makes for a easy destillation.