Preparation of Alcohols
Methods of synthesizing alcohols from alkenes (hydration, hydroboration-oxidation), carbonyl compounds (reduction), and Grignard reagents.
What is Preparation of Alcohols?
In the addition of an protic acid HX to an alkene or alkyne, the hydrogen atom attaches to the carbon atom with the greater number of hydrogen atoms already attached to it, and the halide or other group attaches to the carbon atom with fewer hydrogen atoms.
Key formula / rule: Acid-Catalyzed Hydration of Alkene
Key points
- Describe the methods for synthesizing primary, secondary, and tertiary alcohols.
- Explain the mechanisms and regioselectivity of alkene hydration reactions.
- Predict the products of reduction of aldehydes, ketones, and esters.
- Illustrate the synthesis of alcohols using Grignard reagents.
Common exam trap
Confusing Markovnikov and anti-Markovnikov addition in alkene hydration.
Definitions
- Term
Markovnikov's Rule
- Meaning
In the addition of an protic acid HX to an alkene or alkyne, the hydrogen atom attaches to the carbon atom with the greater number of hydrogen atoms already attached to it, and the halide or other group attaches to the carbon atom with fewer hydrogen atoms.
- Term
Anti-Markovnikov Addition
- Meaning
The regioselectivity of an addition reaction where the substituent with the lower electronegativity adds to the more substituted carbon atom and the substituent with the higher electronegativity adds to the less substituted carbon atom. In hydroboration-oxidation, the hydroxyl group adds to the less substituted carbon.
- Term
Grignard Reagent
- Meaning
An organomagnesium halide (RMgX) used as a nucleophile in organic synthesis, particularly for forming new carbon-carbon bonds.
Learning objectives
Describe the methods for synthesizing primary, secondary, and tertiary alcohols.
Explain the mechanisms and regioselectivity of alkene hydration reactions.
Predict the products of reduction of aldehydes, ketones, and esters.
Illustrate the synthesis of alcohols using Grignard reagents.
Formulae
- Name
Acid-Catalyzed Hydration of Alkene
- Note
Follows Markovnikov's rule.
- Expression
RCH=CH2 + H2O --(H+)-→ RCH(OH)CH3
- Name
Hydroboration-Oxidation of Alkene
- Note
Anti-Markovnikov addition.
- Expression
RCH=CH2 --(1. BH3.THF, 2. H2O2, OH-)-→ RCH2CH2OH
- Name
Reduction of Aldehyde
- Note
Forms a primary alcohol.
- Expression
RCHO --(NaBH4 or LiAlH4)-→ RCH2OH
- Name
Reduction of Ketone
- Note
Forms a secondary alcohol.
- Expression
RCOR' --(NaBH4 or LiAlH4)-→ RCH(OH)R'
- Name
Reaction of Formaldehyde with Grignard Reagent
- Note
Forms a primary alcohol.
- Expression
HCHO + RMgX --(1. Dry Ether, 2. H3O+)-→ ROHCH2R
- Name
Reaction of Aldehyde with Grignard Reagent
- Note
Forms a secondary alcohol.
- Expression
R'CHO + RMgX --(1. Dry Ether, 2. H3O+)-→ R'CH(OH)R
- Name
Reaction of Ketone with Grignard Reagent
- Note
Forms a tertiary alcohol.
- Expression
R'COR'' + RMgX --(1. Dry Ether, 2. H3O+)-→ R'C(OH)(R)R''
- Name
Reduction of Ester
- Note
Forms two primary alcohols (or one primary alcohol and a primary alcohol from the R' group).
- Expression
RCOOR' --(LiAlH4, Dry Ether, then H3O+)-→ ROH + R'CH2OH
Prerequisites
Structure and nomenclature of alcohols
Reactivity of alkenes
Structure and reactivity of carbonyl compounds (aldehydes and ketones)
Grignard reagents
Oxidation and reduction reactions
Common mistakes
Confusing Markovnikov and anti-Markovnikov addition in alkene hydration.
Incorrectly identifying the type of alcohol (primary, secondary, tertiary) formed from different carbonyl compounds and Grignard reagents.
Overlooking the role of hydrolysis after Grignard reagent addition.
Using NaBH4 for ester reduction (it is ineffective).
Keywords
Alcohols
Synthesis
Alkene Hydration
Hydroboration-Oxidation
Carbonyl Reduction
Grignard Reagent
Markovnikov
Anti-Markovnikov
Primary Alcohol
Secondary Alcohol
Tertiary Alcohol
Practice preview
Consider the reaction of 1-methylcyclohexene with B2H6 followed by H2O2/NaOH. What is the major product?…
hard
What is the product formed when propanal is reduced by NaBH4?…
easy
Which of the following statements is INCORRECT regarding the preparation of alcohols?…
medium