Difficulty: Intermediate | Prerequisites: Part 1 of these notes (nomenclature, acidity), alkene reactions, carbonyl basics
Alcohols can be made from alkenes (acid-catalysed hydration, hydroboration-oxidation, oxymercuration), from carbonyls (catalytic hydrogenation, NaBH4, LiAlH4), and by adding carbon nucleophiles to carbonyls (Grignard reagents). The choice of reagent determines both the regiochemistry (Markovnikov vs. anti-Markovnikov) and the selectivity (which functional groups get reduced).
Markovnikov addition
Addition of H-OH across a double bond where the -OH attaches to the more substituted carbon. In simple terms, the hydrogen goes to the carbon that already has more hydrogens.
Anti-Markovnikov addition
The opposite regiochemistry: -OH ends up on the less substituted carbon. This is what hydroboration-oxidation gives you.
Hydroboration-oxidation
A two-step reaction (BH3, then H2O2/NaOH) that adds H and OH across a double bond with anti-Markovnikov regiochemistry and syn stereochemistry.
Oxymercuration-demercuration
A two-step sequence (Hg(OAc)2/H2O, then NaBH4) that gives Markovnikov addition of water to an alkene without carbocation rearrangements.
Di-hydroxylation
Adding two -OH groups across a double bond to form a 1,2-diol (vicinal diol). Think of it as turning a C=C into a C(OH)-C(OH).
NaBH4 (sodium borohydride)
A mild hydride reducing agent. Reduces aldehydes and ketones to alcohols, reduces esters only slowly, and does not reduce carboxylic acids. Safe to use in water or alcohol solvents.
LiAlH4 (lithium aluminium hydride, LAH)
A powerful hydride reducing agent. Reduces all carbonyl types, including esters and carboxylic acids, to alcohols. Must be used in dry, non-protic solvents (Et2O or THF) because it reacts violently with water.
Grignard reagent (R-MgX)
An organomagnesium halide that acts as a carbon nucleophile (effectively R:-). Formed by reacting an alkyl or aryl halide with Mg in Et2O. Adds to carbonyls to build new C-C bonds and yield alcohols after acid workup.
1. Acid-catalysed hydration (H3O+ / H2O)
Markovnikov addition: -OH goes to the more substituted carbon.
Proceeds through a carbocation intermediate, so rearrangements are possible.
The mechanism: protonation of the alkene forms a carbocation, water attacks as a nucleophile, then loss of a proton gives the alcohol.
2. Hydroboration-oxidation (BH3, then H2O2/NaOH)
Anti-Markovnikov addition: -OH ends up on the less substituted carbon.
Syn addition (both H and OH add from the same face of the double bond).
No carbocation intermediate, so no rearrangements.
3. Oxymercuration-demercuration (Hg(OAc)2/H2O, then NaBH4)
Markovnikov addition, like acid-catalysed hydration.
Key advantage: no carbocation rearrangements, because the intermediate is a mercurinium ion rather than a free carbocation.
4. Di-hydroxylation (OsO4 or KMnO4)
Adds two -OH groups across the double bond to give a 1,2-diol (vicinal diol).
Syn addition: both -OH groups add to the same face.
Converts internal alkenes to diols and terminal alkenes to 1,2-diols.
Aldehydes (RCHO) and ketones (RCOR') are both carbonyl compounds. Reducing the C=O adds H across it, yielding an alcohol.
1. Catalytic hydrogenation (H2, metal catalyst)
At high pressure, both the C=O and any C=C double bonds are reduced.
At low pressure, a C=C may be reduced selectively, leaving the C=O intact.
2. Sodium borohydride (NaBH4)
Delivers H:- (hydride) to the carbonyl carbon.
Reduces aldehydes → primary alcohols, ketones → secondary alcohols.
Reduces esters only slowly; does not reduce carboxylic acids.
Safe, can be used in water or ethanol solvent, and the hydride source is not very basic.
With alpha,beta-unsaturated enones, NaBH4 can selectively reduce the C=O while leaving the C=C double bond intact (1,2-addition).
3. Lithium aluminium hydride (LiAlH4, LAH)
Also delivers H:-, but much more reactive and more basic than NaBH4.
Reduces all carbonyl types: aldehydes, ketones, esters, and carboxylic acids.
Esters receive two equivalents of hydride at the original carbonyl carbon, yielding a primary alcohol (plus departure of the -OR leaving group as an alkoxide).
Must use dry, non-protic solvents (Et2O or THF). LAH reacts violently with water, alcohols, and any acidic proton.
Workup with water (or dilute acid) protonates the aluminium alkoxide intermediate to give the free alcohol.
NaBH4 vs. LiAlH4 selectivity summary:
Aldehydes: both reduce (YES / YES)
Ketones: both reduce (YES / YES)
Esters: NaBH4 slowly, LiAlH4 yes
Carboxylic acids: NaBH4 no, LiAlH4 yes
The trade-off: NaBH4 is more selective but less reactive. LiAlH4 is less selective but more powerful.
Formation:
R-X + Mg (in Et2O) → R-MgX
The carbon bonded to Mg is effectively a carbanion (R:-, stabilised by coordination to Mg2+).
You can form 1°, 2°, 3°, vinyl, and aryl Grignard reagents.
Reactions with carbonyls:
The Grignard reagent (nucleophilic carbon) attacks the electrophilic carbonyl carbon.
Acid workup (H3O+) protonates the resulting alkoxide to give the alcohol.
With aldehydes: the Grignard R group adds to RCHO, yielding a secondary alcohol after workup. (Formaldehyde gives a primary alcohol.)
With ketones: addition gives a tertiary alcohol.
With esters: two equivalents of Grignard add to the same carbonyl carbon. The first addition kicks out the -OR' leaving group to give a ketone, and the second equivalent adds to that ketone. The product is a tertiary alcohol with two identical R groups from the Grignard.
With carboxylic acids: an acid-base reaction occurs instead of nucleophilic addition. The acidic -OH protonates the Grignard (R-MgX + RCOOH → RH + RCOOMgX). No alcohol is formed.
Critical constraints:
Grignard reagents are incompatible with acidic protons (OH, SH, CO2H, NH, H2O). Any of these will quench the carbanion.
Must use completely dry solvents.
Real-world connection: Grignard reactions are one of the most important C-C bond-forming reactions in synthetic organic chemistry. Industrial pharmaceutical synthesis routinely uses Grignard additions to build complex alcohol intermediates.
Students often confuse Markovnikov and anti-Markovnikov. Acid-catalysed hydration and oxymercuration both give Markovnikov products. Hydroboration-oxidation is the anti-Markovnikov method.
A common mistake is assuming NaBH4 and LiAlH4 are interchangeable. They are not. NaBH4 cannot reduce carboxylic acids at all, and only slowly reduces esters. LiAlH4 reduces everything but requires anhydrous conditions.
Students forget that Grignard reagents react with acidic protons. If the substrate has an -OH, -SH, or -CO2H group, the Grignard will be quenched before it can do nucleophilic addition. You must protect those groups first.
Confusing acid workup with the reaction itself is common. The Grignard addition happens in Et2O (step 1). The acid workup (H3O+, step 2) just protonates the alkoxide. These are separate steps.
⚠️ "Choose the correct reagent" questions are a staple. Given a target alcohol, you must pick the right alkene + hydration method, or the right carbonyl + reducing agent, or the right Grignard + carbonyl.
⚠️ NaBH4 vs. LiAlH4 selectivity is a favourite exam topic. Know the table: which reducing agent works on which substrate.
⚠️ Grignard + ester = two equivalents of R add. This is a frequently tested trap; students often assume only one equivalent adds.
⚠️ Grignard + carboxylic acid = acid-base reaction only (no addition to the carbonyl). Another common exam trap.
True or False: Hydroboration-oxidation gives Markovnikov addition of water.
Fill in the blank: NaBH4 reduces aldehydes and ketones but does not reduce ______.
True or False: LiAlH4 can be used safely in water as a solvent.
Fill in the blank: When a Grignard reagent reacts with a carboxylic acid, the result is an ______ reaction, not nucleophilic addition.
True or False: Two equivalents of Grignard reagent add to an ester.
Answers: 1. False (it gives anti-Markovnikov). 2. Carboxylic acids. 3. False (LAH reacts violently with water; use Et2O or THF). 4. Acid-base. 5. True.
Q: You need to convert 1-methylcyclohexene to 1-methylcyclohexanol with Markovnikov regiochemistry and no rearrangements. Which method do you use?
A: Oxymercuration-demercuration (Hg(OAc)2/H2O, then NaBH4). It gives Markovnikov addition without the risk of carbocation rearrangement that acid-catalysed hydration would bring.
Q: What product forms when propanal (CH3CH2CHO) is treated with (1) NaBH4 in EtOH, then (2) H3O+?
A: 1-Propanol (CH3CH2CH2OH). NaBH4 reduces the aldehyde to a primary alcohol.
Q: A molecule contains both an aldehyde and an ester group. You want to reduce only the aldehyde to an alcohol, leaving the ester intact. Which reagent do you choose?
A: NaBH4 in ethanol. It reduces the aldehyde readily but reacts with the ester only very slowly, giving you the selectivity you need.
Q: What alcohol is produced when phenylmagnesium bromide (C6H5MgBr) reacts with formaldehyde (HCHO), followed by H3O+ workup?
A: Benzyl alcohol (C6H5CH2OH). The Grignard adds its phenyl group to the formaldehyde carbonyl, and acid workup protonates the alkoxide.
Q: Why can you not perform a Grignard reaction on 4-hydroxybutyl bromide (HOCH2CH2CH2CH2Br) directly?
A: The -OH group has an acidic proton that will quench the Grignard reagent as it forms. You must protect the -OH (e.g. as a TMS ether) before forming the Grignard.
Alkene hydration reactions build on electrophilic addition from the alkene chapter. Carbonyl reduction connects forward to the full carbonyl chemistry unit (aldehydes, ketones, carboxylic acid derivatives). Grignard reagents appear again in ester and carboxylic acid chapters as a workhorse for C-C bond formation. Protecting-group strategy (TMS ethers) introduced here is fundamental to multi-step synthesis problems throughout the rest of the course.
Related Terms / Search Tags
preparation of alcohols, acid-catalysed hydration, Markovnikov addition, anti-Markovnikov, hydroboration-oxidation, oxymercuration-demercuration, di-hydroxylation, vicinal diol, catalytic hydrogenation, NaBH4, sodium borohydride, LiAlH4, lithium aluminium hydride, LAH, hydride reduction, aldehyde reduction, ketone reduction, ester reduction, Grignard reagent, R-MgX, organometallic, carbon nucleophile, C-C bond formation, selectivity, enone, alpha beta unsaturated, organic chemistry chapter 15