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Organic Chemistry · Alcohols and Phenols

Alcohols from Carbonyl Compounds: Grignard Reaction

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On this page 9 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Check yourself
  8. Study tools
  9. Sources & references

In 30 seconds

Reduction (Topic 4) adds hydrogen to a carbonyl; the Grignard reaction adds carbon instead. A Grignard reagent, RMgX, forms when an alkyl or aryl halide is placed with magnesium metal in anhydrous ether; the carbon–magnesium bond is polarized so the carbon carries partial negative charge and acts as a — a powerful carbon nucleophile. When it meets a carbonyl, the carbon adds to the δ+ carbonyl carbon, and an aqueous delivers an alcohol with a new carbon–carbon bond. The product class depends on the carbonyl: gives a primary alcohol, other aldehydes give secondary alcohols, ketones give tertiary alcohols, and esters (two equivalents) also give tertiary alcohols. This is the single most important method for growing a carbon skeleton.

Why this matters

The Grignard reaction is the classic way to build carbon–carbon bonds, the central problem of organic synthesis. Pharmaceuticals and materials are assembled by connecting smaller fragments, and Grignard chemistry is one of the oldest, most reliable tools for that job (Victor Grignard won the 1912 Nobel Prize for it). Mechanistically it is the prototype for all organometallic additions to carbonyls; strategically, it is the reaction you will most often "disconnect" when planning an alcohol synthesis. It also teaches dry-technique discipline: water, alcohols, and amines destroy Grignard reagents.

The college version

Core Concepts

What a Grignard reagent is, and how it forms

Treating an organohalide R–X with magnesium turnings in anhydrous diethyl ether or THF gives RMgX:

R–X + Mg → RMgX

The reaction involves radical intermediates, but the product behaves as if carbon carried a negative charge: the C–Mg bond is covalent yet polarized, Cδ-–Mgδ+. Halide reactivity follows RI > RBr > RCl; aryl and vinyl halides form Grignards more sluggishly than alkyl halides, usually needing THF and sometimes heat. Anything with an O–H or N–H bond quenches the reagent (a strong base abstracts the proton), so everything must be kept dry.

The addition: mechanism in words

The electron-rich carbon of RMgX attacks the electron-poor carbonyl carbon; the π pair of the C=O moves onto the oxygen, and magnesium coordinates to it, forming a (R'–C(R)(O-)MgX+). The addition is irreversible. Adding aqueous acid or ammonium chloride in the workup protonates the oxygen, delivering the alcohol. Because the reagent's carbon bonds directly to the carbonyl carbon, one new C–C bond is formed — the defining feature of the reaction.

Which carbonyl gives which alcohol

  • Formaldehyde, H2C=O: gives RCH2OH, a primary alcohol.
  • Other aldehydes, R'CHO: give R'CH(OH)R, secondary alcohols.
  • Ketones, R'COR'': give R'R''C(OH)R, tertiary alcohols.
  • Esters, R'CO2R'': two equivalents add — the first gives a ketone, the second a tertiary alcohol — so two R groups land on the alcohol carbon.
  • , the three-membered epoxide: ring opening at the less substituted carbon adds a two-carbon chain, giving a primary alcohol (RCH2CH2OH).

Retrosynthetic thinking: disconnecting the C–C bond

Given a target alcohol, find the carbon bearing the OH and disconnect the bond to the R group that came from the Grignard: that R group is the reagent's fragment, and the rest of the molecule is the carbonyl. A primary alcohol RCH2OH comes from formaldehyde plus RMgX; a secondary alcohol R'CH(OH)R from an aldehyde plus RMgX (two disconnections are possible, since either carbon could have come from the reagent); a tertiary alcohol from a ketone plus RMgX. Practicing both directions — reagent to product, and product back to reagents — is the skill this topic builds.

Constraints and traps

The substrate cannot contain acidic hydrogens (O–H, N–H, terminal alkyne C–H): they would quench the reagent before addition. That is why alcohols, amines, and terminal alkynes must be protected (Topic 8) when a Grignard reaction is planned elsewhere in the molecule. Dry glassware, dry solvent, and an inert atmosphere are required.

How It Works / Step-by-Step Process

  1. Identify the alcohol carbon and its hydrogen count (primary, secondary, tertiary) — this fixes the carbonyl partner.
  2. Choose RMgX so that R is the group you want to add (it becomes bonded to the carbonyl carbon).
  3. Form the reagent from R–X + Mg in anhydrous ether/THF, or use a commercial solution.
  4. Add the carbonyl under dry conditions; the alkoxide forms immediately.
  5. Work up with aqueous acid or NH4Cl to protonate the alkoxide; isolate the alcohol. If the substrate has acidic O–H, N–H, or terminal alkyne C–H, protect first (Topic 8).

Common Confusions

Do Not ConfuseWithDifference
Grignard carbon (nucleophile)Magnesium (electrophile)The δ- carbon attacks the carbonyl carbon; Mg is a spectator counterion
Ketone + RMgXAldehyde + RMgXKetones give tertiary alcohols; aldehydes give secondary
Formaldehyde + RMgXOther aldehydes + RMgXFormaldehyde gives primary alcohols; higher aldehydes give secondary
Ester + 1 equivalentEster + excess GrignardEsters need two equivalents to reach a clean tertiary alcohol
AlkoxideAlcoholThe direct product is the magnesium alkoxide; the workup gives the alcohol
Grignard in waterWater in the workupWater destroys the reagent — protic solvents are forbidden during formation and addition; water is added only in the workup
Reduction (Topic 4)Grignard additionBoth convert ketones to alcohols; reduction adds H, Grignard adds R (new C–C bond)
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

A Grignard reagent is a molecule with a carbon end that is very hungry for electrons — like a strong magnet for the carbon of a carbonyl. When the two meet, the hungry carbon grabs the carbonyl's carbon, and after adding some water you get an alcohol with both pieces joined. A whole new carbon attaches, like snapping a brand-new brick onto a LEGO model — the model is bigger than before.

Worked example

Example 1: A tertiary alcohol from a ketone, with a yield calculation

Problem: 2-Phenyl-2-propanol, (CH3)2C(OH)C6H5, is prepared from acetophenone, CH3COC6H5, and methylmagnesium bromide. If 2.00 g of acetophenone (molar mass 120.15 g/mol) reacts with excess CH3MgBr and 1.85 g of product (molar mass 136.19 g/mol) is isolated, what is the percent yield?

Step 1 — Moles of acetophenone:

n = 2.00 g120.15 g/mol = 0.0166 mol

Step 2 — Theoretical yield (1:1 mole ratio):

mtheoretical = 0.0166 mol × 136.19 g/mol = 2.27 g

Step 3 — Percent yield:

% yield = 1.85 g2.27 g × 100 = 81.5%

Answer: 81.5%. Mechanistically, CH3- adds to the ketone carbon, forming the magnesium alkoxide; the workup protonates it to the tertiary alcohol.

Example 2: A secondary alcohol from an aldehyde — and its retrosynthesis

Problem: Propose a Grignard synthesis of 1-phenyl-1-propanol, C6H5CH(OH)CH2CH3.

The OH-bearing carbon is attached to C6H5, CH2CH3, and H — a secondary alcohol. Disconnect either carbon bond: Route A — benzaldehyde, C6H5CHO, plus ethylmagnesium bromide, CH3CH2MgBr (ethyl adds to the carbonyl carbon). Route B — propanal, CH3CH2CHO, plus phenylmagnesium bromide, C6H5MgBr (phenyl adds). Both are valid; choose based on availability. In Route A, the ethyl carbanion equivalent attacks the carbonyl carbon; the alkoxide forms; workup gives the secondary alcohol.

Example 3: Chain extension with ethylene oxide

Problem: How would you convert ethylmagnesium bromide into 1-butanol, and why does the epoxide route work?

Add CH3CH2MgBr to ethylene oxide in ether. The nucleophilic carbon attacks the less substituted (less hindered) carbon of the three-membered ring; the ring opens, and the alkoxide CH3CH2CH2CH2O-MgBr forms. Aqueous workup gives 1-butanol, CH3CH2CH2CH2OH. The epoxide route adds exactly two carbons — the standard way to extend a Grignard by a two-carbon unit; formaldehyde plus propylmagnesium bromide would give the same product.

Key takeaways

  • RMgX is a carbon nucleophile (carbanion equivalent); Cδ-–Mgδ+.
  • Formaldehyde → primary alcohol; other aldehydes → secondary; ketones → tertiary; esters → tertiary (two R groups); ethylene oxide → primary with a two-carbon extension.
  • Mechanism: nucleophilic addition to the carbonyl carbon → magnesium alkoxide → aqueous workup gives the alcohol.
  • Water, alcohols, amines, and terminal alkynes quench the reagent — anhydrous conditions are mandatory.
  • A new C–C bond is formed: the only alcohol preparation that grows the skeleton.
  • Retrosynthesis: disconnect the new C–C bond to identify the carbonyl partner and the RMgX fragment.

Check yourself

5 review questions from the chapter. Try each one, then open the answer.

  1. What alcohol classes come from formaldehyde, other aldehydes, ketones, and esters reacting with a Grignard reagent?

    Show answer

    Formaldehyde → primary; other aldehydes → secondary; ketones → tertiary; esters → tertiary (two R groups added).

  2. In the mechanism, what is the immediate product before workup, and what does the workup do?

    Show answer

    A magnesium alkoxide, R'–C(R)(O-)MgX+; the aqueous workup protonates the oxygen to give the alcohol. (Water must be excluded until then — it would quench the reagent.)

  3. Which two retrosynthetic disconnections produce 1-phenyl-1-propanol?

    Show answer

    Benzaldehyde + ethylmagnesium bromide, or propanal + phenylmagnesium bromide.

  4. What product forms when ethylene oxide reacts with ethylmagnesium bromide?

    Show answer

    1-Butanol, CH3CH2CH2CH2OH — ring opening at the less substituted carbon adds a two-carbon chain.

  5. If 2.00 g of acetophenone gives 1.85 g of 2-phenyl-2-propanol, what is the percent yield? (Molar masses: 120.15 and 136.19 g/mol.)

    Show answer

    Moles of acetophenone = 2.00/120.15 = 0.0166 mol; theoretical product = 0.0166 × 136.19 = 2.27 g; percent yield = (1.85/2.27) × 100 = 81.5%.

Keep learning

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Study tools & related lessonsKey vocabulary · Related

Key vocabulary

Grignard reagent (RMgX)
An organomagnesium compound whose carbon acts as a carbanion
Carbanion equivalent
A species that behaves as if carbon carried a negative charge
Magnesium alkoxide
The C–O-MgX adduct formed before workup
Anhydrous conditions
Complete absence of water
Workup
Aqueous acid/ammonium chloride added after the addition
Formaldehyde
H2C=O, the simplest aldehyde
Ethylene oxide
The three-membered epoxide ring

Sources & references

  1. openstax.org — Organic Chemistry

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

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