10.6 Reactions of Alkyl Halides: Grignard Reagents

Formation and Reactivity of Grignard Reagents

Grignard reagents are among the most useful tools in organic synthesis because they let you form new carbon-carbon bonds. They're made by reacting alkyl or aryl halides with magnesium metal, producing a compound with a highly polar carbon-magnesium bond. That polarity makes the carbon both a strong nucleophile and a strong base.

Formation of Grignard reagents

Grignard reagents have the general formula $RMgX$, where $R$ is an alkyl or aryl group and $X$ is a halogen (Cl, Br, or I). To form one, you react the corresponding organohalide with magnesium metal in an anhydrous ether solvent like diethyl ether or THF.

The ether solvent plays two roles: it coordinates to the magnesium atom (stabilizing the reagent), and it must be completely dry. This is critical because Grignard reagents react immediately with water and other protic solvents, destroying the reagent before you can use it.

Halide reactivity order for formation:

• $RI$ > $RBr$ > $RCl$ (iodides react fastest)
• Fluorides ($RF$) typically do not form Grignard reagents

The overall transformation is straightforward:

$R-X + Mg \xrightarrow{\text{dry ether}} R-MgX$

The resulting carbon-magnesium bond is highly polar. Magnesium is electropositive, so it donates electron density toward the more electronegative carbon. This gives the carbon a partial negative charge, which is exactly what makes Grignard reagents reactive.

Properties of carbon-magnesium bonds

The polarized $C-Mg$ bond means the carbon carries significant partial negative charge ($\delta^-$). You can think of it as behaving almost like a carbanion ($R:^-$), though it's not a free carbanion in solution. This gives Grignard reagents two key modes of reactivity:

As nucleophiles, they attack electrophilic carbon centers. The most important examples are additions to carbonyl groups (aldehydes, ketones, esters), which form new C–C bonds. Less sterically hindered and more electron-rich $R$ groups tend to be more nucleophilic.

As bases, they deprotonate acidic hydrogens. Any proton with a $pK_a$ below about 44–45 (the approximate $pK_a$ of the conjugate acid of the Grignard carbanion) can be removed. In practice, this means:

• Alcohols ($ROH$), water, and thiols ($RSH$) are deprotonated rapidly, forming magnesium salts like $ROMgX$ or $RSMgX$
• Terminal alkynes ($HC\equiv CR$, $pK_a \approx 25$) are deprotonated to give alkynyl Grignard reagents ($RC\equiv CMgX$), which are themselves useful nucleophiles
• Amines ($R_2NH$) can also be deprotonated, forming $R_2NMgX$

This basicity is why anhydrous conditions are non-negotiable. Even trace moisture will protonate the Grignard reagent, converting it to a useless hydrocarbon ($RH$) and a magnesium salt.

Reactivity of Grignard reagents

When a Grignard reagent encounters a molecule with multiple functional groups, the most acidic proton reacts first. Understanding this hierarchy helps you predict what will happen:

1. Water and alcohols ($pK_a \approx 15{-}16$): React rapidly and irreversibly. This is a destruction pathway, not a useful reaction. It produces $RH + Mg(OH)X$ or $ROMgX$.
2. Terminal alkynes ($pK_a \approx 25$): Deprotonated to form new alkynyl Grignard reagents. This is actually a useful reaction for making acetylide nucleophiles.
3. Aldehydes and ketones: React via nucleophilic addition to the $C=O$, forming a new C–C bond. After aqueous workup, you get an alcohol. This is the most synthetically important reaction of Grignard reagents.
4. Esters: React with Grignard reagents through two successive additions, ultimately giving tertiary alcohols (after workup). These reactions may be slower and sometimes require excess reagent.
5. Alkyl halides and ethers: Generally unreactive under standard Grignard conditions.

Organometallic Chemistry and Related Reactions

Grignard reagents belong to the broader class of organometallic compounds, defined by having at least one carbon-metal bond. A few related concepts are worth knowing:

• Carbanion character: The carbon in $RMgX$ has high electron density and behaves like a carbanion equivalent, though it's stabilized by coordination to magnesium and the ether solvent.
• Schlenk equilibrium: In solution, Grignard reagents exist in a dynamic equilibrium between $RMgX$, $R_2Mg$, and $MgX_2$. The position of this equilibrium depends on the solvent and concentration, but for most purposes in an intro organic course, you can treat the reagent as $RMgX$.
• Wurtz reaction: A coupling reaction where two equivalents of an alkyl halide react with sodium metal to produce a symmetrical alkane ($R-R$). It's limited because mixing two different alkyl halides gives a statistical mixture of products.
• Transmetalation: The transfer of an organic group from one metal to another (e.g., from $Mg$ to another metal like $Cu$ or $Zn$). This is used to make organocuprate reagents ($R_2CuLi$), which have different and complementary reactivity to Grignard reagents.