Acetone

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Reagents Solvents Lab Basics

Ketone Polar Aprotic Miscible

Acetone: The Versatile Solvent for Synthesis and Cleaning

Exploring the most common ketone in the laboratory and its indispensable role in the Finkelstein reaction.

Oct 29, 2023 Chemca Editorial

Chemical Identity: $(CH_3)_2CO$

Acetone (Propan-2-one) is the simplest and smallest ketone. It is a colorless, highly volatile, and flammable liquid that is uniquely miscible with both water and most organic solvents.

• Boiling Point: 56.05°C
• Dielectric Constant ($ \epsilon $): 20.7 (Moderate Polarity)
• Solvent Type: Polar Aprotic

The Finkelstein Reaction: Acetone's Specialized Role

In organic synthesis, Acetone is the classic solvent for the Finkelstein Reaction, which converts an alkyl chloride or bromide into an alkyl iodide.

The Solubility Trick: The reaction relies on the fact that Sodium Iodide ($NaI$) is soluble in Acetone, but Sodium Chloride ($NaCl$) and Sodium Bromide ($NaBr$) are not. As the $S_N2$ reaction proceeds, the salt byproduct precipitates out of the solution, driving the reaction forward via Le Chatelier’s Principle.

R-Cl + NaI (in Acetone) $\rightarrow$ R-I + NaCl(s) $\downarrow$

Why Polar Aprotic Matters

Like DMSO and THF, Acetone lacks an $O-H$ group. This means it cannot hydrogen-bond with nucleophiles.

This makes it an excellent choice for $S_N2$ reactions where you want the nucleophile to remain "active" and not be caged by a solvent shell. While it is less polar than DMSO, its low boiling point makes it much easier to remove from the reaction mixture during workup.

The "Golden Rule" of Glassware

Every student knows the final step of washing glassware: The Acetone Rinse. Because Acetone is miscible with water and has a very high vapor pressure, it effectively "carries away" residual water and organic residues, leaving the glass bone-dry and streak-free in seconds.

Comparison: Acetone vs. Other Aprotic Solvents

Solvent	Polarity ($ \epsilon $)	B.P. (°C)	Key Advantage
Acetone	20.7	56	Cheap, easy removal, glassware cleaning.
DMSO	46.7	189	Highest $S_N2$ acceleration.
THF	7.6	66	Excellent for Grignards & Organometallics.

Mechanism Check

Why does the Finkelstein reaction ($R-Cl + NaI \rightarrow R-I + NaCl$) work specifically well in Acetone?

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