What Is 2-Methylpropanoyl chloride

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Last updated: April 15, 2026

Quick Answer: 2-Methylpropanoyl chloride, also known as isobutyryl chloride, is an organic compound with the formula (CH₃)₂CHC(O)Cl. It is a colorless, pungent-smelling liquid used primarily as an acylating agent in organic synthesis. The compound has a molecular weight of 104.56 g/mol and boils at approximately 118–120°C.

Key Facts

2-Methylpropanoyl chloride has the chemical formula (CH₃)₂CHC(O)Cl
It is also commonly referred to as isobutyryl chloride in industrial and laboratory settings
The compound has a boiling point of 118–120°C under standard atmospheric pressure
It is highly reactive with water, undergoing rapid hydrolysis to form isobutyric acid
2-Methylpropanoyl chloride is used in the synthesis of pharmaceuticals, agrochemicals, and perfumes

Overview

2-Methylpropanoyl chloride, systematically named 2-methylpropanoyl chloride, is an organochlorine compound belonging to the acyl chloride family. It is structurally derived from isobutyric acid by replacing the hydroxyl group with a chlorine atom, resulting in high reactivity toward nucleophiles. This compound is widely used in synthetic organic chemistry due to its ability to introduce the isobutyryl functional group into larger molecules.

Commonly known as isobutyryl chloride, it appears as a colorless to pale yellow liquid with a sharp, irritating odor. It is moisture-sensitive and must be stored under anhydrous conditions to prevent decomposition. Its molecular weight is 104.56 g/mol, and it has a density of approximately 1.02 g/cm³ at room temperature.

Chemical formula: The compound has the molecular formula (CH₃)₂CHC(O)Cl, reflecting its branched-chain structure derived from isobutane.
Physical state: At room temperature (25°C), 2-methylpropanoyl chloride exists as a volatile liquid that fumes in moist air due to hydrolysis.
Boiling point: It boils at 118–120°C, making it suitable for distillation under reduced pressure to avoid decomposition.
Solubility: It is insoluble in water but miscible with common organic solvents such as ethanol, ether, and benzene.
Reactivity: It reacts violently with water, releasing hydrogen chloride gas and forming isobutyric acid as a byproduct.

How It Works

2-Methylpropanoyl chloride functions primarily as an acylating agent, transferring the isobutyryl group (CH₃)₂CHC(O)– to nucleophiles such as amines, alcohols, and thiols. Its high electrophilicity at the carbonyl carbon makes it ideal for forming amides, esters, and thioesters in controlled laboratory settings.

Nucleophilic attack: The carbonyl carbon is highly electrophilic, allowing amines to attack and form N-isobutyrylated derivatives used in drug synthesis.
Hydrolysis: Upon contact with moisture, it undergoes rapid hydrolysis, producing isobutyric acid and corrosive hydrogen chloride gas.
Reaction with alcohols: It reacts with alcohols to form isobutyrate esters, which are used in fragrances and flavoring agents.
Use in Friedel-Crafts acylation: It can acylate aromatic rings in the presence of AlCl₃ catalyst, yielding ketone derivatives for industrial applications.
Storage considerations: Must be stored under inert atmosphere (e.g., nitrogen) in sealed containers to prevent moisture exposure.
Handling precautions: Requires use of fume hoods and PPE due to its corrosive nature and toxic fumes.

Comparison at a Glance

Below is a comparison of 2-methylpropanoyl chloride with related acyl chlorides based on physical and chemical properties:

Compound	Molecular Weight (g/mol)	Boiling Point (°C)	Reactivity with Water	Common Use
2-Methylpropanoyl chloride	104.56	118–120	Violent	Pharmaceutical synthesis
Acetyl chloride	78.50	51–52	Very rapid	Ester production
Propionyl chloride	92.55	80–82	Rapid	Agrochemicals
Butyryl chloride	102.57	99–102	Rapid	Perfume intermediates
Benzoyl chloride	140.57	197–198	Moderate	Dye manufacturing

The table highlights that 2-methylpropanoyl chloride has a higher boiling point than smaller acyl chlorides like acetyl chloride but is less volatile than aromatic variants like benzoyl chloride. Its branched structure influences both physical behavior and steric effects in reactions, making it less reactive than linear analogs in some nucleophilic substitutions. This balance of stability and reactivity makes it valuable in fine chemical synthesis.

Why It Matters

2-Methylpropanoyl chloride plays a critical role in the development of complex organic molecules, particularly in the pharmaceutical and specialty chemical industries. Its ability to selectively introduce the isobutyryl group enables the synthesis of bioactive compounds with tailored properties.

Drug intermediates: Used in synthesizing antibiotics and anti-inflammatory agents where the isobutyryl moiety enhances metabolic stability.
Perfume synthesis: Forms esters that contribute to fruity, floral notes in high-end fragrance formulations.
Polymer modification: Acts as a chain terminator in certain polyamide syntheses to control molecular weight.
Agrochemicals: Key reagent in producing herbicides and fungicides with improved environmental persistence.
Laboratory research: Frequently used in peptide coupling and protecting group chemistry due to its selectivity.
Safety regulations: Classified as corrosive and moisture-sensitive, requiring adherence to OSHA and GHS handling guidelines.

Due to its versatility and reactivity, 2-methylpropanoyl chloride remains an essential building block in modern organic synthesis, bridging industrial-scale production and laboratory innovation.