1-Chloro-3-methyl-2-butene: Properties, Uses & Lab Guide

1-Chloro-3-methyl-2-butene is a reactive allylic chloride widely used in organic synthesis as a prenylating agent for the introduction of the five-carbon isoprene unit into target molecules.

Common Name

Prenyl chloride

Molecular Formula

C5H9Cl

Molecular Weight

104.58 g/mol

CAS Number

503-60-6

Functional Class

Allylic chloride

Primary Application

Prenylation in terpenoid and natural product synthesis

Aladdin SKU

C189147

Chemical Identity and Physical Properties

1-Chloro-3-methyl-2-butene, commonly known as prenyl chloride or dimethylallyl chloride, carries the CAS registry number 503-60-6 and the molecular formula C₅H₉Cl (MW 104.58 g/mol). Its IUPAC name reflects the allylic relationship between the chlorine leaving group at C-1 and the trisubstituted double bond at C-2/C-3, with gem-dimethyl substitution at C-3.

The compound is typically a colorless to pale-yellow mobile liquid with a pungent odor. The allylic position of the chlorine atom makes it significantly more reactive toward substitution and elimination than a simple primary alkyl chloride, a property that underpins its utility as a synthetic building block. The compound is sensitive to moisture and should be stored under inert gas at low temperature to minimize hydrolysis and polymerization.

Synthesis Reactions and Applications

The principal value of 1-chloro-3-methyl-2-butene in synthetic chemistry is its role as a prenylation (dimethylallylation) reagent. The prenyl group (3-methyl-2-butenyl) is a ubiquitous five-carbon motif found in thousands of terpenoids, flavonoids, and other biologically active natural products. Introduction of this unit via alkylation, allylation, or transition-metal-catalyzed coupling is a frequent strategic step in total synthesis.

Alkylation reactions: Under basic conditions, prenyl chloride alkylates carbon, oxygen, nitrogen, and sulfur nucleophiles. C-alkylation of stabilized carbanions (malonates, beta-ketoesters, enolates) affords prenylated building blocks for terpenoid synthesis. O-prenylation of phenols and alcohols can be performed with mild bases such as potassium carbonate in polar aprotic solvents (DMF, acetone).

Organometallic chemistry: Reaction with magnesium in anhydrous THF or ether yields prenylmagnesium chloride (a Grignard reagent), which adds to aldehydes, ketones, and esters to produce homoallylic alcohols. Transmetalation to zinc, indium, or other metals provides additional selectivity options.

Transition-metal catalysis: Prenyl chloride can participate in palladium-catalyzed allylic substitution (Tsuji–Trost reactions) when activated with appropriate ligands, enabling enantioselective prenylation of pro-chiral nucleophiles.

Natural product synthesis: The reagent has been employed in the synthesis of prenylated flavonoids, cannabinoids, indole alkaloids, and monoterpenoid scaffolds, making it an indispensable reagent for medicinal chemistry programs targeting terpenoid natural products.

Safe Handling, Storage, and Regulatory Considerations

Hazard profile: As a reactive allylic chloride, 1-chloro-3-methyl-2-butene is a flammable liquid and a lachrymatory irritant. It is corrosive to skin and mucous membranes and may release hydrogen chloride upon hydrolysis. Appropriate personal protective equipment (chemical-resistant gloves, safety goggles, and lab coat) must be worn at all times during handling.

Ventilation: All manipulations should be performed inside a well-functioning fume hood. Inhalation of vapors should be strictly avoided.

Storage: Store in tightly sealed, amber glass containers under an inert atmosphere (nitrogen or argon) at 2–8 °C or as specified on the product certificate of analysis. Keep away from moisture, light, heat, and incompatible materials such as strong bases, strong oxidizers, and reactive metals.

Waste disposal: Dispose of waste material in accordance with local, regional, and national regulations for halogenated organic solvents. Do not pour down the drain.

Emergency procedures: In case of skin contact, wash immediately with copious water for at least 15 minutes and seek medical attention. In case of spill, absorb with inert dry material and collect into appropriate labeled waste containers.

Quality, Purity, and Specification Considerations

For synthetic applications, reagent purity is critical because allylic chloride impurities or over-chlorinated byproducts can interfere with sensitive nucleophilic reactions, Grignard formations, and transition-metal-catalyzed processes. When selecting a lot of 1-chloro-3-methyl-2-butene, researchers should request or review the certificate of analysis (CoA) for the following parameters:

• Purity (GC): High-purity grades (≥95% or greater) are recommended for research applications. The CoA should confirm purity determined by gas chromatography.
• Appearance: Clear, colorless to pale-yellow liquid free from visible particulates or cloudiness, which could indicate moisture contamination.
• Water content: Low water content (Karl Fischer) is important for moisture-sensitive reactions such as Grignard reagent formation.
• Inhibitor status: Some lots may contain a stabilizer to prevent polymerization; confirm compatibility with intended chemistry.

Aladdin Scientific supplies 1-chloro-3-methyl-2-butene under SKU C189147, with full CoA documentation available upon request. Researchers are encouraged to conduct small-scale test reactions when using a new lot for critical synthetic steps.

Frequently asked questions

What is the difference between 1-chloro-3-methyl-2-butene and 3-chloro-3-methyl-1-butene?

Both compounds share the molecular formula C5H9Cl and are interconvertible allylic isomers that can equilibrate via an SN1 mechanism through a common allylic carbocation. 1-Chloro-3-methyl-2-butene (prenyl chloride) has the double bond internal and the chlorine at the primary position, while 3-chloro-3-methyl-1-butene has a terminal double bond and a tertiary chlorine. In ionizing solvents the two isomers interconvert rapidly, so reaction conditions must be controlled carefully to achieve regioselective substitution.

How should I prepare a prenylmagnesium chloride Grignard reagent from 1-chloro-3-methyl-2-butene?

Use freshly activated magnesium turnings in rigorously dried THF or diethyl ether under an inert atmosphere. Add the prenyl chloride slowly and with gentle warming to initiate the reaction; the allylic nature of the substrate makes initiation straightforward but may also lead to Wurtz coupling side products if addition is too rapid. Titrate the resulting Grignard solution before use to determine exact concentration.

What solvents and bases are recommended for O-prenylation of phenols using prenyl chloride?

Potassium carbonate (K2CO3) in acetone or DMF is the most commonly employed system for O-prenylation of phenols and provides good yields under mild conditions (room temperature to 60 °C). Cesium carbonate in DMF can improve yields for less reactive phenols. Reactions should be monitored by TLC or HPLC to avoid over-alkylation or competing C-prenylation.

How should 1-chloro-3-methyl-2-butene be stored to maximize shelf life?

Store the reagent in a tightly sealed amber glass bottle under an inert gas blanket (nitrogen or argon) at 2–8 °C, protected from light and moisture. Under these conditions the compound is stable for an extended period; however, each lot should be inspected for discoloration or cloudiness before use, as these indicate decomposition or contamination.

Is 1-chloro-3-methyl-2-butene suitable for Tsuji–Trost allylation reactions?

Yes, prenyl chloride can serve as an electrophilic allyl source in palladium-catalyzed Tsuji–Trost reactions, though the gem-dimethyl substitution introduces steric effects that influence regioselectivity of nucleophilic attack on the π-allyl palladium intermediate. Careful optimization of ligand, base, and solvent is recommended to achieve the desired regio- and enantioselectivity.

References

1. March, J. Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 4th ed.; Wiley-Interscience: New York, 1992.
2. Clayden, J.; Greeves, N.; Warren, S. Organic Chemistry, 2nd ed.; Oxford University Press: Oxford, 2012.
3. Tsuji, J. Palladium Reagents and Catalysts: New Perspectives for the 21st Century; Wiley: Chichester, 2004.
4. Aladdin Scientific. Product Certificate of Analysis for 1-Chloro-3-methyl-2-butene, SKU C189147.

Products are supplied for research and development use only. Not for use in humans, animals, diagnosis, or therapy.