Isobutylene CAS 115-11-7

What is Isobutylene?

Isobutylene is the branched four-carbon olefin 2-methylpropene, formula C₄H₈, with a double bond on a carbon that carries two methyl groups. At normal temperature it is a colourless, flammable gas with a mild petroleum-like odour, and it liquefies under modest pressure.

Its structure explains its chemistry. The double bond reacts readily with acids to form a stable tertiary carbocation, which makes isobutylene prone to cationic polymerisation and to forming tertiary ethers and alcohols. It polymerises to butyl rubber and polyisobutylene, reacts with methanol to give MTBE, and hydrates to tert-butanol.

Isobutylene is supplied as a liquefied gas in grades set by purity and by the other C4 compounds present. Polymer grade for butyl rubber and polyisobutylene is typically 99% or better, with very low butadiene, 1-butene and water, while chemical grade for ether or alcohol synthesis may be lower.

Commercially, isobutylene comes from the C4 stream of steam crackers and FCC units, from isobutane dehydrogenation, and from the cracking of MTBE or tert-butanol. Because it is often held inside an integrated chain, merchant supply serves the users that sit outside it.

Why isobutylene specifically

• Cationic polymerisation. The branched structure gives a stable carbocation, so isobutylene polymerises readily with Lewis acids to high-molecular-weight polymer.
• Butyl rubber. Copolymerised with a little isoprene it gives butyl rubber, the leading material for gas-tight inner liners in tyres.
• Fuel ethers. Reaction with methanol or ethanol gives MTBE and ETBE, high-octane gasoline components.
• Methacrylate and antioxidant routes. Tert-butanol from isobutylene leads to MMA, and alkylation of phenols gives widely used hindered phenol antioxidants.

How Isobutylene is Manufactured

Isobutylene is separated from mixed C4 streams, or made on purpose. The route decides the impurity profile and the cost of reaching polymer grade.

• Raffinate-1 from the C4 stream. After butadiene is extracted from steam-cracker C4, the remaining raffinate-1 carries about 40 to 50% isobutylene, together with 1-butene, 2-butenes and butanes.
• MTBE route. Isobutylene is reacted selectively with methanol over an acidic resin to MTBE, which is easily separated from the other C4 components.
• MTBE cracking. MTBE is then cracked back over an acid catalyst to high-purity isobutylene and methanol, giving a polymer-grade product.
• Tert-butanol dehydration. Alternatively, tert-butanol, from the hydration of isobutylene or from propylene oxide co-product routes, is dehydrated back to isobutylene.
• On-purpose isobutane dehydrogenation. Isobutane is dehydrogenated over a catalyst, which gives a clean isobutylene stream where C4 supply is short.
• Polishing and drying. Distillation and adsorbents remove butadiene, oxygenates, sulphur and water, which poison cationic catalysts at low ppm levels.

Typical Commercial Specification

Figures below are representative of polymerisation-grade isobutylene. Tell us your polymerisation process and we will advise on the grade and the impurity limits you should specify.

Physical & Chemical Properties

Behaviour in the process

Isobutylene's tendency to form a tertiary carbocation drives most of its chemistry and its handling:

• Acid-catalysed polymerisation. Traces of strong acid or Lewis acid can start rapid, exothermic polymerisation, so storage and transfer equipment must be free of acidic contamination.
• Selective ether formation. It reacts faster than other butenes with alcohols over acid catalysts, which is how it is separated chemically in the MTBE route.
• Tertiary alcohol formation. Hydration over acid catalysts gives tert-butanol, a base for MMA and other products.
• Alkylation of aromatics. Reaction with phenols and cresols gives hindered phenols such as BHT and tert-butylphenols.
• Heavier than air and liquefiable. Escaped vapour pools at low points. Storage as a pressurised liquid is standard.

Isobutylene vs the Other C4 Olefins

Isobutylene shares a supply stream with the other butenes and with butadiene, but it is bought for a distinct set of reactions.

Stated plainly: isobutylene is the C4 olefin that polymerises cationically and forms tertiary products, which is why it is not interchangeable with the linear butenes. Where isobutylene is not available, tert-butanol or MTBE can be converted back to it at the cost of an extra process step.

Industries We Supply Isobutylene To

The same liquefied gas goes to a butyl rubber plant, a fuel-ether unit and an antioxidant works. What changes is the grade and the impurity limits each process requires.

Where Isobutylene is Used

Isobutylene is bought for polymers, fuel components and tertiary chemistry. These are the end uses our customers buy it for.

In butyl rubber manufacture

Isobutylene is copolymerised with 1 to 2 percent isoprene at very low temperature, around −100 °C, in methyl chloride with an aluminium chloride initiator system. The low temperature gives high molecular weight, and the isoprene supplies the double bonds needed for vulcanisation. Water and oxygenates in the monomer feed deactivate the initiator, so feed drying and purity are critical.

In polyisobutylene manufacture

Cationic polymerisation with boron trifluoride or aluminium chloride gives PIB. Conventional PIB has a mixture of end groups, while highly reactive PIB with a high vinylidene content is the base for modern fuel and lubricant dispersants. Feed purity and temperature control decide molecular weight and end-group structure.

The 1-butene problem

This deserves its own section because isobutylene and 1-butene boil within a fraction of a degree of each other and cannot be separated by ordinary distillation. For polymer use, residual 1-butene lowers molecular weight and wastes comonomer capacity, so buyers limit it and rely on chemical separation routes. Confirming the isobutylene source and route is a sensible part of any qualification.

Quality Control & Testing

Every lot is released against a batch-specific Certificate of Analysis based on gas chromatography, with trace impurities reported to the limits you specify.

• Isobutylene content — by gas chromatography, the primary grade measure.
• 1-Butene, 2-butenes and butanes — reported to confirm the C4 profile and separation route.
• 1,3-Butadiene — measured at low ppm, relevant to polymerisation and safety.
• Water — by moisture analysis, critical for cationic polymerisation.
• Oxygenates (methanol, tert-butanol, MTBE) — which act as chain-transfer agents and catalyst poisons.
• Sulphur compounds — measured to low ppm for catalyst-sensitive uses.
• Peroxides and inhibitor status — checked where the specification and storage history require it.

Common Process Problems & Likely Causes

When isobutylene causes trouble, it is usually water, oxygenates or the butene profile. This table is the shortcut our technical desk works through before anyone opens a complaint file.

Packaging, Storage & Shipping

• Standard packing. ISO tank containers, road tankers and rail cars for pressurised liquid, with cylinders for small quantities.
• Pressure control. Held as a liquid at roughly 2.6 bar at 20 °C, in tanks rated for the highest expected vapour pressure with relief devices.
• Storage. Outdoors or in well-ventilated, fire-resistant areas, away from heat and ignition sources, in bunded and earthed tanks.
• Avoid acid contamination. Dedicated or thoroughly cleaned equipment is needed. Traces of acid can trigger exothermic polymerisation.
• Low-point awareness. Because the vapour is heavier than air, avoid pits and trenches near storage and use low-level gas detection.
• Segregation. Keep apart from oxidisers, halogens and strong acids.
• Shelf life. Stable in clean, sound containers. Re-check water and peroxides after long storage.
• Transport classification. UN 1055 isobutylene, Class 2.1 flammable gas.

Hazards & Safe Handling

Isobutylene is an extremely flammable liquefied gas and a simple asphyxiant with mild anaesthetic effects at high concentration. Fire, pressure and cold from flashing liquid are the main hazards; it is not classified as carcinogenic.

• Fire and explosion. Flammable range about 1.8 to 9.6% in air. Control ignition sources, fit gas detection and earth all equipment.
• Heavy vapour. Gas pools at low points and can travel to ignition sources. Ventilate low spaces and monitor.
• Exothermic polymerisation. Acidic contamination can start rapid polymerisation with heat and pressure rise. Keep equipment clean.
• Pressure hazard. Tanks and cylinders can rupture in fire or if overfilled. Use pressure relief and protect from heat.
• Asphyxiation. High concentrations in enclosed spaces displace oxygen. Ventilate and monitor before entry.
• Cold burns. Liquid releases flash and freeze skin. Use insulated gloves and a face shield.
• PPE. Flame-resistant clothing, safety glasses, cold-protective gloves and portable gas detection.
• Environment. Readily volatile and not significantly bioaccumulative. Vent only through approved flare or recovery systems.

Regulatory & Compliance Notes

Isobutylene is a widely traded commodity with well-established regulatory handling. Because obligations differ by territory and use, the notes below are orientation rather than legal advice — confirm your own position with your regulatory adviser.

• REACH and chemical inventories. Isobutylene is registered under EU REACH and listed on major inventories, with the supplier's role confirmed in the quotation.
• GHS classification. Flammable gas (Category 1) and gas under pressure. The SDS carries the full hazard statements.
• Dangerous goods transport. UN 1055, Class 2.1. ADR, IMDG, IATA and DOT requirements apply to tank design, labelling and routing.
• Process safety. Storage and use fall under major-hazard and flammable-gas rules in many jurisdictions, including zoning and hazardous area classification.
• Downstream product rules. Uses such as pharmaceutical stopper rubber, food-contact PIB and fuel ethers carry their own regulatory obligations on the finished product.
• Quality agreements. Polymer producers commonly expect documented specification, test methods and change control. Tell us what your agreement requires.

We Supply Isobutylene Worldwide

Chelora Chemicals is a supplier and export partner, not a single-plant trader. We ship Isobutylene from India and from partner sources to customers across Asia, Europe, the Middle East, Africa, and North and South America, handling the export documentation, dangerous-goods paperwork and freight so the material arrives ready to use.

• Incoterms. EXW, FOB, CIF, CFR and DAP quoted to suit your purchasing terms and freight arrangements.
• Ports. Regular shipments through Nhava Sheva (JNPT) and Mundra, with other ports arranged where the dangerous-goods rules and routing allow.
• Documentation. Commercial invoice, packing list, certificate of origin, COA, SDS and the dangerous-goods declaration issued as a complete set.
• Flexible volumes. From cylinder quantities through ISO tanks up to repeat bulk loads on a supply contract.
• Supply continuity. Because we are independent of any one producer, a plant turnaround or a shipping disruption at one source does not have to become your stock-out.

Buying Isobutylene from Chelora

We are an independent sourcing and supply partner. For isobutylene that means settling three things at enquiry: the grade and water or oxygenate limits, the butene profile your process tolerates, and the dangerous-goods route to your site — because for this product the impurity profile is as much a part of the specification as the purity.

• Documentation on every lot. Batch-specific COA with isobutylene content, butene profile, water and oxygenates, plus SDS and dangerous-goods paperwork.
• Analysis before commitment. Representative analysis shared ahead of a commercial order so your lab can qualify the material.
• Quantities. From cylinders through to full ISO tank and bulk loads on contract supply.
• Grade range. Polymer grade and chemical grade, with tighter limits on water, butadiene or oxygenates where your process needs them.
• Inspection. Independent third-party inspection available at loading when the contract requires it.
• Related C4 products. 1-Butene, 1,3-butadiene, ethylene and propylene alongside isobutylene from one supplier.

How ordering works

• Send the enquiry. Product, grade or purity, quantity, destination port and the Incoterm you buy on. If you have a spec sheet, attach it — it saves a round trip.
• We quote. Price, lead time, packing format and a sample COA, normally within one working day.
• Qualify the supply. For first orders or critical applications, we share the analysis and handling documentation so your technical and safety teams can sign off before you commit.
• Confirm the order. Proforma invoice issued; payment terms agreed against the volume and the relationship.
• Allocation and QC. Material allocated and released against a batch-specific COA, with an independent inspection where the contract calls for it.
• Ship and document. Filled, labelled and loaded with the full export and dangerous-goods document set, and tracked through to arrival.

Related Olefins & Petrochemical Feedstocks

Buyers of isobutylene often source other C4 olefins alongside it — rubber and fuel-ether producers typically buy butadiene, butenes or methanol on the same programme. Consolidating them cuts freight cost and gives you a single point of contact for documentation.