William Kovarik
Radford University
and
Matthew E. Hermes
Kennesaw State University

Fuels and Society C: How Lead was Finally Removed from Gasoline

10. Methyl t-Butyl Ether (MTBE) Chemistry

11. .Organics in Water

Back to: Start

MTBE

MTBE: MTBE, C₅H₁₂O, is a flammable liquid, molecular weight 118D, boiling point 55 degrees Celsius. It is prepared by an addition reaction between two of the most common organic substances, isbutylene and methanol. In the pure state, these materials are inert in the presence of each another. But in the presence of hydrogen ion (H⁺) as a catalyst, the materials combine to form MTBE.

CH₃OH + CH₂=C(CH₃)₂+ H^{+ -->} CH₃OC(CH₃)₃ + H⁺

Production: Methanol is produced commercially from "synthesis gas", a mixture of CO and hydrogen that is itself obtained from natural gas - methane. The isobutylene for the MTBE process is a product of petroleum cracking.

Although the process to manufacture MTBE looks simple as shown by the equation above, in practical application, the process requires us to manage flows of materials and the evolution of or the requirement for heat. Chemical manufacturing plants are built under the guidance of chemical engineers whose training focuses on the material and energy outcomes in these large, continuously producing plants.

Catalysts and Mechanisms of Reaction: Chemists speculate on the specific pathways that the atoms and molecules take as they transform from one species to another. The "mechanism" of a chemical reaction is a proposed pathway that depicts, step-by-step, how the reactants come together and transform to products:

Isobutylene:

CH₂=C(CH₃)₂

is a molecule that has an elctron rich region, as expressed by the double lines. This is a representation of another representatioin - one we learned in first semester General Chemistry - that some carbon compounds have C-C bonds with so called sp₂ hydridization between carbons.

A positively charged proton, H⁺ , is attracted to the electron rich center and forms a bond to give a new, positively charged but lower energy species,

CH₃C⁺(CH₃)₂

This new ionic species, a cation, then finds its own electron rich species to attack - the unbonded electron pair on the oxygen of the methanol molecule. The result is a new charged species - one that readily loses a proton to give MTBE. The mechanism is sketched below.

The net result is regeneration of the proton catalyst.

MTBE Properties: MTBE has no hydrogens attached to oxygen or nitrogen - it cannot form a hydrogen bond to other molecules. Since it is not ionic and does not have a large dipole moment, MTBE is a low boiling liquid.

However, MTBE has an unbonded electron pair on its oxygen atom. Therefore substances that have a hydrogen bonded to oxygen can bond to MTBE! This is why MTBE, although soluble in petroleum at all combinations, also has solubility in water, H₂O. Therefore, any MBTE that gets to the ground will be dissolved in rain water or other aqueous systems.

Therefore we must consider MTBE chemistry in water.