Butenes

 Butenes

There are three isomeric butenes, and all are gases:

• CH₃CH₃CH=CH₂, but-1-ene (b.p. -6.1^oC);
• CH₃CH=CHCH₃, but-2-ene (b.p. 1^oC);
•  (CH₃)₂C=CH₂, isobutene (b.p. -6.6^oC).

All the butenes are obtained from cracked petroleum. The 1- and 2-butenes are used for the preparation of s-butanol (q.v.), and isobutene for t-butanol (q.v.). But-2-ene differs from its isomers in that it exhibits geometrical isomerism. Isobutene differs from its isomers in that it reacts with chlorine at room temperature to give mainly substitution products, substitution occurring in the allyl position. Thus 3-chloro-2-methylprop-1-ene is the main product, and is accompanied by a small amount of the addition product 1,2-dichloro-2-methylpropane.

Mechanistic studies have shown that the substitution product is not obtained by direct substitution Reeve (1952), using isobutene labeled at position 1 (¹⁴C) did not obtain labeled formaldehyde on ozonolysis of the monochloroisobutene. Since the dichloride is stable under the conditions of the experiment, a mechanism which is in accord with these facts is: