Bromination of fluoro-alkanes. Part 3.—Methane, fluoroform and fluoro-ethanes

Abstract

The competitive brominations of the following compounds have been studied in the vapour phase: CH₄, CF₃H, C₂H₆, C₂H₅F, CH₃CHF₂, CH₃CF₃, CF₃CH₂F, CF₂HCF₂H and n-C₃F₇H. Arrhenius parameters have been obtained for all observed reactions involving abstraction of H by Br. Our previous result that D(CF₃—H)–D(CH₃—H)= 2.2±0.5 kcal mole^–1 is confirmed and it is recommended that D(CF₃—H)= 106.0 kcal mole^–1 at 298°K. Upper limits are given for C—H bond dissociation energies in the other fluoroalkanes studied.

Fluorine atoms attached to a given carbon atom have relatively little effect on abstraction of H from that carbon atom but the hydrogen atoms on the adjacent carbon atom are greatly de-activated. The reasons for this are discussed.