Stereoelectronic effects in brominations of cyclopropylarenes and 9-alkylanthracenes

The free radical bromination of several cyclopropylarenes has been studied. The abstraction of a cyclopropyl hydrogen by bromine atom, which to date has been an unrecognized process, is demonstrated in this study. Specifically, when a cyclopropyl group is attached to the 9-position of an anthracene, an unprecedented hydrogen abstraction product, the corresponding cyclopropyl bromide, is obtained. This is believed to be due to stereoelectronic effects. Molecular mechanics calculations and X-ray crystallography have been used to demonstrate that 9-cycIopropylanthracene, unlike other cyclopropylarenes, is effectively locked in a conformation which places the a-cyclopropyl C-H bond in alignment with the p-orbitals of the aromatic system. This proper alignment activates the a-cyclopropyl hydrogen for abstraction by bromine atom. The relative reactivities of several 9-alkylanthracenes towards bromine atom are established, namely: 9-methyI- > 9-cyclopropyI- > 9-ethyl- >> 9-isopropylanthracene. Semi-empirical molecular orbital theory and molecular mechanics calculations have been utilized to demonstrate that the relative reactivities are not a function of bond dissociation energies but rather a function of the size of the dihedral angle between the a C-H bonds and the plane of the central ring of the various 9-alkylanthracenes in their lowest energy conformations. The absolute rate constants for the abstraction of hydrogen by bromine atom from 9-methyl-, 9-cyclopropyl-, and 9·ethylanthracene are estimated to be 1.1 x 10⁸ M⁻¹ sec⁻¹, 3.8 x 10⁷ M⁻¹ sec⁻¹ and 7.2 x 10⁶ M⁻¹ sec⁻¹ respectively. The value for the primary hydrogen/deuterium isotope effect for the abstraction of hydrogen by bromine atom from 9-cyclopropylanthracene is determined to be 2.6. All of the above observations lend support to the importance of stereoelectronic effects in the free radical bromination of the cyclopropylarenes and 9-alkylanthracenes.

Name thesis:degree_name

Ph. D.

Level thesis:degree_level

doctoral

Discipline thesis:degree_discipline

Chemistry

Department dc:contributor.department

Chemistry

Grantor dc:publisher

Virginia Polytechnic Institute and State University

Year dc:date.issued

1989

Author dc:creator

• Hakim, Mas Rosemal

Chair dc:contributor.committeechair

• Tanko, James M.

Committee members dc:contributor.committeemember

• McGrath, James E.
• Gibson, Harry W.
• Taylor, Larry T.
• Dorn, Harry C.

Statement dc:rights

• In Copyright

Licence dc:rights.uri

• http://rightsstatements.org/vocab/InC/1.0/

Language dc:language.iso

en_US

Handle dc:identifier.uri

http://hdl.handle.net/10919/54185

OAI identifier oai:identifier

oai:vtechworks.lib.vt.edu:10919/54185