Theoretical Approach to Substituent Effects. Structures and Stabilities of Carbanions XCH2-

Addy Pross, Douglas J. DeFrees, Beverly A. Levi, Steven K. Pollack, Leo Radom, Warren J. Hehre

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The effect of substituents (X) on the structures and stabilities of XCH2- anions for X = Li, BeH, BH2, CH3, NH2, OH, F, CN, NO2, CH3CH2, CH2=CH, HC=C, CF3, CHO, and C6H3 has been examined by using ab initio molecular orbital theory. Full optimizations have been carried out on anions XCH2 and corresponding neutral parents XCH3 with the 4-31G basis set; improved energies have been obtained by use of a larger basis set (6-31G*) and by incorporation of electron correlation (MP2). A strong stabilizing effect in XCH2“ is observed for π-accepting substituents (X = Li, BeH, BH2, CN, NO2, CH2=CH, HC=C, CF3, CHO, and C6H6). For most of the systems in this category, the preferred anion structure is planar. For π-donor substituents (X = NH2, OH, F), a weak stabilizing interaction takes place which may be attributed to the π-accepting nature of these groups. Only small effects are observed for the CH3 and CH3CH2 substituents. These results generally conform to the accepted views on carbanion stability derived from solution data. A comparison is presented of the stabilities of anions derived from XCH3 by deprotonation at CH3 on the one hand and at X on the other. The former site of deprotonation is generally favored by electropositive or unsaturated substituents while the latter is favored by electronegative substituents and by C=CH.

Original languageEnglish
Pages (from-to)1693-1699
Number of pages7
JournalJournal of Organic Chemistry
Issue number8
StatePublished - 1 Jan 1981
Externally publishedYes

ASJC Scopus subject areas

  • Organic Chemistry


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