A Guidebook to Mechanism in Organic Chemistry

Stereochemistry of Carbanions

The stability of the ion is reflected in the acidity of cyclopentadiene

itself, demonstrating the readiness with which the latter is prepared

to lose a proton in order to attain a more stable state as a carbanion.

The quasi-aromaticity cannot be demonstrated by electrophilic sub­

stitution, for attack by X® would merely lead to direct combination

with the ion, but true aromatic character (Friedel-Crafts reactions,

etc.) is shown by the remarkable series of extremely stable compounds

such as ferrocene (X), (obtained by attack of (IX) fin metallic halides

such as FeCla) *

in which the metal atom is held by ir bonds in a kind of molecular

'sandwich* between two cyclopentadienyl structures.

STEREOCHEMISTRY OF CARBANIONS

Thequestionof whether a simple carbanion of theformR 3 Ce is planar
or pyramidal (like a tertiary amine with which it is isoelectronic)

owes its considerable stability to the fact that, in the system, a total of

six w electrons is available and these can distribute themselves so as

to form delocalised ir orbitals covering all five carbon atoms, leading

to the quasi-aromatic structure (IXa):