In some cases, the stereochemistry of the starting material can prevent the formation of the Zaytsev product. For example, when menthyl chloride is treated with sodium ethoxide, the Hofmann product is formed exclusively, but in very low yield:
This result is due to the stereochemistry of the starting material. E2 eliminations require anti-periplanar geometry, in which the proton and leaving group lie on opposite sides of the C-C bond, but in the same plane. When menthyl chloride is drawn in the chair conformation, it is easy to explain the unusual product distribution.
Formation of the Zaytsev product requires elimination at the 2-position, but the isopropyl group – not the proton – is anti-periplanar to the chloride leaving group; this makes elimination at the 2-position impossible. In order for the Hofmann product to form, elimination must occur at the 6-position. Because the proton at this position has the correct orientation relative to the leaving group, elimination can and does occur. As a result, this particular reaction produces only the Hofmann product.