# E2 Reactions in Cyclic Structures

In my textbook, the percentages of anti-eliminations are given which are quite contrary to expectations. The reason is also not given quite clearly. Why does the preference for anti-periplanarity change and what are the reasons for the disparity between the different cyclic structures?

\begin{array}{lr} \text{Ring Size} & \%\text{ SYN Elimination} \\ \hline \text{Cyclobutyl} & 90 \\ \text{Cyclopentyl} & 46 \\ \text{Cyclohexyl} & 4 \\ \text{Cycloheptyl} & 37 \end{array}

The values are for $\ce{(CH2)_nC-HY}$.

• You need to build some models or look at some computed examples. It's really hard to achieve a conformation where the hydrogen atom and the leaving group are anti-periplanar in some cases. – Zhe Aug 11 '17 at 17:45

If we examine the dihedral angles, we notice that an axial substituent is anti-periplanar to the adjacent two carbons’ hydrogens. This means, it can easily anti-eliminate in both directions. No substituent whether equatorial or axial has any ecliptic hydrogen in either direction. This means, that a syn-specific elimination, which requires an ecliptic configuration, cannot occur out of the chair conformation. The $4~\%$ syn-elimination the table references must occur from other, minor configurations such as the boat.