Introduction to Heterocyclic Chemistry 11 Box 1.13 Cyclohexane Conformers Cyclohexane Boat conformer Chair conforme (for convenience,hydrogen atoms are not shown) iperi Scheme 1.1 that there is a preference for an equatorial N-H (ie.H is larger than the The eneray difference between lone pair),and this preference is consolidated as the size of the N sub- equatorial N-H and axial N-H in stituent increases. 1.5-3.1 min vou of the Components of Ring Strain Angle and torsional strain are major components of the total ring strain in fully reduced cyclic compounds.For cycloalkanes(see Table 1.2),the smaller the ring,the larger the overall strain becomes.What may appear at first to be surprising is that medium-sized rings containing 8-11 atoms Table 1.2 Ring strain in cycloalkanes Number of atoms Total strain Number of atoms Total strain in the ring (kJ mol1) in the ring (kJ mol) 3 115 52 1 21314 41 5Introduction to Heterocyclic Chemistry 1 1 0 N I H Piperidine Ring H I .d - mN.H inversion Pyramidal inversion /I pramipal inversion II - Ring v inversion H I Scheme 1.1 that there is a preference for an equatorial N-H (i.e. H is larger than the The energy difference between lone pair), and this preference is ,consolidated as the size of the N sub￾stituent increases. equatorial N-H and axial N-H in piperidine is estimated to be 1.5-3.1 kJ mol-1 in favour of the Components of Ring Strain Angle and torsional strain are major components of the total ring strain in fully reduced cyclic compounds. For cycloalkanes (see Table 1.2), the smaller the ring, the larger the overall strain becomes. What may appear at first to be surprising is that medium-sized rings containing 8-1 1 atoms Table 1.2 Ring strain in cycloalkanese Number of atoms Total strain Number of atoms Total strain in the ring (kJ mol-l) in the ring (kJ mol-l) 115 110 26 0.5 26 41 53 10 11 12 13 14 15 52 47 17 21.5 8 8 equatorial form. In piperidine the energy for N inversion is ca. 25.5 kJ mol-I