ORG-20

Resonance energy snapshot

E_res = E_Kekulé − E_actual. Extra stability of a delocalised π system.

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AromaticityResonance energy

Governing equation

E_{\mathrm{res}}=E_{\mathrm{Kekul\acute{e}}}-E_{\mathrm{obs}}

$E_{obs} \in \left[-800, 0\right]$

where

E_{Kek}: Kekulé enthalpy (kJ/mol)
E_{obs}: Observed enthalpy (kJ/mol)
E_{res}: Resonance energy (kJ/mol)

The figure, part by part

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①

Governing law — red

E_{\mathrm{res}}=E_{\mathrm{Kekul\acute{e}}}-E_{\mathrm{obs}}

$E_{obs} \in \left[-800, 0\right]$

$E_{obs} = -800$ $E_{res} = -440$

Along the curve

②

Domain of the variable — blue

E_{obs}_{\min} = -800,\quad E_{obs}_{\max} = 0

$E_{obs} \in \left[-800, 0\right]$

$E_{obs} = -800$ $E_{obs} = -800$

Along the curve

③

Response curve — green

E_{\mathrm{res}}=E_{\mathrm{Kekul\acute{e}}}-E_{\mathrm{obs}}

$E_{obs} \in \left[-800, 0\right]$

$E_{obs} = -800$ $E_{res} = -440$

Along the curve

④

Rate of change — amber

\frac{\mathrm{d}E_{res}}{\mathrm{d}E_{obs}}

$E_{obs} \in \left[-800, 0\right]$

$E_{obs} = -795$ $E_{res}^{\prime} = 1$

Along the curve

Lecture brief

Historical brief

Hammett (1937) and Taft linear free-energy, E-factor green metrics, Woodward–Fieser UV and Claisen equilibria are how organic chemistry became predictive. The lab is substituent, waste and tautomer. This sheet (ORG-20 — Resonance energy snapshot) is the form associated with Resonance energy. Working symbols: $E_{Kek}$ , $E_{obs}$ $\rightarrow$ $E_{res}$ . Benzene's hydrogenation is ~150 kJ/mol less exothermic than three cyclohexene bonds — the textbook resonance energy.

Purpose

Purpose: compute

E_{res}

from

E_{Kek}

E_{obs}

in Organic chemistry via

E_{\mathrm{res}}=E_{\mathrm{Kekul\acute{e}}}-E_{\mathrm{obs}}

E_res = E_Kekulé − E_actual. Extra stability of a delocalised π system. Use it when a real organic chemistry question must be answered in SI before a code check.

Live realistic example

In symbols

Live case. Given

E_{Kek} = -360.000\,\mathrm{kJ/mol}

E_{obs} = -208.000\,\mathrm{kJ/mol}

, the governing relation

E_{\mathrm{res}}=E_{\mathrm{Kekul\acute{e}}}-E_{\mathrm{obs}}

yields

E_{res} = 152.0\,\mathrm{kJ/mol}

. Enter a Kekulé reference enthalpy and the observed one, kJ/mol. Move a slider: the numbers are this situation, not a canned story.

Calculator

Inputs

Kekulé enthalpy E_{Kek}-360 kJ/molObserved enthalpy E_{obs}-208 kJ/mol

Outputs

Resonance energy E_{res}152.0 kJ/mol

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Narration of this film

Enter a Kekulé reference enthalpy and the observed one, kJ/mol.

Benzene's hydrogenation is ~150 kJ/mol less exothermic than three cyclohexene bonds — the textbook resonance energy.

Reading speed

Watch on YouTube