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Home >626-64-2>Q&A>Phenol vs 4-hydroxypyridine - Enol Content [duplicate]

Phenol vs 4-hydroxypyridine - Enol Content [duplicate]

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Fernando Answered Oct 25 2021

Your solution manual correctly predict that phenol has a higher enol content due to aromaticity. For example, you won't see strong carbonyl peak around 1700 cm?1 in FT-IR, which proves keto-form does not exists. It is 100% enol-form as phenol.

On the other hand, 4-pyridone is a different story. The dominance form, whether it is 4-pyridone or 4-hydroxypyridine, would be depend on which phase the compound is in. For example, the Ref.1 states that:

In the gas phase, there is a preference of the hydroxy form in both 2- and 4-hydroxypyridine, while in a nonpolar solvents such as cyclohexane and chloroform, the two tautomers exist in comparable amounts. However, the two equilibria are shifted entirely to the right in favor of the oxo-form, 2- and 4-pyridone, respectively in polar solvents as well as in the crystalline state.

Thus, it is safe to say, it is only in the gas phase, 4-hydroxypyridine dominate. However, in liquid phases, both form exist where predominance is depending on the solvent. In solid phase, 4-pyridone dominate. Since, both phenol and 4-pyridone are solid at ambient conditions, phenol has more enol form than 4-pyridone, which exist in keto-form as a solid.

Note that a public database set like Tautobase, implemented in DataWarrior, may be used to train yourself, too.

Reference:

  1. Jiali Gao, Lei Shao, "Polarization Effects on the Tautomeric Equilibria of 2- and 4-Hydroxypyridine in Aqueous and Organic Solution," J. Phys. Chem. 1994, 98(51), 13772–13779 (https://doi.org/10.1021/j100102a051).
  2. Oya Wahl, Thomas Sander, "Tautobase: An Open Tautomer Database", Chem. Inf. Model. 2020, 60, 1085–1089 (doi 10.1021/acs.jcim.0c00035).

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Encyclopedia
4-Hydroxypyridine CAS No: 626-64-2 MF: C5H5NO
Density 1.111 g/cm3
Melting Point 150-151°C(lit.)
Boiling Point 230-235°C12mm Hg(lit.)
Vapour 0.265mmHg at 25°C
Refractive Index 1.513
Flash Point 221 °C
HS Code 2933399090
Phenol CAS No: 108-95-2 MF: C6H6O
Density 1.07
Melting Point 40-42℃
Boiling Point 182℃
Vapour 0.09 psi ( 55 °C)
Refractive Index 1.5418
Flash Point 79℃
HS Code 29071100
4-HYDROXYPYRIDINE CAS No: 108-96-3 MF: C5H5NO
Density 1.1418 (rough estimate)
Melting Point 150-151 °C(lit.)
Boiling Point 230-235 °C12 mm Hg(lit.)
Vapour 0.265mmHg at 25°C
Refractive Index 1.5939 (estimate)
Flash Point 107.8oC
HS Code 2933 39 99
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