Physical Properties of Phenols

What is the order of acid strength for phenols, carboxylic acids, ethanol & water?

RCOOH (strongest) > Phenol > Water > Ethanol (weakest)

Stability of Conj. Base Reasoning: Why is phenol more acidic than water & ethanol?

• Lone pair of electrons on O atom of phenoxide ion delocalissed into the benzene ring.

• Disperses the negative charge on O atom of phenoxide

• Stabilizes the phenoxide ion more, making it less likely to accept a proton.

  • Therefore, making phenoxide a stronger acid.

Strength of O-H bond Reasoning: Why is phenol more acidic than water & ethanol?

• Lone pair of electrons on O atom can delocalize into the benzene ring

• O-H bond becomes more polarized and hence is weakened

  • Allowing it to break more easily & release H+

• POE lies rightwards, hence phenol is more acidic

Stability of conj. base reasoning: Why is ethanol weaker than phenol?

• Negative charge is localized on O atom of ethanoxide ion

• Electron donating alkyl (ethyl) group intensifies the negative charge on O of ethanoxide ion.

• Destabilizes the ethanoxide ion

  • More likely to accept the proton

• POE lies leftwards; weaker acid

Strength of O-H bond reasoning: Why is ethanol a weaker acid than phenol?

• Electron donating alkyl group increases electron density on O atom

• O-H bond is less polarized, thus is stronger

  • H+ is released less easily.

• POE lies leftwards; ethanol is weaker acid

Effect of substituents on acid strength of phenol

• Electron WITHDRAWING groups (Cl, NO2) — INcrease acid strength of phenol

• Electron DONATING groups (CH3, alkyl groups) — DEcrease acid strength of phenol

How do electron withdrawing groups increase the acidity of phenol?

• Electron withdrawing groups disperse the -ve charge on O atom of conj base ion.

• Stabilizing the conj. base and making it less susceptible to accepting the proton.

• Thus, increasing acid strength of phenol.

How do electron donating groups weaken acidity of phenol?

• Intensifies -ve charge on O atom of conj. base ion

• Destabilizing the conj. base ion and making it more susceptible to accepting the proton.

• Thus, decreasing the acid strength of phenol.