4-Nitrophenol is used to manufacture drugs, fungicides, insecticides, and dyes and to darken leather. Acute (short-term) inhalation or ingestion of 4-nitrophenol in humans causes headaches, drowsiness, nausea, and cyanosis (blue color in lips, ears, and fingernails).

p-Nitrophenyl Phosphate (PNPP) is a non-proteinaceous, non-specific substrate used to assay protein, alkaline and acid phosphatases.

Phospahtases are enzymes that hydrolyze phosphate groups from a wide variety of organic substrates (phosphate esters), producing an alcohol and phosphoric acid. They are found in all cells and usually are classified as either acid phosphatases or alkaline phosphatases.

This compound belongs to the class of organic compounds known as nitrophenols. These are compounds containing a nitrophenol moiety, which consists of a benzene ring bearing both a hydroxyl group and a nitro group on two different ring carbon atoms.

4-nitrophenol appears as a white to light yellow crystalline solid. Contact may severely irritate skin and eyes. Poisonous by ingestion and moderately toxic by skin contact.

So, 4-nitrophenol can donate H+ much more easily than phenol where no such effects are present. This makes 4-nitrophenol more acidic. 4-Nitrophenol is more acidic.

The yellow color of the 4-nitrophenolate form (or 4-nitrophenoxide) is due to a maximum of absorbance at 405 nm (ε = 18.3 to 18.4 mM−1 cm−1 in strong alkali).

Ortho and para nitrophenols are more acidic than phenol. Draw the resonance structures of corresponding phenoxide ions. The electron withdrawing groups are more effective in increasing the acidic strength at the para-position relative to the ortho position because of greater dispersal of charge on oxygen atom.

Thus, ortho and para-nitrophenols are more acidic than m-nitrophenol is a little less acidic than p-nitrophenol because of intramolecular hydrogen bonding which makes the loss of a proton difficult to remove.

As hyperconjugation effect can operate through ortho and para positions, therefore meta-cresol is a stronger acid than ortho and para cresols. However, due to the stronger +I effect at ortho position than that at para position(+I effect decreases with distance), ortho-cresol is a weaker acid than para cresol.

Ortho-nitrophenol is more acidic than ortho-methoxyphenol. The presence of the nitro group, which is an electron withdrawing group, at the ortho position decreases the electron density in the O-H bond. Also, the o-nitrophenoxide ion formed after the loss of protons is stabile due to resonance.

Ortho nitrophenol is more acidic than ortho methoxyphenol because nitro group is an electron withdrawing and it will increase +ve charge on the oxygen atom to make it more acidic whereas OCH3 group is an electron releas1ng group and it will decrease +ve charge on the oxygen atom, thus making it less acidic and hence …

O-Nitrophenol forms intramolecular H bond whereas molecules of P-Nitrophenol get associated through intermolecular H bond. During boiling, the strong intermolecular H bonding increases the boiling point but intramolecular H bonding cannot do so. Therefore, O-Nitrophenol is more steam volatile than P-Nitrophenol.

Phenol is more acidic than cyclohexanol and acyclic alcohols because the phenoxide ion is more stable than the alkoxide ion. In an alkoxide ion, the negative charge is localized at the oxygen atom. Phenoxide ion is therefore resonance stabilized.

Phenols have a hydroxyl group directly linked to the ring, whereas alcohols, as non-aromatic compounds, have a hydroxyl group linked to the main chain. The difference is one is cyclic, and the other is non-cyclic.

The reason the benzyl alcohol is less reactive than phenol has to do with the pi electrons on the oxygen. in the phenol molecule, the extra electrons can contribute to the aromatic pi system, but in benzyl alcohol they can’t because they are next to a sp3 carbon which doesn’t allow any overlap.

(i) Phenol is more acidic than methanol. (ii) The C-O-H bond angle in alcohols is slightly less than the tetrahedral angle (109∘28′).