Chemistry · Alcohols, Phenols And Ethers · NEET
Oxygen is electronegative and does pull some electrons (-I effect). But oxygen also has lone pairs. One lone pair overlaps with the ring's pi system and pushes electron density INTO the ring (+R or +M effect). For phenol, this electron donation (+R) is stronger than the electron pull (-I). So the net effect is that the ring becomes electron-rich, which is called 'activated'. An electron-rich ring attracts electrophiles more strongly, so reaction is faster. This is why it matters for NEET: they test whether you know +R beats -I here.
When you draw the resonance structures of phenol, the negative charge (extra electron density) lands on the ortho and para carbons, not the meta carbons. Electrophiles attack where electrons are, so they attack ortho and para. So phenol is an 'ortho/para director'. Remember: the -OH lone pair can only push charge to o and p positions, never to meta.
Benzene has an even ring and no extra electron push. Phenol's -OH keeps feeding electrons into the ring. More electron density means the ring grabs electrophiles much faster. Real proof: phenol reacts with bromine WATER at room temperature and needs NO catalyst (no FeBr3), while benzene needs a Lewis acid catalyst and stronger conditions. If a NEET question says 'reacts with bromine water without a catalyst', the answer is usually phenol or aniline (both activated).
Bromine water is a polar solvent. In water, phenol ionises a little to phenoxide, which is even more activated, so all three free positions (2 ortho + 1 para) get brominated, giving a white precipitate of 2,4,6-tribromophenol. In a non-polar solvent like CS2 or CHCl3 at low temperature (273 K), phenol stays neutral and less reactive, so only ONE bromine goes in, mainly at para, giving p-bromophenol. NEET loves testing this solvent difference.
With DILUTE nitric acid at low temperature, phenol gives a mixture of o-nitrophenol and p-nitrophenol (only one -NO2 goes in). With CONCENTRATED HNO3 plus concentrated H2SO4, all three positions react and you get 2,4,6-trinitrophenol, called picric acid. So: dilute = mono (o + p mix); concentrated = tri (picric acid). Do not mix these two up in the exam.
o-Nitrophenol has the -OH and -NO2 next to each other, so it forms an INTRAmolecular hydrogen bond (a ring inside one molecule, called chelation). This makes it more volatile (steam-volatile, lower boiling). p-Nitrophenol has the groups far apart, so it forms INTERmolecular hydrogen bonds between molecules, making it less volatile (higher boiling). Because their volatilities/boiling points differ, they can be separated by steam distillation or fractional distillation. NEET has asked both answers in different years, so read the exact wording of the options given.
The most suitable method of separation of a 1:1 mixture of ortho- and para-nitrophenols is
In the reaction C6H5OH + conc. HNO3 / conc. H2SO4 -> X + Y, where X and Y are o-nitrophenol and p-nitrophenol, the suitable method to separate X and Y is:
In the reaction C6H5OH + CHCl3 + NaOH -> salicylaldehyde, the electrophile involved is
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. The -OH lone pair donates electrons into the ring (+R effect), making the ring electron-rich. An electron-rich ring attracts electrophiles faster, so phenol reacts much faster than benzene and often needs no catalyst.
Ortho and para. The resonance of the -OH lone pair places extra electron density on the ortho and para carbons, so electrophiles attack there. Phenol never directs to meta.
A white precipitate of 2,4,6-tribromophenol forms, with no catalyst needed, because the polar aqueous medium makes phenol very reactive. In a non-polar solvent like CS2 at low temperature, only p-bromophenol forms.
Dilute HNO3 gives a mono-nitration mixture of o- and p-nitrophenol. Concentrated HNO3 with concentrated H2SO4 gives 2,4,6-trinitrophenol (picric acid).
o-Nitrophenol has intramolecular H-bonding, so it is more volatile. p-Nitrophenol has intermolecular H-bonding, so it is less volatile. This difference in volatility/boiling point allows separation by steam or fractional distillation.
