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A short class with Sarah and her students. The other ways are under More.
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Here is a bottle of phenol that has been on the shelf for months. What do you notice?
The crystals are pinkish, though the book says phenol is colourless.
Pure phenol is colourless, but air and light slowly oxidise it to coloured products. Why must we handle it with gloves?
Because it is corrosive and poisonous. It can burn the skin.
Right. Its strong smell is also familiar from old disinfectants, which were sold as carbolic acid.
Toluene and phenol have molar masses of 92 and 94. Toluene boils at 384 K. What about phenol?
If the masses are similar, maybe about the same?
It boils at 455 K, about seventy kelvin higher. Why?
Phenol has an –OH group, so its molecules hydrogen-bond with one another. Toluene has only van der Waals forces.
Exactly. And the same –OH group lets phenol dissolve in water to some extent.
But not as well as ethanol, because the benzene ring is non-polar and large.
o-Nitrophenol melts at about 45 °C and p-nitrophenol at about 114 °C. They have the same formula. What explains the gap?
In the ortho compound, the –OH and –NO₂ groups are next to each other, so the hydrogen bond forms inside the molecule.
Right. So its molecules hardly hold on to one another. What about the para compound?
Its groups are too far apart, so it hydrogen-bonds to neighbouring molecules instead.
And that is why steam distillation separates them: the ortho isomer goes over with the steam.
phenol is a colourless crystalline solid that turns pink on exposure to air and light.
intermolecular hydrogen bonds give phenol (455 K) a far higher boiling point than toluene (384 K).
phenol is moderately soluble in cold water, more in hot water, and less soluble than ethanol because of its large benzene ring.
o-nitrophenol has an intramolecular hydrogen bond, so it melts lower, dissolves less and is steam volatile.
steam distillation separates o-nitrophenol from p-nitrophenol.
Phenol (molar mass 94) boils at 455 K, while toluene (molar mass 92) boils at 384 K, because only phenol forms hydrogen bonds.
o-Nitrophenol melts at about 45 °C and distils with steam, while p-nitrophenol melts at about 114 °C and does not.
An old bottle of phenol crystals is pink rather than colourless because the phenol has been slowly oxidised by air.
Ortho holds its own hand tight, so steam can carry it off light; para links to friends around, so it stays upon the ground.
A bottle of phenol was left open on a sunny shelf. By the end of the term its white crystals had blushed pink, because the air had slowly oxidised them.
O-I-V - Ortho is Intramolecular and Volatile in steam.
P.H.E.N. - Pink in air, Hydrogen bonds, Elevated boiling point (455 K), Not very soluble in cold water.
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What is the physical state of phenol at room temperature?
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Why does phenol turn pink on standing?
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Why does phenol boil much higher than toluene?
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Which nitrophenol is volatile in steam?
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Why is phenol less soluble in water than ethanol?
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C₆H₅OH, the simplest phenol, in which an –OH group is attached directly to a benzene ring.
An attraction between a hydrogen atom bonded to F, O or N and a lone pair on another such atom, in the same or a neighbouring molecule.
Hydrogen bonding between different molecules, which raises melting and boiling points.
Hydrogen bonding within a single molecule, as in o-nitrophenol.
Separation of a compound that is volatile in steam by passing steam through the mixture.
The methylphenols, which are colourless liquids or low-melting solids.
An old name for phenol, used when it was sold as a disinfectant.
Phenol is a colourless crystalline solid with a characteristic smell that slowly turns pink in air. Because its –OH group forms hydrogen bonds, it boils far higher than hydrocarbons of similar molar mass and dissolves moderately in water, and hydrogen bonding also explains the striking differences between o-nitrophenol and p-nitrophenol.
Simple phenols are colourless liquids or low-melting solids; phenol itself melts at about 41 °C, boils at 455 K and turns pink on standing because air slowly oxidises it. Intermolecular hydrogen bonding gives phenol a much higher boiling point than toluene (384 K), whose molar mass is almost the same. Phenol is moderately soluble in cold water and more soluble in hot water: its –OH group hydrogen-bonds with water, but its benzene ring is non-polar. In o-nitrophenol the hydrogen bond forms within each molecule, so it boils lower, dissolves less in water and is volatile in steam, while p-nitrophenol forms hydrogen bonds between molecules; this difference is used to separate the two by steam distillation.
Like alcohols, phenols form intermolecular hydrogen bonds through their –OH groups, so they boil much higher than hydrocarbons or haloarenes of similar or even greater molar mass.
Toluene and phenol have almost the same molar mass, yet phenol boils about 70 K higher; even the heavier chlorobenzene boils well below phenol.
The two nitrophenols formed when phenol is nitrated show how the position of a group changes physical properties.
When phenol is nitrated with dilute nitric acid, the mixture is steam-distilled: o-nitrophenol distils over with the steam and p-nitrophenol stays behind.