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A short class with Sarah and her students. The other ways are under More.
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Ethanol and methoxymethane have the same formula, C₂H₆O. Ethanol boils at 351 K. Guess the boiling point of methoxymethane.
Probably about the same, since the molar mass is the same.
It boils at 248 K, more than a hundred kelvin lower. What does ethanol have that methoxymethane lacks?
An O–H bond. Ethanol molecules can hydrogen-bond with each other.
Exactly. Methoxymethane is polar but has no hydrogen on its oxygen. And propane, at 231 K, has only van der Waals forces.
So hydrogen bonding makes the biggest difference of all.
Methanol boils at 338 K and butan-1-ol at 391 K. Why the rise?
Bigger molecules have more surface area, so the van der Waals forces are stronger.
Good. Now butan-1-ol boils at 391 K but 2-methylpropan-2-ol at only 355 K. Same formula. Why?
The branched molecule is more compact, so there is less surface area in contact.
Right. Branching lowers the boiling point but, as we will see, it raises the solubility in water.
Ethanol mixes with water in any proportion. What holds them together?
Hydrogen bonds between the –OH of ethanol and water molecules.
Then why is hexan-1-ol almost insoluble, when it also has an –OH group?
Its long alkyl chain is non-polar. One –OH group cannot pull such a big chain into water.
Exactly. It is a tug of war: the –OH pulls the molecule into water, and the alkyl group pulls it out.
–OH groups bond to neighbouring molecules, giving alcohols high boiling points and good solubility in water.
ethanol boils at 351 K, far above methoxymethane (248 K) and propane (231 K) of similar molar mass.
boiling point rises with chain length and falls with branching.
ethane-1,2-diol (470 K) and glycerol (about 563 K) boil much higher than ethanol and are viscous.
lower alcohols are miscible with water; solubility falls as the alkyl group grows and rises with branching.
Ethanol (351 K) and methoxymethane (248 K) are isomers of formula C₂H₆O, and the difference in boiling point comes from hydrogen bonding in ethanol.
Butan-1-ol (391 K) boils higher than its branched isomer 2-methylpropan-2-ol (355 K), but the branched isomer is the one that mixes completely with water.
Glycerol, with three –OH groups, is a syrupy liquid that boils at about 563 K and mixes with water in all proportions.
Longer chain, the boiling's higher; branch it out, the boiling's lower.
The –OH group and the alkyl tail went swimming. In methanol the –OH group dragged the short tail into the pool easily, but in hexanol the long tail kept floating back to the bank.
O-H means Hold Hands - alcohol molecules hold hands through hydrogen bonds, so they are hard to separate.
C.B.O. - Chain up, boiling up; Branch up, boiling down; OH groups up, boiling way up.
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Why does ethanol boil at a much higher temperature than propane?
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How does branching affect the boiling point of isomeric alcohols?
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Why are lower alcohols soluble in water?
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Why is hexan-1-ol almost insoluble in water?
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Why is glycerol so viscous?
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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.
Weak attractions between molecules that increase with molecular size and surface area.
The –OH group, which makes alcohols polar and able to form hydrogen bonds.
Able to mix in all proportions to form a single solution.
The resistance of a liquid to flow.
An alcohol with more than one –OH group, such as ethane-1,2-diol or glycerol.
Compounds with the same molecular formula but different structures.
Alcohols boil at much higher temperatures than hydrocarbons or ethers of similar molar mass, and dissolve in water far better than hydrocarbons, because their –OH groups form hydrogen bonds. This lesson explains their physical state, boiling points, solubility and viscosity, and how chain length, branching and the number of –OH groups change them.
An alcohol molecule has a polar –OH group, which forms hydrogen bonds, and a non-polar alkyl group, which does not, and its physical properties come from the balance between the two. Hydrogen bonding between alcohol molecules gives them high boiling points: ethanol boils at 351 K, while methoxymethane and propane, of almost the same molar mass, boil at 248 K and 231 K. Boiling points rise with chain length and with the number of –OH groups but fall with branching. Lower alcohols mix with water in all proportions because they hydrogen-bond with water molecules, but solubility falls as the alkyl group grows. The lesson also covers state, density, viscosity and the toxicity of methanol.
An alcohol molecule, R–OH, has two parts:
Almost every physical property of an alcohol is decided by the balance between these two parts.
Methoxymethane has polar C–O bonds but no O–H bond, so its molecules cannot hydrogen-bond with one another; propane has only weak van der Waals forces.
Rise with chain length. Methanol 338 K, ethanol 351 K, propan-1-ol 370 K, butan-1-ol 391 K. Larger molecules have larger surface areas and stronger van der Waals forces.
Fall with branching. Among the isomeric butanols: butan-1-ol 391 K, 2-methylpropan-1-ol 381 K, butan-2-ol 373 K and 2-methylpropan-2-ol 355 K. Branching makes a molecule more compact, which reduces its surface area and weakens the van der Waals forces between molecules. An –OH on a more substituted carbon is also more crowded, so it hydrogen-bonds less effectively; among isomers, boiling points therefore tend to follow the order 1° > 2° > 3°. An –OH on a more substituted carbon is also more crowded, so it hydrogen-bonds less effectively; among isomers, boiling points therefore tend to follow the order 1° > 2° > 3°. An –OH on a more substituted carbon is also more crowded, so it hydrogen-bonds less effectively; among isomers, boiling points therefore tend to follow the order 1° > 2° > 3°.
Rise with the number of –OH groups. Ethanol boils at 351 K, ethane-1,2-diol at 470 K and propane-1,2,3-triol (glycerol) at about 563 K, with decomposition. Each extra –OH group adds more hydrogen bonding.
Hydrogen bonding also makes alcohols more viscous than hydrocarbons. Glycerol, with three –OH groups per molecule, is a thick, syrupy liquid.