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chris2009317 2013-7-7 08:49 PM

soluble in water

which member of each of the following pairs of compound would you expect to be more soluble in water?

A) butane and 1-butanoic acid

B) 1-nonanol and 1-propanol

C) 1,2 Butanediol and 1-butanol

chungkin81 2013-7-7 09:03 PM

which member of each of the following pairs of compound would you expect to be more soluble in water?

A) butane and 1-butanoic acid
Hint:
To answer this question, you need to think about:
What intermolecular force is present in molecules of butane?
What intermolecular force is present in molecules of 1-butanoic acid?
If a molecule that can form hydrogen bond with water, it is more soluble. So which one can form hydrogen bond with water?

B) 1-nonanol and 1-propanol
Hint:
Both 1-nonanol (9-C) and 1-propanol  (3-C) are alcohol, they can form intermolecular hydrogen bond. But which one has a shorter carbon chain? The longer the carbon chain, the less soluble would be the alcohol. So the one has shorter carbon chain is more soluble.

C) 1,2 Butanediol and 1-butanol
Hint:
1,2-butanediol has two hydroxyl (-OH) groups but 1-butanol has one hydroxyl group only.
So which one can form more hydrogen bonds with water?
The one can form more hydrogen bonds with water would be more soluble in water.

chris2009317 2013-7-7 10:18 PM

i don't know when to use hydrogen bond , dipole-dipole ,van der waal's force

chungkin81 2013-7-7 10:43 PM

Hydrogen bond:
When hydrogen is bonded to very electronegative atoms (like N, O, F). The interaction between this H atom and the lone pair electrons on N, O or F is called hydrogen bond
It is usually found in alcohol, carboxylic acid and amide.

Dipole-dipole:
When the electronegativity values of two atoms are different. The more electronegative one is slightly negative and the less electronegative one is slightly positive. This is called dipole.
For example, the interaction between H-Cl and another H-Cl molecule is dipole-dipole interaction.

Dispersion forces (or London forces):
It happens in non-polar molecules or monoatomic noble gas atom. This is due to a time moment that the distribution of electrons is uneven, making an area slightly negative and an area slightly positive. This is called induced dipoles.
This would induce another one to have slightly positive and slightly negative, so this is called induced dipole-induced dipole interactions.
Usually this is mainly dominant found in non-polar molecules and noble gas.

chris2009317 2013-7-7 11:01 PM

A) 1-butanoic acid
B) 1-propanol
C) 1,2 Butanediol

Correct?

chungkin81 2013-7-7 11:07 PM

Yes.
You need to explain why by looking at the hint and write the full answer when you face this type of questions.
Of course you need to think of the reason yourself when you face those similar questions in future.

[[i] 本帖最後由 chungkin81 於 2013-7-7 11:09 PM 編輯 [/i]]
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