Question about boiling point(melting poit) and water solubility

[ssodamy]

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Q. Which of the following would be most soluble in water?

a. CH3CH2OH
b. CH3CH2CH2OH
c. acetone
d. H2C=CHCH3
e. CH3OCH2CH3

Q2. Which of the following has the highest melting point?

a. trans-2-butene
b. cis-2-butene
c. n-butane
d. 1-propanol
e. 1-pentanol


The answer for Q1 is (a) and for Q2 is (e)

I know both are related to the hydrogen bonding.

Why is the pattern for melting point and the water solubility different?

One is bigger size in alkyl portion of the alcohol, but other one is smaller size in alkyl portion.

Could you explain the reason?

 

[indigenoustw]

One reason I could think of is basic hydrophobic/hydrophilic principle. Obviously water is hydrophilic, and by having a shorter hydrophobic chain, it minimizes "stacking" of the hydrophobic chain (less VDW attraction forces) so that there are more possibilities that water can solvate ethanol molecules. The same applies to question 2 except now you want more stacking of the hydrophobic chain since stacking will enhance the rigidity of the molecules (by VDW forces) and raise the melting point of molecules (more energy needed to break those VDW attractions). So 1-pentanol has more carbon chains than 1-propanol, hence the answer e.

Hope this helps.

 

[LetsGo2DSchool]

Q1. Solubility is all about polarity. The only possible choices are A and B. A is going to be more soluble because it has a shorter hydrocarbon chain (which is nonpolar). If either one of these OH groups were attached to a super duper long hydrocarbon chain, then neither would become soluble.

 

[Jay0689]

I think the dif is that for solubility, large alkyl chains are bad while for melting point, large alkyl chains raise it.

To have good solublity in water, the molecule must be attracted to water. To have a high melting point, the molecules just need to be attracted to each other. So the larger they are the better... Doesn't matter if the parts that are sticking together are non-polar or polar... Like attracts like.

I think you understand the first one. For the second one, just think that the H bonding is good for intermolecular forces in a solid, but so is the large size.

 

[LetsGo2DSchool]

Q2. D and E are the two possible choices because they have H-bonding which is not present in the other choices. E will have higher melting point than D because it is a larger molecule and thus has greater London dispersion forces.

 

[JohnnnyD]

Q1. Solubility is all about polarity. The only possible choices are A and B. A is going to be more soluble because it has a shorter hydrocarbon chain (which is nonpolar). If either one of these OH groups were attached to a super duper long hydrocarbon chain, then neither would become soluble.

I think this is correct, but I think that you are suppose to look at H-bonding before polarity when determining solubility. Still tho, A and B are both capable of H-bonding with water and thus will both be soluble. And since A has a shorter hydrocarbon chain, as you already stated, it will be more soluble.

You're answer is correct, I just think H-bonding should be looked at first when determining solubility in water...right?

 

[LetsGo2DSchool]

I think this is correct, but I think that you are suppose to look at H-bonding before polarity when determining solubility. Still tho, A and B are both capable of H-bonding with water and thus will both be soluble. And since A has a shorter hydrocarbon chain, as you already stated, it will be more soluble.

You're answer is correct, I just think H-bonding should be looked at first when determining solubility in water...right?

I don't think H-bonds play that much of a role in solubility. H-bonds affect intermolecular attractions and thus will have an effect on boiling and melting points.

Since there are two types of solvents: polar (such as water) and nonpolar (such as benzene) and likes dissolve likes...

The more polar the solute, the more easily it will dissolve in a polar solvent.
The more nonpolar the solute, the more easily it will dissolve in a nonpolar solvent.