[iam_psyche]

how can each pair of compunds may be distinguished from each other, based on their solubility behavior?

a CH3NH2 and CH3(CH2)5CH2NH2

b. CH3CHO and HOCH2CHO

c. Benzylamine and benzyl alcohol

[macman104]

a)  Compare alkyl chain, how does this affect solubility in polar solvents
b)  Compare groups, how is the overall polarity of the molecule affected?  Which one is more polar?
c)  One is more basic than the other, which one will become a salt (have a cation/anion pair) when in an acidic environment?

[iam_psyche]

1. CH3NH2 is a small, polar molecule and can hydrogen bond with water.
CH3(CH2)5 CH2NH2 can also hydrogen bond with water via the amino group but is much less polar due to the presence of a much longer, non-polar, hydrocarbon chain.

2. CH3CHO is small and polar, and can hydrogen bond with water via the O on the C=O group. HOCH2CHO is also small and polar but, in addition to hydrogen bonding with water via the O on the C=O group, can also hydrogen bond with water via the -OH group.

3. Compounds containing the large, non-polar, benzene rings tend not to be very soluble in water.
Alcohols and amines can both form hydrogen bonds with water, but in alcohols the hydrogen bonding is stronger.

[macman104]

Pretty good answers!  The only thing is that for the last one, I don't know which you could say is going to be dissolve better.  However, like I said, can you predict if one is more basic than the other?  What happens if you put both in an acid, will one form a cation that is going to then be soluble in water?

[Vidya]

better solubility in water with less non-polar part and high tendecy to form hydrogen bonds with water.
So on this basis you can decide which one is going to be better.