Predict which member of each group is most soluble in water, a...

Question

Predict which member of each group is most soluble in water, and explain the reasons for your predictions.

a. butan-1-ol, pentan-1-ol, or propan-2-ol

Accepted Answer

All right. Hi, everyone. So this question is asking us to predict which member is the least soluble in water. Pen 10 2, all but 1020 or HEP 1010 explain. All right. So before we start talking about the forces that contribute to solubility in something like water, let's first get a sense for what these three molecules look like. First off here, we have Penan to all which is a five carbon parent chain with a hydroxy group or oh group on carbon number two of that parent chain. On the other hand, we have B 1020 which is pretty similar in appearance. Instead, we have a four carbon chain. However, with a hydroxy substituent on carbon two and our last member here is HEPT 10 which is a seven carbon parent chain with the hydroxy substituent. Now on carbon number one at the very end of the parent chain. So with this in mind, now that we have a sense for what these compounds look like. Let's talk about intermolecular forces and what factors contribute to solubility when it comes to discussions about solubility, a very common principle that you may have heard in the past is this principle of like dissolves like. And so what that means if you recall is that molecules or substances in general of similar polarities are going to mix well with each other. Therefore, they're going to be soluble. So because water is a polar solvent, then molecules that are polar are going to be more soluble in water. But what determines polarity recall that when you consider the structure of water, there's a very noticeable dipole moment when we consider the bonds between oxygen and hydrogen. Because of the difference in electronegativity between oxygen and hydrogen, the dipole is going to skew towards oxygen because the electrons in the covalent bond between oxygen and hydrogen are going to gravitate towards oxygen due to that increased electron negativity. And so that imbalance that uneven shearing of electrons in a covalent bond is what we describe as the net dipole. So in this case, we have two dipoles going towards oxygen. So to understand the polarities of our three starting materials, right, our three alcohols here, we have to consider the presence or lack thereof of any potential diles. However, because all of these compounds happen to be alcohols that contain o age groups, there is going to be a dipole for each of them because first off, right, when we consider the bond between carbon and oxygen in each of these three compounds, oxygen being more electron still is going to attract the electrons of the covalent bond more so towards itself. So we still see a dipole there. And once again, there's another dipole between oxygen and hydrogen skewed towards oxygen. And we can repeat the same thing if we consider our remaining two molecules. So the net dipole is going towards oxygen in all three of these compounds. All three of these cases now recall also that because these alcohols contain hydrogen atoms bound to electron elements such as oxygen or nitrogen or fluorine, these molecules are capable of hydrogen bonding with water. In particular, since that's the solvent we're focusing on for this example. Now, even though all of these molecules have a dipole moment and can exhibit h bonding, there's still going to be a difference in their relative polarities because of the hydrocarbon chains that they have. Because hydrocarbon chains, if you recall are considered nonpolar because of the lack of a significant difference in electronegativity between carbon and hydrogen. And the fact that there's no difference in electronegativity at all between carbon atoms connecting to each other. So because of this, the larger the carbon chain happens to be right, the larger the non polar region of that compound happens to be, which is going to negatively impact its solubility in a polar substance like water. So this effect is especially going to be noticeable if we consider the structure of HEP tan one, all that happens to have the largest carbon chain and therefore of the three molecules that we're comparing in this video. Hep 1010 has the largest non polar region, which means that that's going to have the greatest impact on its solubility in water, which is to say that it's going to be the least soluble and there you have it. So the answer here is hep 10 1 all will be the least soluble in water. Now, as for the explanation, I'm going to go ahead and briefly pause this recording so I can write it out in a sentence. All right. So here I resumed the recording and the explanation reads as follows. Hep 10 1 all has the greatest number of carbon atoms and the largest hydrophobic region. And so with that being said, thank you so very much for watching. And I hope we found this video helpful.

Predict which member of each group is most soluble in water, and explain the reasons for your predictions.
a. butan-1-ol, pentan-1-ol, or propan-2-ol

Key Concepts

Hydrophilicity and Hydrophobicity

Alcohol Functional Group

Chain Length and Solubility

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