Orgo TBR Chapter 1, 2013 version-help pls

[LabratABQ]

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So i am not sure why I have so much trouble with this problem, TBR version 2013, Homework passage #5, Q #1, page 78. The question is:

Which of the following MOST likely to be a solid at room tempt?
A. 1-Butanol
B. Dipropyl Ether
C. Pentanal
D. Octanoic Acid.

The table provides melting point and boiling point.

The answer is D and I cant logic myself into why? I see why A, B, C are out, cause following the trend of this provided table, the melting point gets only more negative, meaning that less likely to be a solid at room tempt, it melts before it gets pass that point.

But then according to this table:
HCO2H: bp of 101c and mp 8c
H3CCO2H bp 118c, and mp 17c
H3CCH2CO2H: bp 141c, mp -21c <<<<< this is what throws me off, I thought the trend was going up from formic acid (8c) to acetic acid (17c), but then when it comes to propionic acid, it went all the way down to -21c, how would you go about predicting octanoic acid? Do I just assume that because the BP of three substances are increasing as well the the MP of the first two are increasing to I can neglect the neg temp of the third?

 

[inasensegone]

I think this has more to do with the long alkyl chain and the molecular weight. Octanoic acid is clearly the heaviest compound listed (C8H16O2). The general trend with alkanes is that as molecular weight goes up, melting point also increases. While Octanoic acid isn't an alkane, the longer the the alky group bonded to the carboxylic acid, the more alkane properties it takes on. This is also why Octanoic acid would be expected to have a low solubility. You may also consider fatty acids. Fatty acids are just long carboxylic acids. With saturated fatty acids (no double bonds in the alkyl chain) these are solids are room temperature (like animal fat).

Basically, I wouldn't take the difference between acetic acid (ethanoic acid) and propanoic acid to indicate a general trend. You should be aware that melting point increases with molecular weight and this is I'm sure what the question is testing.

Side note: I have the 2012 TBR OChem book. I don't have this question. Any idea how this version stacks up to older versions? Just curious.

 

[LabratABQ]

I think this has more to do with the long alkyl chain and the molecular weight. Octanoic acid is clearly the heaviest compound listed (C8H16O2). The general trend with alkanes is that as molecular weight goes up, melting point also increases. While Octanoic acid isn't an alkane, the longer the the alky group bonded to the carboxylic acid, the more alkane properties it takes on. This is also why Octanoic acid would be expected to have a low solubility. You may also consider fatty acids. Fatty acids are just long carboxylic acids. With saturated fatty acids (no double bonds in the alkyl chain) these are solids are room temperature (like animal fat).

Basically, I wouldn't take the difference between acetic acid (ethanoic acid) and propanoic acid to indicate a general trend. You should be aware that melting point increases with molecular weight and this is I'm sure what the question is testing.

Side note: I have the 2012 TBR OChem book. I don't have this question. Any idea how this version stacks up to older versions? Just curious.

You know the new book has modified passages and all, I'm using both the PDFs and the actual book just i can get used to the screen and all...I didnt not find this passage on the 2012 whatsoever. I understand that alkyl molecular weight concept, but then their explanation is wacky...What I am confused about is why the propionic acid drops in both melting point and even density...do you think the last one is a propionic acid as well?

For instance, they changed their discrete questions, completely changed, not modified, but changed. Also chapter one, they cut down about 10 pages....

 

[inasensegone]

Interesting about the re-formatting. Would have been nice to have to read less 🙂

Oh yeah! Dimerization. Smaller Carboxylic acids will exhibit a greater degree of dimerization and this affects the melting point. Here check out this link. (Under physical properties of Carboxylic acids) This is what it says:

"Unbranched acids made up of an even number of carbon atoms have melting points higher than the odd numbered homologs having one more or one less carbon. This reflects differences in intermolecular attractive forces in the crystalline state."

 

[LabratABQ]

Thank you! I wonder if I just have to ignore the change in the last mp of propionic acid since everything else seemed to increased. One thing though, I think they are testing for our ability to think critically rather than previously acquired knowledge, i.e. the ability to interpret the provided info and use that to answer the questions. How do i know? they tell you that in the answer part hahaha. Ya they do cut it down sometimes i feel like they go way way way too detailed.

 

[Oso]

Basically, I wouldn't take the difference between acetic acid (ethanoic acid) and propanoic acid to indicate a general trend. You should be aware that melting point increases with molecular weight and this is I'm sure what the question is testing.

I agree. Think about how octanoic acid has the longest hydrocarbon chain --> more van der waals interactions (intermolecular forces) --> higher MP

That's why for hydrocarbons, increasing molecular weight (ie. longer chain) tends to increase MP.

 

[LabratABQ]

I agree. Think about how octanoic acid has the longest hydrocarbon chain --> more van der waals interactions (intermolecular forces) --> higher MP

That's why for hydrocarbons, increasing molecular weight (ie. longer chain) tends to increase MP.

Thank you!

 

[No Limits]

Thank you!

Late to add anything but I agree with @Oso