Cou
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Preparation of sec-butyl propionate with H2SO4 catalyst
INTRODUCTION
The lab procedure in Vogel's Textbook of Practical Organic Chemistry, 5th ed, calls for use of anhydrous HCl as the catalyst when preparing sec-butyl
acetate by Fischer esterification (1). HCl gas would be generated and bubbled into the alcohol until saturated. Presumably HCl is used instead of
H2SO4 to avoid dehydration of the butan-2-ol to but-2-ene.
I questioned if this extra step of gas generation is really necessary, when I could just use sulfuric acid catalyst as usual for Fischer
esterifications, which is more convenient (Scheme 1). Secondary alcohols are generally compatible with sulfuric acid in esterifications, unlike
tertiary alcohols, which are highly prone to elimination to an alkene. Even if any but-2-ene was formed, it would be easily removed in simple
distillation because its boiling point is only 3.7 °C (2).
Esterification of secondary alcohols require long reflux times, over 10 hours, due to steric bulk of the alcohol slowing the reaction (1).
Scheme 1. Preparation of sec-butyl propionate.
METHODS
To a 50 mL round bottom flask was added sec-butyl alcohol (20. mL / 0.22 mol), propionic acid (20. mL / 0.27 mol), and with rapid stirring, conc.
sulfuric acid (0.7 mL / 0.01 mol / 5% mol equiv). The mixture was refluxed for 10 hours.
After reflux, the mixture was poured into a separatory funnel. First it was washed with a large arbitrary volume of water (about twice the volume).
Next, the ester was washed with saturated sodium carbonate solution until gas production stopped. Next, it was washed with an equal volume of water.
Lastly, it was washed with saturated sodium chloride. The crude ester was dried with anhydrous CaCl2 pellets in a beaker, then transferred into a 50
mL round bottom flask. The ester was purified by simple distillation.
RESULTS
The ester boiled 132.7 °C (literature value 133 °C) (3). I measured its density at 0.8412 g/mL (lit. value 0.8633 g/mL) (4). Yield was 14.30 g /
17.00 mL / 0.1098 mol / 49% yield (I did not use an excess of either reactant because both are equally valuable for me).
Its aroma is the same as sec-butyl acetate. Ethereal, solventy, grapey, nothing special.
CONCLUSION
Sulfuric acid can be used as a catalyst to prepare esters of sec-butyl alcohol.
While it has been shown that sec-butyl esters can be prepared with H2SO4 catalyst, I haven't used yield-increasing techniques to show that a high
yield is possible. I will try that soon with another sec-butyl ester, probably sec-butyl butyrate.
If preparing esters of longer-chain secondary alcohols, such as pentan-2-ol, could there be a risk of rearrangement, such as formation of pentan-3-yl
esters? H2SO4 adsorbed onto anhydrous MgSO4 can be used as a catalyst to prepare tert-butyl esters(5). This reaction is unsuccessful with H2SO4 alone.
The reaction is initiated by dehydration of tert-butyl alcohol followed by addition of carboxylic acid to the resulting isobutylene. If pentan-2-ol
dehydrates to pent-2-ene, could there be a risk of the carboxylic acid adding to this alkene, resulting in a side product of pentan-3-yl acetate
(Scheme 2)? Without MgSO4 it's unlikely.
Why do you think Vogel's called for HCl as a catalyst, when it seems unnecessary? It also calls for HCl as the catalyst in formation of cyclohexyl
esters, and says use of H2SO4 results in a "very impure product". Could that be unnecessary too?
Scheme 2. Hypothetical rearrangement in Fischer esterification of pentan-2-ol with acetic acid.
REFERENCES
1) Furniss, B.S.; Hannaford, A.J.; Smith, P.W.G.; Tatchell, A.R.; Aliphatic Compounds. Vogel's Textbook of Practical Organic Chemistry, 5th ed;
Longman Scientific & Technical: New York, NY, 1989; pp 700-1.
2) GESTIS Substance Database; Physical and Chemical Properties; ZVG No. 490242; http://gestis-en.itrust.de/nxt/gateway.dll/gestis_en/490242....
3) PhysProp Database.
4) (2) Zavgorodnii, S. V.; Trudy Voronezh. Gosudarst. Univ. (Acta Univ. Voronegiensis) 1938, V10(No. 2), P41-99.
5) Otera, J; Nishikido, J; Reaction of Alcohols with Carboxylic Acids and their Derivatives. Esterification: Methods, Reactions, and Applications, 2nd
ed; Wiley-VCH: Weinheim, Germany, 2010; pp 10.
[Edited on 9-24-2020 by Cou]
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HeYBrO
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Good job on the synthesis, seems like you can almost open a perfume factory with all of these esters!
Just some general comments: not sure what is happening with your conclusion- did your experiment prove your hypothesis? State it clearly with the
implications- sorry I just found it a bit confusing with the all the questions. Also, your propionic acid molar quantity has a typo (mol not "mL"),
and with your catalyst you either give mol % or equivalents, there isn't such a thing as "% mol equiv". Normally it is convention to put the BP and
yield in the method not the results. BTW, if you use stoichiometric excess of the acid, you should be able to recover the salt after base wash and
then re-acidify and distill to recover it.
Nice!
[Edited on 23-9-2020 by HeYBrO]
[Edited on 23-9-2020 by HeYBrO]
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Cou
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This was just a test run to see if the reaction would even work, so I didn't bother with techniques to improve yield.
I didn't think it was practical to recover the excess acid, but now I see how it can be done. Boiling down the sodium propionate salt to a solid would
evaporate off all alcohols, then you could follow the procedure for making glacial acetic acid from sodium acetate. I'll be trying this again with
sec-butyl butyrate, except with a large excess of acid to check if yield is still reduced with H2SO4.
Edit: with carboxylic acids butyric acid or heavier, you can use a soxhlet extractor with molecular sieves to extract water from a toluene azeotrope,
which increases yield a great amount. Nurdrage has a video on this
[Edited on 9-23-2020 by Cou]
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Pumukli
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Cyclohexanol is a bit different from other secondary alcohols because it easily dehydrates to cyclohexene. Refluxing it with sulfuric acid can
definitely lead to this. This might be the reason Vogel said it can yield an impure product.
I found that refluxing cyclohexanol with phosphoric acid is a cleaner route to cyclohexene, the reaction mixture is lighter brown. (With sulfuric:
it's almost black.)What happens with HCl I don't know.
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Fery
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Hi Cou, I do not want to open separate thread but I know how much you are concerned about tertiary alcohol esterification, so I post it here, found
something interesting in a book available from here:
http://rushim.ru/books/praktikum/Monson.pdf
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