The Borderline between Reactivity and Pre‐reactivity of Binary Mixtures of Gaseous Carboxylic Acids and Alcohols

Abstract

By mixing primary and secondary alcohols with carboxylic acids just before the supersonic expansion within pulsed Fourier transform microwave experiments, only the rotational spectrum of the ester has been observed. However, when formic acid was mixed with tertiary alcohols, adducts have been formed and their rotational spectra have been easily measured. Quantum mechanical calculations have been performed to interpret the experimental evidence. In the present study esterification takes place without catalyst.To attain a satisfactory conversion a catalyst is generally needed, and yet the employment of one of the reactants in excess is necessary. [2] We discovered a very simple method to obtain esters from a gas phase 1:1 mixture of carboxylic acids and primary and secondary alcohols without need of catalyst. The details are reported below.Several molecular complexes involving carboxylic acids [3][4][5][6][7][8][9][10][11][12] have been investigated by rotational spectroscopy, in order to understand the nature of their non-covalent interactions and to have information on their internal dynamics and on their conformational equilibria. Most attention has been paid to the complexes of carboxylic acids, mainly to their dimers [3] and to their adducts with water. However, no MW studies of complexes between carboxylic acids and alcohols are reported. For this reason, we thought to investigate the prototype of this kind of complex, that is formic acid-methyl alcohol. Unexpectedly, we were not able to assign its rotational spectrum. Such a failure could be due, among to other reasons, to the complications related to the low V 3 barrier underlying the barrier to internal rotation of the methyl group, as well to the inversion of the methyl group from above to below the formic acid plane. In order to understand what was going on, we decided to start the investigation of the adducts carboxylic acids-alcohols from the adduct formic acid with a series of primary, secondary and tertiary alcohols. In details, we made supersonic expansions, with ca. 1% of carboxylic acid and 1% of alcohol in He for the following combinations: HCOOH-CH 3 OH, HCOOH-C 2 H 5 OH, HCOOH-(CH 3 ) 2 CHOH and HCOOH-(CH 3 ) 3 COH, that is formic acid mixed with methyl alcohol and with primary, secondary and tertiary alcohols, respectively.In all cases but the last one, it was not possible to observe the spectra of the adducts, but strong rotational transitions that we discovered to belong to the esters. Then we replaced linear alcohols with cyclic alcohols, like cyclohexanol (secondary) and 1-methylcyclopropanol (tertiary). Again, for the secondary alcohol we could observe only the rotational spectrum of the ester, and for the tertiary alcohol only that of the adduct. Finally, we exploited the replacement of HCOOH with carboxylic acids with stronger (CF 3 COOH) and weaker (pivalic acid) acidity. In the first case we observed only the ester, while in the second case the experiment did not succeed, because pivalic acid was rapidly obstructing our nozzl...

The Barrier to Proton Transfer in the Dimer of Formic Acid: A Pure Rotational Study

“…Plenty of data have been obtained on the hydrogen‐bonded dimers involving proton tunneling, the Ubbelohde effect and conformational equilibria . HCOOH (formic acid; FA) is the prototype of the carboxylic acids family and for this reason it is involved in most of the investigations of carboxylic acids bonded to molecules containing other functional groups, such as its adducts with H 2 O, N(CH 3 ) 3 , CO 2 , the simplest aldehyde CH 2 O, the simplest amide H(CO)NH 2 , ethers, ketones, freons, azines and its reactivity with alcohols . An interesting feature of the dimer of FA, (FA) 2 , is the concerted transfer of the two hydroxylic protons, a motion in a double minimum potential.…”

“…[1] HCOOH (formic acid; FA)isthe prototype of the carboxylic acids family and for this reason it is involved in most of the investigations of carboxylic acids bonded to molecules containing other functional groups, such as its adducts with H 2 O, [2a] N(CH 3 ) 3 , [3] CO 2 , [4] thesimplest aldehyde CH 2 O, [5] thesimplest amide H(CO)NH 2 , [6] ethers, [7] ketones, [8] freons, [9] azines [10] and its reactivity with alcohols. [11] An interesting feature of the dimer of FA,( FA) 2 ,i st he concerted transfer of the two hydroxylic protons,amotion in ad ouble minimum potential. It can generate tunneling doubling observable in molecular spectra, and makes them useful to studies of the tunneling process.T he precise tunneling splittings (DE 01 )m easured by microwave (MW) spectroscopy for the acrylic acid dimer [1g] and for the formic acid-benzoic acid bi-molecule [1h] have been reproduced by model calculations with barriers at or close to values obtained ab initio.In Figure 1w es how the proton transfer motion in the formic acid dimer along as ymmetric double minimum potential energy profile.…”

Interactions between Ketones and Alcohols: Rotational Spectrum and Internal Dynamics of the Acetone–Ethanol Complex

“…With water [4] and HCl, [5] the OH groups in alcohols have been found to act as proton acceptors,f orming OÀH···O or ClÀH···O HBs.W ith ammonia, [6] amines, [7] ethers, [8] and aldehydes, [9] the OH groups in alcohols act as proton donors. [11] Ar otationals tudy of the ethanol-cyclobutanonec omplex showedt hat the alcohol plays ad ual role as protond onor and acceptor, forming an etwork of one OÀH···O HB and two CÀH···O weakHBs. [10] In heterodimers, it seems that there is preference for the role of protond onor/acceptor:O nly two isomers with ethanolacceptor/methanol-donor structuresh ave been observed, which suggests ethanol-donor-methanol-acceptor interactions are energetically disfavored.…”

“…Interesting observations have been made in the rotational spectra of the complexes formed between alcohols and carboxylic acids: By mixing primary and secondary alcohols with carboxylic acids just before the supersonic expansion only the rotational spectra of the esters, which are formed by gaseous esterification, were observed. However, when formic acid was mixed with tertiary alcohols, adducts were formed and their rotational spectra could be easily measured . A rotational study of the ethanol–cyclobutanone complex showed that the alcohol plays a dual role as proton donor and acceptor, forming a network of one O−H⋅⋅⋅O HB and two C−H⋅⋅⋅O weak HBs …”

Carboxylic Acids, Reactivity with Alcohols and Clustering with Esters: A Rotational Study of Formic Acid–Isopropylformate

“…We have to outline, however, that we came to this investigation accidentally. When trying to assign the pulsed jet Fourier transform microwave (FTMW) spectrum of the adduct FA-2-propanol by expanding a binary mixture of HCOOH and 2-propanol in He, we discovered that isopropylformate was formed by the esterification reaction instead . However, when HCOOH was in excess in the binary mixture, it was possible to observe and assign the rotational spectrum of FA-IPF.…”