Chemistry

B. L. Dyatkin, E. P. Mochalina, and Academician I. L. Knunyants

Condensation of Formaldehyde with Perfluoroolefins—Tetrafluoroethylene, Hexafluoropropylene, and Trifluorochloroethylene

A consequence of the electrophilic character of the double bond in fluoroolefins is, as is known, the exceptional ease of their reactions with various nucleophilic reagents—alcohols, mercaptans, amines, organomagnesium compounds, and others. At the same time, reactions with electrophilic reagents that are typical of hydrocarbon olefins are greatly impeded and therefore have been studied to a considerably lesser extent. Such reactions include, in particular, condensation with formaldehyde in the presence of strong acids, which, in the case of ordinary olefins, leads either to unsaturated alcohols or to 1,3-glycols or their derivatives—meta-dioxanes (the Prins reaction):

\[ \ce{CH2O + HCR'=CR''R''' ->[H^+] HOCH2CR'=CR''R'''} \]

\[ \ce{CH2(OH)2 + CRR'=CR''R''' ->[H^+] HOCH2CRR'-CR''R'''OH ->[CH2O]} \quad \begin{array}{c} \ce{CRR'}\\[-2pt] /\ \ \backslash\\[-2pt] \ce{H2C}\quad \ce{CR''R'''}\\[-2pt] \ce{O}\quad\ce{O}\\[-2pt] \backslash\ /\ \\[-2pt] \ce{CH2} \end{array} \]

If \(R''\) and \(R'''\) are halogens, then β-hydroxy acids or their derivatives are formed, for example \((^1)\):

\[ \ce{CCl2=CCl2 + CH2O + H2O -> [HOCH2CCl2CCl2OH] ->[H2O] HOCH2CCl2COOH.} \]

In 1949, Coffman, Raasch, and co-workers carried out the condensation of formaldehyde with tetrafluoroethylene in the presence of 80% sulfuric acid and obtained, in low yield, \(\alpha,\alpha\)-difluorohydracrylic acid, isolated in the form of its ethyl ester \((^{2,3})\). In 1952, McBee and co-workers repeated this reaction and obtained the ethyl ester of \(\alpha,\alpha\)-difluorohydracrylic acid in 20% yield \((^4)\).

Using 1,2-difluoro-1,2-dichloroethylene as an example, we established that in this case chlorosulfonic and fluorosulfonic acids are very effective catalysts of the Prins reaction, whereas in the presence of sulfuric acid of various concentrations the reaction could not be carried out \((^5)\). It seemed of interest to study these catalysts further and to apply them in extending the Prins reaction to a number of other fluorine-substituted olefins. In the present work we report results on the study of the interaction of formaldehyde with tetrafluoroethylene, hexafluoropropylene, and trifluorochloroethylene in the presence of chlorosulfonic acid.

It turned out that when a mixture of tetrafluoroethylene, paraformaldehyde, and chlorosulfonic acid is heated to \(100^\circ\), condensation proceeds according to the general scheme of the Prins reaction, with formation of \(\alpha,\alpha\)-difluorohydracrylic acid, isolated in the form of its ethyl ester. The yield of the latter was 62.6%, which was a quite clear confirmation of the greater activity of chlorosulfonic acid in comparison with sulfuric acid.

Condensation of paraformaldehyde with hexafluoropropylene in the presence of chlorosulfonic acid at 130–150° gives, in 41% yield, α-fluoro-α-trifluoromethylhydracrylic acid:

\[ \mathrm{CF_2{=}CFCF_3 + CH_2O + H_2O \rightarrow [HOCF_2{-}CFCH_2OH] \rightarrow HOCH_2CFCOOH} \]
\[ \begin{array}{cc} & \mathrm{|} \\ & \mathrm{CF_3} \end{array} \qquad \begin{array}{cc} & \mathrm{|} \\ & \mathrm{CF_3} \end{array} \]

The formation of this compound is consistent with the polarization of the double bond in hexafluoropropylene,

\[ \overset{\delta+}{\mathrm{CF_2}}=\overset{\delta-}{\mathrm{CF}}-\mathrm{CF_3}. \]

The double bond of trifluorochloroethylene, \(\mathrm{CF_2{=}CFCl}\), is polarized in such a way that the partial negative charge is located on the carbon atom of the CFCl group. This polarization has been demonstrated by numerous nucleophilic-addition reactions to \(\mathrm{CF_2{=}CFCl}\), as a result of which, ultimately, only derivatives of fluorochloroacetic acid are formed \((^{6-8})\). Thus one should have expected that condensation of formaldehyde with trifluorochloroethylene would lead to α-fluoro-α-chlorohydracrylic acid:

\[ \mathrm{HOCH_2^{+}+\overset{\delta-}{CFCl}=\overset{\delta+}{CF_2} \rightarrow [HOCH_2CFClCF_2] \xrightarrow{+\,H_2O} HOCH_2CFClCOOH.} \]

This acid, in the form of its ethyl ester, was indeed isolated in a yield of 19.8% of theory. However, along with this, ethyl α,α-difluorohydracrylate was obtained in a yield of 30.5%; its formation cannot be explained otherwise than as the result of electrophilic attack on the \(\mathrm{CF_2}\) group, i.e., on the carbon atom that is less negatively polarized than the carbon atom of the CFCl group in trifluorochloroethylene.

Earlier, Knunyants, Shokina, and Li observed the phenomenon of dual orientation in the addition reaction of iodine chloride to trifluorochloroethylene \((^9)\):

\[ \mathrm{CF_2{=}CFCl + JCl \rightarrow CF_2J{-}CFCl_2 + CF_2Cl{-}CFClJ.} \]

However, it remains unknown whether this reaction proceeds by an ionic or a radical mechanism. In the case of condensation with formaldehyde, orientation opposite to the polarity is observed for an evidently ionic process—the reaction of electrophilic addition at a multiple bond. Apparently, this phenomenon may be explained by competition between polar and steric factors. The effective radius of \(\mathrm{F}\) is 1.25 Å, and that of \(\mathrm{Cl}\) 1.58 Å; thus the direction of the reaction corresponding to the polarity encounters greater steric hindrance. Meanwhile, for nucleophilic-addition reactions to trifluorochloroethylene the action of the polar and steric factors coincides, and therefore the orientation in these reactions should be, and is, strictly unambiguous.

Experimental Part

Condensation of formaldehyde with tetrafluoroethylene. Ethyl ester of α,α-difluorohydracrylic acid. Into a steel autoclave of 250 ml capacity were placed 14.5 g (0.48 mole) of paraformaldehyde and 140 g (1.2 mole) of chlorosulfonic acid. After evacuation and cooling with liquid nitrogen, 35 g (0.35 mole) of tetrafluoroethylene was condensed into the autoclave, after which the mixture was shaken at 100° for 7 h. After cooling, the reaction mass was poured onto 100 g of ice. The organic layer was separated and treated with sodium carbonate solution until evolution of carbon dioxide ceased. The substance insoluble in the carbonate solution was extracted with ether and discarded, and the aqueous portion was combined with the previously separated aqueous layer (the medium was strongly acidic) and continuously extracted with ether for several hours. The ethereal extract was dried over magnesium sulfate, and the ether was distilled off (finally under vacuum). The resulting α,α-difluorohydracrylic acid was esterified with ethyl alcohol in the presence of a catalytic amount of fluorosulfonic acid, with azeotropic removal of water with dichloroethane. Obtained were 33.8 g (62.6%)

ethyl ester of α,α-difluorohydracrylic acid with b.p. 66–69° at 6 mm Hg. By repeated distillation a substance was obtained with b.p. 69.5° at 7 mm Hg, $n_D^{20}$ 1.3840, $d_4^{20}$ 1.2612. $MR$ found 28.57; calculated for $\mathrm{C_5H_8F_2O_3}$ 28.75. Literature data ($^3$): b.p. 58–61° at 6 mm Hg, 181° at 760 mm Hg, $n_D^{25}$ 1.3830. The number $H_{\text{act}}$ found 1.02; 1.03; calculated for $\mathrm{C_5H_8F_2O_3}$ 1.00.

Amide of α,α-difluorohydracrylic acid was obtained by treating the ester with an aqueous-alcoholic solution of ammonia. M.p. 98–99° (from water).

$\mathrm{C_3H_5F_2NO_2}$. Found, %: C 28.56; 28.63; H 3.97; 4.00; F 31.39; 31.30; N 10.70; 10.70
Calculated, %: C 28.81; H 4.02; F 30.38; N 10.20

Condensation of formaldehyde with hexafluoropropylene. α-Trifluoromethyl-α-fluorohydracrylic acid. A mixture of 28 g (0.186 mole) of hexafluoropropylene, 10.7 g (0.36 mole) of paraformaldehyde, and 75 g (0.64 mole) of chlorosulfonic acid was shaken in a steel autoclave of 250 ml capacity at 140–150° for 7 h. After cooling, the unreacted perfluoropropylene (11 g) was distilled off, the reaction mass was poured into a mixture of 150 g of potassium chloride and 100 ml of conc. hydrochloric acid; the resulting mixture was boiled with stirring for 4 h. After cooling to room temperature, the precipitate was filtered off and washed with water. The filtrate was continuously extracted with ether for several hours, the extract was dried over magnesium sulfate, and the ether was distilled off (at the end, in vacuum). A little dry benzene was added to the residue on cooling. 8.4 g (41.3%, based on the perfluoropropylene that reacted) of α-trifluoromethyl-α-fluorohydracrylic acid was obtained, m.p. 107–108° (from benzene). Neutralization equivalent found 174.3; 178.1; calculated for $\mathrm{C_4H_4F_4O_3}$ 176.1.

$\mathrm{C_4H_4F_4O_3}$. Found, %: C 27.46; 27.21; H 2.25; 2.44; F 43.50; 43.10
Calculated, %: C 27.28; H 2.28; F 43.20

Aniline salt, m.p. 164–165° (from a benzene–alcohol mixture).

$\mathrm{C_{10}H_{11}F_4O_3N}$. Found, %: N 5.26; 5.35
Calculated, %: N 5.20

Ethyl ester of α-trifluoromethyl-α-fluorohydracrylic acid was obtained in 61% yield by esterifying the acid with ethanol in the presence of boron trifluoride etherate. B.p. 76.5–77° at 11 mm Hg, $n_D^{20}$ 1.3650, $d_4^{20}$ 1.3590. $MR$ found 33.55; calculated 33.64.

$\mathrm{C_6H_8F_4O_3}$. Found, %: C 35.17; 35.21; H 3.93; 3.81
Calculated, %: C 35.31; H 3.95

Amide of α-trifluoromethyl-α-fluorohydracrylic acid was obtained from the ethyl ester by the action of an aqueous-alcoholic solution of ammonia. M.p. 117–118° (from benzene).

$\mathrm{C_4H_5F_4O_2N}$. Found, %: F 42.72; 43.04; N 8.44; 8.32
Calculated, %: F 43.44; N 8.00

Condensation of formaldehyde with trifluorochloroethylene. A mixture of 45 g (0.39 mole) of trifluorochloroethylene, 14.2 g (0.48 mole) of paraformaldehyde, and 148 g (1.27 mole) of chlorosulfonic acid was shaken at 120–130° for 7 h. After cooling, the reaction mass was treated as in the case of tetrafluoroethylene. As a result of fractionation, the following were obtained:

1. Ethyl ester of α,α-difluorohydracrylic acid. 18.2 g (30.5%), b.p. 64–64.5° at 5 mm Hg, $n_D^{20}$ 1.3862. From the ester the amide was obtained

$\alpha,\alpha$-difluorohydracrylic acid, with the melting point and the melting point of a mixed sample with an authentic specimen being 98–99°.

1. Ethyl ester of $\alpha$-fluoro-$\alpha$-chlorohydracrylic acid. 13 g (19.8%), b.p. 82° at 6 mm Hg, $n_D^{20}$ 1.4249, $d_4^{20}$ 1.3032. $MR$ found 33.47; calculated 33.48.

\[ \begin{aligned} &\mathrm{C_5H_8FClO_3}. &&\text{Found \%: } \mathrm{C}\ 35.41;\ 35.52;\ \mathrm{H}\ 4.78;\ 4.78;\ \mathrm{F}\ 11.59;\ 11.91\\ &&&\text{Calculated \%: } \mathrm{C}\ 35.22;\ \mathrm{H}\ 4.73;\ \mathrm{F}\ 11.14 \end{aligned} \]

Amide of $\alpha$-fluoro-$\alpha$-chlorohydracrylic acid was obtained from the ethyl ester by the action of an aqueous-alcoholic solution of ammonia. M.p. 101–102° (from dichloroethane).

\[ \begin{aligned} &\mathrm{C_3H_5FClNO_2}. &&\text{Found \%: } \mathrm{C}\ 25.57;\ 25.52;\ \mathrm{H}\ 3.80;\ 3.70;\ \mathrm{F}\ 14.31;\ 14.72\\ &&&\text{Calculated \%: } \mathrm{C}\ 25.46;\ \mathrm{H}\ 3.56;\ \mathrm{F}\ 13.43 \end{aligned} \]

Institute of Organoelement Compounds
Academy of Sciences of the USSR

Received
23 III 1961

REFERENCES

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