CH646681A5 - PROCESS FOR THE PREPARATION OF DI-N-PROPYLACETIC ACID AND ITS SALTS. - Google Patents

Description

The main object of the present invention is to eliminate these two disadvantages. The process according to the invention consists, essentially, in eliminating the use of sodium nitrite, by simply determining the formation of dipropylacetic acid by further hydrolysis, but with lowering of the sulfuric acid concentration of the reactive environment, according to the diagram:

20

25

c3h7

2 \: h-conh2 /

c3h7

c3h7

2 \: h-cooh + (nh4) 2 so4

h2s04 q TT

(60%) L'37

year)

Swiss Patent No. 621108 relates to a process for the preparation of dipropylacetic acid, from dipropylaceto-nitrile, according to a reaction scheme comprising three successive stages.

- In a first step, hydrolysis of the dipropyl-acetonitrile is carried out in a concentrated sulfuric medium, to obtain the dipropyl-acetamide:

C, H,

\ h2o

; ch-c e = n!

/ h + (80%)

c3h7>

>

c3h7 c3h7

- In a second step, without intermediate isolation, the dipropylacetamide is subjected to a diazotization with sodium nitrite, in an aqueous sulfuric medium:

In other words, the method according to the present invention. after step (I) of the aforementioned patent, to introduce into the reaction medium the quantity of water necessary and sufficient to bring the concentration of sulfuric acid to 60%, and to continue heating at reflux, preferably around 125-128 ° C, for about 4 to 6 h, which determines the quantitative transformation of the amide into acid.

In addition to eliminating the nitrite and simplifying the working conditions, this process makes it possible to obtain the dipropylacetic acid free from the starting nitrile and especially from dipropylacetamide. As will be seen in more detail in the example which follows, the productivity concerning the production of dipropylacetic acid is multiplied by 2.5 40 and the yield of the reaction increases from 92 to 98%.

Furthermore, the present invention aims to improve this stage of the process, making it possible to achieve an even purer acid.

This improvement consists in replacing the rectification of the reaction mixture by a steam stripping of the acid. 45 It has thus been found that this steam entrainment plays a selective role with respect to the residual impurities and, in particular, not only of the nitrile and of the residual amide, but also of the valero-nitrile and of the valeric acid. the content of which may be reduced by half.

In addition, steam entrainment is preferable to rectification, since it involves less investment and represents an H — CO — NH2 (I) economic advantage.

The following three examples, given by way of illustration, provide some operational details confirming the interest of the invention:

c3h,

c3h7 c3h7 <

NOT /

CH-CO-NH2 + NaN02 + H2SO,

(II)

\

c3h7

/

H-COOH + N2 H20 + NaHS04

Example 1 (laboratory scale):

In a Pyrex reactor of 1000 cm3, equipped with an agitator, a thermometer, an introductory bulb replaceable with 60 dip tube and a right reflux condenser modifiable in distillation at atmospheric pressure, maintained in a bath controlled by thermostat, 72 g of purified water are placed, then slowly introduced with stirring 478 g of 92% sulfuric acid, limiting the temperature to 60 ° C.

65 125.2 g of dipropylaceto-nitrile (1 mol) are then introduced at one time, the temperature is brought to 100 ° C. and maintained there for 1 h, after which time the hydrolysis of the nitrile to amide is practically total.

185 g of purified water are also introduced at once, bringing the concentration of sulfuric acid to 60%. The mixture is brought to reflux (125-130 ° C) and the reflux is maintained for 5 h.

The condensation system is then modified in a distillation apparatus at atmospheric pressure, and the entrainment is carried out by introduction of steam by the plunger. The process is continued for 2 Zi h, which corresponds to the cessation of demixing in the distillate, and to a consumption of entrainment water of 1750 g.

In this respect, the operation is preferably carried out at a constant volume in the reactor, which improves the productivity and makes it possible to complete the hydrolysis. The decantation is perfectly clean, which makes it possible to limit the volume of organic phase collected, by carrying out discontinuous withdrawals of the aqueous phase, and to avoid the use of an extraction solvent.

The organic phase is dried under vacuum by azeotropy under vacuum (20 mmHg, 20 ° C). 138.5 g of dipropylacetic acid are collected (yield 96.2%), which can be further purified by flash distillation.

Example 2 (pilot scale):

In an enamelled reactor, equipped with a 2001 gauging tank under load, a glass superstructure allowing reflux, an agitator and a bottom valve, receiving water vapor under pressure 6 kg / cm2, water and brine, 0.657 kg of purified water is charged and then, with stirring and maintaining the temperature at 50-60 ° C, 2.0851 (3.753 kg) of 94% sulfuric acid.

The concentration of the acid, which must be 80 ± 1% by weight, is checked after 15 min of stirring. The temperature is then brought to 100 ° C., and 1 kg of dipropylacetonitrile N is introduced, while controlling the reaction exothermicity by circulation of water in the jacket of the reactor, so as to maintain the temperature at 100 ° C.

At the end of the introduction, the temperature and stirring are maintained for 1 h, then 1.470 kg of purified water are added, while still maintaining the temperature by slight heating, which brings the concentration of sulfuric acid at 60%.

Then heated to 125-128 ° C to bring to reflux, which is maintained for 6 h.

It is cooled to approximately 30 ° C. and the stirring is stopped, and it is left to settle for 30 min, then the lower sulfuric mother liquors, which represent 3.51, or approximately 5 kg, are drawn off via a flask. of glass serving as a viewfinder, with the possibility of phlegm. These mother liquors have an acidimetric titer close to 59%, expressed as sulfuric acid. It is then washed at approximately 30 ° C for 30 min using 1.333 kg of purified water, it is decanted for 30 min and the lower aqueous phase (1.4 kg) is drawn off, then the phlegm (of variable importance) in view of their filtration to recover the dipropylacetic acid, and finally the organic phase consisting of dipropylacetic acid with approximately 1% water (1.12 kg, ie a yield of 97-98%).

It was possible, on an experimental basis, to control the efficiency of the steam entrainment stage on the purity of the acid obtained with respect to the presence of valeric acid formed in the second stage. The following example summarizes the conditions for such a check.

646,681

Example 3:

An experimental medium at the end of hydrolysis (end of step III) is reconstituted in the following manner:

- Dipropylacetic acid 144 g (1 mol) with valeric acid content = 0.35%.

- 60% sulfuric acid, 730 g.

To this medium are added, for the purposes of the experiment, known quantities of valeric acid, so as to bring its concentration to various values, ranging from 0.35 to 1%.

This medium is subjected to a steam entrainment, keeping the volume constant and the temperature at 130 ° C. The volume of water required for training is 1800 cm3 per mole driven. The yield of isolated dipropylacetic acid is practically quantitative.

Decantation is then carried out at ambient temperature and drying over Na2SO4, and the content of valeric acid is checked analytically.

The results are collated in Tables A and B.

Table A gives, for four tests, for variable initial contents of valeric acid, the corresponding final contents.

Table B allows, on the same test, corresponding to an initial content of 1% of valeric acid, to follow the evolution of the valeric acid content of the product collected by entrainment.

It appears from these results that the steam entrainment according to the present additive makes it possible to lower the content of valeric acid by approximately 50%. This characteristic is of primary importance when we consider that the slightest traces of valeric acid can render the final product unusable because of its odor.

Table A

Test No

Initial content of valeric acid (%)

Valeric acid content of dipropylacetic acid collected by entrainment (%)

1

0.35

0.2

2

0.60

0.3

3

0.80

0.4

4

1.00

0.5

Table B

Initial content of crude acid before training: 1% = 1.44 g

Successive fractions

Weight of fraction (g)

Title of product collected in valeric acid (%)

Corresponding weight of valeric acid (g)

I

36

0.9

0.324

II

34

0.6

0.204

III

42

0.2

0.084

IV

32

0.1

0.032

Total

144

0.644

Recovery yield of valeric acid: 44.72%.

3

5

10

15

20

25

30

35

40

45

50

R

Claims (2)

646 681 2 CLAIMS 1. Process for the preparation of di-n-propylacetic acid of formula: O ch3-ch2-ch2 ch3-ch2-ch2 \ / CH-C-OH and its non-toxic alkali and alkaline earth metal salts, characterized in that: - Sodium n-propylate in n-propanol medium is added in a single step to a reaction medium consisting of a cyanacetic ester of general formula: CN h2c COOR in which R represents an alkyl radical having from 1 to 4 carbon atoms, and n-propyl bromide or iodide, the alkyl ation reaction taking place at reflux, the crude ester obtained is saponified with a solution of sodium or potassium hydroxide from 10 to 20%, the salt thus obtained is acidified with a strong acid, which gives the crude di-n-propylcyanacetic acid which is decarboxylated by heating at a temperature between 140 and 190 ° C, which provides the di-n-propylacetonitrile, - the di-n-propylacetonitrile obtained is hydrolyzed using an aqueous sulfuric acid solution of 75 to 80% at a temperature of 80 to 140 ° C to obtain the crude di-n-propylacetamide which is hydrolyzed using an aqueous solution of sulfuric acid, the concentration of which is lowered to 60% by adding water and heating at reflux for 4 to 6 h, which provides the di-n-propylacetic acid which is made react, if desired, with an alkali metal hydroxide or an alkaline earth metal oxide to obtain the corresponding salt. 2. Method according to claim 1, characterized in that, at the end of the hydrolysis, the dipropylacetic acid formed is recovered by steam entrainment. - In a third step, the dipropylacetic acid is collected by rectification of the reaction mixture. This process has several drawbacks: - Firstly, it involves the use of sodium nitrite, a toxic compound (LD50 p.o. on rats: 180 mg / kg). In addition, this use of nitrite leads to the presence of nitrites and nitrates in the final product which affect the purity of this product, in addition to valeric acid, resulting from the presence of valero-nitrile as an impurity in the starting product. , which contributes to a very unpleasant odor of the final product. - Secondly, the operating conditions in a concentrated sulfuric medium affect both the yield and the productivity of the process.