Method for preparing acrolein cyanohydrins

1. A method for preparing a compound of formula (I)

the method comprising:

(a) reacting at least one compound of the formula (II)

with hydrocyanic acid and at least one base B, whereby a crude product CP comprising the compound of formula (I) and hydrocyanic acid is obtained; and

b) at least partially removing hydrocyanic acid from the crude product CP by subjecting the latter to stripping, whereby a pure product comprising the compound (I) is obtained, wherein the pure product has a reduced content of hydrocyanic acid compared to CP,

wherein the stripping is carried out at a pressure of <1 bar,

wherein optionally an inert stripping gas is used in countercurrent flow, and

wherein R 1 and R 2 are each independently hydrogen, an alkyl group, phenyl, or benzyl.

2. The method according to claim 1 , wherein

R 1 and R 2 are each independently hydrogen, an alkyl group having 1 to 6 carbon atoms, phenyl, or benzyl.

3. The method according to claim 1 , wherein

R 1 and R 2 are each hydrogen.

4. The method according to claim 1 , wherein

the base B is at least one member selected from the group consisting of ammonia, a trialkylamine, and an ammonium salt.

5. The method according to claim 1 , wherein

said a) reacting is conducted at a temperature of from 50° C. to 80° C.

6. The method according to claim 1 , wherein

the hydrocyanic acid in said a) reacting is present in an amount of >1 molar equivalent, based on the amount of all compounds of the formula (II) present in said a) reacting.

7. The method according to claim 1 , wherein

the amount of base B in said a) reacting is 0.01 to 5% by weight, based on the sum of the weights of hydrocyanic acid present in said a) reacting and all compounds of the formula (II) present in step said a) reacting.

8. The method according to claim 1 , wherein

the crude product CP is mixed prior to the at least partially removing of b) with at least one acid so that, after mixing with the acid, CP has a pH of <6.9.

9. The method according to claim 8 , wherein

the at least one acid has a pKa of ≤1.

10. The method according to claim 8 , wherein

the acid is at least one member selected from the group consisting of mineral acid, an alkylcarboxylic acid, and an aromatic sulfonic acid.

11. The method according to claim 1 , wherein

a temperature of ≤85° C. is maintained during the stripping in the at least partially removing of b).

12. The method according to claim 1 , wherein

the crude product CP during the stripping in the at least partially removing of b) is passed at least partially over a bed of packing elements or over a structured packing.

13. The method according to claim 1 , further comprising, after b):

c) reacting the pure product comprising the compound (I) obtained in b), wherein the pure product has a reduced content of hydrocyanic acid compared to CP, with a compound of the formula (IV)

to obtain a compound of the formula (III)

wherein R 3 , R 4 are each independently an (halo)alkyl group, an (halo)aryl group, a (halo)aralkyl group, or a (halo)cycloalkyl group,

wherein X is oxygen or sulfur, and

wherein n=0 or 1.

14. The method according to claim 13 , wherein the reaction in said c) reacting is conducted at a temperature of 50° C. to 105° C.

15. The method according to claim 13 , wherein

each of R 3 and R 4 is, independently, a (halo)alkyl group having 1 to 12 carbon atoms, a (halo)aryl group having 6 to 10 carbon atoms, a (halo)aralkyl group having 7 to 10 carbon atoms, or a (halo)cycloalkyl group having 4 to 10 carbon atoms.

16. The method according to claim 13 , wherein

at least one free-radical-forming substance is present in said c) reacting.

17. The method according to claim 16 , wherein

the free-radical-forming substance is a free-radical initiator of the formula (V)

wherein R 5 is methyl, ethyl, 2,2-dimethylpropyl, or phenyl,

wherein R 6 , R 7 are each independently an alkyl group having 1 to 10 carbon atoms, and

wherein R 8 is hydrogen or an alkyl group having 1 to 10 carbon atoms.