Method for producing polyalcohols

A process for the preparation of a methylolalkanal of the formula II

• wherein R is a methylol group or an alkyl group of from 1 to 22 carbon atoms or an aryl of aralkyl group of from 6 to 22 carbon atoms, by condensation of aldehyde having from 2 to 24 carbon atoms with formaldehyde and a tertiary amine catalyst, which process is carried out step-wise and comprises
• a) in a first step, reacting the aldehyde with 2 to 8 times the molar amount of formaldehyde in the presence of the tertiary amine,
• b) in a second step, separating the reaction mixture
• i) into a bottom fraction comprising, in addition to the compounds of formula II, an incompletely methylolated compound of the formula III
• and a distillate stream comprising unconverted or partially converted starting materials, or
• ii) into an aqueous phase and an organic phase, by means of a phase-separating equipment,
• and recycling the distillate stream or the organic phase to the first stage, and
• c) in a third stage, subjecting the bottom fraction or the aqueous phase
• i) to a catalytic reaction or thermal reaction, to convert the compound of the formula III to the compound of formula II and to a corresponding methylene compound of formula IV
• wherein R′ is hydrogen or the radical R, and
• ii) distilling the reaction product, wherein an overhead stream comprising the compound of the formula IV and unconverted formaldehyde is recycled to the first stage, and
• the compound of the formula II is isolated from the bottom fraction.