a process for preparing compounds of the general formula : r -- c . tbd . c -- c -- c ═ ch . sub . 2 r represents an h atom or an optionally substituted hydrocarbyl group , by reacting a compound r -- c . tbd . ch with co in the presence of a pd compound , an organic phosphine and a protonic acid having a pk a below 3 . 0 . compounds of formula i in which r represents an optionally substituted hydrocarbyl group are novel .