Of all aldehydes, formaldehyde is produced on the largest scale, about 6,000,000 tons per year. It is mainly used in the production of resins when combined with urea , melamine , and phenol (., Bakelite ). It is a precursor to methylene diphenyl diisocyanate ("MDI"), a precursor to polyurethanes .  The second main aldehyde is butyraldehyde , of which about 2,500,000 tons per year are prepared by hydroformylation . It is the principal precursor to 2-ethylhexanol , which is used as a plasticizer .  Acetaldehyde once was a dominating product, but production levels have declined to less than 1,000,000 tons per year because it mainly served as a precursor to acetic acid , which is now prepared by carbonylation of methanol . Many other aldehydes find commercial applications, often as precursors to alcohols, the so-called oxo alcohols , which are used in detergents. Some aldehydes are produced only on a small scale (less than 1000 tons per year) and are famously used as ingredients in flavours and perfumes such as N°5 from CHANEL . These include cinnamaldehyde and its derivatives, citral , and lilial .