Coming back to aldehydes, they are formed by partial oxidation of primary alcohols and form carboxylic acids when they are further oxidized. Whereas, secondary alcohols oxidize to give ketones. Aldehydes also vary in smell. Low molecular weight aldehydes, e.g., formaldehyde and acetaldehyde, have sharp, unpleasant odors; higher molecular weight aldehydes, e.g., benzaldehyde and furfural, have pleasant, often flowery odors and are found in the essential oils of certain plants. Aldehydes are used for the manufacture of synthetic resins, e.g. bakelite, and for making dyestuffs, flavorings, perfumes, and other chemicals. Some are used as preservatives and disinfectants.