Лако-красочные материалы — производство

One of the most important perfume ingredients made from benzene is

2-phenylethanol. The synthesis of this, and related compounds, is shown in Scheme 4.50. 2-Phenylethanol is a major component of rose oils and is widely used in perfumery for its blending qualities. In tonnage terms, it is one of the most important of all perfumery ingredients. The original production method involved Friedel-Crafts addition of ethylene oxide to benzene, which is fairly efficient despite some addition of ethylene oxide to the product that results in a small amount of poly ethoxy lated derivatives. The major disadvantage of this route is the safety issue of handling ethylene oxide and benzene. Both reagents can be handled safely, but the engineering required to do so adds to the process costs.

Another possibility is to hydrogenate styrene oxide (the production of styrene from benzene is described below). This route gives a very high-quality product and the intermediate, styrene oxide, can also be used to produce other fragrance materials. Rearrangement of the epoxide gives phenylacetaldehyde, which is a potent green note. Phenylacetaldehyde is responsible for the characteristic green top note of narcissus and is used to create a narcissus-like effect in floral perfumes. Addition of alcohols to styrene oxide gives the correspond­ing acetals of phenylacetaldehyde. The most important of these is the dimethyl acetal, known in the industry by the acronym PADMA. This material has much of the green character of the aldehyde, but is chemically more stable. Many esters of 2-phenylethanol are used in
perfumery, the acetate, isobutyrate and phenylacetate in particular. These are prepared by esterification of the alcohol.

HO OH

Scheme 4.51 shows the process known to the bulk chemicals industry as SMPO, the styrene monomer-propylene oxide process. Styrene is used in polymers, and propylene oxide derivatives have a wide variety of uses, including as surfactants and in anti-freeze. For the bulk industry, the process is as follows. Addition of ethylene to benzene gives ethylbenzene, which undergoes air oxidation to give the hydro­peroxide. Reaction of this with propylene, in the presence of a suitable catalyst, gives styrallyl alcohol and propylene oxide. Styrallyl alcohol is readily dehydrated to styrene.

Perfumery interest in this process is four-fold. Firstly, one of the major products, styrene, is a starting material for perfume ingredients, as described above. Secondly, the other major product, propylene oxide, is also a precursor for a number of fragrance materials and for dipropylene glycol, one of the major solvents used in perfumes. Thirdly, the intermediate, styrallyl alcohol, is the starting material for a number of esters used in perfumery, the acetate in particular. Fourthly, but by no means least important, the crude styrallyl alcohol contains a small trace of 2-phenylethanol. Since the SMPO process is run on a scale far beyond that of the perfume industry, what is a small
trace to the polymer business represents a very significant proportion of the world requirement for 2-phenylethanol as a perfume ingredient. The problem is that of odour quality.

It is extremely difficult to purify this by-product 2-phenylethanol to the odour quality of that produced by either of the above routes. However, most of the companies, such as ARCO in the USA and Sumitomo in Japan, who run the SMPO process can produce 2- phenylethanol of a quality which can be used in perfumery (often in collaboration with a perfumery company). The amount of 2-phenyl­ethanol available from this route is dictated by the demand for styrene and propylene oxide, the market value is dictated largely by material from the other two routes and all three run in economic balance.

Scheme 4.52

Scheme 4.52 shows the preparation of the hydrocinnamic aldehydes, another family of materials derived from benzene and which possess fresh, white-floral notes reminiscent of muguet (lily of the valley) and cyclamen. One of these, Lilial® or Lilistralis®, can also be prepared from toluene, and that route is also described in this section for comparison. Addition of isobutene or propylene to benzene gives t — butylbenzene and cumene, respectively. Addition of acrolein or metha — crolein diacetates to these gives, after hydrolysis of the intermediate enol ester, the corresponding hydrocinnamaldehydes. It is also possible to prepare the hydrocinnamaldehydes from the substituted toluene by oxidation to the corresponding substituted benzaldehyde, followed by aldol condensation and hydrogenation. The oxidation of the substi­tuted toluenes to the corresponding substituted benzaldehydes can be carried out either by electrochemical oxidation or by chlorination- hydrolysis. This, however, is not the case for oxidation of /7-cymene (p — isopropyltoluene), since in this case the isopropyl group is more reactive than the methyl group.

The substituted benzaldehydes can also be prepared by formylation of the corresponding alkyl benzenes. In Scheme 4.52, all of the possibilities are shown for the preparation of Lilial®. The most important route commercially is that which proceeds from toluene through electrochemical oxidation of t-butyltoluene followed by aldol reaction and hydrogenation. Lilial® is an intermediate for a herbicide, and so some producers gain the advantage of scale in their production costs by making both products. Cumene is produced on a vast scale as a precursor for acetone and phenol, which makes it the most sensible starting material for cyclamen aldehyde. As with so many perfumery materials, this family of aldehydes demonstrates how various synthetic routes are possible; the ones chosen depend on a fine balance of technical, economic and strategic factors.