Page 21 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

P1: GLQ Revised Pages
Encyclopedia of Physical Science and Technology EN001F-4 May 7, 2001 16:19

Acetylene 73

CH 3 CH 3
2C 2 H 5 MgBr
HO CH 2 CH C C CH BrMgCH 2 CH C C CMgBr

CH 3
CH 2 CH C CHO

CH 3 CH 3
CH 2 CH C CH C C C CHCH 2 OH

OH
SCHEME 1

X. VINYL ETHERS methylisopropenyl ether, and (4) allylic isomerization
with loss of carbon dioxide. This four-step sequence is
The reaction of alcohols with acetylene to form vinyl repeated a number of times in the synthesis of both vi-
ethers, commonly termed vinylation, takes place readily tamins A (trans-retinol) and E (dl-α-tocopherol). Some
at 150–180 C using basic catalysts such as alkali metal of the intermediate products such as methylheptenone,
◦
hydroxides or alkoxides: linalool, pseudoionone, β-ionone, citral, geranylacetone,
and nerolidol, are valuable flavor–perfumery chemicals.
ROH + C 2 H 2 → RO CH CH 2
This circumstance increases the overall versatility and
The vinylation process can be carried out at both at- profitability of the process by providing diversification.
mospheric pressure and under elevated pressures with an The reaction sequence in Fig. 7 illustrates the following
acetylen–nitrogen atmosphere. The formation of methyl transformations:
vinyl ether from methanol and acetylene can be carried
Acetone + acetylene → methylbutynol
out continuously at atmospheric pressure and 180 C. The
◦
GAF Corporation has produced a variety of vinyl ethers → methylbutenol → methylheptenone
derived from methoanol, ethanol, n-butanol, isobutanol,
→ dehydrolinalool → linalool
and decanol. The vinyl ethers find their most important
use as the copolymers derived from methyl vinyl ether This route and related routes are high-yield, high-
and maleic anhydride, known as the Gantrez AN series. selectivity steps of basic importance to the final economics
Applications of these products and other polyvinyl ethers of the processes for vitamins A and E. The second se-
are summarized in Section VI.A. quence, shown in Fig. 8, illustrates two processes for the
production of citral from the alkynol DHL. The DHL–
acetic anhydride route involves earlier technology, which
XI. FLAVOR AND FRAGRANCE may have been replaced by the direct isomerization of
COMPOUNDS AND VITAMINS DHL, the second process. Large amounts of citral are
A AND E also produced by SCM-Glidden via the oxidative dehy-
drogenation of geraniol.
The reaction flow sheets in Figs. 7–13 summarize the The flow sheet shown in Fig. 9 illustrates the following
rather lengthy chemistry and technology developed since process sequences:
1955 for the production of polyterpene compounds. This
DHL or citral → pseudoionone → α- and β-ionones;
chemistry is characterized by change and is highly propri-
etary. Prominent companies in this field are Hoffmann–La linalool → geranylacetone → dehydronerolidol
Roche and BASF. The choice of a given synthetic route de-
→ nerolidol
pends on cost and availability of raw materials in addition
to manufacturing sites. Citral and DHL compete with one another in the prepa-
The synthesis sequence pioneered by Roche and ration of pseudoionone, the precursor of the ionones.
used for more than 30 years involves (1) ethynylation, Lemongrass oil and myrcene (from turpentine) are nat-
(2) semihydrogenation, (3) reaction with diketene or ural sources of citral. β-Ionone is a very important