Chemistry Reference
In-Depth Information
The distributions of these isomers in pine trees depends on the thermal history of the
rosin [6, 7]. One of the most obvious changes is the large increase in the more stable
dehydroabietic acid from 32% to 52% [6]. This is mainly due to the decomposition,
dehydrogenation isomerisation and disproportionation reactions taking place during
fractionation [8-11]. Generally, the rosin acids can be divided into two subgroups:
the pimaric type acid, characterised by both methyl and vinyl substituents at the C
7
position, and the abietic type acid [5], which bears only a single isopropyl group
at this position as shown in
Figure 2.1
. The numbering system used in this review
adheres to the rules for naming rosin acids as shown in
Figure 2.1
.
19
18
6
5
7
8
20
17
13
14
4
3
12
11
2
9
1
10
16
COOH
15
Figure 2.1
Chemical structure of rosin acids
The structures of some of the important rosin acid isomers (a) including abietic (b)
levopimaric (c), palustric (d), neoabietic (e), dehydroabietic (f), dihydroabietic (g)
and tetrahydroabietic (h) acids are shown in
Table 2.1
. The distribution of these
isomers found in pine trees varies depending on their geographic location and,
perhaps equally importantly, on the thermal history of the rosin [6,7]. Gum rosin
has a total abietic type acid content of 60-65%, but Finnish tall oil rosin has only
39-47%. It has been shown that the rosin acid composition between crude tall oils
and tall oil rosin changes substantially during the high temperature fractionation
process. Furthermore, some of the rosin acids which were not originally present in
crude tall oil are found after distillation [6]. This is mainly due to the decomposition,
dehydrogenation, isomerisation and disproportionation reactions taking place during
fractionation [8-11]. The melting points of rosin acids and the compositional data
for different rosins are given in
Table 2.2.
