Chemistry Reference
In-Depth Information
Along with fats and proteins, they are among the
main constituents of food.
ethane-1,2-diol
HO(CH
2
)
2
OH
benzene-1,4-dicarboxylic acid
HOOC(C
6
H
4
)COOH
Starch occurs in potatoes, rice and wheat. Glucose,
from which starch is polymerised, belongs to
a group of simple carbohydrates known as
monosaccharides
. They are sweet to taste and
soluble in water. Starch belongs to the more
complicated group of carbohydrates known as
polysaccharides
. Starch does not form a true
solution and it does not have a sweet taste. With
iodine it gives an intense blue colour (nearly black),
which is used as a test for starch or iodine itself
(Figure 15.20).
HO(CH
2
)
2
OCO
(C
6
H
4
)COOH
2
O
ester link
This ester link is the same linkage as in fats.
Generally, polyesters have the structure
O
O
O
O
C
C
O
O
C
C O
O
Like nylon, Terylene can be turned into yarn, which
can then be woven. Terylene clothing is generally
softer than that made from nylon but both are hard
wearing. Because an ester link is formed during the
polymerisation, Terylene is known as a
polyester
.
Fats possess the same sort of linkage as Terylene
but have different units. For a further discussion see
p. 238.
Questions
1
Draw the structure of the repeating units found in:
a
nylon
b
Terylene.
2
Explain the differences between an addition polymer
and a condensation polymer.
●
Some biopolymers
Starch
Starch is a
biopolymer
or
natural polymer
. It is a
condensation polymer of glucose, a type of sugar. It
is often produced as a way of storing energy and is
formed as a result of photosynthesis in green plants.
carbon
Figure 15.20
A dark blue-black colour is produced when dilute iodine
solution is applied to starch, for example in a potato.
photosynthesis
+ water
glucose + oxygen
dioxide
sunlight/
chlorophyll
Hydrolysis of starch
Starch can be broken down in two ways, both of
which take place in the presence of water. Hence
the reactions are known as
hydrolysis
reactions.
Hydrolysis of starch is the key reaction that enables
us to use this energy source. If starch is boiled
for about one hour with dilute hydrochloric acid,
it is broken down into its monomers, glucose
molecules.
(in green plants)
6CO
2
(
g
) + 6H
2
O(
l
)
C
6
H
12
O
6
(
aq
) + 6O
2
(
g
)
glucose →
starch
+
water
n
C
6
H
12
O
6
(
aq
) → (C
6
H
10
O
5
)
n
(
s
)
+
n
H
2
O(
l
)
Starch can be represented by
O
O
O
O
Both starch and glucose are
carbohydrates
, a class
of naturally occurring organic compounds which can
be represented by the general formula (CH
2
O)
x
.
dilute acid
starch
+
water
glucose
heat
(C
6
H
10
O
5
)
n
(
s
) +
n
H
2
O(
l
)
n
C
6
H
12
O
6
(
aq
)
