Biology Reference
In-Depth Information
Chain Length
Fatty acids commonly have a chain of 4 to 36 carbons (usually un-branched and even
numbered), which may be saturated or unsaturated. Chains of carbon length 4 to 6 are
referred to as short chain; 8 to 10 as medium chain; and 12 to 24 (or longer), as long chain.
Short chain fatty acids are biochemically more closely related to sugars than to fats and so
are not major components of membranes. Most membrane fatty acyl chains are 14, 16, 18,
20, 22, or 24 carbons in length. While present, odd carbon chain fatty acids are far less abun-
dant. Preponderance of even carbon number fatty acids is the result of their biosynthesis
from 2-carbon acetyl units. Acyl is a general term meaning an esterified fatty acid of unspec-
ified chain length.
Saturated Fatty Acids
Acyl chains may contain from 0 to 6 double bonds
[9]
. Fatty acids with no double
bonds belong to the class known as saturated. Saturated fatty acids, unlike unsaturated
fatty acids, cannot be modified by hydrogenation or halogenation. They are also highly
resistant to oxidation since they have no susceptible double bonds. Saturated fatty acids
are named for the saturated hydrocarbon with the same number of carbons. The series of
even-carbon, saturated fatty acids from C-2 to C-24 are shown in
Tabl e 4. 3
. Listed are the
fatty acids' melting points (T
m
s) that, with the exception of acetic acid, increase with
chain length, and their water solubility, that decreases with chain length. Also note
TABLE 4.3
The Series of Even Carbon Saturated Fatty Acids from C-2 to C-24 with Melting Point and Water
Solubility.
Melting Point (
o
C)
Water Solubility (g/l, 20
o
C)
Systematic Name
Trivial Name
Structure
Acetic
Acetic
2:0
16.7
infinite
Butanoic
Butyric
4:0
7.9
infinite
Hexanoic
Caproic
6:0
3.4
9.7
Octanoic
Caprylic
8:0
16.7
0.7
Decanoic
Capric
10:0
31.6
0.15
Dodecanoic
Lauric
12:0
44.2
0.055
Tetradecanoic
Myristic
14:0
53.9
0.02
Hexadecanoic
Palmitic
16:0
63.1
0.007
Octadecanoic
Stearic
18:0
69.6
0.003
Eicosanoic
Arachidic
20:0
75.3
insoluble
Docosanoic
Behenic
22:0
79.9
insoluble
Tetracosanoic
Lignoceric
24:0
84.2
insoluble
