In chemistry, especially biochemistry, a fatty acid is a carboxylic acid (or organic acid), often with a long aliphatic tail (long chains), either saturated or unsaturated. Depending on the context, fatty acids may be assumed to have at least 8 carbon atoms, e.g., caprylic acid (octanoic acid). Most of the natural fatty acids have an even number of carbon atoms, because their biosynthesis involves acetate which has two carbon atoms.
Industrially, fatty acids are produced by the hydrolysis of the ester linkages in a fat or biological oil (both of which are triglycerides), with the removal of glycerol.
Reduction of fatty acids yields fatty alcohols.
Several fatty acid molecules
Saturated fatty acids do not contain any double bonds or other functional groups along the chain. The term "saturated" refers to hydrogen, in that all carbons (apart from the carboxylic acid [-COOH] group) contain as many hydrogens as possible. In other words, the omega (ω) end contains 3 hydrogens (CH3-) and each carbon within the chain contains 2 hydrogens (-CH2-).
Saturated fatty acids form straight chains and, as a result, can be packed together very tightly, allowing living organisms to store chemical energy very densly. The fatty tissues of animals contain large amounts of long-chain saturated fatty acids. In IUPAC nomenclature, fatty acids have an -oic acid suffix. In common nomenclature, the suffix is usually -ic.
Some saturated fatty acids are:
- Acetic: CH3COOH
- Butyric: CH3(CH2)2COOH
- Lauric (dodecanoic acid): CH3(CH2)10COOH
- Myristic (tetradecanoic acid): CH3(CH2)12COOH
- Palmitic (hexadecanoic acid): CH3(CH2)14COOH
- Stearic (octadecanoic acid): CH3(CH2)16COOH
- Arachidic (eicosanoic acid): CH3(CH2)18COOH
This is a computer generated image of Dodecanoic Acid, a fatty acid
Unsaturated fatty acids are of similar form, except that one or more alkene functional groups exist along the chain, with each alkene substituting a singly-bonded " -CH2-CH2-" part of the chain with a doubly-bonded "-CH=CH-" portion (that is, a carbon double bonded to another carbon).
The two hydrogen atoms (H) that are bound to the doubly-bonded carbon atoms (C) can occur in a cis or trans configuration.
cis: A cis configuration means that the two hydrogen atoms are on the same side of the chain. Because of the polarization of the hydrogen atoms, the hydrogen atoms repel each other and cause the chain to bend. The more double-bonds the chain has in the cis configuration, the more bent it is. When a chain has many cis bonds, it becomes quite curved. For example, oleic acid, with one double bond, has a "kink" in it, while linoleic acid, with two double bonds, has a more pronounced bend. Alpha-linolenic acid, with three double bonds, forms a hooked shape.
trans: A trans configuration, by contrast, means that the two hydrogen atoms occur on opposite sides of the chain. As a result, they don't cause the chain to bend much, and their shape is similar to the straight saturated fatty acids.
In most naturally occurring unsaturated fatty acids, each double bond has 3n carbon atoms after it, for some n, and all are cis bonds. Most fatty acids in the trans configuration (trans fats) are unnatural and the result of human processing.
The differences in geometry between these various types of unsaturated fatty acids, as well as between saturated and unsaturated fatty acids, plays an important role is biological processes, and in the construction of biological structures (such as cell membranes).
There are two different ways to make clear where the double bonds are located in molecules. For example:
- cis/trans-Delta-x or cis/trans-Δx: The double bond is located on the xth carbon-carbon bond, counting from the carboxyl terminus. The cis or trans notation indicates whether the molecule is arranged in a cis or trans conformation. In the case of a molecule having more than one double bond, the notation is, for example, cis,cis-Δ9,Δ12.
- Omega-x or ω-x : A double bond is located on the xth carbon-carbon bond, counting from the ω (methyl carbon) end.
Example of unsaturated fatty acids:
- Alpha-linolenic acid: CH3CH2CH=CHCH2CH=CHCH2CH=CH(CH2)7COOH
- Docosahexaenoic acid
- Eicosapentaenoic acid
- Linoleic acid: CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH
- Arachidonic acid CH3(CH2)4CH=CHCH2CH=CHCH2CH=CHCH2CH=CH(CH2)3COOH
- Oleic acid: CH3(CH2)7CH=CH(CH2)7COOH
- Erucic acid: CH3(CH2)7CH=CH(CH2)11COOH
Alpha-linolenic, docosahexaenoic, and eicosapentaenoic acids are examples of omega-3 fatty acids. Linoleic acid and arachidonic acid are omega-6 fatty acids. Oleic and erucic acid are omega-9 fatty acids. Stearic and Oleic acid are both 18 C fatty acids. They differ only in that stearic acid is saturated with hydrogen, while oleic acid is an unsaturated fatty acid with two fewer hydrogens.
Essential fatty acids are the polyunsaturated fatty acids, linoleic acid and alpha-linolenic acid, which are the parent compounds of the omega-6 and omega-3 fatty acid series, respectively. They are essential in the human diet since they cannot be synthesized by the body. Humans can easily make saturated fatty acids or monounsaturated fatty acids with a double bond at the omega-9 position, but do not have the enzymes necessary to introduce a double bond at the omega-3 or omega-6 position. As a result, these fatty acids must be obtained from food sources; hence, they are "essential."
The essential fatty acids are very important to the human immune system and to help regulate blood pressure, since they are used to make compounds such as prostaglandins. The brain is also highly enriched in derivatives of linolenic and alpha-linoleic acids. Changes in the levels and balance of these fatty acids caused with a typical Western diet of processed food and high intensity agriculture has been associated with depression and behavioral change, including violence. Change to a more natural diet or consumption of supplements to compensate for dietary imbalance is associated with a reduction in violent behavior, and increases attention span. This finding has been replicated in studies within schools as well as a double blind study in a prison.
A trans fatty acid (commonly shortened to trans fat) is an unsaturated fatty acid molecule that contains a trans double bond between carbon atoms, which makes the molecule less kinked compared to fatty acids with cis double bonds. Research suggests a correlation between diets high in trans fats and diseases like atherosclerosis and coronary heart disease.
Fatty acids can be bound or attached to other molecules, like triglycerides or phospholipids. When they are not attached to other molecules, they are known as "free" fatty acids.
The uncombined fatty acids or free fatty acids may come from the breakdown of a triglyceride into its components (fatty acids and glycerol).
Free fatty acids are an important source of fuel for many tissues since they can yield relatively large quantities of ATP. Many cell types can use either glucose or fatty acids for this purpose. However, heart and skeletal muscle prefer fatty acids. On the other hand, the brain cannot use fatty acids as a source of fuel, relying instead on glucose, or on ketone bodies produced by the liver from fatty acid metabolism during starvation, or periods of low carbohydrate intake.
Formic acid and acetic acid are miscible with water and dissociate to form reasonably strong acids (pKa respectively 3.77 and 4.76). Longer chain fatty acids do not show a great change in pKa: nonanic acid, for example, has a pKa of 4.96. However, as the chain length increases the solubility of the fatty acids in water decreases very rapidly, so that the longer chain fatty acids have very little effect on the pH of a solution. The significance of their pKa values therefore only has relevance to the types of reaction that they take part in.
Even those fatty acids that are insoluble in water will dissolve in warm ethanol, and can be titrated with sodium hydroxide solution using phenolphthalein as an indicator to a pale pink endpoint. This analysis is used to determine the free fatty acid content of fats, i.e. the proportion of the triglycerides that have been hydrolyzed.
Fatty acids react just like any other carboxylic acid, which means they can undergo esterification, hydrolysis, and acid-base reactions. Unsaturated fatty acids can additionally undergo addition reactions, most commonly hydrogenation, which is used to convert vegatable oils into margarine. With partial hydrogenation, unsaturated fatty acids can be isomerized from cis to trans configuration.
Fatty acids at room temperature undergo a chemical change known as autoxidation. The fatty acid breaks down into hydrocarbons, ketones, aldehydes, and smaller amounts of epoxides and alcohols. Heavy metals present at low levels in fats and oils promote autoxidation. Fats and oils often are treated with chelating agents such as citric acid.
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