Editing Fatty acid (section)

==Reactions of fatty acids==
Fatty acids exhibit reactions like other carboxylic acids, i.e. they undergo [[esterification]] and acid-base reactions.

===Transesterification===
All fatty acids [[transesterification|transesterify]].  Typically, transesterification is practiced in the conversion of fats to fatty acid methyl esters.  These esters are used for biodiesel.  They are also hydrogenated to give fatty alcohols.  Even vinyl esters can be made by transesterification using [[vinyl acetate]].<ref>{{cite journal |doi=10.15227/orgsyn.030.0106 |title=Vinyl Laurate and Other Vinyl Esters |journal=Organic Syntheses |date=1950 |volume=30 |page=106|first1=Daniel|last1=Swern|first2=E. F.|last2=Jordan, Jr }}</ref>

===Acid-base reactions===
Fatty acids do not show a great variation in their acidities, as indicated by their respective [[Acid dissociation constant|p''K''<sub>a</sub>]]. [[Nonanoic acid]], for example, has a p''K''{{sub|a}} of 4.96, being only slightly weaker than acetic acid (4.76). As the chain length increases, the solubility of the fatty acids in water decreases, so that the longer-chain fatty acids have minimal effect on the [[pH]] of an aqueous solution. Near neutral pH, fatty acids exist at their conjugate bases, i.e. oleate, etc.

Solutions of fatty acids in [[ethanol]] can be [[titration|titrated]] with [[sodium hydroxide]] solution using [[phenolphthalein]] as an indicator. This analysis is used to determine the free fatty acid content of fats; i.e., the proportion of the triglycerides that have been [[hydrolysis|hydrolyze]]d.

Neutralization of fatty acids, like [[saponification]], is a widely practiced route to [[metallic soap]]s.<ref>{{Ullmann |last1=Schumann |first1=Klaus |last2=Siekmann |first2=Kurt |year=2000 |doi=10.1002/14356007.a24_247 |title=Soaps}}</ref>

===Hydrogenation and hardening===
[[Hydrogenation]] of unsaturated fatty acids is widely practiced. Typical conditions involve 2.0–3.0 MPa of H{{sub|2}} pressure, 150&nbsp;°C, and nickel supported on silica as a catalyst. This treatment affords saturated fatty acids. The extent of hydrogenation is indicated by the [[iodine number]]. Hydrogenated fatty acids are less prone toward [[rancidification]]. Since the saturated fatty acids are [[melting point|higher melting]] than the unsaturated precursors, the process is called hardening. Related technology is used to convert vegetable oils into [[margarine]]. The hydrogenation of triglycerides (vs fatty acids) is advantageous because the carboxylic acids degrade the nickel catalysts, affording nickel soaps. During partial hydrogenation, unsaturated fatty acids can be isomerized from ''cis'' to ''trans'' configuration.<ref name="Ullmann Fatty Acids"/>

More forcing hydrogenation, i.e. using higher pressures of H{{sub|2}} and higher temperatures, converts fatty acids into [[fatty alcohol]]s. Fatty alcohols are, however, more easily produced from simpler fatty acid [[ester]]s, like the [[fatty acid methyl ester]]s ("FAME"s).

===Decarboxylation===
[[Ketonic decarboxylation]] is a method useful for producing symmetrical ketones from carboxylic acids.  The process involves reactions of the carboxylic acid with an inorganic base. Stearone is prepared by heating [[magnesium stearate]].<ref>{{cite journal |author=A. G. Dobson and H. H. Hatt|doi=10.15227/orgsyn.033.0084|title=Stearone |journal=Organic Syntheses |year=1953 |volume=33 |page=84}}</ref>

===Chemistry of saturated vs unsaturated acids===
The reactivity of saturated fatty acids is usually associated with the carboxylic acid or the adjacent methylene group  By conversion to their acid chlorides, they can be converted to the symmetrical fatty ketone laurone ({{chem2|O\dC(C_{n}H_{(2n+1)})2}}).<ref>{{cite journal |author=J. C. Sauer|doi=10.15227/orgsyn.031.0068 |title=Laurone |journal=Organic Syntheses |date=1951 |volume=31 |page=68 }}</ref>  Treatment with [[sulfur trioxide]] gives the α-sulfonic acids.<ref>{{cite journal |doi=10.15227/orgsyn.036.0083 |title=α-Sulfopalmitic Acid |journal=Organic Syntheses |date=1956 |volume=36 |page=83 |first1=J. K. |last1=Weil |first2=R. G. |last2=Bistline, Jr. |first3=A. J. |last3=Stirton}}</ref>

The reactivity of unsaturated fatty acids is often dominated by the site of unsaturation.  These reactions are the basis of ozonolysis, hydrogenation, and the iodine number. Ozonolysis (degradation by ozone) is practiced in the production of [[azelaic acid]] ((CH{{sub|2}}){{sub|7}}(CO{{sub|2}}H){{sub|2}}) from [[oleic acid]].<ref name="Ullmann Fatty Acids"/>