PHVF partially hydrogenated vegetable fats
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PHVF partially hydrogenated vegetable fats
Hydrogenation is a class of chemical reactions which result in an addition of
hydrogen (H2) usually to unsaturated organic compounds. Typical substrates
include alkenes, alkynes, ketones, nitriles, and imines. Most hydrogenations
involve the direct addition of diatomic hydrogen (H2) but some involve the
alternative sources of hydrogen, not H2: these processes are called transfer
hydrogenations. The reverse reaction, removal of hydrogen, is called
dehydrogenation.
The classical example of a hydrogenation is the addition of hydrogen on
unsaturated bonds between carbon atoms, converting alkenes to alkanes. Numerous
important applications are found in the petrochemical, pharmaceutical and food
industries. Health concerns associated with the hydrogenation of unsaturated
fats to produce saturated fats and trans fats is an important aspect of current
consumer awareness. Hydrogenation differs from protonation or hydride addition
(e.g. use of sodium borohydride): in hydrogenation, the products have the same
charge as the reactants.
The hydrogenation process
Hydrogenation has three components: the unsaturated substrate, the hydrogen (or
hydrogen source) and, invariably, a catalyst. The largest scale technological
uses of H2 are the hydrogenation and hydrogenolysis reactions associated with
both heavy and fine chemicals industries. Hydrogenation is the addition of H2 to
unsaturated organic compounds such as alkenes to give alkanes and aldehydes to
give alcohols. Hydrogenolysis is the cleavage of C-X (X = O, S, N) bonds by H2
to give C-H and H-X bonds. Large-scale applications of hydrogenolysis reactions
are associated with the upgrading of fossil fuels. Hydrogenation and
hydrogenolysis reactions require metal catalysts, often those composed of
platinum or similar precious metals. It is a curious fact that under mild
conditions, H2 reacts directly with no organic compound in the absence of such
catalysts.
The addition of H2 to an alkene affords an alkane in the protypical reaction:
RCH=CH2 + H2 → RCH2CH3 (R = alkyl, aryl)
Catalysts
With rare exception, no reaction below 480 °C occurs between H2 and organic
compounds in the absence of metal catalysts. The catalyst simultaneously binds
both the H2 and the unsaturated substrate and facilitates their union. Platinum
group metals, particularly platinum, palladium, rhodium and ruthenium, are
highly active catalysts. Highly active catalysts operate at lower temperatures
and lower pressures of H2. Non-precious metal catalysts, especially those based
on nickel (such as Raney nickel and Urushibara nickel) have also been developed
as economical alternatives but they are often slower or require higher
temperatures. The trade-off is activity (speed of reaction) vs. cost of the
catalyst and cost of the apparatus required for use of high pressures.
Two broad families of catalysts are known - homogeneous and heterogeneous.
Homogeneous catalysts dissolve in the solvent that contains the unsaturated
substrate. Heterogeneous catalysts are solids that are suspended in the same
solvent with the substrate or are treated with gaseous substrate. In the
pharmaceutical industry and for special chemical applications, soluble
""homogeneous"" catalyst are sometimes employed, such as the rhodium-based
compound known as Wilkinson's catalyst, or the iridium-based Crabtree's
catalyst.
The activity and selectivity of catalysts can be adjusted by changing the
environment around the metal, i.e. the coordination sphere. Different faces of a
crystalline heterogeneous catalyst display distinct activities, for example.
Similarly, heterogeneous catalysts are affected by their supports, i.e. the
material upon with the heterogeneous catalyst is bound. Homogeneous catalysts
are affected by their ligands. In many cases, highly empirical modifications
involve selective "poisons." Thus, a carefully chosen catalyst can be used to
hydrogenate some functional groups without affecting others, such as the
hydrogenation of alkenes without touching aromatic rings, or the selective
hydrogenation of alkynes to alkenes using Lindlar's catalyst. For prochiral
substrates, the selectivity of the catalyst can be adjusted such that one
enantiomeric product is produced.
Hydrogenolysis
The catalytic hydrogenation of organic sulfur compounds to form gaseous hydrogen
sulfide (H2S) is very widely used in petroleum refineries, petrochemical plants
and other industries to desulfurize various final products, intermediate
products and process feedstocks by converting sulfur compounds to gaseous
hydrogen sulfide which is then easily removed by distillation. The gaseous
hydrogen sulfide is subsequently recovered in an amine treater and finally
converted to elemental sulfur in a Claus process unit. In those industries,
desulfurization process units are often referred to as hydrodesulfurizers (HDS)
or hydrotreaters (HDT). In the petroleum refining and petrochemical industries,
cobalt-molybdenum or nickel-molybdenum catalysts are commonly used for
hydrogenation and hydrogenolysis catalysts.
Mechanism of reaction
Because of its technological relevance, metal-catalyzed “activation” of H2, has
been the subject of considerable study, focusing on the reaction mechanisms of
by which metals mediate these reactions. First of all isotope labeling using
deuterium can be used to determine the regiochemistry of the addition:
RCH=CH2 + D2 → RCHDCH2D
Essentially, the metal binds to both components to give an intermediate
alkene-metal(H)2 complex. The general sequence of reactions is:
binding of the hydrogen to give a dihydride complex ("oxidative addition"):
LnM + H2 → LnMH2
binding of alkene:
LnM(η2H2) + CH2=CHR → Ln-1MH2(CH2=CHR) + L
transfer of one hydrogen atom from the metal to carbon (migratory insertion)
Ln-1MH2(CH2=CHR) → Ln-1M(H)(CH2-CH2R)
transfer of the second hydrogen atom from the metal to the alkyl group with
simultaneous dissociation of the alkane ("reductive elimination")
Ln-1M(H)(CH2-CH2R) → Ln-1M + CH3-CH2R
Preceding the oxidative addition of H2 is the formation of a dihydrogen complex.
Hydrogen sources
The obvious source of H2 is the gas itself, often under pressure. Hydrogen can
also be transferred from hydrogen-donor molecules, such as hydrazine,
dihydronaphthalene, dihydroanthracene, isopropanol, and formic acid. Transfer
hydrogenation can be metal catalysed. Hydrogenation does proceed from some
hydrogen donors without catalysts, examples being diimide and aluminium
isopropoxide.
Temperatures
The reaction is carried out at different temperatures and pressures depending
upon the substrate. Hydrogenation is a strongly exothermic reaction. In the
hydrogenation of vegetable oils and fatty acids, for example, the heat released
is about 25 kcal per mole (105 kJ/mol), sufficient to raise the temperature of
the oil by 1.6-1.7 °C per iodine number drop.
Hydrogenation in the food industry
Types of Fats in Food
Unsaturated fat
Monounsaturated fat
Polyunsaturated fat
Trans fat
Omega: 3, 6, 9
Saturated fat
Interesterified fat
See Also
Fatty acid
Essential fatty acid
Hydrogenation is widely applied to the processing of vegetable oils and fats.
Complete hydrogenation converts unsaturated fatty acids to saturated ones. In
practice the process is not usually carried to completion. Since the original
oils usually contain more than one double bond per molecule (that is, they are
poly-unsaturated), the result is usually described as partially hydrogenated
vegetable oil; that is some, but usually not all, of the double bonds in each
molecule have been reduced. This is done by adding hydrogen atoms which bond to
the carbon, thus occupying a place in the outer orbital of the carbon which
would have otherwise been used to bond with the next carbon in the fatty acid
chain.
Hydrogenation results in the conversion of liquid vegetable oils to solid or
semi-solid fats, such as those present in margarine. Changing the degree of
saturation of the fat changes some important physical properties such as the
melting point, which is why liquid oils become semi-solid. Semi-solid fats are
preferred for baking because the way the fat mixes with flour produces a more
desirable texture in the baked product. Since partially hydrogenated vegetable
oils are cheaper than animal source fats, are available in a wide range of
consistencies, and have other desirable characteristics (e.g., increased
oxidative stability (longer shelf life)), they are the predominant fats used in
most commercial baked goods. Fat blends formulated for this purpose are called
shortenings.
Health implications
trans fat
A side effect of incomplete hydrogenation having implications for human health
is the isomerization of the remaining unsaturated carbon bonds. The cis
configuration of these double bonds predominates in the unprocessed fats in most
edible fat sources, but incomplete hydrogenation partially converts these
molecules to trans isomers, which have been implicated in circulatory diseases
including heart disease (see trans fats). The catalytic hydrogenation process
favors the conversion from cis to trans bonds because the trans configuration
has lower energy than the natural cis one. At equilibrium, the trans/cis isomer
ratio is about 2:1. Food legislation in the US and codes of practice in EU has
long required labels declaring the fat content of foods in retail trade, and
more recently, have also required declaration of the trans fat content.
In 2006, New York City adopted the US’s first major municipal ban on most
artificial trans fats in restaurant cooking.
History
The earliest hydrogenation is that of platinum catalyzed addition of hydrogen to
oxygen in the D?bereiner's lamp, a device commercialized as early as 1823. The
French chemist Paul Sabatier is considered the father of the hydrogenation
process. In 1897 he discovered that the introduction of a trace of nickel as a
catalyst facilitated the addition of hydrogen to molecules of gaseous carbon
compounds in what is now known as the Sabatier process. For this work Sabatier
won half of the 1912 Nobel Prize in Chemistry. Wilhelm Normann was awarded a
patent in Germany in 1902 and in Britain in 1903 for the hydrogenation of liquid
oils using hydrogen gas, which was the beginning of what is now a very large
industry world wide. The commercially very important Haber-Bosch process
(ammonia hydrogenation) was first described in 1905 and less so Fischer-Tropsch
process (carbon monoxide hydrogenation) in 1922. Another commercial application
is the oxo process (1938), a hydrogen mediated coupling of aldehydes with
alkenes. Wilkinson's catalyst was the first homogeneous catalyst developed in
the 1960s and Noyori asymmetric hydrogenation (1987) one of the first
applications in asymmetric synthesis. A 2007 review article advocated the use of
more hydrogenations in C-C coupling reactions like the oxo process
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