Name: soap formation from triglycerides
Uploaded: 2020-03-26T07:22:28.000Z
Duration: 4 min 10 s
Channel: JoVE

Les huiles végétales et les graisses animales sont des lipides naturellement présents dans les plantes ou les animaux. Les huiles et les graisses se présentent souvent sous forme de triglycérides, qui sont formés d’une molécule de glycérol et de trois acides gras. Un acide gras est un acide carboxylique faible dont le groupe fonctionnel est une chaîne hydrocarbonée. Dans le triglycéride, les trois acides gras sont liés à la molécule de glycérol par des liaisons esters. Un triglycéride a donc trois groupes esters, ce qui fait de la molécule un triester.
Un ester est un type de molécule organique qui ressemble à un acide carboxylique, l’hydrogène de l’hydroxyle étant remplacé par un groupe alkyle ou aryle. Ce groupe est appelé R-prime pour montrer qu’il peut être différent du groupe R de l’autre côté. Dans un triglycéride, R-prime est le squelette du glycérol, et chaque groupe R est la chaîne d’un acide gras.
Les acides gras avec des doubles liaisons dans leurs chaînes carbonées sont connus sous le nom d’acides gras insaturés. Ces doubles liaisons ont tendance à être dans la conformation cis, ce qui met une courbure dans la chaîne. Les triglycérides avec ces chaînes courbées ne peuvent pas s'emboîter étroitement, de sorte que les forces d'attraction entre eux sont généralement à longue portée et faibles.
Les acides gras qui n’ont que des liaisons simples entre les atomes de carbone sont saturés. Ces chaînes non courbées permettent aux triglycérides de s’agglomérer étroitement, ce qui permet des interactions intermoléculaires plus fortes et à courte portée. Cela donne aux triglycérides saturés un point de fusion plus élevé que les triglycérides insaturés, qui sont généralement liquides à température ambiante.
Les triglycérides peuvent être reconvertis en acides gras et en glycérol en hydrolysant les trois esters. L’hydrolyse des esters est une réaction qui rompt une liaison ester avec une molécule d’eau ou un ion hydroxyde pour former un acide carboxylique et un alcool. Une utilisation courante de l’hydrolyse des esters est de créer des savons, qui sont les sels d’acides gras des triglycérides. Ce processus s’appelle la saponification.
Dans cette réaction, les ions hydroxyde attaquent chacun des trois esters carbonyles du triglycéride, produisant trois molécules d’acide gras et une molécule de glycérol. Les carboxylates d’acides gras s’associent au contre-ion de la base, qui est généralement du sodium ou du potassium.
Mais comment fonctionne le savon ? La longue chaîne carbonée est lipophile et hydrophobe, ce qui signifie qu’elle est attirée par les graisses mais pas par l’eau. Le groupe carboxylate est hydrophile, c’est-à-dire qu’il est attiré par l’eau. Lorsque vous mélangez du savon et de l’eau, les longues chaînes carbonées ont tendance à interagir les unes avec les autres et à éviter l’interaction avec les molécules d’eau, tandis que les groupes carboxylates préfèrent interagir avec l’eau. En fin de compte, les molécules de savon forment des grappes appelées micelles, avec les carboxylates hydrophiles tournés vers l’extérieur.
Maintenant, supposons qu'il y ait de la graisse hydrophobe sur vos mains. Le simple fait de se rincer les mains à l’eau n’enlèverait pas la graisse. Lorsque vous lavez avec du savon, la graisse interagit avec les chaînes de carbone du savon. Au fur et à mesure que les molécules de savon forment des micelles, elles amènent la graisse dans le noyau hydrophobe en l’enfermant dans une coque hydrophile qui peut ensuite être rincée à l’eau.
Dans ce laboratoire, vous effectuerez une réaction de saponification pour créer du savon à partir d’huile de noix de coco et d’hydroxyde de sodium. Vous testerez ensuite la tolérance de votre savon à l’eau dure en comparant la capacité moussante de l’eau déminéralisée, de l’eau du robinet et d’une solution de chlorure de calcium, qui imite l’eau extrêmement dure.

hydrolyse d’un ester

<h4>Triglycérides</h4>
Les triglycérides sont des molécules triesters composées d’un squelette glycérol lié à 3 acides gras. Ceux-ci forment la base de nombreux lipides biologiques que l’on trouve dans les huiles et les graisses végétales. Les lipides isolés de la matière végétale sont à la base des huiles végétales, tandis que les lipides extraits d’animaux sont utilisés comme saindoux ou sources générales de graisses.
Une molécule d’acide gras a une longue chaîne carbonée – entre 12 et 20 carbones – avec un acide organique faible à une extrémité. Certains acides gras ont des doubles liaisons dans la chaîne carbonée. Ce sont des graisses « insaturées ». En raison de la double liaison dans la chaîne carbonée, ces acides gras ont généralement une forme de chaîne courbée, car la liaison a tendance à adopter la conformation cis plutôt que la conformation trans.
Les molécules de triglycérides d’un acide gras insaturé sont incapables de s’empiler uniformément, ce qui entraîne moins de forces d’attraction entre les molécules. Ainsi, les graisses insaturées sont généralement liquides à température ambiante. En revanche, les graisses saturées, c’est-à-dire celles qui n’ont que des liaisons uniques dans leur chaîne carbonée, sont capables de s’empiler parce que leur chaîne est de forme plus régulière. Les graisses saturées ont donc des forces d’attraction plus fortes entre les molécules et, par conséquent, des points de fusion plus élevés. Ainsi, les graisses saturées ont tendance à être solides à température ambiante.
<h4>Réaction d’hydrolyse</h4>
Les triglycérides, comme d’autres molécules possédant des groupes esters, sont capables de subir une hydrolyse. L’hydrolyse est un type de réaction chimique où une molécule d’eau rompt une liaison. Dans le cas d’une hydrolyse d’ester, le nucléophile – l’eau ou un ion hydroxyde – attaque le carbone carbonyle du groupe ester pour rompre la liaison ester. Un fragment de la molécule d’origine gagne l’ion hydrogène de l’eau pour former un alcool, tandis que l’autre fragment gagne le groupe hydroxyde pour former un sel de l’acide carboxylique.
Dans des conditions normales, seules quelques réactions d’hydrolyse se produisent sans l’ajout d’un acide ou d’une base forte. L’acide ou la base forte agit comme un catalyseur pour accélérer la réaction, et elle est récupérée à la fin. La réaction d’hydrolyse est réversible. On l’appelle une réaction de condensation car elle produit de l’eau comme produit secondaire.
L’hydrolyse d’un triglycéride est l’un des plus anciens exemples de réaction d’hydrolyse, car il est utilisé depuis des siècles pour fabriquer du savon. La réaction s’appelle la saponification.
<h4>saponification </h4>
Lors de la saponification, les ions hydroxyde attaquent les groupes carbonyles dans les trois esters du triglycéride. Par conséquent, l’addition stœchiométrique de trois moles d’hydroxyde de sodium à une mole de triglycéride entraînera l’hydrolyse des trois groupes esters. Cette réaction produit trois moles de molécules de savon et une mole de glycérol. Souvent, la réaction sera effectuée avec de l’hydroxyde de sodium comme réactif limitant pour minimiser la quantité de triglycérides n’ayant pas réagi, ce qui peut réduire l’effet desséchant du savon.
Une molécule de savon a une longue chaîne carbonée hydrophobe avec un anion carboxylate hydrophile polaire à l’autre extrémité. La différence de solubilité le long de la molécule permet sa capacité de nettoyage. Dans l’eau, les longues chaînes carbonées des molécules de savon vont être attirées les unes vers les autres et se réorganiser pour éviter les molécules d’eau polaires. Cela oriente les têtes de carboxylate polaires vers les molécules d’eau, formant une structure sphérique connue sous le nom de micelle. Le centre est hydrophobe et l’extérieur est hydrophile, ce qui permet aux micelles d’être solubles dans les solutions aqueuses.
Les propriétés micellaires du savon sont essentielles à sa capacité de nettoyage. Le simple fait de rincer la graisse ou l’huile d’une surface ne permet pas de l’enlever. En effet, la graisse est hydrophobe et non soluble dans l’eau. Cependant, lorsque du savon est utilisé, la graisse pénètre à l’intérieur de la micelle. L’extérieur de la micelle est hydrophile, elle peut donc être rincée à l’eau.
Un inconvénient important du savon est son intolérance à l’eau dure. L’eau dure est une solution aqueuse contenant du magnésium, du fer et des ions calcium sous forme de minéraux. La « dureté » de l’eau dépend de la concentration de ces ions. En présence de cations métalliques, les molécules de savon interagissent avec les molécules et les font tomber de la solution, formant un précipité salin plus communément appelé écume de savon.
<h4>références</h4>
<ol>
	<li style="font-size :17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). Chimie et réactivité chimique. Belmont, Californie : Brooks/Cole, Cengage Learning.</li>
	<li style="font-size :17px">Silberg, M.S. (2015). Chimie : la nature moléculaire de la matière et du changement. Boston, MA : McGraw Hill.</li>
	<li style="font-size :17px">Streiwiester, A., Heathcock, C.H., Kosower, E.M. (1998). Introduction à la chimie organique. Upper Saddle River, NJ : Prentice Hall.</li>
</ol>

Hydrolyse d’un ester

Triglycérides
Les triglycérides sont des molécules triesters composées d’un squelette glycérol lié à 3 acides gras. Ceux-ci constituent la base de nombreux ...

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soap formation from triglycerides

Triglycerides are lipids made of a glycerol backbone linked to three fatty acids. They are common in vegetable oils and animal fats. Plant lipids are the basis of vegetable oils, while animal lipids are used as lard or general sources of fats.

A fatty acid has a long carbon chain, usually between 12 and 20 carbons, with a weak organic acid at one end. Some fatty acids contain double bonds. These are unsaturated fats. The double bond usually gives the chain a bent, cis shape. As a result, unsaturated triglycerides do not stack evenly and have weaker attractions between molecules. They are usually liquid at room temperature. Saturated fats have only single bonds. Their chains are more regular, so they stack more easily. This gives them stronger attractions and higher melting points, which is why they are often solid at room temperature.

Triglycerides can also undergo hydrolysis because they contain ester groups. Hydrolysis is a reaction in which water breaks a bond. In ester hydrolysis, a nucleophile such as water or a hydroxide ion attacks the carbonyl carbon of the ester. One part of the molecule becomes an alcohol after gaining a hydrogen ion from water. The other part becomes a salt of a carboxylic acid after gaining the hydroxide group. Under normal conditions, only a few hydrolysis reactions occur without a strong acid or base. The acid or base acts as a catalyst and is recovered at the end. This reaction is reversible. It is also called a condensation reaction because water is formed as a side product.

The hydrolysis of triglycerides is one of the oldest examples of this chemistry and has been used for centuries to make soap. This reaction is called saponification. During saponification, hydroxide ions attack the carbonyl groups in the three ester bonds of the triglyceride. With three moles of sodium hydroxide for one mole of triglyceride, all three ester groups can be hydrolyzed. The products are three moles of soap molecules and one mole of glycerol. In practice, sodium hydroxide is often the limiting reagent so less triglyceride is left over. That helps reduce the drying effect of the soap.

Soap Formation from Triglycerides

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Soap Formation from Triglycerides

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