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

植物油和动物脂肪是天然存在于植物或动物体内的脂质。油和脂肪通常以甘油三酯的形式存在，由一分子甘油和三分子脂肪酸形成。脂肪酸是一类弱羧酸，其官能团为烃链。在甘油三酯中，三个脂肪酸通过酯键与甘油分子相连。因此，甘油三酯含有三个酯基，使其成为一种三酯化合物。
酯是一类有机分子，其结构类似于羧酸，但羟基中的氢原子被一个烷基或芳基取代。该基团标记为 R'，以表明它可能与另一侧的 R 基团不同。在甘油三酯中，R' 是甘油骨架，而每个 R 基团则是脂肪酸的碳链。
碳链中含有双键的脂肪酸被称为不饱和脂肪酸。这些双键通常呈cis构型，使碳链发生弯曲。具有这种弯曲链的甘油三酯无法紧密排列，因此它们之间的吸引力通常为长程且较弱。
碳原子之间仅以单键连接的脂肪酸为饱和脂肪酸。这类不带弯曲的链状结构可使甘油三酯紧密排列，从而形成更强的短程分子间相互作用。因此，饱和甘油三酯的熔点高于不饱和甘油三酯，后者在室温下通常呈液态。
甘油三酯可通过水解其三个酯键重新转化为脂肪酸和甘油。酯的水解是一种化学反应，即酯键与水分子或氢氧根离子反应，生成羧酸和醇。酯水解的一个常见应用是制造肥皂，肥皂即甘油三酯中脂肪酸的盐类。该过程称为皂化反应。
在此反应中，氢氧根离子攻击甘油三酯中的三个酯羰基，生成三个脂肪酸分子和一个甘油分子。脂肪酸的羧酸根与碱提供的反离子（通常为钠或钾）结合。
但肥皂是如何起作用的呢？长碳链具有亲脂性和疏水性，意味着它容易与脂肪结合，但不易与水结合。而羧酸根则具有亲水性，意味着它容易与水结合。当你将肥皂与水混合时，长碳链倾向于彼此相互作用，并避免与水分子接触，而羧酸根则更倾向于与水接触。最终，肥皂分子会形成称为胶束的微小聚集体，其中亲水的羧酸根朝向外侧。
现在，假设你的手上沾有疏水性的油脂。仅用水冲洗双手是无法去除这些油脂的。当你使用肥皂洗手时，油脂会与肥皂分子的碳链发生相互作用。随着肥皂分子形成胶束，油脂被带入其疏水性核心，并被包裹在亲水性外壳内，从而可以随水冲洗而被清除。
在本实验中，你将通过皂化反应，利用椰子油和氢氧化钠制备肥皂。随后，通过比较所制肥皂在去离子水、自来水以及氯化钙溶液（模拟极硬水）中的起泡能力，测试其对硬水的耐受性。

酯的水解

<h4>甘油三酯</h4>
甘油三酯是由甘油骨架与3个脂肪酸结合形成的三酯分子。它们是许多存在于植物油和脂肪中的生物脂质的基础。从植物材料中分离出的脂质构成了植物油的基础，而从动物中提取的脂质则用作猪油或脂肪的一般来源。
脂肪酸分子具有一个长碳链——包含12至20个碳原子——一端带有一个弱有机酸。某些脂肪酸的碳链中含有双键，这些即为“不饱和”脂肪。由于碳链中存在双键，这些脂肪酸通常呈现弯曲的链状结构，因为双键倾向于采取cis构型而非trans构型。
不饱和脂肪酸中的甘油三酯分子无法均匀堆叠，导致分子间的吸引力较弱。因此，不饱和脂肪通常在室温下呈液态。相比之下，饱和脂肪酸（即其碳链中仅含单键）由于碳链形状更规则，能够有效堆叠。因此，饱和脂肪分子之间的吸引力更强，熔点也更高。因此，饱和脂肪在室温下通常呈固态。
<h4>水解反应</h4>
甘油三酯与其他含有酯基的分子一样，能够发生水解反应。水解是一种化学反应，其中水分子断裂化学键。在酯类水解过程中，亲核试剂——水或氢氧根离子——进攻酯基的羰基碳，从而断裂酯键。原始分子的一个片段获得水中的氢离子，形成醇；而另一个片段则获得羟基，形成羧酸盐。
在正常条件下，仅有少数水解反应可以在不添加强酸或强碱的情况下发生。强酸或强碱作为催化剂可加速反应，并在反应结束时被回收。水解反应是可逆的，因其副产物为水，故也称为缩合反应。
甘油三酯的水解是水解反应最古老的实例之一，几个世纪以来一直被用于制造肥皂。该反应被称为皂化反应。
<h4>皂化</h4>
在皂化反应中，氢氧根离子攻击甘油三酯中三个酯基的羰基。因此，将三摩尔氢氧化钠按化学计量比加入一摩尔甘油三酯，可使三个酯基发生水解。该反应生成三摩尔肥皂分子和一摩尔甘油。通常，反应中会将氢氧化钠作为限量试剂，以尽量减少未反应的甘油三酯含量，从而降低肥皂的干燥效应。
肥皂分子具有一条长的疏水性碳链，另一端带有一个极性的亲水性羧酸根阴离子。分子两端溶解性的差异使其具有清洁能力。在水中，肥皂分子的长碳链会相互吸引并重新排列，以避开极性的水分子。这使得极性的羧酸根头部朝向水分子，形成一种称为胶束的球形结构。胶束内部为疏水性，外部为亲水性，因而可在水溶液中溶解。
肥皂的胶束特性对其清洁能力至关重要。仅用水冲洗表面的油脂或油污无法将其去除，因为油脂具有疏水性且不溶于水。然而，当使用肥皂时，油脂会进入胶束内部，而胶束的外部具有亲水性，因此可以通过水冲洗而被清除。
肥皂的一个显著缺点是其对硬水的不耐受性。硬水是指含有镁、铁和钙等以矿物质形式存在的金属离子的水溶液。水的“硬度”取决于这些离子的浓度。在金属阳离子存在的情况下，肥皂分子会与这些离子相互作用，导致其从溶液中析出，形成一种盐类沉淀物，通常称为皂垢。
<h4>参考文献</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). 化学与化学反应性。加利福尼亚州贝尔蒙特：Brooks/Cole, Cengage Learning。</li>
	<li style="font-size:17px">Silberg, M.S. (2015). 化学：物质的分子本质与变化。马萨诸塞州波士顿：McGraw Hill。</li>
	<li style="font-size:17px">Streiwiester, A., Heathcock, C.H., Kosower, E.M. (1998). 有机化学导论。新泽西州上萨德尔河：Prentice Hall。</li>
</ol>

甘油三酯  
甘油三酯是由一个甘油骨架与3个脂肪酸结合形成的三酯分子。它们是多种生物体脂的主要成分……

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Hydrolysis of an Ester

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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