Name: purifying crystals by solubility
Uploaded: 2020-03-26T07:22:27.000Z
Duration: 3 min 12 s
Channel: JoVE

تحتوي معظم المنتجات على شوائب. تتمثل إحدى طرق تنقية هذه المنتجات في إعادة التبلور. تبدأ إعادة التبلور بإذابة المنتج غير النقي في مذيب ساخن كاف لتشكيل محلول مشبع ، حيث يتم إذابة أكبر قدر ممكن من المذاب في المذيب. أي مذاب إضافي لن يبدو أنه يذوب. 
يتم استخدام مذيب ساخن لأن الذوبان يزداد عادة مع درجة الحرارة. مع زيادة درجة الحرارة ، تزداد كمية المذاب التي يمكن إذابتها في المذيب. عندما يبرد المحلول ، تقل قابلية ذوبان المنتج ، وتتجمع جزيئات المذاب معا لتشكيل بلورات صغيرة مستقرة تسمى النوى. 
هذه هي الخطوة الأولى من التبلور ، والتي تسمى التنوي. يحدث نمو بلوري إضافي على النوى لأن الجزيئات المذابة لها تقارب أكبر للانضمام إلى بلورات المذاب الموجودة بدلا من تكوين بلورات جديدة. يتم ترك الشوائب القابلة للذوبان في المحلول. 
يمكن أن يحدث التبلور تلقائيا أو يمكن تشجيعه عن طريق خدش الجزء الداخلي من القارورة ، أو تحريك المحلول ، أو إضافة بلورة بذرة للمركب ، وكلها توفر سطحا لمزيد من النمو. 
لا تعمل إعادة التبلور إلى بلورات كبيرة نقية وذات شكل منتظم إلا عند استخدام مذيب مناسب. يجب أن يكون المركب غير قابل للذوبان في المذيب في درجة حرارة الغرفة وقابل للذوبان في درجات حرارة عالية. من الناحية المثالية ، يجب أن تكون الشوائب إما غير قابلة للذوبان في المذيب عند درجة حرارة عالية أو قابلة للذوبان في المذيب في درجة حرارة الغرفة. 
إذا كانت الشوائب غير قابلة للذوبان في المذيبات الساخنة ، يتم ترشيحها قبل التبلور. بعد إعادة التبلور ، يتم ترشيح البلورات وغسلها بمذيب بارد لإزالة الشوائب من الأسطح. الآن ، يمكن تحليل نقاء البلورات. 
عند اختيار مذيب ، ضع في اعتبارك أنه كلما زاد الاختلاف في الذوبان بين درجات الحرارة المرتفعة ودرجات الحرارة المنخفضة ، زادت احتمالية خروج المذاب من المحلول أثناء تبريده لتكوين بلورات. معدل التبريد مهم أيضا في إعادة التبلور. 
يفضل التبريد السريع تكوين العديد من مواقع التنوي ونمو بلورات أصغر ، في حين أن التبريد يؤدي ببطء إلى تكوين عدد أقل من مواقع التنوي ونمو بلورات أقل وأكبر ولكن أكثر نقاء. لذلك ، يفضل التبريد البطيء. 
في هذا المختبر ، ستعيد بلورة مركبين عضويين غير نقيين ، الأسيتانيليد و عبر - حمض السيناميك ، ثم تقييم نقاء المركبات المستردة من خلال مقارنة نطاق نقطة انصهارها بالقيم الموجودة في الأدبيات.

إعادة التبلور

إعادة التبلور
في كثير من الأحيان ، يكون المنتج المطلوب للتفاعل الكيميائي جزءا من خليط تفاعل أكثر تعقيدا ، والذي يمكن أن يتكون من المذيب والمواد الأولية والشوائب. يعد تعلم كيفية تنقية المركبات العضوية بشكل صحيح تقنية قيمة في الكيمياء العضوية. تسخر إعادة التبلور الاختلافات في قابلية ذوبان المركب المطلوب والشوائب في المذيب لتنقية المنتج المطلوب كمادة صلبة. هناك ثلاث طرق قياسية للتنقية: التقطير والاستخراج وإعادة التبلور. 
الذوبان
قابلية ذوبان المادة هي الحد الأقصى للكمية التي تذوب في حجم ثابت لمذيب معين عند درجة حرارة معينة. المواد المذابة المختلفة لها قابليات ذوبان مختلفة وتذوب في مذيبات مختلفة. قد يكون للمواد المذابة خصائص مميزة يمكن استغلالها لإعادة التبلور. تظهر المركبات أحد السلوكيات التالية في المذيب. أولا ، يمكن أن يكون المركب غير قابل للذوبان أو له قابلية ذوبان منخفضة جدا في المذيب في جميع درجات الحرارة. ثانيا ، يمكن أن يكون المركب قابلا للذوبان في المذيب في درجات حرارة أعلى. ثالثا ، يمكن أن يكون المركب قابلا للذوبان في المذيب في جميع درجات الحرارة. 
العامل الرئيسي في تحديد ما إذا كان المذاب يذوب في المذيب ويشكل محلولا هو قوة ونوع القوى بين الجزيئات بين المذاب والمذيب. القاعدة العامة هي "مثل يذوب مثل" ، مما يعني أن المواد ذات الأنواع المتشابهة من القوى بين الجزيئات تذوب في بعضها البعض. على سبيل المثال ، المواد القطبية مثل ملح الطعام (NaCl) ، تذوب جيدا في المياه القطبية. 
عامل رئيسي آخر يحسن الذوبان هو درجة الحرارة. بالنسبة للعديد من المواد ، تزداد قابلية الذوبان بشكل كبير في درجات الحرارة المرتفعة. هذا يرجع إلى حقيقة أن الطاقة الحركية المتزايدة في درجات الحرارة المرتفعة تكسر القوى المذابة بين الجزيئات التي تحافظ على الجزيئات معا. يظهر هذا في الحياة اليومية. على سبيل المثال ، نحن نعلم أن ملح الطعام (NaCl) يذوب جيدا في الماء. ومع ذلك ، يذوب المزيد في درجات حرارة أعلى منه في درجات الحرارة المنخفضة. 
من الناحية النوعية ، يعتبر المحلول غير مشبع إذا لم يتم الوصول إلى الحد الأقصى لكمية المذاب المذابة بعد. عندما يذوب الحد الأقصى الممكن للمذاب ، يكون المحلول مشبعا. يحتوي المحلول الفائق التشبع على مذاب مذاب أكثر من الحد الأقصى للكمية الممكنة في ظل الظروف النموذجية. 
<h4>إعادة التبلور </h4>
تستفيد إعادة التبلور من الاختلافات في الذوبان بين المنتج المطلوب والملوثات في درجات الحرارة العالية. تتمثل الخطوة الأولى لإعادة التبلور في إذابة خليط المنتج في الحد الأدنى من المذيبات الساخنة التي لا تزال تنتج محلول مشبع - ولكن ليس مفرط التشبع. بعد ذلك ، يتم تبريد المحلول إلى درجة حرارة الغرفة ، مما يقلل من قابلية ذوبان كل من المركب المطلوب والشوائب. 
عندما يبرد المحلول ، يبدأ تبلور المكون النقي ، في حين أن الشوائب التي لا تزال قابلة للذوبان لا تبدأ في ذلك. يحدث هذا عندما يكون المكون محل الاهتمام في تركيز أعلى بكثير من الشوائب. أولا ، في مرحلة التنوي ، يبدأ المذيب التكتل العشوائي للجزيئات المذابة ، مكونا أول بلورة تسمى البذرة أو النواة. بعد ذلك ، في مرحلة نمو الجسيمات أو التبلور ، تتم إضافة المزيد من الجزيئات إلى البذرة ، لتشكيل بلورة. تحتوي البلورة على المركب النقي ، بينما تبقى الشوائب في المذيب. 
يستمر التنوي بشكل أسرع من نمو الجسيمات في محلول مفرط التشبع. مع المزيد من البذور ، تكون كل بلورة أصغر. وبالتالي ، إذا كان المحلول مشبعا ، بدلا من التشبع المفرط ، يتشكل عدد أقل من البذور ، مما ينتج عنه بلورات أكبر. يتيح تسخين المحلول إلى درجة حرارة أعلى قبل التبريد إلى درجة حرارة الغرفة إذابة تركيز أعلى من المذاب ، مما يقلل من التشبع الفائق. بالإضافة إلى ذلك ، يؤدي التبريد السريع إلى تنوي سريع ، وتشكيل العديد من البلورات الصغيرة ومحاصرة الشوائب داخل البلورات. يفضل التبريد البطيء لتحقيق بلورات أقل وأكبر. 
بمجرد أن يبرد المحلول إلى درجة حرارة الغرفة ، وتتشكل البلورات ، يتم ترشيح المحلول باستخدام الترشيح الفراغي. ثم يسمح للبلورات بالجفاف. يتم حساب النسبة المئوية للاسترداد بقسمة كتلة المنتج المستعاد على كتلة المنتج الخام. 
<img src="/CDNSource/lm/labs/58/58_Concepts_1.jpg">
نادرا ما يكون الانتعاش 100٪ ، حيث أن قابلية ذوبان المركب في درجات حرارة منخفضة تتحكم في مقدار المركب المتبلور. 
<h4>اختيار مذيب</h4>
لكي يكون التبلور فعالا ، يجب استخدام المذيب الأمثل. يجب أن يكون للمنتج المطلوب قابلية ذوبان منخفضة في المذيب المحدد في درجة حرارة الغرفة ولكن قابلية ذوبان عالية في المذيب عند درجة حرارة أعلى. من الناحية المثالية ، يجب أن تكون الشوائب قابلة للذوبان في المذيب في جميع درجات الحرارة. وبالتالي ، عندما يضاف الخليط إلى المذيب عند درجة حرارة عالية ، يذوب المنتج والشوائب المطلوبة بسهولة. 
عندما يتم تبريد المحلول ، تقل قابلية ذوبان المنتج المطلوب ويبدأ التبلور في الحدوث ، مكونا منتجا منقا. في بعض الأحيان ، قد تظل الشوائب غير قابلة للذوبان في المذيب المفضل ، حتى في درجات الحرارة المرتفعة. يمكن أن يؤدي ترشيح الجاذبية الساخنة للمحلول الذي يحتوي على منتج مذاب إلى إزالة الشوائب الصلبة. يمكن بعد ذلك إعادة بلورة المنتج عن طريق تبريد العينة. 
من الناحية المثالية ، يجب أن يكون المذيب المراد استخدامه قادرا على الغليان عند درجة حرارة أقل بكثير من نقطة انصهار المنتج المطلوب. يجب أن يكون المذيب أيضا خاملا ولا يتفاعل مع المنتج المنقى المطلوب. 
<h4>المراجع</h4>
<ol>
	<li style="font-size:17px">كوتز ، جي سي ، تريتشيل جونيور ، بي إم ، تاونسند ، جي آر (2012). الكيمياء والتفاعل الكيميائي. بلمونت ، كاليفورنيا: بروكس / كول ، Cengage Learning. </li>
	<li style="font-size:17px">سيلبربيرج ، MS (2009) الكيمياء: الطبيعة الجزيئية للمادة والتغيير. بوسطن ، ماساتشوستس: ماكجرو هيل. </li>
	<li style="font-size:17px">هاريس ، العاصمة (2015). التحليل الكيميائي الكمي. نيويورك ، نيويورك: دبليو إتش فريمان وشركاه. </li>
</ol>

إعادة التبلور
في كثير من الأحيان ، يكون المنتج المطلوب للتفاعل الكيميائي جزءا من خليط تفاعل أكثر تعقيدا ، والذي يمكن أن يتكون من المذيب ، ...

Education

JoVE Lab Manual

Chemistry

الكيمياء

Recrystallization

purifying crystals by solubility

Purifying crystals by solubility is a key organic chemistry technique. Recrystallization uses differences in solubility to separate a desired solid product from starting materials, solvent, and impurities. It is one of the three standard purification methods, along with distillation and extraction.

Solubility is the maximum amount of a substance that can dissolve in a fixed volume of solvent at a given temperature. Different solutes dissolve in different solvents, and their behavior can vary widely. A compound may stay insoluble or only slightly soluble at all temperatures, it may dissolve better at high temperature, or it may remain soluble at all temperatures. The main reason is the strength and type of intermolecular forces between the solute and the solvent. The rule “like dissolves like” applies here, so substances with similar intermolecular forces tend to dissolve in each other. For example, polar table salt (NaCl) dissolves well in polar water. Temperature also matters. In many cases, solubility rises as temperature rises because added kinetic energy helps break the forces holding solute molecules together. A solution is unsaturated when more solute can still dissolve. It is saturated when it has reached the maximum amount. A supersaturated solution contains more dissolved solute than expected under normal conditions.

Recrystallization uses those solubility differences at high temperature and low temperature. The crude product is first dissolved in a minimal volume of heated solvent. The goal is a saturated solution, but not a supersaturated one. The solution is then cooled to room temperature. As it cools, the solubility of both the desired compound and the impurity drops. The pure compound begins to crystallize first, while impurities that are still soluble remain in the liquid. This works best when the compound of interest is present at a much higher concentration than the impurity.

Crystal formation happens in two stages. In nucleation, the solvent helps solute molecules gather into the first tiny crystal, called a seed or nucleus. In particle growth, more molecules attach to that seed and the crystal gets larger. The crystal contains the pure compound, while the impurity stays in the solvent. In a supersaturated solution, nucleation happens faster than particle growth, so more seeds form and the crystals stay small. If the solution is only saturated, fewer seeds form and the crystals become larger. Heating the solution more before cooling can reduce supersaturation by dissolving a higher concentration of solute. Slow cooling is also preferred because rapid cooling makes many small crystals and can trap impurity inside them.

After the crystals form, the solution is filtered by vacuum filtration and the crystals are allowed to dry. The percent recovery is found by dividing the mass of the recovered product by the mass of the crude product. Recovery is rarely 100% because the compound’s low-temperature solubility limits how much can crystallize. Choosing the right solvent is important for successful crystallization. The desired product should be poorly soluble at room temperature but highly soluble when hot. Ideally, impurities stay soluble at all temperatures, so they remain in solution as the product crystallizes. If some impurities do not dissolve even when hot, hot gravity filtration can remove them before the sample is cooled again. The solvent should also boil well below the melting point of the desired product and should not react with it.

Purifying Crystals by Solubility

lab techniques

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Boiling Point and Vapor Pressure

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recrystallization

Purifying crystals by solubility is a key organic chemistry technique. Recrystallization uses differences in solubility to separate a desired solid product from starting materials, solvent, and impurities. It is one of the three standard purification methods, along with distillation and extraction. Solubility is the maximum amount of a substance that can dissolve in a fixed volume of solvent at a given temperature. Different solutes dissolve in different solvents, and their behavior can vary...

extraction

Separating Mixtures with Solvent Extraction

Solvent extraction is a common way to separate and isolate a target compound in organic chemistry. In this process, a solute moves from one phase to another so it can be separated from unreacted starting materials or impurities. Extraction is also useful when a product is dissolved in a reaction solvent that is hard to remove by evaporation, such as a solvent with a high boiling point. There are three main types of extraction. In solid-liquid extraction, the solute moves from a solid phase...

simple distillation

Separating Liquids by Boiling Point

Separating liquids by boiling point is the goal of distillation. A miscible mixture contains liquids that mix evenly, and it can be heated so that the more volatile liquid evaporates first. Volatility is a liquid’s tendency to vaporize, and boiling begins when bubbles form in the liquid phase. In a closed container, liquid and vapor can reach equilibrium, which means the rate of vaporization equals the rate of condensation. The pressure from the vapor above the liquid is called vapor pressure.

steam distillation

Steam Distillation and Low-Boiling Separation

Steam distillation is a separation method for temperature-sensitive organic compounds. It works best when the organic compound is immiscible in water, meaning the two liquids do not mix. The method helps remove the organic molecule from a non-volatile contaminant without using the higher temperatures needed for many other separations. In steam distillation, the immiscible mixture is heated until both water and the volatile organic compound distill together. The vapor rises to a condenser,...

thin-layer chromatography

TLC Separation by Polarity

Thin-layer chromatography (TLC) separates compounds in a mixture by polarity. It uses the same basic parts as other chromatography methods: a stationary phase, a mobile phase, and the solute. In TLC, the stationary phase is a thin coating on a plate instead of material packed into a column. The stationary phase in most TLC systems is polar silica gel, a form of silicon dioxide. Its surface has OH groups that can form hydrogen bonds. The sample is first spotted on a pencil start line with a...

column chromatography

Separating Mixtures by Polarity

Column chromatography separates compounds in an organic chemistry lab by using differences in physical properties. It is especially useful for microscale separations when less than 10 grams of material is available for analysis. The method can separate a mixture based on solubility, polarity, hydrophobicity, size, and charge. Any chromatography system has three main parts: the stationary phase, the mobile phase, and the sample mixture. In column chromatography, the stationary phase is usually...

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,...

synthesis of luminol

Luminol Chemiluminescence in Forensics

Luminol chemiluminescence shows how a chemical reaction can produce light and help with blood detection. Chemical reactions may be endothermic or exothermic, depending on whether they absorb or release heat as reactants become products. The enthalpy of a reaction is the energy difference between reactants and products, or ΔH. A positive ΔH means the reaction is endothermic. A negative ΔH means it is exothermic. Many reactions do not happen in a single step. They can pass through intermediates...

esterification

Ester Formation and Everyday Fragrances

Esters are organic compounds with a carbonyl group attached to an oxygen that connects to another alkyl or aryl group. Their general formula is RCOOR'. They are often made by reacting a carboxylic acid with an alcohol in an esterification reaction. In this process, the hydroxyl group of the acid is replaced by an alkyl or aryl group. Many simple esters have familiar fruit and flower smells because their structures are small and less complex. Formic acid and methanol can combine to form methyl...

identification of unknown aldehydes and ketones

identification-of-unknown-aldehydes-and-ketones

Carbonyl Tests for Aldehydes and Ketones

Aldehydes and ketones are carbonyl compounds, which means they contain a C=O functional group. A ketone has two alkyl or aryl groups attached to the carbonyl carbon. Acetone is the simplest ketone, with two methyl groups. An aldehyde is similar, but it has at least one hydrogen attached to the carbonyl carbon. Formaldehyde is the simplest aldehyde, and acetaldehyde has one hydrogen and one methyl group. The carbonyl carbon in both aldehydes and ketones is electrophilic because the oxygen atom...

uv-vis spectroscopy of dyes

How Dyes Absorb Visible Light

Dyes absorb visible light because their molecular structure lets them interact with specific wavelengths. When light hits a substance, some light is absorbed and the rest is reflected or transmitted. The color we see depends on the wavelength that is reflected. A blue substance reflects blue light, around 430 to 480 nm, and absorbs the complementary orange region, around 590 to 630 nm. Compounds that do not absorb in the visible range appear colorless to the human eye. Light also has energy and...

identifying alcohols

Alcohol Tests for Primary, Secondary, and Tertiary

Alcohol tests can help sort alcohols by structure and reactivity. Alcohols are organic molecules with a hydroxyl group attached to an alkyl or aryl group, written as ROH. If the carbon with the hydroxyl group has one R group, the alcohol is primary. If it has two R groups, it is secondary. If it has three R groups, it is tertiary. Alcohols can also be aromatic when they contain a benzene ring. Phenol is the simplest aromatic alcohol. To identify an alcohol, chemists can use its chemical...

chemistry

Comprehensive, curriculum-focused videos for both instructors and students in introductory chemistry lab courses.

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Learn how recrystallization uses solubility, temperature, and slow cooling to purify organic solids and separate impurities from crystals.

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Purifying Crystals by Solubility

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