meltingpointrecrys-091013090749-phpapp01 - Melting Points...

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Unformatted text preview: Melting Points Melting Background Information Background Melting point: temperature at which solid is in equilibrium with its Melting liquid Melting point range: span of temperatures from the point at which Melting the crystals first begin to liquefy to the point at which the entire sample is liquid Pure compounds usually melt over a narrow temperature range of 1Pure 2°C Presence of soluble impurity almost always decreases and Presence broadens the melting point range Melting Point of Mixture (X and Y) Melting Eutectic point Eutectic temperature: lowest possible melting point for a mixture of Eutectic 2 compounds Eutectic composition: composition at which both solid X and solid Eutectic Y are in equilibrium with the liquid mixture (at eutectic temperature) Taking a Melting Point: Packing Taking Pack the capillary tube by pressing the open end gently into a sample of the compound to be analyzed. (Capillary tubes break easily!) The solid should fill the tube to a depth of ~2 mm. Tap the bottom of the capillary on a hard surface so that the crystals pack down into the bottom of the tube. Melting Point Apparatus: Mel-Temp Melting Pack the crystals at the bottom of the tube and place the tube in the slot behind the eye-piece on the Mel-Temp. Turn on the Mel-Temp and turn the knob to adjust heating rate (not too fast or too slow). Watch through eyepiece and record temperature at which the crystals first begin to liquefy and record temperature at which the entire sample is liquid (mp range). Experiment: Part I - Practice Melting Range Experiment: Use only benzoic acid (mp = 122-123°C). Use benzoic Use proper size of sample (not more than 2 mm in capillary) just Use enough to see!!! Do not move the Mel-Temp to your bench or remove the probe. Do Increase Temperature by 10°C/min until 10-15°C below the Increase known mp range, then reduce rate to 1-2°C/min. Allow mp apparatus to cool down naturally after experiment (no Allow wet paper towels or ice – may use air) Experiment: Part III - Identifying Your Unknown Experiment: Record the code of your unknown. Record of unknown Take 2 melting points (2 different samples): Take 1st: taken quickly to approximate range (~10°C/min) taken 2nd: taken slower (1-2°C/min) when 10°C below mp range approximated taken from 1st run Choose the known compound that could match the mp of your unknown Choose known unknown (table 1 - p. 39). Mix known vs unknown on a watch glass (1:1 mixture) thoroughly by Mix known grinding with a spatula. grinding Take mp of the mixture: Take >If mp is the same as mp of unknown, unknown is identified. >If mp of mixture is different from mp of unknown, repeat process with another compound. Part III - Identifying Your Unknown Unknown mp = 133-135°C Unknown ⇒ Benzoin (mp = 135-137°C) or trans-Cinnamic Acid (mp = 133-134°C) trans Mix Unknown and Benzoin mp = 135-137°C mp <<133°C ⇒ Unknown is Benzoin ⇒ Unknown is not Benzoin Mix Unknown and trans-Cinnamic Acid trans mp <<133°C mp = 133-134°C ⇒ Unknown is not trans-Cinnamic Acid trans ⇒ Unknown is trans-Cinnamic trans Acid Waste Handling Waste Don’t discard capillaries in the trash. Put them in the collection Don beaker labeled “3221 Melting Capillaries”. No glass in the trash!! No Any unused unknown is to be returned to the TA in the original Any sample vial (properly capped). Purifying Acetanilide by Purifying Recrystallization Definition Definition Recrystallization: purification process used to remove Recrystallization: impurities from organic compounds which are solid at room temperature. Principle: Principle: Solubility of a compound in a solvent increases with temperature. Solubility As the solution cools, crystals form and grow. As Molecules in a crystal have a greater affinity for molecules of the Molecules same kind than for impurities. Choosing a Recrystallizing Solvent The compound being purified must be insoluble in solvent at The insoluble room temp but soluble in boiling solvent. Solvent’s boiling point must be lower than the compound’s Solvent lower compound melting point. An abundant quantity of crystals must be recoverable from the An abundant cool solvent. A B C Room Temperature Soluble Not Soluble Not Soluble Solvent’s Boiling Point Soluble Soluble Soluble Cold Solvent Soluble Not Soluble Not Soluble Mass Recovered NA 0.85 g 0.63 g Note: 1. Be careful that your compound does not melt before the solvent boils. Why? – Oiling Out 2. Make sure that you record your observations in a chart like the one shown here. Step 1 – Choosing the Right Solvent for Acetanilide Step Water Ethanol Acetone (polar) (polar) (polar) Petroleum Ether Record whether the acetanilide is soluble or not at room temperature. Record acetanilide Heat the solution(s) to boiling (in a hot water bath). Record whether the Heat acetanilide is soluble or not in each hot solvent. acetanilide Let them cool at room temperature first, and then in an ice water bath. Let Record your observations – Does crystal form?. observations Choose the appropriate solvent by consulting with your TA and go to the next step. Step 2 – Dissolving the Compound Step Dissolve the compound in a MINIMUM volume of boiling solvent. Dissolve MINIMUM After all of the compound dissolves, add 5% excess to prevent After premature crystallization. (For example: if 10 mL of boiling solvent is premature required to just dissolve a compound, 0.5 mL will be added) 1- Heat some solvent to boiling. Place the solid to be recrystallized in an Erlenmeyer flask. 3- Swirl the flask 2- Add small amount (1 mL) of the hot solvent into the flask. 4- Place the flask on the hot plate to keep the solution warm (Use a Pasteur pipet) 5- Repeat steps 2 to 4 until you have dissolved the compound, and then add the 5% excess. Step 3 - Decolorizing Step Hot & Ready! Decolorizing the solution Decolorizing Add a spatula tip of activated carbon after the solution after is removed from the heat and has stopped boiling !!! is Stir in the carbon and then allow it to sit briefly. Hot Filtration!!! Hot The hot solution is filtered (in this step) by gravity hot gravity filtration through a stemless funnel containing a fluted filtration stemless fluted filter paper. Keep the paper and funnel warm to avoid crystals forming during filtration. Be quick!!! at the hot filtration step to avoid premature premature crystallization in the filter. Step 4 – Forming Crystals Step Recrystallizing Pure Compound Recrystallizing Slowly cool the solution at room temperature before placing in an iceSlowly water bath to maximize crystal formation. TIP: If crystals do not form, induce crystallization yourself: If - scratching the inside of the flask with a glass stirring rod - or concentrating by evaporation (last resort!). concentrating Prepare ice cold solvent for washing step! Step 5 – Collecting, Washing, and Drying the Crystals Vacuum filtration: Pre-weigh your filter paper and record mass! Vacuum filtration Pre Step 6 – Recovery Analysis (Next Week) Sandwich your crystals between two filter Sandwich papers. Let your crystals dry for 1 week in your lab drawer. in Next Week: Measure and record the mass and Measure mass melting point. melting Submit your crystals to the TA in a Submit labeled vial (your name, compound’s name, mass of compound, and MP). mass of recrystallized compound, g % Recovery = ————————————————— x 100 mass of crude compound, g ...
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