Melting Point&Recrys

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Melting Point&Recrys

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