Presentation on theme: "Organic Chemistry Lab 315 Fall, 2014. DUE DATES Today –At beginning of lab -- Melting Pt. and Ref. Index Report –At end of lab -- copy of laboratory notebook."— Presentation transcript:
DUE DATES Today –At beginning of lab -- Melting Pt. and Ref. Index Report –At end of lab -- copy of laboratory notebook pages for today's experiment (do not turn in your pre-lab notes or in- class notes) Next Week –Nothing. I will return M.pt./RI reports. –There is a big reading assignment for the quiz. –Download and print the Powerpoint “UV-vis, MS, IR.ppt” in the Spectroscopy section of our class website. Two Weeks –Due at beginning of lab -- Crystallization Report
Just Ahead Next Week –We may meet in a different room for a long lecture (and quiz). –I will e-mail you about in any change in meeting place.
Today You can look at your M.pt./RI notebook copy, but you cannot keep it. Please return it to me today. – Make a note of errors you made on your own notebook copy. –There is a grading scheme for the notebook copy on my desk. Please look at it so you will understand how it was graded. –Work a bit on the Purpose. Saying that the purpose of last week’s lab was to “take a melting point of a solid and determine the ref. index of a liquid” does not quite capture the essence. –Manual asked for some statements about observations – they need to be included in notebook
Today The quiz (and all future quizzes) are a maximum of 50 points. – Notice that most of the quiz questions came directly from the assigned reading and video. –There is an answer key on my desk that you can look at.
In Lab Today Crystallization (recrystallization) of a solid sample –Solid compounds synthesized or used as reactants in the Organic laboratory usually need to be purified (recrystallized). Next week a melting point range will be measured to ascertain the purity of the isolated compound
An Ideal Recrystallization Solvent Should: dissolve all of the solid when the solvent is hot (boiling). dissolve none of the solid when the solvent is at room temperature. have different solubilities for the solid and the impurities. have a lower boiling point than the melting point of the solid. have a fairly low boiling point
Calculating the approximate amount of solvent to use at the boiling point: 4-Methoxybenzoic acid has the highest solubility in the MeOH-soluble list at the b.pt., 5g/100ml. If your unknown sample weighs 2.5 g, it will require this much solvent:
Steps in Recrystallization 1.Use an Erlenmeyer flask. Dissolve crude solid in minimum amount of hot solvent. This must be done with small volume increments, and fairly rapidly so the solvent does not cool or evaporate.
Steps in Recrystallization 2. Let solution cool to room temperature, so that crystal lattice reforms slowly and impurities are not trapped. (Still in Erlenmeyer. Cover with watch glass.) 3. Cool solution in an ice bath for further crystallization, ~ 15 min. The mixture must be a “filterable” slurry. If it is not, heat to boiling and add a bit more solvent.
Steps in Recrystallization 4. Vacuum filter the pure solid, leaving impurities in solution. Wash the flask and crystals with a small amount of ice-cold solvent. Some of the original substance will also remain dissolved (impossible to achieve 100% recovery). 5. Let purified solid air dry on a watch glass to remove traces of solvent. Transfer to a beaker to store until next week.
Notes Make sure the balance is displaying ±0.001g See Pavia, p. 666 for pictures of crystallization steps and p. 671 for list of steps. –Use hot water in a large beaker as a hot bath for heating methanol solvent and solution. Add boiling stones for smooth boiling. –If you forget to add them in the beginning, NEVER add stones to a hot solution. Cool solution first. Get ice from the stock room to make an ice bath. –Use the ice bath to cool solvents for washing crystals, and to cool solutions below room temperature. DO NOT start over without permission. –Let me know if you have a problem.
Notebook-keeping Record quantities of all substances used. Keep a running total of solvent added. Estimate quantity if solvent has evaporated during heating. Write your procedure as you do it. Please space your writing in the notebook to make it readable! When doing calculations, pay attention to significant figures.