CH361/361H Week 1 Lecture. Laboratory Notebook Keeping, Melting Point Determination, Recrystallization, & Yields. Identify unknown.

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1 sodium salt of unknown aromatic carboxylic acid + soluble impurities + insoluble impurities total mass:? g insoluble impurities mass:? g colored dye(s) mass: ~ 20 mg dissolve in water, filter aq. sol n of unk. sodium salt + sol. impurities decolorize, filter aq. sol n of unk. sodium salt + sol. impurities conc. HCl; filter account for all masses; calculate %recovery CH361/361H Week 1 Lecture Identify unknown carboxylic acid Laboratory Notebook Keeping, Melting Point Determination, Recrystallization, & Yields mp, MW, pk a, derivative mp water-soluble imp. crude unk. carboxylic acid mass:? g recrystallize pure unk. carboxylic acid mass:? g melting point ( mp )

2 Experiment I verview: Prep n and Charact n of an Unknown Carboxylic Acid impure sodium salt of an aromatic carboxylic acid pure free aromatic carboxylic acid amide derivative R Na R H R N H R REPRT 1 + insoluble impurities + soluble impurities major activities in experiment I preparation of the free acid from its sodium salt purification of the acid by recrystallization measurement of melting point of pure acid determination of molecular weight by end-point titration determination of pk a by potentiometric titration preparation of an amide derivative What is the identity of your randomly assigned carboxylic acid? establish by measuring properties, rationalization, and comparing to values published in literature

3 Safety Tips: Use of Gloves and Avoiding Contamination wear gloves when handling chemicals BUT, take them off when leaving the lab to avoid tracking contamination - VIDE wash hands BEFRE using restroom (and after as usual!) avoid contaminating screen of smart phone, consider using a stylus the balance room can become messy very quickly! clean up after yourself

4 Proper Maintenance of a Laboratory Notebook your lab notebook is the primary record of your experimental data, use it to record not only procedures/numerical data but any observations that are potentially relevant; you may, and should, cross-reference other materials (e.g., CH361 lab manual); see Mohrig Chapter 3 use a pen not a pencil, do not erase or obliterate mistakes, rule through neatly; make sure original page is properly duplicated (just to the page immediately beneath) entries must be dated and signed/counter-signed off on at the end of day; turn in duplicate pages for grading at end of Friday s lab each week

5 Proper Maintenance of a Laboratory Notebook

6 Development of a Purification Strategy: Exploit Properties of Na Salt vs. Free Acid ionic compound high solubility in H 2 polar non-ionic compound sparing solubility in H 2 Na R + insoluble impurities + soluble impurities R H + related impurities precipitate (solid) insoluble impurities (a) dissolve in H 2 (b) vacuum filtration R Na (a) acidify w. HCl (b) vacuum filtration NaCl + some soluble impurities filtrate (liquid) residue (solid) + soluble impurities filtrate (liquid) this week!

7 sodium salt of unknown aromatic carboxylic acid + soluble impurities + insoluble impurities total mass:? g dissolve in water, filter account for all masses; calculate %recovery insoluble impurities mass:? g aq. sol n of unk. sodium salt + sol. impurities decolorize, filter Identify unknown carboxylic acid colored dye(s) mass: ~ 20 mg aq. sol n of unk. sodium salt + sol. impurities conc. HCl; filter mp, MW, pk a, derivative mp water-soluble imp. crude unk. carboxylic acid mass:? g recrystallize pure unk. carboxylic acid mass:? g melting point ( mp )

8 Melting Point Determination melting point: temperature at which solid and liquid are in equilibrium melting range: range of temps. over which a solid sample becomes liquid melting point determination serves two purposes: 1)identity 2)purity Alq3, tris(8-hydroxyquinolinato)aluminum

9 Melting Point Determination Two-component (A & B) phase diagram eutectic point: point on phase diagram that represents particular ratio of 2-component mixture with the lowest melting point Figure from Laboratory Techniques in rganic Chemistry, 4 th Edition by Mohrig, Alberg, Hofmeister, Schatz, Hammond

10 Melting Point Determination practical considerations Preparation of sample tube 1-2 mm compound tightly packed never re-use sample tubes Rate of temperature increase first time: fast increase thereafter: 1-2 o C/minute consult chart for voltage settings Melting point standards choose standard with mp +/- 10 o C your sample heat standard sample alongside your sample Pitfalls decomposition sublimation rate of heating is too fast

11 Melting Point Determination practical considerations 72 o C 73 o C 74 o C 75 o C VIDE CLIP Images downloaded from:

12 Recrystallization Why?? need pure compound to conduct analytical tests very common lab technique How? exploit temperature-dependence of solubility dissolve in minimum vol. of appropriate solvent heat to ~boiling point of solvent slowly cool filter impurities dissolved in filtrate ( mother liquor ) solid, crude sample (unk. Ar-C 2 H + imp.) minimal solubility at room temp. all solids dissolve at high temp. imp. stay dissolved; unk. Ar-C 2 H crystallizes pure crystals of unk. Ar-C 2 H

13 Recrystallization What makes an appropriate solvent for recrystallization? very low solubility at low temp; very high solubility at high temp impurities are soluble at all temps gives good recovery (>70%) and significant purification does not co-crystallize with compound of interest chemically inert Figure from Laboratory Techniques in rganic Chemistry, 4 th Edition by Mohrig, Alberg, Hofmeister, Schatz, Hammond

14 Recrystallization practical considerations Progressive approach: 1)solvent selection on micro-scale (~20 mg) 2)medium batch (~ mg) 3)large batch (~3 5 g) also involves hot filtration Solvent selection sample must be completely dry! careful, detailed observations are crucial! begin with 20 mg in ~0.1 ml solvent; incrementally add more solvent if > 1.0 ml solvent required, solvent is not suitable Solvent choices water ethanol (CH 3 CH 2 H) hexane (CH 3 CH 2 CH 2 CH 2 CH 2 CH 3 ) toluene (methylbenzene) Mixed solvent systems water/ethanol hexane/toluene

15 Recrystallization practical considerations Common pitfalls : 1) crystals do not form seed crystal scratching too much solvent? 2) crystals are of insufficient purity was solution cooled slowly? inappropriate solvent sample melting at boiling point of recrystallization solvent 3) low recovery filtrate (mother liquor) contains compound of interest too much solvent? insufficient cooling

16 Recrystallization Crystallization that occurs with slow cooling: Crystallization that occurs with fast cooling: Images downloaded from:

17 Tasks for Week 1: Decolorize Na Salt, Precipitate Free Acid (Trial Recrystallizations) polar non-ionic compound sparing solubility in H 2 R H + related impurities precipitate (solid) oluble purities ss:? g colored dye(s) mass: ~ 20 mg water-soluble imp. aq. sol n of unk. sodium salt + sol. impurities decolorize, filter aq. sol n of unk. sodium salt + sol. impurities conc. HCl; filter crude unk. carboxylic acid mass:? g recrystallize Ide car melting point ( mp ) Wednesday Wed. (/ Fri.) Friday decolorize filtrate using activated charcoal, 2-3 times might be necessary, try different brands of charcoal in 2 nd (/3 rd ) round for maximum effect acidify Na salt with conc. HCl to precipitate the free acid; isolate acid by vacuum filtration, suck dry for ca min, then leave to air dry in hood until next period oven dry precipitated free acid (after a trial mp determination); TIME ALLWING, begin trial recrystallizations with small amounts of acid