E28 EXTRACTION OF ORGANIC COMPOUNDS Separation using acid-base properties, acylation

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1 E28 EXTRATIN F RGANI MPUNDS Separation usin acid-base properties, acylation THE TASK To use the acid/base properties of oranic compounds to separate mixtures. THE SKILLS By the end of the experiment you should be able to: reconise how the common oranic functional roups react with stron acids and bases, desin separation strateies for oranic compounds based on this, use a separatin funnel safely. THER UTMES You will develop an appreciation of the techniques used in oranic and pharmaceutical science to separate the compounds present in complex mixtures. INTRDUTIN The separation of pure components from a complex mixture is a problem that is central to practical oranic chemistry, and one that cannot usually be solved by usin variations in a sinle property such as boilin point, or solubility in a sinle solvent. However, by takin advantae of the presence of acidic and basic roups, it is sometimes possible to achieve clean separations of mixtures usin a separatin funnel, an oranic solvent such as ether, and a sequence of extractions with stron acids and bases. This separation technique uses the property that chared species are soluble in water (a polar solvent) and neutral or unchared species are soluble in oranic (non-polar) solvents. To understand this procedure fully, it is necessary to revise some acid/base theory as it applies in ranic hemistry. Acids A typical acid, HB, can dissociate accordin to the followin equation: HB B + H 1 The equilibrium constant for this reaction is called the acid dissociation constant and is iven by: K a = [B ][H ] [HB] The stroner acids have the hiher values of K a. Indeed, for stron acids, reaction 1 proceeds essentially 100% to the riht and thus K a is very lare. The vast majority of oranic acids, however, have K a values smaller than 10 3 M. In order to avoid dealin with these small numbers, it is common practice to tabulate acid strenths as pk a values. pk a = lo 10 K a The weakest acids have the smallest K a values and hence have the larest pk a values. onversely, the stron acids have the small pk a values. E28-1

2 E28-2 Any oranic molecule that contains at least one hydroen atom can be considered to be an acid. The vast majority of such compounds have pk a values reater than 18 and have essentially no tendency to dissociate at all. Their importance as acids in oranic chemistry is because of the reactions that their conjuate bases (which are very stron bases) undero. Table E28-1 ives many examples of oranic acids and their conjuate bases. Table E28-1 Some ranic Acids and their onjuate Bases Acid lass of compound (functional roup) K a /M pk a onjuate base H 3 H carboxylic acid H 3 2 HH carboxylic acid H 2 H 3.90 carboxylic acid 10 H H 3 H phenol N H phenol N H 3 H 2 H alcohol H 3 H 2 H H alkyne H H arene (aromatic rin) H 3 H 3 alkane H 3 H 2 Potentially, any acid can react with any base in a proton-transfer reaction. Whether or not any detectable reaction occurs is dependent on the relative strenths of the acid and base involved. The hydroxide ion is a stron base and will undero an acid/base reaction with any acid that has a pk a less than 14, viz. the carboxylic acids and phenols. The reaction can be represented as: HB + H B + H 2 The hydroencarbonate ion is a weaker base than the hydroxide ion and will only undero reaction with relatively stron acids, specifically those that have pk a values less than 6.35, viz. the carboxylic acids. The reaction can be represented as: HB + H 3 B + H ()

3 E28-3 Bases Just as any compound that contains a hydroen atom can be thouht of as an acid, any oranic compound that contains a lone pair of electrons can be considered as a base. Such a species can potentially accept a proton from an acid accordin to one of the followin equations. L + BH HL + B L + BH HL + B Just as the relative strenths of acids are ranked, it is common practice to tabulate base strenths as pk b values, or even more commonly as the pk a values of the conjuate acids. The stronest bases have the smallest pk b values and hence their conjuate acids have the larest pk a values. Table E28-2 ives many examples of bases and their conjuate acids. Table E28-2 Some Bases and their onjuate Acids in ranic hemistry Base lass of base K b /M pk b onjuate acid NH 3 ammonia NH 4 H 3 H 2 NH 2 aliphatic amine H 3 H 2 NH 3 pk a of conjuate acid NH 2 aromatic amine (aniline) NH N NH 2 aromatic amine (aniline) N NH phenoxide ion H 9.98 H 2 water H H 3 H 2 H alcohol H 3 H 2 H 2 2 H 3 H 2 H 2 H 3 ether H 3 2 H carboxylate ion hydroxide ion H 3 H 2 H 2 H 3 H H 3 H H H 3 H 2 alkoxide ion H 3 H 2 H 18 NH 2 amide ion NH 3 34 H 3 H 2 carbanion H 3 H 3 42

4 E28-4 Potentially, any base can react with any acid in a proton-transfer reaction. Whether or not any detectable reaction occurs is dependent on the relative strenths of the acid and base involved. When the acid is stron (e Hl), reaction will occur except with the weakest of bases, those with pk b values reater than All the bases listed in Table E28-2, except the alcohol and the ether, will react with 2 M hydrochloric acid to yield their conjuate acids. Similarly, when the acid is water, which is a very weak acid, only the bases that are stroner than H (those with pk b values less than 1.74) will react. When workin with such species in the laboratory, special precautions must be taken to ensure that dry reaction conditions are maintained, often to the extent of excludin water vapour in the atmosphere. The bases in Table E28-2 that are stroner than H and hence unstable in the presence of water are H 3 H 2, NH 2 and H 3 H 2. Extraction The aim of this experiment is to separate a mixture of three aromatic, water-insoluble compounds usin their acid/base properties. The compounds are an amine, a carboxylic acid and a neutral compound, and the separation depends upon the followin properties. (1) The aromatic AMINE used is a weak base (pk b ~ 9). The amine can be protonated by dilute hydrochloric acid (2 M) to form a chared species that is then extracted from ether into water. H 2 N H 2 H 3 H H 3 N H 2 H 3 ether soluble water soluble Amines are often oily compounds that slowly decompose. Hence, for isolation and storae purposes, they are usually converted to a more stable derivative - frequently an amide. n addition of a solution of sodium acetate (pk a of acetic acid is 4.76) the aqueous solution is buffered to a point where a substantial proportion of the amine is unprotonated, and thus available for acetylation by acetic anhydride. H 3 H3 H 2 N + H 2 H 3 H H 3 + H 3 N H H 2 H 3 ethyl p-acetamidobenzoate ether soluble

5 E28-5 (2) The aromatic ARBXYLI AID used is a weak acid (pk a ~ 4) and so can be extracted from ether by 2 M NaH. The carboxylate anion is water soluble and ether insoluble. H ether soluble H water soluble (3) The NEUTRAL compound is naphthalene, a low-meltin solid (m.p. rane ) that is soluble in ether, but insoluble in water. ne of its uses is as mothballs. SAFETY naphthalene Materials Nature benzoic acid ethyl p-aminobenzoate ethyl p-acetamidobenzoate naphthalene sodium acetate acetic anhydride sodium hydroxide hydrochloric acid sodium sulfate diethyl ether ethanol Indicate, by sinin, that you have understood the information in the safety table. I understand the safety information Demonstrator s Initials

6 E28-6 LAB-WRK The sequence of operations is best summarised usin a flow chart. omplete the chart. H H 2 N H 2 H 3 dissolve in ether add 2 M NaH Ether layer (top) Aqueous layer (lower) add 2 M Hl add 10 M Hl Ether layer Aqueous layer add sodium acetate and acetic anhydride H 3 N H H 2 H 3

7 E28-7 1) Reread the instructions in Appendix A7 on how to use a separatin funnel. heck the tap of your 250 ml separatin funnel to ensure that it does not leak and the tap moves smoothly. Set up the funnel in a rin holder attached to the stand on your bench. Label three 100 ml conical flasks as ARBXYLI AID, AMINE and NEUTRAL MPUND. Holdin a separatory funnel durin shakin; Always point the stem away from others durin ventin 2) Weih out 4 of the mixture which consists of naphthalene ( 50 % by weiht), benzoic acid ( 25 % by weiht) and ethyl p-aminobenzoate ( 25 % by weiht). Dissolve your mixture of compounds in ether, transferrin the solution to the separatory funnel and usin in total about 40 ml of ether. Add 10 ml of 2 M NaH. Stopper the funnel well, invert (ARE! do not shake) and immediately open the tap to release pressure which develops as you shake the ether with the alkaline solution. lose the tap, shake briefly and re-open the tap to relieve pressure. Repeat this procedure twice and return the funnel to the stand and remove the stopper. When the layers have separated run out the alkaline solution (the lower layer) into the flask labelled ARBXYLI AID. Re-extract the ether layer with 5 ml of NaH solution and then 5 ml water addin these extracts to the flask. [Think about why these extractions are made.] Set aside the ARBXYLI AID flask. 3) Use the same extraction procedure described above. Extract the ether with 2 M Hl (ARE! Do not use the 10 M Hl.) usin 10 ml the first time and 5 ml the second and finally 5 ml water. ollect all the aqueous extracts (lower layers) in the flask labelled AMINE and set aside this flask also. The ether layer which contains the neutral compound should be poured out of the top of the separatory funnel into the flask labelled NEUTRAL MPUND. Take care to prevent any water droplets enterin the flask. Add a spatulaful of anhydrous sodium sulfate (dryin aent), swirl and cover the flask until you are ready to isolate the compound.

8 E28-8 4) Take the flask labelled AMINE. Add 2 or 3 boilin chips and warm on a steam bath in the fume hood for a few minutes to boil off any ether dissolved in the aqueous solution. Add 5 of sodium acetate (H 3 Na 3H 2 ) and then 1.5 ml of acetic anhydride from the dispenser stored in the fume hood. (Take your flask to the fume hood and transfer the acetic anhydride there.) Return the flask to your fume hood, mix the contents well and cool in an ice-bath with swirlin. The N-acetyl derivative should crystallise within 5 minutes. After the contents are thorouhly cooled, take the flask back to your bench, collect the amide product usin a Hirsch funnel (see diaram on previous pae) and dry at the pump. Label a clean and dry 100 ml beaker with AMIDE, weih it and transfer your sample into it once the compound is dry. What is the function of the sodium acetate? Mass of AMIDE beaker and sample Mass of AMIDE beaker Mass of ethyl p-acetamidobenzoate 5) While the amide is crystallisin take the ARBXYLI AID flask to the steam bath in the fume hood, add a few boilin chips and boil off the dissolved ether. ool the solution and cautiously add 5 ml of 10 M Hl, slowly with swirlin. ARE! ool in ice if necessary and collect the crystals usin a Hirsch funnel. Wash with a little water and dry the crystals at the pump while you continue the rest of the experiment. Weih a clean and dry 100 ml beaker labelled with ARBXYLI AID and transfer your sample into it once the compound is dry. What is the purpose of addin the hydrochloric acid?

9 E28-9 Mass of ARBXYLI AID beaker and sample Mass of ARBXYLI AID beaker Mass of benzoic acid 6) Take the flask containin the NEUTRAL MPUND and filter the solution throuh a fluted filter paper (to remove the sodium sulfate) into a conical flask. Add a few boilin chips and boil off the ether on the steam bath in the fume hood. The residue should crystallise in the flask on coolin. Purify the compound by recrystallisation from ethanol (15-20 ml). See pae E24-4 for full details on recrystallisation technique. Label a clean and dry 100 ml beaker with NEUTRAL, weih it and transfer your sample into it once the compound is dry. Mass of NEUTRAL beaker and sample Mass of NEUTRAL beaker Mass of naphthalene Demonstrator s Initials After all your compounds have been checked by your demonstrator, dispose of them in the appropriately marked containers in the fume hood.

10 E28-10 PST-WRK omplete the followin flow chart to show how you would separate p-nitrobenzoic acid and aniline. (Hint: Both compounds are soluble in ether, but neither is soluble in water.) 2 N H NH 2 Demonstrator s Initials