Boiling Points and Distillation

Transcription

1 iling Pints and Distillatin The actin f biling is a prcess familiar t anybdy wh has cked pasta r brewed tea. s heat is applied t a pan f water, the temperature f the water increases until it reaches 100 C (212 F). t this temperature, additinal heat causes the water t bubble vigrusly and transfrms liquid water int gaseus water, r steam. Mst rganic liquids will behave in a similar fashin. On heating, the temperature f the liquid increases until a certain temperature called the biling pint is reached. dditinal heating causes the liquid t vaprize accmpanied by vigrus bubbling f the liquid. The biling pint f a substance is a physical prperty f a substance and can be useful fr characterizing that substance. The prcess f heating a substance until it vaprizes, cling the vaprs, and cllecting the cndensed liquid is the basis f a cmmnly used purificatin technique called distillatin. Vapr Pressure and iling Pint If a liquid is placed in an empty, clsed cntainer, sme mlecules at the surface f the liquid evaprate int the empty space abve the liquid. Once vaprized, sme f the mlecules in the vapr cndense int the liquid in a cmpeting prcess. s the space abve the liquid becmes ccupied with mlecules f vaprized liquid, the pressure f the vapr abve the liquid rises until it reaches a certain value. When the pressure stabilizes, the rates f evapratin and cndensatin are equal. The pressure f the vapr under these cnditins is called the equilibrium vapr pressure. The equilibrium vapr pressure f a liquid is temperature dependent. s the temperature f the liquid is raised, mre mlecules vaprize and the equilibrium vapr pressure increases. The graph in Figure 11 shws the relatinship between vapr pressure and temperature fr three substances, dichlrmethane, water, and d-limnene. Figure 11. Vapr pressure-temperature diagram fr dichlrmethane, water and d-limnene. The nrmal biling pint f a substance is defined as the temperature at which the vapr pressure f that substance equals atmspheric pressure, 760 mmhg. The nrmal biling pints f dichlrmethane, water, and d-limnene are, respectively, 40.2 C, 100 C, and 175 C. If the barmetric pressure is less than 760 mmhg, the temperature at which a substance bils will be less than the nrmal biling pint. When measuring biling pints, it is imprtant t recrd the barmetric pressure at the time f the measurement. In the chemical literature, there are several different methds f reprting biling pints with the barmetric pressure. Fr example, the biling pint f 2,5-hexanedine at 754 mmhg is 194 C and is

2 reprted in the fllwing frmats: bp C, bp 194 C/754 mmhg, bp 194 C (754 mmhg), r 194 C 754. If a biling pint is reprted withut a pressure it is assumed t be the nrmal biling pint and the pressure is 760 mmhg. T cmpare an bserved biling pint t ne reprted in the chemical literature, it is ften necessary t cmpensate fr differences in pressure. The fllwing frmula prvides a gd estimate t crrect an bserved biling pint t a nrmal biling pint: t (760 - p) (t + 273) where t is the crrectin in degrees Centigrade t be added t the bserved biling pint t at barmetric pressure p. nther rule f thumb fr estimating biling pints near 760 mmhg states that the bserved biling pint will differ frm the nrmal biling pint by 0.5 fr every 10 mmhg difference in pressure. Distillatin f a Single Vlatile Liquid The vaprizatin f a liquid and cndensatin f the resulting vapr is the basis fr a methd f purificatin called distillatin. Organic liquids cntaining very small amunts f impurities, nn-vlatile substances, r substances with very high biling pints are easily purified by simple distillatin. typical simple distillatin setup is shwn in Figure 12. It cnsists f a flask cntaining the material t be distilled, an adapter t hld a thermmeter and t cnnect the flask t a water-cled cndenser, and a flask t hld the cndensed material. Figure 12. Simple distillatin apparatus

3 Distillatin pparatus The distillatin flask. The distillatin flask is a rund-bttm flask and is ften referred t as the pt. The liquid t be distilled shuld fill the distillatin flask t 1/2 t 2/3 f its capacity. If the flask is t small, the liquid is likely t bump r fam ver int the receiving flask withut vaprizing. If the flask is t large, a substantial amunt f the liquid may be lst as vapr filling the flask. This residual vlume is called the take-up vlume. T prmte even biling f the liquid, pieces f prus clay plate r silicn carbide chips are added t the liquid befre starting t heat the liquid. The irregular chips prvide sites fr bubbles f vapr t frm. n alternative methd fr prmting even biling is t agitate the liquid with a magnetic stirrer as it is being heated. CUTION: Never add biling chips r a stir bar t a ht liquid. This can cause a seemingly calm liquid t bil suddenly and vilently. The distilling adapter. The adapter cnnects the distillatin flask, the cndenser, and the thermmeter. This type f adapter is ften referred t as a distillatin head. The grund glass jints must be lined up and mated tightly t avid leakage f the vaprized liquid. Leakage will result in lss f sme f the liquid and will pllute the labratry envirnment. The psitin f the thermmeter is adjusted s the bulb is belw the adapter sidearm cnnected t the cndenser. In rder t get reliable readings n the thermmeter during the distillatin, the vaprs f the heated liquid must ttally surrund and cntact the thermmeter bulb. The cndenser. The cndenser cls the vapr causing it t reliquify and directs this cndensate t the receiving flask. The mst cmmn type f cndenser is the water-jacketed type shwn in Figure 12. The water supply is cnnected t the cndenser with rubber hses. The water flws in the lwer hse cnnectin (mst remte frm the distillatin flask) and ut the upper hse cnnectin. efre turning the water flw n, check the hse cnnectins carefully t ensure that they are secure and will nt pp ff. n extra margin f security can be gained by twisting wire arund the hse cnnectins. The water flw is adjusted s there is a slw, cnstant flw f cld water t the cndenser. During the distillatin f very high biling liquids, it is cmmn practice t cl the cndenser with air instead f water. The thermal shck t glassware frm a large temperature difference between cld water and ht vapr can crack the glassware. The receiving flask. The cntainer t cllect the liquified vapr is called the receiver. It may be a rund-bttm flask, an Erlenmeyer flask, a bttle, r a graduated cylinder. In Figure 12, the receiver is cnnected t the cndenser with an adapter. The sle purpse f the adapter in this case is t direct the cndensate frm the cndenser t the receiver. One imprtant feature f the setup shuld be nted at this pint the system is pen t the atmsphere. If the liquid being cllected has a lw biling pint, it is gd practice t cl the receiving flask with a cld water bath. Heat is slwly applied t the distillatin flask. The amunt f heat t apply is determined by the rate f distillatin. The liquid shuld gently bubble and vaprize. s vapr rises frm the liquid, it mves up the apparatus raising the temperature f the apparatus. The vapr will fill the distillatin flask and mst f the distillatin head. The thermmeter bulb shuld be cmpletely surrunded by the vapr. If vapr creeps past the thermmeter bulb withut cntacting it, the measured biling pint will be lw. The vapr cndenses in the cndenser and drips int the receiving flask. Typically, the liquid shuld drip int the receiving flask at a rate f abut 10 drps per minute. If the rate f distillatin is t rapid, the heat applied t the distillatin flask must be decreased. With t rapid a rate, the measured biling pint is likely t be inaccurate and the purity f the distilled liquid will be cmprmised. The behavir f the measured biling pint during the curse f a simple distillatin is graphed in Figure 13. s the liquid vaprizes and the vapr cmes int cntact with the thermmeter bulb, the temperature rises. The temperature stabilizes at the biling pint and mst f the liquid distills. The temperature drps when there is n liquid left in the distillatin flask.

4 Figure 13. iling pint f a pure substance as a functin f amunt f liquid distilled The first material that distills befre the temperature stabilizes is called the frerun. The frerun will cntain any lw biling impurities and is usually discarded after checking its purity. The material cllected after the temperature stabilizes is the purified substance. Usually the temperature stabilizes slightly belw the literature value fr the biling pint and then slwly creeps up. When the temperature begins t drp, the distillatin is halted by remving the heat frm the distillatin flask. s a safety precautin, distillatins are never carried ut t dryness. The residual liquid plus the liquid frm the take-up vlume is the price ne pays fr purity. The biling pint f the cllected material is actually a range rather than a pint. Distillatin f Mixtures There are tw general types f mixtures t cnsider, mixtures f miscible liquids and mixtures f immiscible liquids. Their behavir n distillatin is very different frm ne anther and each type has very well defined uses in rganic chemistry. Miscible liquids are sluble in each ther in all ratis. Immiscible liquids d nt disslve in ne anther t any extent. Water is immiscible with mst rganic substances and, fr ur purpses, will always be ne f the cmpnents in a mixture f immiscible liquids. Mixtures bey Daltn's law f partial pressures which states that vapr pressure abve a mixture is equal t the sum f the vapr pressures f the individual cmpnents. Fr example, fr a tw cmpnent mixture: P P + P ttal where P and P are the partial pressures f cmpnents and respectively. The difference in the behavir f the tw types f mixtures n distillatin arises frm the differences in partial pressures. Mixtures f miscible liquids. In a mixture f miscible substances, the partial pressure f a cmpnent depends n the vapr pressure f the pure cmpnent and the relative amunt f the cmpnent in the mixture. This relatinship is stated as Rault's law the partial pressure f a cmpnent in an ideal slutin is equal t the vapr pressure f the pure cmpnent multiplied by its mle fractin:

5 The mle fractin f cmpnent, X is defined as: P P X X number f mles f ttal number f mles in mixture Fr a mixture cntaining nly tw cmpnents, and, the mle fractin is: X number f number f mles f mles f + number f mles f Cmbining Daltn's law f partial pressures and Rault's law t a mixture f and, the vapr pressure f the mixture is: P P X + P ttal Since the vapr pressures f the pure cmpnents increase with temperature, the vapr pressure abve the mixture will increase. When the vapr pressure f the mixture reaches 760 mmhg, the mixture will bil. Fr ideal slutins, i.e., slutins that bey Rault's law, the biling pint f the mixture will be between the biling pints f the pure cmpnents. Withut ging int the mathematical explanatin, the imprtant cnsequence f Rault's law is that the vapr abve a biling mixture is enriched in the lwer biling cmpnent. y carrying ut a distillatin carefully, it is pssible t cllect prtins f the distillate, which are cnsiderably enhanced in the amunt f lwer biling cmpnent and higher biling cmpnent. If the difference in the biling pints f the tw cmpnents is large, a careful distillatin can separate the mixture int its tw cmpnents. In the figure belw, are shwn tw curves representing the temperature f the distilling vapr as a functin f the vlume f distillate. The curve n the left represents an idealized distillatin in which the lwer biling cmpnent distills cmpletely and then the higher biling cmpnent distills. The curve n the right represents a mre empirical curve in which the distillate at the beginning f the distillatin is enhanced in the amunt f the lwer biling cmpnent and the distillate at the later stages f the distillatin is enriched in the higher biling cmpnent. X

6 y careful cntrl f temperature and by using clumns designed t increase the surface area that the distilling vaprs cme in cntact with, it is pssible t make the empirical curve mre clsely resemble the curve n the left than the ne n the right. This technique is referred t as fractinal distillatin. The apparatus fr a fractinal distillatin (Figure 14) is similar t the apparatus fr a simple distillatin with an extensin added t increase the path the vapr has t travel. This extensin is called a fractinating clumn. There are many types f fractinating clumns that are used in fractinal distillatin. They are all similar in that the surface area, which cntacts the distilling vapr, is increased. The larger the surface area cntacted by the vapr, the mre efficient the clumn is in separating the cmpnents. There are clumns which are pen, clumns with glass indentatins called Vigruex clumns, and clumns which are lsely packed with glass, metal r ceramic material (Figure 15). The fractinating clumn is ften insulated t keep the temperature f the clumn nearly cnstant. If the temperature f the clumn fluctuates widely, it is difficult t maintain a slw, cnstant distillatin rate. Sphisticated fractinating clumns have regulated heating cils built int them. Figure 14. Fractinal Distillatin Setup

7 Figure 15. Types f fractinating clumns: a) Vigruex b) pen c) packed clumn Mixtures f immiscible liquids. In an immiscible mixture, the partial pressure f a cmpnent is equal t the vapr pressure f the pure cmpnent. Fr an immiscible mixture Daltn's law becmes: P P + P ttal where P and P are the vapr pressures f pure and pure. When a mixture f immiscible liquids is heated, it will bil at a temperature which is less than the biling pint f either f the cmpnents. th cmpnents will be present in the vapr. Fr a specific example cnsider a mixture f limnene and water. t a little ver 97 C the vapr pressure f water is 695 mmhg and the vapr pressure f limnene is 65 mmhg. Since the sum f the vapr pressures equals 760 mmhg, the mixture bils. The mixture will cntinue t bil at this temperature as lng as any limnene is present in the mixture. When all f the limnene is gne, the biling pint rises t 100 C, the biling pint f pure water. On cndensing the vapr, the limnene and water, being immiscible in ne anther, separate int tw phases. This technique is referred t as steam distillatin and can be an effective methd fr islating rganic materials frm cmplex mixtures. The earliest islatins f rganic substances frm natural materials were dne using steam distillatin. One distinct advantage f steam distillatin is the lwer temperature required t islate the rganic substance. In the example with limnene, limnene is vlatilized at 97 C whereas its nrmal biling pint is 175 C. The universal gas law, PV nrt, als hlds fr partial pressures: and P V P V n n RT RT

8 where P and P are the partial pressures f and in the vapr and n and n are the number f mles f and in the vapr. Since the vlume V, temperature T, and gas cnstant R are the same fr bth and and the partial pressures f and are equal t the vapr pressures f pure and pure, the rati f the vapr pressures f and is equal t the rati f the number f mles f and in the vapr: P n P n Since the numbers f mles f each cmpnent are equal t the weight f the cmpnent divided by its mlecular weight, the rati becmes: P m / MW P m / MW where m and m are the weights f and and MW and MW are the mlecular weights f and. pplying this t the limnene example, the amunt f water required t distill 0.5 g f d-limnene can be calculated. P P m m water lim nene / / MW MW water lim nene 695 m /18 water /136 mwater 0.7 g Figure 16. Steam Distillatin Setup

9 The setup shwn (Figure 16) is a mdified simple distillatin apparatus wherein a claisen head is added in between the biling flask and the distilling adapter. The purpse f this is t prvide a secnd pening int the system t accmdate a surce f steam r the additin f water. In actual practice, cnsiderably mre water is usually distilled t ensure that all f the rganic material has distilled ver. One way f determining when all f the rganic material has distilled is t check the cndensate. If it is clear and is nly ne phase, the steam distillatin is cmplete.