Micro-Scale distillation

A distillation is one of the most commonly used techniques to purify liquids (in some cases also solids if their boiling points are not too high i.e., phenol, antimony(III) chloride). For instance, it is used to refine alcohol from the fermentation process or to purify water (=distilled water) or separate different hydrocarbons in raw oil (i.e., kerosine, gasolines, naphtha, etc.). In general, there are two important processes involved in this technique.

1. Vaporization = phase transfer from liquid to vapor (gas)

2. Condensation = phase transfer from vapor to liquid

The vaporization process requires heat, which makes it endothermic (ΔH>0), while the condensation process releases heat (=exothermic, ΔH<0). The entropy increases in the vaporization process because the volume occupied from the vapor is much larger (~1000 times) compared to the liquid (i.e., one mole of water is 18 mL of liquid but about 30600 mL at 100 °C as steam). The opposite applies to the condensation process.

There are different forms of distillation depending on the type of compounds that are being separated from each other.

I. Simple Distillation (in micro-scale and macro-scale)

This type of distillation is carried out at a pressure of one atmosphere (1 atm). The mixture usually only contains one volatile component i.e., salt solution, or two compounds that have boiling points that are sufficiently far away from each other i.e., diethyl ether (b.p.=34.6 ^oC) and toluene (b.p.=111 ^oC). Since many organic compounds oxidize relatively easily in air, the boiling point should be below 150 ^oC to avoid thermal decomposition (i.e., oxidation) of the compound during the distillation.

II. Vacuum Distillation

This technique is usually used if the compound is either not very stable (=decomposes before its boiling point or close to it) or for compounds that have boiling points above 150 ^oC. For instance, N,N-dimethyl formamide (DMF), which is a commonly used solvent in organic chemistry, has a normal boiling point of 153 ^oC, but starts to slowly decompose into dimethylamine and carbon monoxide well below its normal boiling point. The decomposition is catatyzed by acidic or basic impurities. Therefore, the literature recommends to distill this compound below 90 ^oC, which requires reduced pressures (i.e., 76 ^oC at 30 torr).

This technique is generally not used for a low boiling solvent if a standard cooling system (=cold water) is used, because the compound does not condense anymore. An aspirator or house vacuum line is often used as source for the reduced pressure (p~30-50 torr). For a better vacuum, a rotary pump is used (p~10^-2 torr). But this also requires other equipment and precautions as well. See more here.

As can be seen in the table below, the boiling points will decrease by about 60 ^oC for most of these compounds if the pressure is decreased from 760 torr to 40 torr. The effect is a little larger if the normal boiling point was higher like in the case of methyl benzoate. The slope of the vapor pressure as the function the temperature is different for each compound, but many compounds have similar slopes (Trouton's Rule: ΔS_vap= 85-88 J/(mol*K)). Compounds that display intermolecular hydrogen bonds (i.e., alcohols) exhibit higher values for ΔS_vap. In general, differences in boiling point arise primarily from the different heats of vaporization.

For compounds that exhibit a large ΔH_vap, the effect is more pronounced than for compounds with low ΔH_vap. Note that the enthalpy of vaporization is temperature dependent and is zero at the critical temperature. The vapor pressure of a compound can be estimated using a nomograph as shown in the SKR (page 112 (Spring 16)). A more accurate estimation can be done using the simplified Antoine equation

log(p)=A-B/(C+T)

with A, B and C being the Antoine parameters and T in ºC. The full equation contains six parameters.

Compound	1	5	10	20	40	60	100	200	400	760	A	B	C	ΔH_vap(t_b)	ΔS_vap(t_b)
Acetic acid	-17.2	+6.3	17.5	29.9	43.0	51.7	63.0	80.0	99.0	118.1	7.27642	1326.65	183.913	23.70	60.6
Acetone	-59.4	-40.5	-31.2	-20.8	-9.4	-2.0	7.7	22.7	39.5	56.5	7.31414	1315.67	240.479	29.10	88.4
Acetonitrile	-47.0	-26.6	16.3	-5.0	7.7	15.9	27.0	43.7	62.5	81.8	7.54662	1583.98	257.887	29.75
Benzaldehyde	26.2	51.8	62.0	76.3	90.1	99.6	112.5	132.0	154.1	179.0	7.25411	1734.80	217.931	42.50
Benzene	-36.7	-19.6	-11.5	-2.6	7.6	15.4	26.1	42.2	60.6	80.1	6.81404	1090.43	197.146	30.72	87.1
Benzoic acid	96.0	118.6	132.1	146.7	162.6	172.8	186.2	205.8	227.0	249.2	7.41844	1824.74	152.886	50.63
Carbon disulfide	-73.8	-54.3	-44.7	-34.3	-22.5	-15.3	-5.1	10.4	28	46.5	7.21790	1303.79	254.394	26.74
Carbon tetrachloride	-50.0	-30.0	-19.6	-8.2	+4.3	12.3	23.0	38.3	57.8	76.7	7.01144	1278.54	232.888	29.82
Chloroform	-58.0	-39.1	-29.7	-19.0	-7.1	+0.5	10.4	25.9	42.7	61.3	7.11148	1232.79	230.213	29.24	87.5
Cyclohexane	-45.3	-25.4	-15.9	-5.0	6.7	14.7	25.5	42.0	60.8	80.7	6.89019	1200.95	218.815	29.97	84.7
Cyclohexanol	21.0	44.0	56.0	68.8	83.0	91.8	103.7	121.7	141.4	161.0	7.14994	1405.48	168.370	45.92
1,2-Dichloroethane	-44.5	-24.0	-13.6	-2.4	10.0	18.1	29.4	45.7	64.0	82.4	7.29525	1407.85	235.480	31.98
Dichloromethane	-70.0	-52.1	-43.3	-33.4	-22.3	-15.7	-6.3	8.0	24.1	40.7	7.11464	1152.41	232.442	28.06
Diethyl ether	-74.3	-56.9	-48.1	-38.5	-27.7	-21.8	-11.5	2.2	17.9	34.6	7.04631	1112.55	232.657	26.52
Diethyl succinate	54.6	83.0	96.6	111.7	127.8	138.2	151.1	171.7	193.8	216.5	7.88220	2229.64	228.805	50.57
1,2-Dimethoxyethane	-48.0	-22.0	-15.3	1.3	10.7	19.7	31.8	47.1	70.8	82	7.30039	1418.59	236.928	32.42
Dimethylformamide	7.2	31.0	42.6	55.5	69.9	79.0	91.5	110.0	131.0	153.0	7.24128	1597.92	213.457	39.41
Dimethylsulfoxide	32.2	57.8	70.6	84.7	100.4	110.3	123.8	144.0	166.9	190.9	7.23168	1733.52	207.580	43.70
1,4-Dioxane	-35.8	-12.8	-1.2	12.0	25.2	33.8	45.1	62.3	81.8	101.1	7.29969	1485.85	234.976	34.16
Dipropyl succinate	77.5	107.6	122.2	138.0	154.8	166.0	180.3	202.5	226.5	250.8	8.20708	2636.26	244.154
Ethanol	-31.3	-12.0	-2.3	8.0	19.0	26.0	34.9	48.4	63.5	78.4	8.13484	1662.48	238.131	38.56	109.7
Ethyl acetate	-43.4	-23.5	-13.5	-3.0	9.1	16.6	27.0	42.0	59.3	77.1	7.25963	1338.46	228.608	31.94
Ethyl benzoate	44.0	72.0	86.0	101.4	118.2	129.0	143.2	164.8	188.4	213.4	7.50855	2047.03	228.940	44.64
Heptane	-34.0	-12.5	-2.1	9.3	22.3	30.3	41.8	58.3	78.0	98.4	7.04605	1341.89	223.733	31.77	85.4
Hexane	-53.9	-34.5	-25.0	-14.1	-2.3	5.4	15.8	31.6	49.6	68.7	6.98950	1216.92	227.451	28.85	84.4
Isopropanol	-26.1	-7.0	+2.4	12.7	23.8	30.5	39.5	53.0	67.8	82.5	7.83056	1483.30	217.413	39.85
Methanol	-44.0	-25.3	-16.2	-6.0	5.0	12.1	21.2	34.8	49.9	64.7	8.09126	1582.91	239.096	25.21	104.3
Methyl benzoate	39.0	64.4	77.3	91.8	107.8	117.4	130.8	151.4	174.7	199.5	7.32570	1825.16	211.583	44.02
Methylethyl ketone	-48.3	-28.0	-17.7	-6.5	6.0	14.0	25.0	41.6	60.0	79.6	7.20103	1325.15	227.093	31.30
Nitromethane	-29.0	-7.9	+2.8	14.1	27.5	35.5	46.6	63.5	82.0	101.2	7.53850	1588.83	239.919	33.99
n-Pentane	-76.6	-62.5	-50.1	-40.2	-29.2	-22.2	-12.6	+1.9	18.5	36.1	7.00877	1134.15	238.678	25.79	83.4
n-Propanol	-15.0	+5.0	14.7	25.3	36.4	43.5	52.8	66.8	82.0	97.8	7.77374	1518.16	213.076	41.44	111.9
Pyridine	-18.9	+2.5	13.2	24.8	38.0	46.8	57.8	75.0	95.6	115.4	7.18740	1463.63	224.598	35.09
Tetrahydrofuran	-55.8	-36.5	-26.8	-16.1	-4.3	3.3	13.6	29.0	46.5	64.9	7.10537	1256.68	232.621	29.81
Toluene	-26.7	-4.4	6.4	18.4	31.8	40.3	51.9	69.5	89.5	110.6	7.13620	1457.29	231.827	33.18	86.5
Water	-17.3	+1.2	11.2	22.1	34.0	41.5	51.6	66.5	83.0	100.0	8.05573	1723.64	2330.76	40.65	109.1

Source: Yaws' Handbook of Antoine Coefficients for Vapor Pressure (2^nd Electronic Edition)

III. Fractional Distillation

This technique is used for mixtures where the boiling points vary by less than 20 ºC from each other i.e., the big distillation setups in the petroleum industry to refine crude oil. This technique can be performed at atmospheric and reduced pressure and requires a more elaborated setup i.e., Vigreux columns (shown in diagram), spinning band columns, etc. If a mixture was distilled using a simple distillation, the boiling point of the mixture would change more gradually. If the same mixture was distilled using a fractionated distillation, the boiling point would increase dramatically after most of the low boiling component was removed. This allows for a cleaner separation of the components in a mixture. The more theoretical plates the column has, the better the separation will be.

1. The round-bottomed flask with the liquid to be distilled (=pot) should not be more than half-filled in order to leave enough room for the liquid to boil. Otherwise the solution will spill out or over during the boiling process.

2. A spin vane, stir bar (both have to be spinning during the heating!) or a boiling stone has to be added to avoid bumping (=spilling over).

3. A good seal between the joints minimizes the loss of target compound during the distillation. It also prevents that the compound drips onto the hotplate and catches on fire.

4. The hotplate will get extremely hot (>300 ºC) if a setting of “10” is used. It will be very difficult to control the distillation since the liquid will foam much more. An appropriate temperature setting has to be used. Often times, a water or oil bath is used for better temperature control. If a heating mantle is used as heat source, it has to be plugged into a variable power control.

5. A liquid boils when a reflux ring is going up the neck of the Hickman head or three-way adapter, and not when it starts to bubble! Often times, liquids dissolve gases that are released prior reaching the boiling point because their solubility decreases. For example, a coke can expands when you store it in a warm/hot place because the CO₂ dissolved in the liquid is desorbed!

6. It is also imperative to remove the drying agent before the liquid is distilled since the drying process is reversible at higher temperatures (see chapter about Drying Solutions).

Questions:

1. Why is it imperative to leave an opening somewhere in the setup if a liquid or solid is distilled?

2. A mixture of bromobutane and water is distilled.

a. Is the boiling point of the mixture going to be above or below the lowest boiling point of the individual compounds?
b. Which of the two compounds will be the major component in the distillate in terms of moles and in terms of grams?

3. Why is toluene or benzene used in azeotropic distillations and not ethanol?

4. Compound A has a boiling point of b.p.=120 ºC, while compound B boils at b.p.=150 ºC.

a. What can be said about the vapor pressures of these compounds?
b. Would you be able to separate the two compounds using the simple distillation setup used in the lab?