CH 236 Study Guide

RECRYSTALLIZATION:

-Impure solid is dissolved in a solvent and then allowed to slowly crystallize out as the solution cools. Impurities are not included in the framework of the crystal. For most compounds, the solubility of the compound increases as the temperature of the solvent increases.

-Steps in the recrystallization of a compound are

  • Find a suitable solvent for recrystallization
  • Dissolve the impure solid in a minimum volume of hot solvent
  • Remove any insoluble impurities by filtration
  • Slowly cool hot solution to crystallize the desired compound from solution
  • Filter the solution to isolate the purified solid compound

-Properties of a good solvent

  • The compound should be very soluble at the boiling point of the solvent and only sparingly soluble in the solvent at room temperature.
  • The unwanted impurities should be either very soluble in the solvent at room temperature or insoluble in the hot solvent.
  • The solvent should not react with the compound being purified
  • The solvent should be volatile enough to be easily removed from the solvent after the compound has crystallized.

-Colored impurities sometimes become difficult to remove from solid mixtures. In these cases, a decolorizing carbon is used to remove the impurities. The impure solid is dissolved in hot solvent, a small amount of decolorizing carbon is added, the solution is stirred and heated a few minutes, and then the solution is filtered to remove colorized carbon.

-IF the crystallization does not begin, two methods can be used in an attempt to induce the crystallization.

  • Scratch the inner wall of the Erlenmeyer flask with a glass stirring rod to release small particles of glass which act as nuclei for crystal growth.
  • Addition of a small crystal of desired compound (a seed crystal) to serve as a template for crystallization of the rest of the compound.

-Crystallization of a solid involves a slow, selective formation of the crystal framework to give a pure compound, while precipitation involves the rapid formation of a solid from a solution that usually produces an amorphous solid containing many trapped impurities in the crystal framework. Therefore, precipitation experiments must also undergo an additional final purification step to give the pure compound.

MELTING POINT:

-The melting point is defined as the temperature at which the solid phase is in equilibrium with the liquid phase. Melting point is measured as a range from the point the solid begins to melt to the point the solid has completely melted and is a clear liquid. To obtain accurate results, the solid must be heated slowly.

-Melting point of a compound is a characteristic physical property that can be useful in identification of a compound.

-Melting point can be useful in identifying the purity of a compound. The presence of impurities in a sample can have two effects:

  • Lower or depress the melting temperature
  • Broaden the melting range

-In a two compound mixture, each of the compounds can act as an impurity to the other, lowering the melting temperature and broadening the melting range. Depending on the melting properties of each of the compounds, one of the compounds, as its concentration increases, will act as an impurity on the other until a minimum melting point is reached. Once this minimum melting point, called the eutectic point, is reached, the other compound begins to act as the impurity.

-Because of the vast number of different organic compounds, a simple comparison of melting points is not a good way of determining identity, as many compounds will have the same melting point. Mixed melting points can be used as a way to determine the identity of a compound. To determine the identity, an unknown compound can be mixed with a known pure compound and the melting point measured. If the two compounds in the mixture are identical, the melting point will be the same as in each individually tested. If the two compounds are not identical, they will act as impurities on one another and the melting point of the mixture will change.

DISTILLATION:

-Distillation is a process in which components of a liquid mixture can be separated. This separation is based on differences in the boiling points of the components. Distillation involves two processes—vaporization followed by condensation.

-To carry out distillation, the liquid mixture is initially heated until it boils. The vapors rise above the liquid and enter a condenser. Once the vapors reach the condenser, they are condensed into a liquid form and the liquid, now known as distillate, is collected. The differences in boiling point of the two components allowed for only one of the components to vaporize while the other remained in liquid form.

-Two major types of distillation were used in lab—simple and fractional distillations. Fractional distillation differs from simple in one major way. The fractional distillation apparatus has an additional fractionating column between the distillation flask and distillation head. This additional fractionating column is needed when a mixture is composed of two liquids with very similar boiling points. Copper threads are placed in the column in order to increase the surface area. Because the vapor from the distillation flask must pass through the fractionating column before reaching the condenser, additional distillation occurs. This additional distillation occurs due the constant condensing and revaporization of the component on the copper threading. Because of these repeated condensations and vaporizations, the vapor is repeatedly undergoing a series of simple distillation as it travels up the column to the condenser.

-The efficiency of a distillation is measured using theoretical plates. One theoretical plate is equal to one simple distillation. Therefore, the more theoretical plates in a fractionating column, the more simple distillations that occur and the better the separation of liquids.

-An azeotrope is a constant-boiling mixture of definite proportion. These are mixtures of miscible liquids that form nonideal solutions in which the volatility of each component in the mixture is affected by the presence of another compound. These properties cause azeotropes to behave as single pure compounds even though they are actually mixtures. Therefore, the two components of an azeotrope cannot be separated using fractional distillation, regardless of the number of theoretical plates in the column.

-When carrying out a distillation, several factors are of vital importance and must be performed correctly to ensure the process gives the desired results.

  • The volume of the mixture being distilled should fill the distilling flask between one-half and two-thirds volume to avoid loss of distillate as vapor in flasks too large and the splashing up of liquid in a flask too small.
  • Bumping is an instantaneous, violent expulsion of vapor from the liquid. In order to avoid bumping during distillation, boiling chips are added to the distilling flask to prevent hot spots from developing in the liquid.
  • Stopcock grease is a lubricant used to ensure the tapered joints of the glassware do not become frozen together.
  • The bulb of the thermometer should be place just below the opening of the sidearm to the condenser to ensure the bulb is bathed in vapor before the vapor reaches the condenser. If the thermometer is not properly placed, inaccurate readings could occur.
  • Distillation should never be carried out in a sealed apparatus. If the apparatus is sealed, pressure could build up and cause an explosion.
  • Do not distill a sample completely to dryness. If no liquid remains to carry away the heat, the flask will become hot and can crack.
  • A rheostat acts as a resistor in circuit. By decreasing the power settings on the rheostat, the resistance of rheostat is increased and therefore less voltage is delivered to the sand bath, causing it to operate at a lower voltage and lower heat. It is important to use the rheostat, as the adjustment of the resistances allows for the controlling of the amount of heat delivered by the sand bath.

-BE ABLE TO DRAW EACH APPARATUS

THIN-LAYER CHROMATOGRAPHY

Chromatography is a tool that can be used to separate components in mixtures. The separation depends on the polar and non-polar affinities of each of the components. The two phases present in thin-layer chromatography are the mobile phase and the stationary phase. The more strongly a compound is adsorbed on the stationary phase, the slower that particular compound will move down the plate. Therefore, it is the difference of affinities for these phases that cause the separation of components, as each component will move at different speeds down the TLC plate.

-For this solid liquid chromatography, the most common stationary phases (adsorbents) used are silica gel and alumina. Silica gel was used in this lab. When organic compounds are placed on the gel, the compounds will stick with varying degrees of strength. Depending on how soluble each organic compound is in the solvent (the mobile phase) and the strength at which is stuck to the stationary phase determines the distance traveled along the plate by the compounds.

-Steps to prepare a TLC plate.

  • A line is lightly drawn on the TLC plate with a pencil about 1 cm from the end of the plate.
  • A microcapillary tube is used to draw up a small amount of the organic samples to be tested. A small dot of each sample is placed on the pencil line.
  • To carry out the chromatography, the TLC plate must be placed in a developing chamber. A small amount of solvent is placed in the chamber and the chamber is lined with a piece of filter paper to keep the inside of the chamber saturated with solvent vapors while the plate is developing. It is important that the solvent level is not higher than the pencil line when the TLC plate is placed inside. Once the solvent chamber is at equilibrium, the TLC plate is carefully stood up inside the chamber and the lid is replaced. The TLC plate is removed once the solvent has traveled near the top of the plate. After the plate dries, the separation can be analyzed.
  • Because the TLC plates are treated with a fluorescent indicator that glows green when under ultraviolet light, an ultraviolet lamp can be used to mark the separation of the compounds on the plate. It is important not to stare directly into the UV lamp, as it can cause serious eye damage.
  • Once the distance traveled by the solvent and each of the organic components, the retention factors can be determined.

The retention factor or Rf = Distance traveled by the compound

Distance traveled by the solvent

- The value determined as the retention factor can be used to determine the identity of unknown compounds by comparing the retention factor of it to the retention factors of known pure substances. However, as retention factors are always between 0 and 1, many organic compounds may have the same retention factors. Therefore, thin-layer chromatography isn’t a completely reliable way of determining the identity of a compound.

- The choice of solvent used is important in TLC. Typically, a solvent of an intermediate polarity is used to ensure that all the various compounds are pulled up the plate to varying degrees. The polarity of the solvent determines the distance traveled by the compounds.

-Common solvents (lowest to highest polarity)-- Hexane, cyclohexane, toluene, dichoromethane, diethyl ether, ethyl acetate, acetone, ethanol, and methanol.

EXTRACTION:

-Extraction is a process which allows for the separation of one compound from a mixture. In order to carry out an extraction, a proper solvent must be determined. For the extraction of caffeine, water makes an excellent solvent, as caffeine is highly soluble in water. By steeping tea with hot water, caffeine passed freely through the filter due to its solubility in water while other components of the tea, which were insoluble in water, were blocked from passage. However, some other components of tea are also soluble in water; therefore, another extraction must be carried out.

-NaCl was dissolved in the caffeine/water mixture from the initial extraction. This solution was then chilled and poured into a separatory funnel. Three 5 mL portions of dichloromethane were then added, one portion at a time, to the separatory funnel. After all three portions were added and the funnel was carefully turned upside down to mix the two solutions, two distinct layers formed in the funnel. Caffeine has a higher affinity for dichloromethane than it does for water. Therefore, the caffeine was extracted from the water layer to the dichloromethane.

-Because dichloromethane is more dense than water, it forms the bottom layer in the separatory funnel.

-Emulsions are a suspension of insoluble droplets of one liquid in another. There is no clear separation between layers and they do not easily separate. An emulsion will likely disperse if the funnel is allowed to sit for a few minutes. To avoid emulsions, the funnel should be slowly inverted and swirled with the addition of dichloromethane. Avoid vigorously shaking. Magnesium sulfate can also break up emulsions.

SUBLIMATION:

-Sublimation is the process by which compounds go directly from the solid phase to the gas phase, skipping the liquid intermediate. Sublimation occurs in compounds that do not display strong intermolecular forces. Sublimation is an extremely valuable purification technique.

-Sublimation has several advantages.

  • No solvent is involved
  • It can be performed at low temperatures, especially under vacuum
  • Loss of material due to handling is kept at a minimum
  • It readily separates nonvolatile impurities

-BE ABLE TO DRAW APPARATUS (On sublimation handout)

-The outer vessel (flask) holds the impure solid. A test tube is filled with ice and placed in the mouth of the flask. A filter adapter around the test tube prevents the loss of vapor from the flask. When the flask is heated in the sand bath, the desired compound vaporizes while the impurities do not. The cold test tube serves as a surface for condensation of the vapors.

-During the procedure, it is important that the sand bath does not exceed 220° C.

-The flask should be tilted occasionally to reduce the amount of caffeine subliming on the sides of the flask.

-When the caffeine begins to sublime, a white solid will form on the test tube. This solid can then be scraped off.

HYDROLYSIS OF T-BUTYL CHLORIDE (KINETICS)

-The purpose of this experiment was the study of the rate of reaction and the effects on the rate that concentration and temperature had.

-The hydrolysis of t-butyl chloride is a first order nucleophilic substitution (SN1).

-Each experiment was carried out three times. The average time of reaction was then calculated and used in the experimental data. Running each experiment three times allowed for a realization of consistency or error in the procedure.

-Acetone is used to rinse the glassware after each test. Acetone is not only a strong cleaning agent, but is also an effective drying agent, due to its readiness to bind to water.

-Results: Concentration Effect: The overall solvent concentration was kept the same, but the volume of the solvent was doubled. The average time for the reaction was slightly higher in this procedure than it was in the normal.

-Temperature- Heat- The addition of heat clearly sped up the reaction greatly. Rate increased proportionally with temperature increase.

-Temperature- Ice- Running the experiment after cooling the reactants caused a significant decrease in reaction time. The data collected was used to calculate values using two major formulas. The value k was calculated using

kt = 2.303log( 1 )

1-% reaction

100

T is the average time calculated from each experimental procedure. The second formula was used to determine a value for 1/T.

Logk = logc - Ea .

4.58T

This equation can be used to create an Arrhenius plot of logk vs. 1/T. By plotting these values, the Eacan be determined, as the slope of the graph will be equal to - Ea /4.58.

ALKENES FROM ALCOHOLS

(Dehydration of 2-methylcyclohexanol):

This reaction is an acid-catalyzed dehydration of an alcohol. It is a first order elimination reaction (E1).

Overall reaction:

The acid catalyzed dehydration of 2-methylcyclohexanol forms1-methylcyclohexene and 3-methylcyclohexene. A third product, methylenecyclohexane, forms due to a 1,2-hydride shift that form a tertiary carbocation.Even so, methylenecyclohexane forms a less substituted alkene, which is unstable, resulting in the reaction reversing. Therefore the methylenecyclohexane does not contribute to the product mixture in any significant way.

MECHANISM:

  • Formation of the tertiary carbocation

  • A product can also form from a hydride shift
  • Rearranged carbocation and products

In this experiment, an alcohol was dehydrated with the aid of phosphoric acid. Water

was eliminated from the molecule and a C=C double bond was formed. Russianchemist Saytzev predicted that for a reaction following E1 mechanism, the more substituted C=Cis favored in the product. In the case of 2-methylcyclohexanol, this means that 1-methylcyclohexene should be formed as the major product. As such, we carried out the dehydration of 2-methylcylclohexanol in H3PO4/H2O, and analyzed thedistribution of alkene products on a GC. The result allowed us to test whether Saytzev’s rule held in this case.

The chromatogram showed two peaks (Peak 1 and Peak 2): Peak 1 showed a smaller mole percentage, shorter retention time, and a lesser area in comparison to Peak 2. With our analysis, we determined that the product represented by Peak 2 was the more stable isomer as well as the preferred reaction product. The product represented by Peak 2 was1-methylcyclohexene, as itrepresented the more stable isomer due to the more highly substituted double bond. Accordingly, the product represented by Peak 1 was 3-methylcyclohexene.