Melting Point and Distillation Techniques in Organic Chemistry

What are two reasons for taking the melting point of a compound?

To identify the compound and to assess its purity.

What is the recommended heating rate through the melting range of a compound?

Heat at a rate of 1-2 °C/min as the melting range is approached.

How much material is needed to take a melting point?

Approximately 1/8-1/4 inch high in a capillary tube.

How does the melting range of an impure compound compare to that of a pure compound?

An impure compound has a melting range that is lower and broader than that of a pure sample.

What is the melting point range of Benzamide?

128-130 °C.

What is the melting point range of Cinnamic Acid?

132-133 °C.

What is the melting point range of Maleic Acid?

135-140 °C.

What is the purpose of mixing an unknown compound with known compounds during the experiment?

To identify the unknown by observing the melting point of the mixture.

What happens to the melting point of a mixture of two compounds?

The melting point is lower and the range is broader than that of the pure compounds.

What is a eutectic mixture?

A specific composition of a mixture that has a sharp melting point.

What should be recorded during the melting point determination?

The temperature at which liquid first appears and the temperature at which the sample is entirely converted to liquid.

What is the cleanup procedure after the experiment?

Put all leftover chemicals into the dry chemical waste container and used melting point plates into the glass waste container.

What effect does rapid heating have on the observed melting point?

Rapid heating can lead to an inaccurate melting point measurement, often resulting in a higher observed melting point.

How can you determine the identity of a sample given its melting point?

Compare the melting point with known values of pure compounds and perform a melting point determination procedure.

What is the first step in the melting point determination procedure?

Fill a capillary tube with a small amount of the compound.

What should be done after the sample is heated to about 20 °C below the suspected melting range?

Decrease the rate of heating to 1-2 °C/min as the melting range is approached.

What is the significance of observing changes during the melting process?

Observations can provide insights into the purity and characteristics of the compound as it melts.

What is the purpose of grinding mixtures of the unknown with known compounds?

To facilitate the melting point determination of the mixtures for identification.

What is the purpose of recrystallization in this experiment?

To purify a sample of acetanilide that contains sand and sodium chloride.

What is the melting point of pure acetanilide?

The melting point is not specified in the notes; students need to determine it experimentally.

What is the difference between crystallization and precipitation?

Crystallization is an effective purification process, while precipitation usually is not.

Why is acetanilide more soluble in ethyl acetate than in hexane?

The note requires drawing the structures of ethyl acetate and hexane to explain the solubility difference.

For effective purification by recrystallization, how must the solubility of the compound compare in solvents?

The compound must be much more soluble in the hot solvent than in the cold solvent, and impurities must be either soluble in hot solvent or insoluble in cold solvent.

What is the first step in the recrystallization procedure?

Place 4 grams of the impure mixture of acetanilide and a boiling stone in a 125 mL Erlenmeyer flask and add approximately 50 mL of water, then boil over a hot plate.

What should you do if acetanilide does not fully dissolve during heating?

Add a little more water to dissolve all of the acetanilide if needed.

What is the purpose of covering the funnel with a watch glass during filtration?

To avoid evaporation and cooling of the solution during hot filtration.

What should be done after the solution cools to room temperature?

Chill the flask in an ice/water bath for about 10 minutes and then filter via vacuum filtration.

What is the final step after filtering the crystals?

Weigh the dry crystals, take a melting point, and calculate the % recovery.

What should be recorded during the experiment?

The appearance of each solution and mixture during the procedures.

What is the maximum % recovery of one crop of crystals based on solubility data?

The maximum % recovery calculation must be shown based on the solubility of acetanilide in water at different temperatures.

What should be suggested if the yield is less than the theoretical value?

Discuss how material was lost and how to improve the percent recovery.

What method can be used to test the purity of acetanilide?

Taking the melting point is a method to test purity.

What should be done to remove colored impurities from a clear, tan solution?

The note does not specify; students should provide a method based on their understanding.

Where should the thermometer bulb be placed during distillation for accurate temperature measurement?

The thermometer bulb should be positioned at the top of the distillation flask, just below the sidearm.

Why is the water inlet at the bottom of the condenser and the outlet at the top?

This configuration allows for efficient cooling, as water enters from the bottom, ensuring that the entire condenser is filled and maximizes heat exchange.

What is the definition of boiling point?

The boiling point is the temperature at which the vapor pressure of a liquid equals the external pressure surrounding the liquid.

What is the potential hazard of a distillation without an atmospheric opening?

Without an opening, high pressure can build up in the glassware, potentially causing it to explode, referred to as becoming a 'bomb'.

Why should a distillation flask never be more than ¾ full before starting a distillation?

Filling the flask more than ¾ full can lead to bumping and overflow of the liquid during boiling, which can contaminate the distillate.

What is the purpose of 'packing' in a fractional distillation column?

Packing increases the surface area for vapor-liquid contact, enhancing separation efficiency during fractional distillation.

What is the procedure for simple distillation in this experiment?

Combine 25 mL of cyclohexane, 35 mL of toluene, and boiling stones in a flask, heat until boiling, and collect distillate at about one drop per second.

What should be recorded during the simple distillation process?

Record the boiling point and total volume for every 5 mL of distillate collected.

What is the next step after collecting 50 mL of distillate in simple distillation?

Discontinue the distillation and dismantle the equipment after it has cooled.

What is the significance of plotting a graph of boiling point versus total volume of distillate?

The graph visually represents the relationship between boiling point and the amount of distillate, helping to identify the boiling points of the components.

What is the function of boiling stones in the distillation flask?

Boiling stones provide nucleation sites to prevent superheating and bumping during boiling.

When should simple distillation be used instead of fractional distillation?

Simple distillation is used when separating liquids with significantly different boiling points, typically greater than 25°C.

What is a potential problem with fractional distillation due to larger apparatus size?

More heat is required, which can lead to thermal decomposition of sensitive compounds.

How can thermal decomposition problems in fractional distillation be mitigated?

Using lower heat settings or employing vacuum distillation can help reduce thermal degradation.

What is an azeotrope?

An azeotrope is a mixture of two or more liquids that has a constant boiling point and composition throughout distillation.

How does the fractionating column improve separation of cyclohexane from toluene?

The fractionating column allows for better separation by providing multiple vaporization-condensation cycles, leading to purer distillate.

What volume from the middle of the distillation should be rejected to collect pure cyclohexane and toluene?

The middle fraction, which contains a mixture of both cyclohexane and toluene, should be rejected to ensure purity.

What compound is isolated in this experiment?

trans-cinnamaldehyde, responsible for the odor of cinnamon.

What is the purpose of steam in the steam distillation process?

Steam acts as an external source of heat to facilitate the distillation.

What is the role of anhydrous magnesium sulfate in this experiment?

It is used to dry the organic layer after extraction.

What is the first step in the procedure for steam distillation?

Break cinnamon sticks into small pieces and place 1 gram into a 100 mL round bottom flask with 50 mL distilled water.

How much water is distilled during the steam distillation process?

35-40 mL of water.

What is done with the distillate after it is collected?

It is transferred to a separatory funnel and washed with 25 mL of dichloromethane.

What happens to the organic layer during the extraction process?

The organic layer (dichloromethane) is saved while the top aqueous layer is discarded.

What is the purpose of washing the dichloromethane extract with saturated aqueous NaCl solution?

To further purify the organic layer by removing impurities.

What indicates that the magnesium sulfate is dry?

Dry MgSO4 remains a fine powder without clumps.

What is done after swirling the MgSO4 in dichloromethane?

The dichloromethane solution is decanted into a round-bottomed flask.

What is the final step in the experiment after decanting the solution?

Remove the solvent using the rotovap.

Where should excess dichloromethane be disposed of?

In the halogenated organic solvent waste container.

What should be done with aqueous solutions and magnesium sulfate after the experiment?

They can be put down the drain.

What is the normal boiling point of trans-cinnamaldehyde?

This value can be found in the CRC handbook or the Aldrich Catalog.

How can trans-cinnamaldehyde be isolated by steam distillation if it decomposes near its boiling point?

Steam distillation lowers the effective boiling point, allowing it to be distilled without decomposition.

What is the vapor pressure of water at 99 C during steam distillation?

733 mm Hg.

What is the atmospheric pressure during the steam distillation at 99 C?

760 mm Hg.

How is the vapor pressure of the compound being steam distilled calculated?

It is calculated by subtracting the vapor pressure of water from the atmospheric pressure.

What is a stereogenic center?

A carbon atom that has four different groups attached to it.

What is chirality in a molecule?

A property where a molecule's mirror image is not identical to the original molecule.

What are enantiomers?

Two substances whose molecular structures are related as an object and its nonsuperimposable mirror image.

What is the difference between chiral and achiral molecules?

Chiral molecules lack a plane or center of symmetry, while achiral molecules have a mirror image that is identical to them.

What does a plane of symmetry indicate about a molecule?

If a molecule has a plane of symmetry, it is achiral.

What are diastereomers?

Stereoisomers that are not related as enantiomers and differ in all properties, chiral and achiral.

What is a meso form?

A molecule with two stereogenic centers that has a plane of symmetry, making it achiral and optically inactive.

What is the R-S convention?

A system used to designate the configuration at a stereogenic center based on the priority of attached groups.

How do you determine the priority of groups attached to a stereogenic center?

By atomic number; higher atomic numbers have higher priority.

What configuration is assigned if the priority groups form a clockwise array?

R (rectus, or right).

What configuration is assigned if the priority groups form a counterclockwise array?

S (sinister, or left).

What is the significance of optical activity in chiral molecules?

Chiral molecules can rotate a beam of plane-polarized light, while achiral molecules cannot.

What happens when two identical groups are attached to a stereogenic center?

The molecule becomes achiral and its mirror images are identical.

What is the effect of the configuration at the stereogenic center in drugs like L-dopa?

The configuration determines the physiological activity; L-dopa is effective, while its mirror image D-dopa is not.

What is the relationship between enantiomers and optical activity?

Enantiomers differ in the direction they rotate plane-polarized light and are optically active.

What is the role of a model in understanding stereoisomerism?

Models help visualize the spatial arrangement of atoms and the properties of chirality and symmetry.

What does it mean for two models to be superimposable?

It means that one model can be placed on top of the other and they match perfectly.

How can you verify if a model is chiral or achiral?

By checking for a plane of symmetry or attempting to superimpose its mirror image.

What is the importance of constructing models with different colored atoms?

Different colors represent different groups, aiding in the understanding of chirality and stereogenic centers.

What is the consequence of having two or more stereogenic centers in a molecule?

It allows for the existence of multiple stereoisomers, including diastereomers.

What is the significance of the Cahn-Ingold-Prelog rules?

These rules provide a systematic way to assign priority to groups attached to stereogenic centers.

What happens to the configuration of a stereogenic center if two groups are interchanged?

The configuration changes from R to S or vice versa.

What is the relationship between meso forms and diastereomers?

Meso forms are a specific type of diastereomer that is achiral due to internal symmetry.

How does the arrangement of groups affect the chirality of a molecule?

The specific arrangement determines whether the molecule is chiral or achiral.

What is caffeine classified as?

Caffeine is classified as an alkaloid, a natural product that contains one or more nitrogen atoms in its structure.

What is the purine ring system found in caffeine similar to?

The purine ring system in caffeine is similar to that found in adenine and guanine, which are purine constituents of DNA and RNA.

What are the biological effects of caffeine?

Caffeine acts as both a stimulant and a diuretic.

Why is caffeine extracted from tea instead of coffee?

Caffeine is extracted from tea because the separation from complex mixtures of other natural products is easier from tea than from coffee.

What is the solubility of caffeine in water at 25°C?

The solubility of caffeine in water at 25°C is 2.2 mg/mL.

What is the role of sodium carbonate in the extraction process?

Sodium carbonate is added to ensure that acidic substances remain water-soluble and that caffeine is present as a free amine.

What happens if you shake the separatory funnel too hard?

Shaking the separatory funnel too hard can cause an emulsion to form.

What is the consequence of not shaking the separatory funnel hard enough?

Not shaking the separatory funnel hard enough may result in inadequate mixing of the organic and aqueous layers.