Orgo Lab Introduction Questions

Which would you expect to have a higher melting point? t-butyl alcohol (2-methyl-2-propanol) or n-butyl alcohol (1-butanol)

I would expect 1-butanol to have a higher boiling point because it is a more compact molecule with no side chains, so the polymers can pack closer together, and more energy would be needed to break them apart.

Which would you expect to have a higher boiling point? 1-butanol (CH3CH2CH2CH2OH) verses 1-propanol CH3CH2CH2OH)

1-butanol has a higher boiling point because it has a longer carbon chain, creating more surface area, meaning more forces holding them together.

How does a liquid-liquid extraction work? Comment on the different properties of the two liquids involved.

A liquid-liquid extraction is a common method used by organic chemists when isolating reaction products. Typically, the reactions are homogenous liquid mixtures and can be extracted with an organic or aqueous solvent. The extraction involves two layers: an organic one and an aqueous one. The technique for a liquid/liquid extraction has four steps to it. The first step is mixing the layers together. The second step is to separate the followings, followed by drying the organic layer. The last step is to remove the solvent using a Pasteur pipette. The ideal solvent has the following properties: it should easily dissolve the substance to be extracted, it should dissolve the substance at room temperature, it should have a low boiling point, it should not be reactive with the solute or other solvents, it should have low flammability and toxicity, and be immiscible in water.

Under what circumstances is it wise to use a mixture of solvents to carry out a recrystallization?

A mixture of solvents can be used when there is not a single solvent available to thoroughly dissolve the solute.

Explain the three types (ignore ion-ion) of intermolecular attractive forces and how they affect boiling point.

All molecules have the weakest form of intermolecular forces, dispersion forces. These forces arise between the instantaneous dipole moments in molecules. Molecules that are only affected by this type of intermolecular force will have a lower boiling point than molecules with other types of intermolecular forces. The next type of intermolecular forces is dipole-dipole forces. These forces are between polar-covalent molecules. the negative end of a polar molecule will be attracted to the positive end of another molecule and vice versa. Dipole-dipole forces are stronger than dispersion forces and will therefore increase boiling point. The last type of intermolecular force is hydrogen bonding. Hydrogen bonding occurs between an H bonded to an O, N, Or F atom and an O, C, N, or F atom on another molecule. This is the strongest form of intermolecular fore and substances that have this type of intermolecular force will have a much higher boiling point.

Explain what the Rf value is and describe its usefulness.

An Rf value is the distance the spot has traveled divided by the distance the solvent travels. It is defined as the retention factor. The quality of the separation can be gauged based on its Rf value. If a value falls between 0.3 and 0.7 it is considered a good separation. The usefulness of these values is that they help us identify substances. If the Rf value for two spots is the same, it then indicates that the two substances might be the same molecule

What is the effect of an insoluble impurity, such as sodium sulfate, on the observed melting point of a compound?

An insoluble impurity will not depress the melting point. There is no effect. However, if it is a solid, one might miss the melting point range of their substance because they would be waiting for their entire sample to liquefy, but the solid insoluble impurity might remain solid.

Based on the number and polarity of the functional groups in aspirin, acetaminophen, ibuprofen, and caffeine, whose structure are on page 184 (5th) or on page 164 (6th), predict which of these compounds has the highest Rf and the lowest Rf.

Based on the number and polarity of the functional groups in aspirin, acetaminophen, ibuprofen and caffeine, I predict that ibuprofen will have the highest Rf value, and caffeine will have the lowest Rf value.

Which would you expect to have a higher boiling point? 1-butanol (CH3CH2CH2CH2OH) or diethyl ether (CH3CH2OCH2CH3)

Both of these molecules are similar in size, and therefore their boiling point would be determined by polarity. Diethyl has two polar C-O bonds, and 1-butanol has two polar bonds, C-O and O-H. Since the O-H bond is more polar, it makes 1-butanol more polar than diethyl ether, and giving it a higher boiling point.

Briefly describe the process of Thin Layer Chromatography. Make it clear in the discussion what the stationary and mobile phases are and how they work together to effect the mobility of a substance in TLC. This is in general terms, not specifics for this lab.

Chromatography is the separation of two or more compounds caused by their molecular interactions with two phases, a stationary and a moving phase. In a lab, cellulose paper is the stationary phase, and a propanol-water mixture is the mobile phase. The samples are spotted near one edge of the paper, and this edge is dipped into the mobile stage. Capillary action is used to draw the solvent through the paper, and the molecules separate based on their interaction with the paper. One specific form of chromatography is thin layer chromatography. The stationary phase is normally a polar solid adsorbent. The network of these adsorbents creates a polar surface. In general, the more polar molecules will have a stronger adsorption on the stationary phase, and therefore the molecules will move more slowly. Those nonpolar compounds will travel with the solvent front. The mobile phase can either be one single solvent or a combination of a few solvents. A polar solvent will carry along the polar substrates, and vice versa. They are as follows: preparing the sample, spotting the TLC plate, picking a solvent, developing the plate, visualizing the results and calculating Rf values

How might freeze-dried foods be prepared?

Freeze-drying is a dehydration technique often used to preserve perishable materials. It process of it involved freezing the material, and then reducing the surrounding pressure to allow the frozen water in the material to sublime directly from a solid to a gas.

Consult a MSDS for concentrated hydrochloric acid. Comment on the blue health hazards and proper method of disposal of hydrochloric acid.

Hydrochloric acid is a strong irritant to skin, eyes, and indigestion. It is slightly hazardous if inhaled and can produce burns if it comes in contact with skin. Inhalation can cause coughing, choking, or shortness of breath. Over-exposure can cause death. To dispose of concentrated hydrochloric acid, it should be neutralized and then can be disposed of with sewage.

The pKa of phenol (C6H5OH) is 10.0. Based on the discussion on page 150 (5th Edit.) or page 144 (6th Edit.) of your text, would you expect phenol to dissolve in bicarbonate solution? Explain your choice using specific numbers.

I would not expect phenol to dissolve in bicarbonate solution. The pKa of bicarbonate is 10.3, whereas the pKa of phenol is 10.0. Sodium bicarbonate is not a strong enough base for phenol to completely dissolve in solution. The saturated bicarbonate has a [OH]- of 3 x 10^-4, and needs 10^-2 for 99% conversion of phenol to its salt.

What will be appearance of a TLC plate if a solvent of too low polarity is used for the development? A solvent of too high polarity?

If a development solvent of too high a polarity is used, all components in the mixture will move along with the solvent and no separation will be observed. The Rf values would be too large. If the solvent is of too low a polarity, then the components will not move enough and similarly, the separation will not occur. The Rf values will be too small.

What problem will ensue if the level of the developing liquid is higher than the applied spot in a TLC analysis?

If the level of the developing liquid is higher than the applied spot in a TLC analysis, then the spots would become submerged, and they would be washed off.

Explain the process of melting. Make sure to include the types of bonds that are breaking when a non-ionic organic compound melts.

Melting happens when the intermolecular forces between molecules break, allowing the molecules to move further apart and take on the characteristics of a liquid. The type of bonds breaking between non-ionic organic compounds are dispersion forces, the weakest form of intermolecular forces. These will have a lower boiling point than molecules that experience dipole-dipole or hydrogen bonding forces. In some cases, a non-ionic compound may break apart hydrogen bonds as well, if the organic molecule has a hydrogen attached to a O, N, or F atom. This molecule would have a much higher boiling point.

Define and explain the process of sublimation. Identify the physical changes involved int he sublimation of a sample and explain when and why chemists use it.

Since temperature and pressure are related, there is a point where, if the pressure is low enough, a substance can exist in a solid, liquid, and gas state. Below this point, a substance is likely to sublime, in which it passes through the phase of turning a liquid and changes from a solid to a gas. The intermolecular forces between the molecules break apart, allowing them to spread out far enough for the substance to take on the characteristics of a gas. In the lab, sublimation happens when the substance heats on a warm surface and then condenses on a cold surface. Sublimation is often used to purify substances because there is less of a chance of losing material than when using a method such as recrystallization.

Explain what is meant by the "boiling point" of a liquid.

The boiling point of a liquid is the temperature at which the intermolecular forces between the molecules of a substance are broken apart and they separate even further, taking on the characteristics of a gas.

From the plot of the boiling versus volume of distillate in the simple distillation of the cyclohexane-toluene distillation mixture, what conclusion can you draw about the homogeneity of the distillate?

The distillate is not perfectly homogenous because the substances did not boil at their exact boiling point. Rather, the sample continued to boil throughout the process, only slowing during the intermediate temperatures between the boiling points of cyclohexane and toluene.

Consult a Material Safety Data Sheet (MSDS) for methanol and comment on the blue-health hazards, red- fire hazard, and proper method of disposal for methanol.

The health hazards of being exposed to methanol include being hazardous if it were to come into contact with skin or eyes, can cause irritation if ingested and can cause respiratory irritation if breathed in. Methanol is able to permeate the skin. Methanol is very flammable, especially if kept around open flames or heat sources. There is a risk of explosion, therefore in order to get rid of a small fire use dry chemical fire or to get rid of a large fire, use alcohol foam, water spray, or fog. Methanol vapor is also explosive. In order to dispose of methanol, it should first be contained in a well-ventilated are until becomes solid and can be incinerated in an EPA-approved facility.

Why must the ice be added to the cold finger after the sample to be sublimed has been "sealed" inside the apparatus?

The ice is added to the cold finger after the substance has been sealed so that there is no condensation on the outer surface of the cold finger. If the ice began melting and water starting condensing on the outside of the tube, and then the tube was placed into the apparatus and sealed off, the results would be affected. The sublimation of the purified substance would be affected because the cold surface would be wet.

In order for a sublimation of an organic solid to actually purify the solid, what can you say about the physical properties of the solid in question as well of the impurities?

The product being sublimed would have to sublime at a higher atmospheric pressure than the substance that the product is being separate from.

When you cook spaghetti and macaroni (separately), why does the spaghetti stick together more than the macaroni?

The spaghetti has more surface area to stick to than the macaroni.

In this lab we are doing two extractions to get impure caffeine. Make sure to identify and distinguish between the two extractions.

The two extractions used in this lab were solid-liquid extraction and liquid-liquid extraction. A solid-liquid extraction allows soluble components to be removed from solids using a solvent. A liquid-liquid extraction involves using a liquid solvent to remove a liquid component from a liquid mixture. The component dissolves preferably in the solvent. In the experiment, the sold-liquid extraction was when we tea was extracted from the tea leaves in the bag, and the liquid-liquid was extracting caffeine from the tea.

Strictly speaking, why is it incorrect to speak of a melting point?

The words "melting point", imply that the substance will melt at once specific temperature. However, it is more accurate to say that the substance has a "melting range", indicating that there is a range of temperatures at which the substance will begin to melt and then completely melt.

Using pKa analysis, explain why this experiment works. (ie. one can separate benzoic acid from 4-tert-butylphenol and from benzil.) Note: organic molecules tend to be soluble in organic solvents and salts of ionic compounds are soluble in water.

This experiment works because the pKa values can help determine which chemicals will react with each other. For example, benzoic acid, which has a pKa of 4.17 is mixed with sodium bicarbonate, which removes the H+ proton from the benzoic acid molecule, thus making sodium benzoate with a sodium ion, water, and carbon dioxide. The low pKa of benzoic acid makes it easy to separate with a weaker base. However, the other chemicals involved, such as phenol which has a pKa of 10 would require a much stronger base to make the substance undergo a similar reaction. The difference in pKa values allow the substance to be separated by preventing the stronger bases from being affected by the weaker base reactions.

Three test tubes, labeled A, B, and C, contain substances with approximately the same melting points. With this lab in mind, how could you prove that the test tubes contain three different compounds?

To prove that there are different substances in the test tubes, two of the samples could be mixed together and the melting points tested. If the melting point is lower than the theoretical melting point, then they are two different substances. If the temperature at which the sample melts remains the same, then they are the same substance.

What properties of a solvent are necessary for a good recrystallization? In other words how does one choose a solvent to use for recrystallization?

When choosing a solvent, the most important thing to remember is that "like dissolves like," meaning that polar compounds dissolve in polar solvents and nonpolar compounds dissolve in nonpolar solvents. A good solvent is very soluble at high temperatures and sparingly soluble at low temperatures. In other words, it will dissolve the solute when the solution is hot, but not when the solution is cold. Another necessary property of a good solvent is that it should either dissolve the impurities very well, or not dissolve them at all. This is to ensure that the impurities do not recrystallize out with the solute. The perfect solvent should also not react with the solute. Lastly, it should be very volatile, as this will help remove in removal from the crystals