LAB 2

Question 1

What are three main physical properties used for the isolation and purification of organic compounds?

Answer 1

Volatility, Solubility, and Adsorption

Question 2

What is recrystallization based on?

Answer 2

The principle that compounds are more soluble in hot solvents than in cold ones

Question 2

What does the method of distillation rely on?

Answer 2

Volatility of liquids

Question 3

Describe liquid-solid extraction

Answer 3

A finely divided solid mixture is stirred with a solvent to selectively dissolve components, followed by mechanical separation of the liquid from undissolved solids

Question 4

In liquid-liquid extraction, what causes partitioning between two immiscible liquid phases?

Answer 4

The differing relative solubility of the component in the two phases

Question 5

How does the behavior of acids differ from bases during extraction?

Answer 5

Acids can exist as uncharged (organic soluble) or charged (water soluble) forms, while bases can exist as charged (water soluble) or uncharged (organic soluble) forms

Question 6

What are the three compounds used in the extraction process of this lab?

Answer 6

Benzoic acid, 3-nitroaniline, and azobenzene

Question 7

How can the purity of a compound be assessed in the lab?

Answer 7

Through melting point determination, chromatography, IR spectroscopy, NMR, or UV spectroscopy

Question 8

What is the purpose of using sodium hydroxide in the extraction of the organic acid?

Answer 8

To convert the uncharged organic acid into its charged conjugate base form, which is soluble in water

Question 9

What is the melting point range indicative of?

Answer 9

The identity and purity of the solid product; pure compounds have a sharp melting point range

Question 10

What are the steps involved in isolating the organic base from the acidic aqueous extracts?

Answer 10

Neutralize the solution with NaOH, cool it, and collect the precipitate by vacuum filtration

Question 11

What should you do if the solid is not totally try before weighing it for yield determination?

Answer 11

The perceived yield will be higher than the actual yield due to the presence of solvent

Question 12

What is an important property of a good recrystallization solvent?

Answer 12

The solvent should dissolve the compound well at high temperatures but poorly at low temperatures

Question 13

What chemical structure change occurs when an organic acid is treated with a strong base?

Answer 13

It converts from the uncharged form to the charged conjugate base form

Question 14

What is the primary technique for purifying solid substances before chromatographic methods?

Answer 14

Recrystallization

Question 15

What happens after dissolving a solid in a hot solvent during recrystallization?

Answer 15

The solution is filtered to remove insoluble impurities, and the desired substance crystallizes after the solution cools

Question 16

What is done if a single recrystallization does not yield a pure substance?

Answer 16

The process is repeated, using the same or a different solvent

Question 17

What is the key to a successful recrystallization?

Answer 17

Choosing the proper solvent, where the compound to be purified is highly soluble in the hot solvent but only slightly solvent in the cold solvent

Question 18

What properties should a suitable solvent for recrystallization have?

Answer 18

• inert (not react w the sample)
• volatile (easily evaporated)
• allow the desired compound to crystallize while leaving impurities dissolved or insoluble

Question 19

Why is it important that the impurities remain soluble or insoluble during recrystallization?

Answer 19

To ensure the impurities don’t crystallize with the desired substance, allowing for effective purification

Question 20

What factors besides solubility are important when selecting a solvent for recrystallization?

Answer 20

Inflammability, toxicity, and cost

Question 21

What is the desired relationship between solubility and recovery in recrystallization?

Answer 21

The lower the solubility of the compound in cold solvent, the higher the recovery of purified material from the crude mixture

Question 22

How does the amount of impurities affect the recrystallization process?

Answer 22

small amounts: impurities may remain dissolved in the solvent

large amounts: may require a second recrystallization for complete separation

Question 23

What observation makes a solvent obviously unsuitable during the solvent test?

Answer 23

If the solid dissolves at room temperature, the solvent is unsuitable

Question 24

What should you do if the solid does not dissolve during the cold solvent test?

Answer 24

Heat the test tube gently, and if partial solution occurs, add more solvent dropwise until complete dissolution or no further dissolution is observed

Question 25

What should you do if no crystals form after cooling a test tube during solvent testing?

Answer 25

Scratch the inside walls of the test tube to induce crystallization

Question 26

How do you determine the ultimate suitability of a solvent for recrystallization?

Answer 26

By collecting the crystals precipitated from the solvent and determining their melting point to check for purity

Question 27

What is the key to preparing the hot solution during recrystallization?

Answer 27

Use the minimum amount of solvent necessary to dissolve the compound at the boiling point to maximize recovery of pure material

Question 28

What is a benefit of using a flask with a fitted condenser during recrystallization?

Answer 28

The condenser return solvent vapor back into the solution, minimizing solvent loss and preventing premature superstauration

Question 29

Why should the solution be slightly below saturation before hot filtration

Answer 29

to minimize difficulties in filtration and avoid premature crystallization

Question 30

Why is decolorizing charcoal used in recrystallization?

Answer 30

To absorb colored impurities that may arise from oxidation, charring, or polymerization

Question 31

When should decolorizing charcoal be added to the solution during recrystallization?

Answer 31

After the solution has been heated but not super-heated, to avoid excessive frothing or boiling over

Question 32

What is the main precaution when using decolorizing charcoal?

Answer 32

Use the smallest affective amount of charcoal, as it can also absorb the desired solute and reduce the yield

Question 33

What is the primary purpose of hot filtration in recrytallization?

Answer 33

To separate the hot solution from any insoluble impurities or decolorizing charcoal before crystallization

Question 34

Why is it important to perform hot filtration quickly?

Answer 34

To prevent premature crystallization in the filter which can clog the funnel

Question 35

What type of filter paper and funnel are best for hot filtration in recrystallization?

Answer 35

A fluted filter paper in a stemless glass funnel, as it increases filtration speed and prevents clogging

Question 36

What should be done if crystals form during hot filtration?

Answer 36

Reheat the solution to dissolve the crystals and proceed with the filtration again

Question 37

How is the crystallization best achieved after filtration in recrystallization?

Answer 37

By allowing the hot filtrate to cool slowly and undisturbed, following by chilling in ice to complete the precipitation

Question 38

What is the effect of slow vs rapid cooling on crystal formation?

Answer 38

slow cooling tends to produce larger, purer crystals, while rapid cooling with stirring produces small crystals

Question 39

What problem can occur if crystallization happens too rapidly?

Answer 39

Co-deposition of impurities or formation of an oily later, which can trap impurities within the solid.

Question 40

What is the main purpose of cold filtration in recrystallization?

Answer 40

To completely separate the purified crystals from the "mother liquor" containing dissolved impurities using suction filtration

Question 41

Why should strong suction be avoided initially during cold filtration?

Answer 41

strong suction can pull finer particles into the pores of the filter paper, clogging it and slowing down the filtration process

Question 42

What should be done if some crystals remain in the flask during cold filtration?

Answer 42

Rinse the flask w a portion of the filtrate (already saturated with solute) rather than fresh solvent to transfer the remaining crystals to the funnel

Question 43

How can melting point determination help in assessing the purity of a crystalline compound?

Answer 43

Pure compounds have sharp melting points with a narrow range, while impure compounds have depressed and broader melting points

Question 44

What is a "mixture melting point" and how is it used?

Answer 44

A test where two samples suspected to be the same are mixed; if they are identical their melting point remains sharp, but if different the melting point is broadened.

Question 45

What factors can influence the observed melting point range of a sample?

Answer 45

- sample purity - crystal size - packing density in capillary tube - heating rate

Question 46

Why is it important to heat the melting point apparatus slowly as the melting point is approached?

Answer 46

Rapid heating can cause the observed melting point to be higher than the true valve, as the temp rises too quickly for even heat transfer

Question 47

What is solvent partitioning in liquid-liquid extractions?

Answer 47

It's the distribution of solutes between two immiscible solvents, typically water and an organic solvent, based on their solubility and partition coefficient

Question 48

What determines which layer is on top in a liquid-liquid extraction?

Answer 48

The less dense solvent forms the upper later, while the denser solvent forms the lower later

Question 49

Why is it important to know the partition coefficient when performing extractions?

Answer 49

The partition coefficient helps predict how much of the solute will distribute between the two solvents, impacting the efficiency of the extraction

Question 50

How can the efficiency of a liquid-liquid extraction be improved?

Answer 50

By increasing the volume of the organic solvent, performing multiple extractions, or selecting a solvent with a high partition coefficient for the desired solute.

Question 51

What are the key criteria for choosing a suitable solvent for extraction?

Answer 51

- The solvent should not react with the solute - be immiscible w the og solution - have a favorable partition coefficient - be easily separated by distillation or evaporation

Question 52

How do you safely perform liquid-liquid extraction in a separatory funnel?

Answer 52

Add the extracting solvent, shake the funnel while venting built-up pressure periodically, and allow the layers to separate before draining

Question 53

How can you tell which later is the aqueous phase in an extraction?

Answer 53

Add a small amount of water to the funnel, it will mix with the aqueous layer, helping to distinguish it from the organic layer