Organic Chemistry Laboratory

Question 1

Boiling chip

Answer 1

Granules of porous alundum which release small bubbles of entrapped air to a solution as it is being heated. These bubbles provide a path by which the vapor of the boiling liquid can be smoothly led to the surface and released.

NOTE: boiling chips can never be used twice and they should never be added to a hot solution (i.e., near its boiling point) since violent frothing will result.

Question 2

Bumping

Answer 2

The sudden violent surge of vapour breaking from the surface of a liquid. If bubbles do not form readily, the temperature of the liquid may rise above its boiling point. The liquid becomes superheated and when a bubble does form, it is large and can push the liquid out of the container. This can be prevented using boiling chips or by stirring.

Question 3

Condenser

Answer 3

An apparatus in which gases are cooled until they become liquid. The direction of the water flow in a condenser is always AGAINST gravity.

Question 4

Ice Bath

Answer 4

A mixture of both ice and water, which is more efficient at cooling than just ice.

Question 5

Melting point

Answer 5

The temperature at which the solid melts. Melting point ranges consistof two temperatures; T₁: the temperature at which the first drop of liquid appears amongst the crystals and T₂: the temperature at which the whole mass of crystals turns to a clear liquid. The purer the compound, the narrower the melting point range (1-2ºC). By mixing two compounds together (mixing melting points) one can verify the identify of a compound.

Question 6

Miscible

Answer 6

Liquids capable of being mixed to form homogenous substances.

Question 7

Mother Liquor

Answer 7

The solution that remains after a crystallization. [NOTE: this consists of the recrystallization solvent saturated with the dissolved solute]

Question 8

Reflux

Answer 8

A process where the heated vapours of a liquid enter a condenser and are cooled back to a liquid state then flow back to the original boiling solution. A ‘good’ reflux rate is estimated as 1 drop of liquid per second off the end of the condenser.

Question 9

Saturated (solution)

Answer 9

A solution with the maximum amount of solute that can be dissolved.

Question 10

Scratching

Answer 10

Using the roughened end of a glass rod to pull a little of the recrystallization solvent above the surface of the liquid while ‘scratching’ the rod against the side of the container to give a rough area for the first crystal to form.

Question 11

Seeding

Answer 11

Placing a pure crystal ‘seed’ of a substance in a saturated solution of this compound to initiate the growth of the crystal lattice.

Question 12

Alkaline

Answer 12

‘basic’; pH > 7: will turn neutral litmus paper blue.

Question 13

Anhydrous

Answer 13

‘without water’; the material where all the water has been removed but will readily absorb any water it contacts

Question 14

Drying (re)agent

Answer 14

inorganic salts that are hydroscopic (readily take up and retain moisture) and can form hydrates; they are used to remove small amounts of water that have been absorbed by organic solvents.

Hydrate: a compound formed by the union of water with some other substance

Question 15

Emulsion

Answer 15

A finely dispersed suspension of one liquid layer in another which happens when dissolved materials in these liquids cause the mutual solubility of the two layers to increase. It often appears as a ‘foamy’ layer between two immiscible liquids.

(The mutual solubility temperature is the highest temperature you can reach before two partially miscible liquids become miscible.)

Question 16

Extraction

Answer 16

The process of removing a desired substance from a solution or solid mixture, based upon differences in solubility. The term ‘extract’ commonly refers to removing compound(s) from an aqueous layer using an organic solvent.

Question 17

Immiscible

Answer 17

Substances unable to be blended, usually liquids like oil and water.

Question 18

Percentage recovery

Answer 18

The final weight divided by the original weight of a substance expressed as a percentage. Often used during purification to determine how much was lost.

Question 19

Successive extractions

Answer 19

Taking your total volume of extracting medium and splitting it into several portions. Each portion is added to a separatory funnel containing the other immiscible solvent, mixing and removing before adding the next portion. This ensures getting the ‘best’ possible separation of the components in a mixture.

Question 20

Percentage yield

Answer 20

The actual yield of a substance divided by the theoretical yield expressed as a percentage (i.e., x 100%)

Question 21

Schlieren lines

Answer 21

Streaks or wavy lines in a solution indicating regions of different densities; (usually indicates that something is dissolving).

Question 22

Quantitative Transfer

Answer 22

The transfer of a substance from one container to another without losing any. This is usually accomplished by rinsing the first container several times with a suitable solvent and adding all of these rinses to the second container.

Question 23

Benchtop Extraction

Answer 23

Recovering a substance that has been ‘spilled’ on the benchtop by scraping up the solid or by soaking up a solid/liquid substance with a cotton saturated in a suitable solvent then extracting the substance from the cotton.

Question 24

Azeotrope

Answer 24

A mixture of 2 or more substances that distills at a constant temperature and with a constant composition, even though separately the components have different boiling points. Usually occurs due to intermolecular interactions (e.g., hydrogen bonds).

Question 25

Boiling Point

Answer 25

The temperature at which the vapour pressureof a liquid is equal to the atmospheric pressure. The boiling point is constant for a pure substance.

Question 26

Condensate

Answer 26

The liquid that results when gas is cooled.

Question 27

Distillate

Answer 27

The condensed liquid obtained from distillation.

Question 28

Distillation

Answer 28

The two-part process where (1) a liquid is boiled (converted into vapour) and (2) the vapour is then condensed to a liquid and collected in a *different* container.

Question 29

Rheostat (Powermite)

Answer 29

An adjustable resistance for regulating electric current; used to regulate the temperature of the thermowells since they have no regulating dials.

Question 30

Refractive Index

Answer 30

The measure of how much the path of light is being refracted by or how fast light travels in a medium; (the ratio of the sine of the angle of incidence to the sine of the angle of refraction). This measurement is dependent upon temperature and the wavelength of light.

Question 31

Volatile

Answer 31

Tending to evaporate under normal temperature and pressure conditions.

Question 32

Compare pure and impure substances. How is a compound purified? What are frequently used purification methods in CHEM 235?

Answer 32

The concept of purity is introduced in many entry-level chemistry courses. A pure substance is one that is made up of only one type of molecule. An impure substance is a mixture of two or more molecules. Therefore, to purify a compound is the process of separating the desired molecules from the undesired molecules in an impure substance. Frequently used purification methods in CHEM 235 are recrystallization (if crude is a solid) or distillation (if crude is a liquid).

Question 33

What principles is the process of recrystallization based on? [3]

Answer 33

1. Different compounds have different solubility in various solvents. 2. The solubility of a compound in a given solvent increases with increasing temperature. 3. The molecule of one substance will rarely fit into the growing crystal lattice of another compound.

Question 34

What is the general scheme for a recrystallization process? [4]

Answer 34

* The impure solid is first dissolved in a minimal amount of hot/boiling solvent to produce a solution that is close to the saturation point. * If the resulting solution is coloured, and the pure compound is known to be colourless, charcoal may be added to adsorb the coloured impurities. * Slowly cooling the solution causes the desired compound to crystallize out of solution, while the soluble impurities remain in solution. Controlled cooling of solution is done at 45º angle. * Finally, the crystals are isolated by vacuum (suction) filtration, dried, and a melting point measured to assess their purity.

Question 35

Describe an ideal recrystallization solvent. [5]

Answer 35

* Cold temperature; solvent dissolves the solute sparingly, or not at all. * Hot (at reflux); solvent dissolves the solute completely. * Impurities either dissolve completely in cold solvent, or are completely insoluble in hot solvent. * The solvent does not react with any components of the reaction. * Boiling point of the solvent is lower than the melting point of the compound so that the compound does not melt/ 'oil out' at reflux.

Question 36

What is the rule of thumb for choosing a solvent?

Answer 36

Try to find a solvent that, at its boiling point, will dissolve ~5-8 grams of solute per 100 mL of solvent.

Question 37

What is the procedure for identifying the correct solvent to use? [2]

Answer 37

1. To a small test tube containing ~0.3 mL of the solvent being tested, add ~10-20 mg of the compound (approximately the tip of spatula). Agitate slightly and look for signs of dissolving. If the compound does not dissolve totally or at all, proceed to step 2. Note: if the compound DOES dissolve at room temperature, this solvent is NOT suitable for a single solvent recrystallization. 2. Put the test tube in a boiling-water bath for one minute. Again, watch for dissolving and/or 'oiling out'. If the compound does not dissolve, you can add more solvent - a few drops at a time - until you reach the maximum of 2 mL. Stop after 2 minutes of boiling, if dissolving has not occurred. Note: if the compound DOES NOT dissolve while the solvent is HOT, this solvent is NOT suitable for a single solvent recrystallization.

Question 38

What happens if you are not able to find a single solvent with ideal characteristics?

Answer 38

In these cases, two miscible solvents can be selected to form a 'solvent pair'. In a solvent pair recrystallization, one solvent dissolves the compound well at all temperatures (very soluble at HOT and COLD), while the other solvent does not dissolve the compound at any temperature (insoluble at both HOT and COLD). By mixing them together, you can create a new solvent system that has the appropriate characteristics for recrystallization.

Question 39

What happens if your solution is blue or red?

Answer 39

This is most likely because you didn't listen to your TA or the Lab Director, and put your pH paper directly into your solution to test the pH. In this case, you are stuck with a difficult decision: ## Footnote a) Do you attempt to use decolourizing charcoal to remove the colour? If you select this approach, you will get marks for appearance, but you may lose your sample altogether. b) Do you submit blue or red crystals? You will lose marks for appearance, but you will have something to show. You also have a story to tell in the lab report.