Lab Final

Question 1

What are some important uses of TLC in Organic chemistry?

Answer 1

1. To establish that two compounds are identical.
2. To determine the number of components in a mixture.
3. To determine the appropriate solvent for a column chromatographic separation.
4. To monitor a column chromatographic separation.
5. To monitor the effectiveness of a separation achieved on a column, recrystallization or extraction.
6. To monitor the progress of a reaction.

Question 2

Identify the advantages of using TLC in the laboratory.

Answer 2

1. Only small amounts of material are necessary.
2. Material is not wasted.
3. Time efficient.

Question 3

Identify the disadvantages from using TLC in the laboratory.

Answer 3

Volatile materials can NOT be used because they will evaporate from the TLC plate.

Question 4

Identify the simple breakdown of steps to perform a TLC. (3 steps)

Answer 4

Spotting, developing, and visualizing.

Question 5

What kind of partitioning technique is TLC?

Answer 5

TLC is a solid-liquid partitioning technique.

Question 6

Define mobile phase.

Answer 6

(aka: eluent or solvent) The mobile phase carries the sample through a solid, backing support (stationary phase).

Question 7

What are some characteristics of the mobile phase in TLC?

Answer 7

It ascends (does not descend), and it is generally in a liquid phase but can also be in the gas phase.

Question 8

Define stationary phase.

Answer 8

(aka-adsorbent) It adsorbs the mixture compunds dissolved within the mobile phase.

Question 9

What are some characteristics of the stationary phase?

Answer 9

For TLC, the stationary phase is found coated along the solid, backing support.

Question 10

Describe the TLC process.

Answer 10

1. A mixture is spotted on a TLC plate.
2. The mobile phase carries it through the stationary phase.
3. The mixtute will then become partitioned between the mobile and stationary phases based on attractions of its components to either one.

Question 11

Identify things that you should never do when running a TLC plate.

Answer 11

1. Applying too much sample-Tailing occurs and a poor separation.
2. Marking the eluent front.
3. Solvent line that is higher than spotted plate- The sample will be dissolved in the eluent.
4. Allowing eluent front to reach top of the plate because Rf calculations will be innaccurate.
5. Uncovering the developing chamber- The contents on TLC plate will evaporate.

Question 12

What is retention factor?

Answer 12

Rf is the ratio to front which expresses the distance traveled by the compound relative to that traveled by the solvent.

     Rf=_distance traveled by substance_                 distance traveled by solvent

Question 13

Is it possible to have the same Rf value for two or more compunds?

Answer 13

Yes.

Question 14

Why is it necessary to mention which solvent (or solvent mixture) has been used to measure Rf?

Answer 14

Because a given compund should travel a fixed distance relative to the distance the solvent travels.

Question 15

Explain the effect water has on the adsorbent? Should plates be hydrated or dehydrated when running a TLC?

Answer 15

Water affects the adsorbent activity by occupying polar sites, therefore, the plates must be dehydrated by heating in a drying oven.

Question 16

What is visualization in regards to TLC?

Answer 16

The separated material must be made visible by some reagent or method that makes separated compunds visible.

Question 17

What were two visualization methods discussed in the lab?

Answer 17

1. I₂ crystals produce I₂ vapors that react with many organic materials to form colored complexes (brown, purple or yellow).
2. UV lamp- Under the UV light many compunds look like bright spots because certain types of compunds shine very bright because they fluoresce.

Question 18

Write the names of two fluorescent indicators.

Answer 18

Zinc and cadmium sulfides. Fluorescent indicators are added to adsorbent in TLC plate, and under UV light, the entire plate fluoresces except where the compounds absorb UV light.

Question 19

What is a binder in TLC?

Answer 19

It binds the adsorbent and the glass plates together. In TLC about 10-13% by weight of gypsum is added as a binder.

Question 20

What is the chemical name of gypsum?

Answer 20

Calcium sulfate.

Question 21

Write the chemical structure of biphenyl, benzophenone, and benzhydrol.

Question 22

Write the name of two commonly used adsorbents in TLC.

Answer 22

1. Silica gel G (Si*xH₂O)
2. Alumina G (Al₂O₃)

Question 23

What is mixed eluents?

Answer 23

Eluents of different polarities can be mixed in 1:1 ratios resulting in a mixture with eluting power in between both solutions.

Question 25

What kind of partitiioning technique is column chromatography?

Answer 25

Solid-liquid partitioning technique.

Question 26

Identify ways the mobile phase is both similar and different in column chromatography when compared to TLC.

Answer 26

It also carries the sample through the stationary phase but it descends down the column. Another difference is that column chromatography used various solvents to achieve separation.

Question 27

What was teh adsorbent used for column chromatography in the lab?

Answer 27

Alumina, 3 g.

Question 28

Identify the simple breakdown steps for running a column chromatography.

Answer 28

1. A mixture is added to an eluent and poured in the column. 2. Interaction with stationary phase occurs in the column. 3. Compounds in mixture move at different rates depending on their difference in attraction for either phase.

Question 29

How is polarity related to the movement of compounds down a chromatographic column?

Answer 29

Polar compounds interact with the stationary phase and travel down the column more slowly. Nonpolar compounds or less polar compounds travel faster down the column.

Question 30

What are some typical problems associated with running a column chromatography?

Answer 30

Column should be evenly packed, free of air bubbles and gaps, and bands should be horizontal.

Question 31

What is channeling?

Answer 31

(aka streaming) Occurs when part of a band advances ahead of the band and is caused by cracks or irregularities in the adsorbent surfaced or by air bubbles in the packing.

Question 32

What causes the process of cracking within a chromatographic column?

Answer 32

When heat is generated (through weak bonds between solvent and Alumina) that evaporates the solvent and creates bubbles that force separation of the column packing.

Question 33

What are two ways to try to avoid problems when running a column chromatography?

Answer 33

Using a support base such as sand.

Question 34

Be able to identify the five different interactions found between the eluent and adsorbent studied in lab. Rank their strengths of interaction.

Answer 34

The more polar the functional group, the stronger the bond to alumina. In order of strongest to weakest interatction: Salt formation \> Coordination \> Hydrogen bonding \> Dipole-dipole \> Van der Waals

Question 35

Arrange the following classes of organic compounds according to their increasing elution sequence: aromatics, olefins, ketones, amines, and acids.

Question 36

# Experiment 9 What are characteristics of a nucleophile for a substitution reaction?

Answer 36

It contains an unshared pair of electrons, it's generally a strong lewis base, it attacks another molecule at a site deficient of electrons, and it has a high affinity for a carbon 2p orbital.

Question 37

# Experiment 9 What are characteristics of the leaving group in a substitution reaction?

Answer 37

It is generally a weak base, acids with low pKa values are good leaving grooups and acids with high pKa values are poor leaving groups.

Question 38

# Experiment 9 How do Pka values of acids relate to whether it is a good leaving group or not?

Answer 38

Acids with low pKa values are good leaving groups and acids with high pKa values are poor leaving groups.

Question 39

# Experiment 9 Depict examples (two each) of good and poor leaving groups.

Answer 39

Iodide, bromide, chloride, sulfonate and water are good leaving groups. Fluoride, thiolate, cyanide, hydroxide, and alkoxyde are poor leaving groups.

Question 40

# Experiment 9 Why is water a good leaving group?

Answer 40

Water is a good leaving group because it is a weak acid and its conjugate base is a strong base.

Question 41

# Experiment 9 What is meant by "back-side" attack?

Answer 41

The nucleophile attacks the carbon atom that results in the inversion of stereochemistry.

Question 42

# Experiment 9 Have an understanding of the concepts (rate law, stereochemistry, the R group, etc.) discussed for both SN-1 and SN2 reactions.

Question 43

# Experiment 9 Which solvent mixture is more polar: 40% 2-propanol or 60% 2-propanol?

Question 44

# Experiment 9 Fill in the blank: SN1 reaction yields---------products whereas ---------------reaction yields optically active products.

Answer 44

Optically active, SN2.

Question 45

What are some characteristics discussed in the lab which define an elimination reaction?

Answer 45

In elimination reactionss, the carbon skeleton loses HX or H2O. Then a double bond is formed. The loss of HX occurs through heterolytic bond cleavage, where X leaves with both electrons from the original R-X bond. This results in the formation of a carbocation.

Question 46

What is the slow step of an E1 reaction?

Answer 46

The ionization of the substrate to give a carbocation.

Question 47

How does the slow step of an E1 reaction relate to an SN1 reaction?

Answer 47

The slow step is the iionization of the substrate to give a carbocation. It's the same in SN1 reactions.

Question 48

The carbocation intermediate is favored by what kind of nucleophile for an E1 reaction? And for SN1?

Answer 48

In an E1 reactions, basic bases are favored. In SN1 reactions, nucleophilic nucleophiles are favored.

Question 49

Why can more than one product sometimes be formed during an E1 reaction?

Answer 49

Because there are various hydrogens available. The major product is favored.

Question 50

What is Saytzeff's Rule?

Answer 50

Favors the formation of the more substituted, more stable alkene. This is becausethe stability of more substituded double bonds are more thermodynamically favored.

Question 51

What functional group did bromine (Br2) test for within the structure of cyclohexene?

Answer 51

Double bond.

Question 52

What is an oxonium ion?

Answer 52

Ion formed when a compound such as an alcohol is protonated.

Question 53

What functional group did ammonium cerium (IV) nitrate [(NH4)2Ce(NO3)6)] test for within the structure of cyclohexene?

Answer 53

Hydroxyl group. (OH)