Lab Midterm Flashcards
(117 cards)
1
Q
Are organic compounds generally more soluble in hot solved or cold solvents?
A
Hot solvents
2
Q
Basic recrystallization process
A
Organic compound is dissolved into a solvent at elevated temperatures to form a nearly saturated solution, followed by gradual cooling. As the solution cools, the compound’s solubility decreases, leading to the formation of crystal precipitates.
Results in a solid product that is purer than the original material
3
Q
Properties of an ideal solvent for recrystallization
A
One where the compound of interest is highly soluble at high temps but sparingly soluble at low temps
Boiling point is high enough to give ample range for heating, but not too high as to slow the necessary evaporation of any remaining solvent. Best temperature range is from 50-80 C
4
Q
Mixed melting point technique
A
Used to further verify the purity and identity of a sample
Roughly equal amounts of your sample and an authenticated pure sample are mixed together, and the melting point range is determined. If the point is sharp and its value unchanged, the unknown and authentic sample are identical. If the two components are different the melting point will be decreased and the range broadened.
5
Q
Are volatile solvents like ether, pentane, and methylene chloride ideal solvents for recrystallization?
A
No
6
Q
You run a mixed melting point analysis on your sample and an authenticated sample. You see the mixed sample’s melting point has decreased and the range has broadened. Are your samples identical in composition?
A
No
7
Q
You run a mixed melting point analysis on your sample and an authenticated sample. You see the mixed sample’s melting point has stayed the same. Are your samples identical?
A
Yes
8
Q
In recrystallization, faster cooling will lead to what quantity and quality of crystals?
A
Fewer and smaller crystals of less purity
9
Q
In recrystallization, slower cooling will lead to what quantity and quality of crystals?
A
More and larger crystals of greater purity
10
Q
What kind of extraction did we do in lab?
A
Liquid-liquid extraction
11
Q
What kind of extraction do you use to make tea?
A
Solid-liquid extraction
12
Q
What apparatus/equipment did we use for extraction?
A
A separatory funnel
13
Q
What type of funnel did we use in the vacuum filtration?
A
A Buchner funnel
14
Q
What organic solvent was used in our extraction lab?
A
Diethyl ether
15
Q
In our extraction lab, which solvent made up the upper layer?
A
The organic solvent, diethyl ether
16
Q
In our extraction lab, which solvent made up the lower layer?
A
The aqueous solvent
17
Q
Are organic solvents usually more or less dense than aqueous solvents?
A
Less dense, forms the upper layer
18
Q
Are chlorinated aqueous solvents more or less dense than normal aqueous solvents?
A
More dense, forms the lower layer
19
Q
Is benzoic acid preferentially soluble to organic solvents or aqueous solvents?
A
Organic solvents
20
Q
What happens to benzoic acid when a base is added?
A
It deprotonates and forms its conjugate base, which is a salt.
It now moves from the organic solvent to the aqueous solvent, where the water stabilizes the salt’s ionic bond
21
Q
What happens to fluorenone upon the addition of an acid?
A
Nothing, no reaction
22
Q
What happens to fluorenone upon the addition of a base?
A
Nothing, no reaction
23
Q
What functional group allows benzoic acid to transition to the aqueous layer?
A
It has a carboxylic acid group (pKa = 4) which is deprotonated upon the addition of a base
24
Q
What functional group allows pseudoephedrine to transition to the aqueous layer?
A
It’s (secondary) amine group is protonated upon the addition of an acid
Note: it does have a hydroxyl group, but it is not strong enough to deprotonate
25
What is the function of the rotovap?
Quickly removes/evaporates organic solvents
26
What is the purpose of the brine wash in the extraction lab?
It removes the water saturated in the ether layer, takes the water percentage down from ~8% to ~2%
27
Your test your solution with pH paper. It turns red. What's the pH of your solution?
Acidic
28
Your test your solution with pH paper. It turns dark purple. What's the pH of your solution?
Basic
29
What should you use to clean glasses that contained fluorenone?
Acetone
30
How flammable is diethyl ether?
Highly
31
What is the purpose of gas chromatography?
To separate compounds on the basis of polarity and boiling point
32
In gas chromatography, how are compounds separated?
Substances are separated between a mobile phase and a stationary phase, due to a difference in affinities between the two phases
33
A compound is moving slowly through the gas chromatography apparatus. Which phase does it have more affinity for?
Less affinity for the mobile phase
More affinity for the stationary phase
34
A compound is moving quickly through the gas chromatography apparatus. Which phase does it have more affinity for?
Less affinity for the stationary phase
More affinity for the mobile phase
35
A compound has more affinity for the stationary phase. At what speed will it move through the gas chromatography apparatus?
Slowly
36
A compound has more affinity for the mobile phase. At what speed will it move through the gas chromatography apparatus?
Quickly
37
A compound has less affinity for the stationary phase. At what speed will it move through the gas chromatography apparatus?
Quickly
38
A compound has less affinity for the mobile phase. At what speed will it move through the gas chromatography apparatus?
Slowly
39
In gas chromatography, what is the stationary phase?
A high boiling liquid or polymer suspended inside metal tubing (the column) in an oven
40
In gas chromatography, what is the mobile phase?
An inert gas
41
What type of column did we use in our gas chromatography lab?
A polar column
42
Gas chromatography: Based on the type of column we used, how long would a compound with a lower boiling point spend in the column?
Less time, not very long
Lower boiling compounds would be less polar, and repelled by the polar column
43
Gas chromatography: Based on the type of column we used, how long would a compound with a higher boiling point spend in the column?
More time, longer
Higher boiling compounds would be more polar, and attracted to the polar column
44
Your gas chromatography sample graph has a new peak in relation to your unknown sample. What does this indicate?
It indicates that the compound that made the peak is not present in my unknown sample
45
Your gas chromatography sample graph has a single peak (your unknown sample), but is much larger than usual. What does this indicate?
It indicates that the compound mixed with my sample is present in my unknown compound
46
Sample A is less polar, has a low boiling point, and a high surface area. How will it appear on your gas chromatography graph?
A tall, thin peak closer to x = 0
47
Sample B is highly polar, has a high boiling point, and has low surface area. How will it appear on your gas chromatography graph?
A shorter, wider peak farther from x = 0
48
What is the X-axis of your gas chromatography graph?
Retention time
49
Using your gas chromatography graph, find the approximate percentage of each compound in the mixture
Use this formula (which isn't super accurate, but it's a good estimation)
Approximate percentage =
(area of peak in question) /
(sum of areas of all peaks)
50
Rank the following substances in order of elution from a gas chromatograph apparatus:
Cyclohexane, o-xylene, toluene, THF
Elutes first:
THF
Cyclohexane
Toluene
O-xylene
Elutes last
51
What three techniques did we use to identify our unknown carboxylic acid?
Melting point analysis
Neutralization equivalent of a titration
Sodium fusion with elemental analysis (three subsequent chemical tests)
52
What does a compounds neutralization equivalent (N.E.) represent?
The Mass of an acid (in grams) that one mole of sodium hydroxide (NaOH) can neutralize
NE = (mg of acid)/(mmol of base)
53
For an acid with a single carboxyl group, what is its NE equal to?
NE will be equal to its molecular weight
54
For an acid with two carboxyl groups, what is its NE equal to?
Half its molecular weight
55
For an acid with three carboxyl groups, what is its NE equal to?
One third its molecular weight
56
What is the purpose of the molten sodium metal in the sodium fusion and chemical test process?
The molten sodium metal is a very strong reducing agent
It combusts the C, H, and O of the organic molecule and converts any remaining sulfur to sulfide ion, nitrogen to cyanide ion, and halogen to halide ions
57
What is added to your sodium fusion sample to determine if sulfur is present?
Lead (Pb2+)
58
What is added to your sodium fusion sample to determine if nitrogen is present?
Iron (Fe3+ or Fe2+)
59
What is added to your sodium fusion sample to determine if a halogen is present?
Silver (Ag+)
60
After sodium fusion, you add lead ions. Your precipitate is black. What does this indicate?
Sulfur is present
61
After sodium fusion, you add iron ions. Your precipitate is prussian blue. What does this indicate?
Nitrogen is present
62
After sodium fusion, you add silver ions. Your precipitate is white. What does this indicate?
Chlorine is present
63
After sodium fusion, you add silver ions. Your precipitate is off white/pale yellow. What does this indicate?
Bromine is present
64
After sodium fusion, you add silver ions. Your precipitate is bright yellow. What does this indicate?
Iodine is present
65
Why is a chair with an axial substituent higher energy than a chair with the same substituent but equatorial?
Gauche (aka 1,3-diaxial) interactions between the axial substituent and the cyclohexane ring make the axial chair higher energy
66
How many gauche interactions does each axial substituent have with the cyclohexane ring?
2
67
What is the purpose of recrystallization in organic chemistry?
To purify organic solids by removing impurities
68
What is the significance of slower cooling during the recrystallization process?
It produces larger and purer crystals
69
What is the role of vacuum filtration in recrystallization?
To separate crystallized compounds from the solution
70
Why is water chosen as a solvent for recrystallization in this experiment?
It has ideal solubility properties for acetanilide
71
In melting point analysis, what happens if the sample is wet?
The observed melting point is lower and inaccurate
72
Is a particular solvent appropriate for recrystallization if the target molecule is highly soluble at room temperature?
No
Recrystallization takes advantage of the temperature dependance of solubility. If it is already soluble at room temperature, it will be difficult to recover the target molecule
73
Why would a solvent with a very low boiling point be inappropriate for recrystallization?
The temperature range for heating is limited, and it is difficult to fully dissolve the crude mixture
74
Would you expect crystals grown more quickly or slowly to be of higher purity?
Slower crystal growth is desirable to avoid trapping impurities
75
How does the rotovap work to efficiently evaporate solvents?
It reduces pressure, heats, and spins the sample
76
The extraction lab used several glassware items, including a separatory funnel, a Buchner funnel, 2 Erlenmeyer flasks, and a round bottom flask. Which one needed to be dry and not contain any water residue before use?
The round bottom flask
77
What was the purpose of the sodium hydroxide in the extraction experiment? Could the lab have worked without it?
The NaOH deprotonated the benzoic acid and allowed it partition into the aqueous phase. The lab wouldn't have worked without it because otherwise the benzoic acid would've stayed in the organic phase
78
Why is ethanol a poor solvent for extraction?
Ethanol is miscible (soluble) in water
79
Why is 1,2-xylene a poor solvent for extraction?
1,2-xylene has a very high boiling point
80
Naphthalene is made of two benzene rings bound by two of the rings carbons (bicyclic). Which phase does it partition to?
Organic phase
81
Nootkatone is a bicyclic alkene with some methyl groups and a ketone. Which phase does it partition to?
Organic phase
82
Quinine is a funky organic alkene with a bicycle ring, a funky nitrogen-infused tricycle ring, and a hydroxyl group. Which phase does it partition to?
Aqueous phase
83
Phenylalanine is an amino acid whose R group is a phenyl ring. Which phase does it partition to?
Aqueous phase
84
What is the primary purpose of performing a melting point determination on the unknown carboxylic acid?
To identify the acid by comparing its melting point with known values
85
Which observation would indicate the presence of nitrogen in the unknown sodium fusion?
A deep blue precipitate appearing after the addition of iron ions
86
In the sodium fusion tests, why is it necessary to acidify the filtrate before performing the chemical tests for sulfur, nitrogen, and halogens?
It adjusts the pH to a level suitable for each specific test reaction
87
What was the mobile phase for our gas chromatography lab?
An inert gas
88
If a chromatogram shows a broad, poorly resolved peak, what most likely caused it?
The sample was overloaded or injected improperly
89
How would the resolution (separation into discrete peaks) of your chromatogram be affected by using a column half as long as usual?
The peaks would be smaller, broader, and more difficult to discern
90
What happens to peak resolution on the chromatogram when column length is reduced?
Resolution decreases
Peaks are smaller and broader
91
What happens to peak resolution on the chromatogram when column length is increased?
Resolution is increased
Peaks are larger/taller and narrower
92
Why would a compound in a mixture not appear on a gas chromatogram?
It's boiling point is too high and it subsequently didn't vaporize
93
What would happen to the retention time of a given compound if the carrier gas flow rate of the gas chromatograph apparatus is increased?
Retention times would be lower
94
What would happen to the retention time of a given compound if the carrier gas flow rate of the gas chromatograph apparatus is decreased?
Retention time is increased
95
You are monitoring the reaction A-->B via periodic gas chromatograph analysis. Initially, you see only one peak in your chromatogram; however over time you find that a new peak is growing while the original peak is shrinking. What is happening?
The initial peak is compound A. As the reaction occurs the amount of A available decreases and the amount of B increases, shrinking A's original peak and growing B's new peak
Reactant A is being converted to product B
96
How are benzene's C-C bonds best described?
They are 1.5 bonds, neither single nor double. They are a resonance hybrid
97
As a cyclohexane substituent, why does iodine have a lower conformational energy cost than chlorine?
Iodine has a larger atomic radius and greater electron shielding.
Iodine is farther away from the cyclohexane ring, and the longer C-I bond length minimizes steric strain
98
How many gauche (aka 1,3-diaxial) interactions does an equatorial substituent have with axial hydrogens?
2
99
How many gauche (aka 1,3-diaxial) interactions does an axial substituent have with axial hydrogens?
2
100
How many ring-substituent gauche interactions does trans-1,2-dimethylcyclohexane have when its methyl groups are in axial positions?
4
101
How many ring-substituent gauche interactions does trans-1,2-dimethylcyclohexane have when its methyl groups are in equatorial positions?
0
102
How many substituent-substituent gauche interactions does trans-1,2-dimethylcyclohexane have when its methyl groups are in axial positions?
0
103
How many substituent-substituent gauche interactions does trans-1,2-dimethylcyclohexane have when its methyl groups are in equatorial positions?
1
104
How many total gauche interactions does trans-1,2-dimethylcyclohexane have when its methyl groups are in axial positions?
4
105
How many total gauche interactions does trans-1,2-dimethylcyclohexane have when its methyl groups are in equatorial positions?
1
106
What is Keq for trans-1,2-dimethylcyclohexane?
Close to zero
The chairs have different numbers of gauche interactions (4 vs 1). This causes a difference in energy which affects Keq
107
What is Keq for cis-1,2-dimethylcyclohexane?
1
Chairs are degenerate, meaning equal in energy. Same number of gauche interactions, no energy difference
108
What is distillation?
The process of vaporizing a liquid into the gas phase, and then condensing the vapor back to the liquid phase, collecting the distillate
109
What is the purpose of distillation?
To separate substances with different boiling points
110
Is the composition of a boiling liquid generally equal to the composition of the vapor above the boiling liquid?
NO!
111
What is the specific purpose of our distillation lab?
To distill a mixture of cyclohexane and toluene and to see how the composition of the distillate varies over time
112
When should you turn on the distillation apparatus?
Only after it's been inspected by the instructor or TA
113
In your distillation apparatus, at what rate should water flow through the condenser?
Slowly, in a trickle
114
Once boiling commences, you should adjust the hot plate temperature to maintain what distillation rate?
One drop every 1-2 seconds
115
How often should you record the temperature during the distillation process?
Once at the first distillate drop, then again at every 2 mL interval
116
When should you stop distillation?
Before the flask becomes completely dry
Without the absorption of heat due to vaporization, the temperature of the flask could rise very quickly
117
What should you do to your heating block after distillation is complete?
Lower it so that the boiling flask does not go dry
