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Lab Midterm Flashcards

1
Q

Write a balanced equation for the reaction between the iodide ion, a peroxide, and the hydrogen ion. What causes the orange or brown color?

A

The reaction is: 2I− + H2O2 + 2H+ → I2 + 2H2O. The orange or brown color is caused by the formation of molecular iodine (I2) in solution.

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2
Q

Why does the horn of the carbon dioxide fire extinguisher become cold when the extinguisher is used?

A

When CO₂ is released from high pressure, it undergoes rapid expansion, causing adiabatic cooling.

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3
Q

Why is water not used to extinguish most fires in an organic laboratory?

A

Water may spread organic solvent fires instead of extinguishing them. Organic solvents are often immiscible with water and can float, continuing to burn.

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4
Q

What are safety issues when handling diethyl ether?

A

Spilling diethyl ether on gloves can pose a fire hazard. Ether is highly volatile and flammable. Pouring ether down the drain is hazardous as it may cause vapors to ignite. Washing hands after removing gloves is good practice, but prolonged skin exposure to ether should be avoided.

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5
Q

What effect would poor circulation of the melting point bath liquid have on the observed melting point?

A

Poor circulation can cause uneven heating, leading to inaccurate or broader melting point ranges.

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6
Q

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

A

Insoluble impurities do not affect the melting point but can make observation difficult.

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7
Q

How could you prove that three test tubes contain different compounds with similar melting points?

A

Perform mixed melting point tests by mixing samples. Different compounds will show a depression in melting point and broader range.

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8
Q

What happens if a sample is heated too rapidly in a melting point experiment?

A

The observed melting point may be inaccurately high due to lag in temperature equilibrium.

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9
Q

Why is it incorrect to speak of a melting ‘point’?

A

Melting occurs over a range of temperatures, not a single point.

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10
Q

What effect does incomplete drying of a sample have on the melting point?

A

Residual solvent lowers and broadens the melting point range.

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11
Q

Why should the melting point sample be finely powdered?

A

Ensures uniform heating and accurate melting point determination.

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12
Q

Estimate the melting point of acetanilide mixed with 10% naphthalene.

A

The melting point will be lower and broader than that of pure acetanilide.

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13
Q

You have an unknown with a melting point of 90°C-93°C. Is it compound A (mp 95.5°C-96°C) or B (mp 90.5°C-91°C)?

A

More likely compound B, as the observed melting point aligns closely with its range.

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14
Q

Why should the melting point bath be heated slowly near the melting point?

A

To prevent overshooting the melting point and ensure accuracy.

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15
Q

Why should the sample be tightly packed in the capillary?

A

Ensures even heat transfer and accurate results.

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16
Q

What happens if an unknown is mixed with pure acetanilide? With benzoic acid?

A

With acetanilide: No change in melting point. With benzoic acid: Depressed and broadened melting point.

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17
Q

Which has a higher boiling point, t-butyl alcohol or n-butyl alcohol? Why?

A

n-Butyl alcohol has a higher boiling point due to stronger hydrogen bonding.

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18
Q

What is the purpose of the side arm in a thermometer adapter?

A

Allows pressure release and prevents buildup of gases.

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19
Q

Why might purified naphthalene appear grayish?

A

Residual charcoal or impurities may cause discoloration despite a correct melting point.

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20
Q

What is the solubility of phthalic acid in boiling water and at 14°C?

A

Requires experimental solubility data.

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21
Q

When to use activated carbon during recrystallization?

A

When colored impurities are present.

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22
Q

Why not use too much activated carbon?

A

Excess removes desired compound along with impurities.

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23
Q

Why use mixed solvents for recrystallization?

A

Optimizes solubility differences for better purification.

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24
Q

Why is gravity filtration used instead of suction filtration for hot solutions?

A

Prevents premature crystallization.

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25
Why use fluted filter paper?
Increases filtration speed.
26
Why use stemless funnels for hot filtration?
Prevents clogging due to premature crystallization.
27
Why use vacuum filtration for final isolation?
More efficient at drying crystals.
28
Why not use wooden applicator sticks in reactions?
Wood can react with chemicals.
29
Why not use beakers for recrystallization?
Beakers allow too much evaporation.
30
What should the boiling point of distillation products match?
Should match one of the pure components if separation is successful.
31
What does a constant boiling point indicate in simple distillation?
Purity is indicated by a constant boiling point.
32
What does a more gradual change in boiling point trends indicate in fractional distillation of cyclohexane-toluene?
Indicates separation efficiency.
33
What does azeotropic behavior show in fractional distillation of ethanol-water?
May show a constant boiling point.
34
What is the effect of nonvolatile vs. insoluble impurities on boiling point?
Nonvolatile increases, insoluble has little effect.
35
Why doesn’t all water vaporize instantly at boiling point?
Continuous heat input is required.
36
What is the effect of condensate returning in fractional distillation?
Enhances separation by repeated vaporization-condensation.
37
Why is a completely closed distillation apparatus dangerous?
Pressure buildup may cause explosion.
38
Why is slow distillation better?
Allows better separation.
39
Why is a packed fractionating column more efficient?
Provides increased surface area for vapor-liquid interactions.
40
Why is it important to maintain equivalent proportions of reagents?
To ensure complete reaction and prevent the formation of unwanted side products.
41
What are expected side products in the reaction?
Expected side products include benzaldehyde and benzalacetone. These can be removed via recrystallization and chromatography.
42
How can evidence for a single geometric isomer or a mixture be obtained?
From melting point analysis and spectroscopic data. A sharp melting point suggests a single isomer, while a broader range suggests a mixture.
43
What do the coupling constants and chemical shifts in the 'H NMR spectrum of dibenzalacetone help determine?
Help determine the geometric isomer formed.
44
How can benzalacetone or benzalacetophenone be synthesized?
By altering the ratio of benzaldehyde to acetone and using different reaction conditions.
45
What does the IR peak at 1651 cm⁻¹ correspond to?
Corresponds to the C=O stretching vibration of the conjugated ketone.
46
How many geometric isomers of dibenzalacetone can be drawn?
Three geometric isomers can be drawn based on cis-trans isomerism around the double bonds.
47
What adds further variation to the isomers of dibenzalacetone?
Single bond cis and trans isomers add further variation. The most stable isomer has the lowest steric hindrance and conjugation.
48
How do molecular mechanics calculations help with isomers?
Help determine the steric energies of the isomers. The most stable isomer has the lowest energy.
49
What do energy minimization calculations for trans-benzalacetone provide insights into?
Provide insights into rotational barriers and conformational stability.
50
Why is TLC separation of cis- and trans-2-butene difficult?
Due to their similar polarity and molecular structures.
51
What happens if the top is left off the developing chamber during TLC?
Causes solvent evaporation, leading to inconsistent Rf values.
52
What effect does a solvent level higher than the applied spot have in TLC?
The compound will dissolve into the solvent pool, affecting separation.
53
What is the expected TLC order on silica gel using dichloromethane?
Expected TLC order (from top to bottom): naphthalene, phenyl acetate, butyric acid.
54
What happens if the solvent front reaches the top of the TLC plate?
Rf values become unreliable due to over-development.
55
What is the increasing order of Rf values?
Increasing order of Rf values: acetic acid < 1-butanol < 2-octanone < acetaldehyde < decane.
56
What happens if too much compound is applied to a TLC plate?
Results in streaking, reducing resolution.
57
What does a closed TLC chamber maintain?
Maintains solvent vapor saturation, preventing uneven development.
58
What happens if the polarity of the solvent is too low or too high in TLC?
Too low polarity solvent results in poor movement of spots; too high polarity causes all spots to move to the top.
59
What may happen to spots with similar Rf values after a second development?
May merge or shift slightly.
60
Which analgesic has a chiral center?
Ibuprofen. One enantiomer is more effective than the other.
61
How is the Rf value of substance B calculated?
Using distance measurements on the TLC plate.
62
What do the peaks in the 'H NMR spectrum of cyclohexene correspond to?
Different proton environments within the molecule.
63
What does dehydration of 2-heptanol yield?
Yields heptene isomers; 2-methyl-1-cyclohexanol produces methylcyclohexene isomers.
64
What does an energy diagram of cyclohexanol dehydration illustrate?
Illustrates energy changes from alcohol to oxonium ion to carbocation to cyclohexene.
65
What do the 'H and 'C NMR spectra of cyclohexene show?
Show similarities due to equivalent molecular environments, but differences in chemical shifts.
66
How does acetic acid act in the hypochlorite oxidation of cyclohexanol?
Acts as a catalyst by maintaining an acidic environment.
67
What follows the order of chemical shifts in the 'C NMR spectra of cyclohexanone and adipic acid?
Follows electronic and structural influences on deshielding effects.
68
What does washing with sodium bicarbonate remove?
Removes acidic impurities.
69
What does washing with sodium chloride solution help with?
Helps in phase separation.
70
What does anhydrous calcium chloride do?
Removes residual water from the organic layer.
71
What do the resonances in the 'H NMR spectrum of methyl benzoate correspond to?
Correspond to protons in the aromatic and ester regions.
72
What do small resonances in Figures 40.9 and 40.10 correspond to?
Correspond to carbons with low electron density, such as quaternary or carbonyl carbons.
73
What effect does an insoluble impurity like sodium sulfate have on melting point?
Has no effect on melting point but may cause inaccurate readings if improperly mixed.
74
How can water be removed from an organic liquid immiscible with water?
Using a separatory funnel and drying agents like sodium sulfate.
75
How can water be removed from a miscible organic liquid?
By distillation or drying agents like molecular sieves.
76
What is the purpose of the 'chaser' in cyclohexene preparation?
Helps remove residual product from the reaction flask.
77
What does sodium chloride help extract?
Helps extract an organic solute by 'salting out' the aqueous phase, reducing solubility of the organic compound in water.
78
What does a steam bath provide?
Provides gentle, even heating, reducing the risk of overheating and decomposition.
79
What is a carcinogen?
Causes cancer (e.g., benzene).
80
What is a teratogen?
Causes birth defects (e.g., thalidomide).
81
What is a mutagen?
Causes genetic mutations (e.g., ethidium bromide).
82
What is a lachrymator?
Irritates eyes (e.g., benzyl chloride).
83
What is an azeotropic mixture?
Has a constant boiling point and composition (e.g., ethanol-water mixture).
84
What does a boiling point diagram show for an azeotropic mixture?
Shows a plateau.
85
Why does fractional distillation provide better separation?
Due to repeated vaporization-condensation cycles, improving purity.
86
Why is a melting point not an exact point?
Because melting occurs over a range due to impurities and sample size.
87
What is a Variac?
A variable transformer used to control heating devices in laboratory setups.
88
Which experimental method would you recommend for the preparation of 1-bromooctane and t-butyl bromide?
For 1-bromooctane, an SN2 reaction using NaBr and H2SO4 in refluxing alcohol is recommended. For t-butyl bromide, an SN1 reaction with HBr and t-butanol under acidic conditions is suitable.
89
Why does the crude product contain certain organic impurities?
Side reactions such as elimination (leading to alkenes) and ether formation can introduce impurities. Additionally, incomplete reactions or residual starting materials may be present.
90
How should the reaction conditions be modified to produce 1-chlorobutane instead?
Replace NaBr with NaCl and adjust reaction conditions to favor an SN2 mechanism, such as using a polar aprotic solvent like acetone.
91
Write a balanced equation for the reaction of sodium bisulfite with bromine.
Br₂ + NaHSO₃ + H₂O → 2HBr + NaHSO₄
92
What is the purpose of refluxing the reaction mixture for 45 minutes? Why not simply boil it in an Erlenmeyer flask?
Refluxing maintains a constant reaction temperature, ensuring complete reaction without loss of volatile reactants, unlike simple boiling, which may lead to evaporation and inconsistent results.
93
Why is it necessary to remove all water from 1-bromobutane before distillation?
Water can form azeotropes, reducing purity and yield. Additionally, residual water can cause hydrolysis or side reactions, leading to undesired products.
94
Write reaction mechanisms showing how 1-butene and di-n-butyl ether are formed.
1-Butene Formation: E2 elimination of 1-bromobutane with a strong base. Di-n-butyl Ether Formation: SN2 reaction where butanol reacts with 1-bromobutane.
95
Why are the bromine-bearing and hydroxyl-bearing carbon atoms the farthest downfield in Figures 16.3 and 16.4?
These carbon atoms experience deshielding due to the electronegativity of Br and OH, leading to an electron-withdrawing effect, which shifts their NMR signals downfield.

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