CHEM LAB EXAM

(EXP 1) Nitrobenzene is much more common that 4-nitrotoluene, and its hazards are better known. Do you think the hazards of 4-nitrotoluene are likely to be very different from or very similar to those of nitrobenzene?

they are similar because they’re both toxic if swallowed, but 4-nitrotoluene is more hazardous since ot can’t touch your skin

(EXP 1) density of ethyl acetate in g/mL

0.90 g/mL

(EXP 1) density of water in g/mL

1 g/mL

(EXP 1) will the density of 5% aqueous NaOH be greater than, less than, or the same as that of pure water?

the density of 5% aqueous NaOH will be greater, but not by much

(EXP 1) For a mixture of 20 mL ethyl acetate and 20 mL of aqueous 5% sodium hydroxide, two layers form. Based on densities, which layer is on the bottom?

the layer on the bottom would be the 20 mL of aqueous sodium hydroxide

(EXP 1) For ethyl acetate/neutral water, which layer would 4-nitrobenzoic acid be in mainly?

the top layer (organic layer, ethyl acetate layer)

(EXP 1) For ethyl acetate/neutral water, which layer would sodium 4-nitrobenzoate be in mainly?

neutral water

(EXP 1) For ethyl acetate/neutral water, which layer would 4-nitrotoluene be in mainly?

ethyl acetate

(EXP 1) How is the 4-nitrobenzoic acid isolated from the basic water solution?

the 4-nitrobenzoic acid is isolated from the basic water solution by adding hydrochloric acid to make it acidic rather than basic

(EXP 1) If you started with 1.00 g of a 1:1 (by weight) mixture of 4-nitrotoluene and 4 nitrobenzoic acid, and after the separation and drying obtained 0.35 g of 4-nitrobenzoic acid, what percent recovery would that represent?

0.35 × 100 = 35%

(EXP 1) Which of the following represent important observations that should be recorded in any experiment?

color changes, dissolving of solids, formation of precipitates, temp changes of solutions, size of glassware used, actual weights and volumes used, spontaneous bubbling of a solution

(EXP 1) what was the net weight of your p-nitrobenzoic acid/4-nitrotoluene mixture?

0.97g

(EXP 1) what was the total volume of ethyl acetate you added?

45mL

(EXP 1) what was the volume of 5% aq NaOH added each time?

15mL

(EXP 1) what is the molarity of 5% NaOH (soln d = 1.055 gmL)?

(5/100) × (1mol/40) * (1.055/1mL) * (1000mL/1L) = 1.32M

(EXP 1) what was the volume of 15% H2SO4 added?

1.68M

(EXP 1) what’s the molarity of 15% H2SO4 solution (soln d = 1.10)?

0.153/0.0909 - 1.80 M

(EXP 1) describe what happened when the sulfuric acid solution was added

the solution in the Erlenmeyer flask turned white

(EXP 1) what is the name of the technique that was used to separate solid from liquid at this point?

vacuum filtration

(EXP 1) how was the precipitate dried?

oven

(EXP 1) what was the net weight of the dried precipitate?

37.31g

(EXP 1) how was the complete drying of the solid verified?

by a constant number

(EXP 1) assuming you recovered every bit of the 4-nitrobenzoic acid you started with, what was the weight of the 4-nitrotoluene present in the original sample?

137.14g

(EXP 1) why was there a “hiss” noise when you first vented the separatory funnel? what caused this?

pressure escaping

(EXP 1) was the first portion of NaOH solution you added sufficient to react with all the carboxylic acid present?

mmoles 4-nitrobenzoic acid: 0.97/167.1 × 1000 = 5.8

mmoles of NaOH in first portion: (1.32 * 0.015) * 1000

(EXP 1) if there was enough NaOH in the first portion to react with all the carboxylic acid, why was the extraction with base done twice?

make sure it’s efficient

(EXP 1) 1-Octanol is immiscible with water. consider the octanol/water distribution constants for a series of ! degree alcohols. Based on this data, what is true?

the relative ease with which an organic compound is extracted out of water depends on the ratio of nonpolar to polar groups in the molecule

(EXP 2) how many dyes/colored compounds are in this experiment?

5

(EXP 2) is it better to apply the compounds as smaller spots or as larger spots?

smaller spots so that there’s separation between the others

(EXP 2) when the TLC plate’s developed with solvent, what property of the different chemicals used to spot the TLC plate leads to spots being in different locations on the TLC plate afterwards?

the polarity of each solvent is different

(EXP 2) why were different solvent compositions used to do the chromatography experiment?

to see how the compounds are affected by polarity

(EXP 2) hexanes & ethyl acetate are the 2 components used to modulate solvent composition. which of these is more polar and which is less polar?

ethyl acetate is more polar

(EXP 2) why is it called “hexanes” and not simply “hexane”?

mixture of C6H14 isomers

(EXP 2) once you put the spotted TLC plate into the solvent, you will see the solvent begin to move up the plate. how do you determine when the process is complete and the TLC plate should be taken out of the solvent?

once it reaches 0.5-1cm from the top

(EXP 2) once you take your TLC plate out of the solvent, where should you draw a line to indicate the solvent front?

where the solvent was reached when dipping the TLC plate

(EXP 2) just looking at the number of polar functional. groups in the five dye molecules used in this experiment, make an attempt to rank them from least polar to most polar here:

azulene, azobenzene, oil red O, disperse blue 14, & quinazarin green

(EXP 2) why was a line draw near the top immediately after the chromatography?

to show when the solvent ended up before evaporating

(EXP 2) when your TLC plate is spotted & ready for chromatography, why is it important, when you put it into the solvent, that the solvent level is below the starting points of the spots (i.e., solvent isn’t at the level of or above the spots)?

the compounds could disappear/wash off

(EXP 2) what is an R_f value?

a distance ratio of a spot going to the solvent front

(EXP 2) why do we calculate R_f values?

to see what the components of a mixture are

(EXP 2) when carefully analyzing mixtures of compounds, what problems is encountered if using a solvent of too low of a polarity?

it wouldn’t move & separate

(EXP 2) when carefully analyzing mixtures of compounds, what problem is encountered if using a solvent of too high of a polarity?

the components will move and NOT separate

(EXP 2) what are the ranks of compounds in order of increasing polarity (slower on the plate = more polar)

Az, AzB, QG, ORO, DB

(EXP 2) based on your results, which statement would be accurate?

the more polar the solvent becomes, the faster all compounds move

(EXP 2) difference in polarity:

DB is more polar but QG has 2 additional phenyl groups on each nitrogen, lone pairs are further delocalized; can’t participate in H-bonding easily

(EXP 2) difference in colors:

benzene ring and chromophase

(EXP 2) why is the nonpolar solvent called “hexanes” and not “hexane”?

it’s a mixture of different hexanes

(EXP 2) “the TLC polarity of a molecule isn’t related to the ratio of polar to nonpolar parts of the structure; rather, it depends simply on how many __ groups there are and how __ those groups are“

polar;polarity

(EXP 2) which compounds were in your unknown?

QG and ORO

(EXP 2) using the video “TLC time lapse video #1“, what can you say about the rate of the TLC development?

rate of development slows down the time

(EXP 2) what do you think happens to the spots if the plate is left in the developing jar for a long time?

the spots will come together then fade away

(EXP 2) what 4 major changes took place after the plate developed in 10% EtOAc was exposed to air & light for about 30 hours?

change of color, spots fading, spots size, & height

(EXP 3) hexanes boiling point

69

(EXP 3) azulene boiling point

242

(EXP 3) trifluoroacetic anhydride boiling point

40

(EXP 3) based on the introduction, azulene is:

aromatic

(EXP 3) the electrophile used in this electrophilic aromatic substitution (EAS) reaction is:

trifluroracetic anhydride

(EXP 3) what is the molarity of pure trifluoroacetic anhydride?

1.511 g/mL * 1000mL /210 = 7.19

(EXP 3) what color is the compound that elutes from the column first?

blue

(EXP 3) what color is the compound that elutes from the column second?

red-orange

(EXP 3) how many pi electrons does azulene have?

10

(EXP 3) give 2 reasons that azulenes are much more reactive than benzene:

less aromatic than benzene, therefore more reactive

(EXP 3) azulenes lose only __ of their aromaticity as they react, unlike benzene, which loses __ of its aromaticity upon initial reaction

part; all

(EXP 3) what did you observe in the first few seconds after addition of the trifluoroacetic anhydride?

the solution turned purple

(EXP 3) what did you observe after the addition of water

two layers formed

(EXP 3) how many micromoles of trifluoroacetic anhydride were in the 8.0 uL used?

8.75

(EXP 3) how many micromoles of azulene did you use?

0.01g/128g/mol * 10⁶ = 78.12

(EXP 3) what was the molar ratio of trifluoroacetic anhydride to azulene?

57.5 / 78.12 = 0.736 → 74%

(EXP 3) from your TLC plate, calculate R_f’s for azulene & trifluoroacetyl azulene in 10% EtAOc in hexanes:

4/5 = 0.3

(EXP 4) briefly explain why it’s possible to perform a steam distillation @100C on relatively high boiling compounds

steams boiling point will be lower than 100 because of partial pressure

(EXP 4) what’s limonene’s boiling point?

176

(EXP 4) which part of the orange peel is to be grated in this experiment?

orange-colored layer

(EXP 4) why is this the layer that we care about?

contains essential oils

(EXP 4) why is it important to not let any of the oil drip off the grater?

it will be lost in the paper

(EXP 4) how is this distillation heated?

bunsen burner

(EXP 4) at what point do you stop the distillation?

after 30 min & collect 50mL of condensed water in a flask

(EXP 4) why do you suppose the procedure calls for adding sodium chloride to the aqueous phase?

it’s used to remove water from the organic layer

(EXP 4) what’s the point of adding the anhydrous magnesium sulfate?

it’ll remove the amount of water from the product mixture

(EXP 4) what information do you expect to get from the GC-MS analysis?

the structure of other terpenoids

(EXP 4) how many g of orange zest was collected?

9.302g

(EXP 4) how manny g of orange zest were added to your flask?

6.24g

(EXP 4) at what temp did you distillation proceed?

100C

(EXP 4) what volume of limonene was evident in your burette?

0.4mL

(EXP 4) why was limonene treated with magnesium sulfate?

to consume any leftover water

(EXP 4) using the published density of limonene, convert the volume of your organic distillate into a wight in grams:

0.4mL * 0.841 = 0.336g

(EXP 4) assuming that the distillate is 100% limonene, use the previous answer to determine the weight percent of limonene contained in the orange peel

0.3364/6.24 × 100 = 5.39%

(EXP 4) using the GC-MS data, list all terpenoids that are found in the chromatogram

beta-pinene, thujene, alpha-pinene, beta-mycrene, & limonene

(EXP 4) draw out the structures of the terpenoids found in the GC-MS chromatogram

.

(EXP 5) boiling point of hexanes:

155°F

(EXP 5) boiling point of benzil:

202.6°F

(EXP 5) boiling point of pentane:

97°F

(EXP 5) does this experiment involve a reaction (changes in bonding)?

no

(EXP 5) what are the 4 types of recrystallization techniques described in the procedure?

hot/cold method, evaporation of solution, layering, & vapor diffusion

(EXP 5) WHY are recrystallizations done?

to move impurities within a compound

(EXP 5) what weight of impure benzil did you start with?

0.210g

(EXP 5) what weight of recrystallization benzil, after drying, did you obtain?

56.88mg

(EXP 5) what percent recovery of benzil did you get? (pure benzil/crude benzil * 100%)

5.688/0.210 × 100 = 27%

(EXP 5) what was the color of you recrystallized product?

red

(EXP 5) what was the color of the solvent you pipetted away from the recrystallized benzil?

dark red

(EXP 5) in your own words, describe 1 thing that can go wrong in a recrystallization

impurities can be formed in the final product if it’s cooled too fast