OChem Lab Final
What possible products would you expect to see from the following reaction?
#1 and #2
What does the EE value tell you?
% tells you how good the separation was of the enantiomer excess. EE = 0 means racemic, EE = 100 means only S
The starting material, 2-methylcyclohexanol, is actually a mixture of four isomers. Name each of the isomers using appropriate stereochemical nomenclature.
(1R, 2S)-2-methylcyclohexanol (1S, 2R)-2-methylcyclohexanol (1S, 2S)-2-methylcyclohexanol (1R, 2R)-2-methylcyclohexanol cis-2-methylcyclohexanol, trans-2-methylcyclohexanol,
Name the following compound:
(3R,4S)-3-bromo-4-iodohexane
What is RF?
Response Factor = distance of compound/distance of solvent
In the Diels-Alder Lab, why do we add hexane to the solution?
So when it crystallizes, it will be a better solvent solution than just the ethyl acetate.
In the distillation lab, which chemicals were in which fractions?
The lower boiling point material would be more prominent in fractions 1 and 2 which is ethyl acetate and the higher boiling point which is n-propyl acetate is in the last 2 fractions
What are the 2 ways we have learned to add water to an alkene?
acid-catalyzed addition and hydroboration-oxidation
Stereoisomerism
mirror images that are not superimposable
What is the lowest number of chiral centers that can exist in any isomer of trichlorocyclobutane?
0
How many E2 products can you get from the following alcohol?
1
what were the names of the reactants in the Diels-Alder lab?
1,3-cyclopentadiene and maleic anhyrdride
Given the following data for the hydration of 1-methylcyclohexene to give a mixture of 1- and 2-methylcyclohexanol and the subsequent analysis of the product mixture on a gas chromatograph (GC), what is the percent yield for this reaction, if you produce 1.53 g of product? (Hint: Refer to the theoretical yield you just calculated in Question #1. Calculate to two significant figures, please.)
1.53g/ 183g *100 = 0.84%
Given the following data for the hydration of 1-methylcyclohexene to give a mixture of 1- and 2-methylcyclohexanol and the subsequent analysis of the product mixture on a gas chromatograph (GC), what is the theoretical yield of this reaction if you started with 2.0 mL of 1-methylcyclohexene? (3 sig figs.)
1.6g* 1mol/1mol * 114.2g/1mol = 183g methylcyclohexanol
How to identify regioselectivity based on percentages?
100% to 0% = complete 80% to 20% = high 45% to 55% = low
Name the following compound:
2-ethyl-1-hexanol
An analysis of a solution of Compound A with a spectrophotometer yielded an absorbance value of 2.575. If the molar absorptivity of this compound was 4802 M-1*cm-1 and if the pathlength of the cell was 1 cm, then what is the concentration of solution?
2.575=4802*c*1 c= 5.36*10^-4 M
What is the Rf value for the compound in lane #1? Give your answer to 2 significant figures.
2.6/6.0 = 0.43
Assume you have a solution of a single enantiomer with a known concentration. You place the solution into a polarimeter cell with a 1 dm length and find the observed optical rotation (numbers showing on the polarimeter) to be x. If you then fill a cell with a 2 dm length with the same solution, what now is the value of the observed optical rotation, in terms of x?
2x
How many constitutional isomers are there for C3H8O?
3
How many lone pairs of electrons need to be added to the drawing of the Lewis dot structure of HC2H3O2?
4
What is the sum of the numbers of the possible E2 products for all 3 of the following molecules?
5
You ran a distillation to separate a mixture of ethanol and propanol. You took samples from various points in the distillation to determine how the distillation was doing. The data below corresponds to one of the fractions you collected. Use the following data to answer the following question. To three significant figures, what is the % propanol in the mixture indicated above?
67.3% Propanol
What is the expected temperature of the oven surrounding the GC column?
> 97 ⁰C (will be higher than the highest bp)
Distillation
A process that separates the substances in a solution based on their boiling points
Beers Law
A=ecl e = molar absorptivity or molar extinction coefficient, units of M-1 * cm-1 c = concentration of the sample, units of M l = path length of the cell, units of cm
What type of hydration reaction did you run, judging by the product ratios in the gas chromatograph?
Acid in water
What occurs in an acid-catalyzed addition of water?
Adds water across the double bond in a Markovnikov fashion. It is slow, incomplete, and goes through a carbocation intermediate.
In the Diels-Alder Lab, why did we need an ice bath?
Because the reaction was highly exothermic and could all just bubble away
How can we separate enantiomers?
By reacting a racemic mixture with chiral reagents to form diasteromers
Which of the following correctly matches the chiral carbon and its configuration?
C4, R
What role is the tartaric acid playing in the reaction for chiral resolution?
Chiral reagent
What is a chiral reagent?
Chiral reagent is used to convert a mixture of enantiomers into diastereomers to analyze the quantities of each enantiomer present within the mixture
You inject a mixture of two liquids on a gas chromatograph. Compound A has a boiling point of 83 ⁰C, and Compound B has a boiling point of 68 ⁰C. Which compound will exit the column last, assuming the column is > 83 ⁰C?
Compound A
Compound A travels through silica gel faster than Compound B. Which has a greater affinity for the stationary phase (silica gel)? What does this tell you about the nature of that compound?
Compound B would have a greater affinity for the stationary phase. This tells me that compound B is very polar.
Explain why cyclopentadiene is more reactive than 1,3-pentadiene in a Diels-Alder reaction.
Cyclopentadiene is more reactive than 1,3-pentadiene in a Diels-Alder reaction because of its formation in a ring. The formation allows it to be in a fixed formation and not move/rotate between the cis and trans structures.
What are some characteristics of diastereomers?
Different physical properties that allow for separation
In the distillation lab, was fractional or simple distillation better and why?
Fractional distillation is better than simple distillation because it is more accurate in terms of its temperatures being closest to the real boiling point temperatures and its n-propyl % is closest to ideal
What is the FALSE statement about thin layer chromatography (TLC) where silica gel composes the stationary phase and ethyl acetate composes the mobile phase? -The solvent level in the developing chamber must be below the starting line. -The mobile phase moves via capillary action. -Highly polar compounds are more likely to move on the TLC plate. -More than one spot in a lane indicates a mixture.
Highly polar compounds are more likely to move on the TLC plate.
In the distillation lab, why do we increase surface area in the pipe?
It allows for better separation for more condensation
What is a FALSE statement about cyclopentadiene?
It can only behave as a dienophile in a Diels-Alder reaction
Why is important to draw the terminal line of the mobile phase as soon as the TLC plate is taken out from the developing chamber?
It evaporates very quickly because it no longer has the nonpolar solvent or nonpolar wick attracting/ causing it to move up the TLC.
How is phosphoric acid a catalyst for the dehydration reaction?
It increases the rate of the reaction occurring, but it won't skew the stereochemistry. It will remain in flask after reaction has occurred.
what are the RF values in the thin layer chromatography lab?
It is the ratio of the distance travelled and it should be between 0 - 1
The solvent is one of the keys to a good recrystallization. What is one criterion listed in your manual that is used to determine a good solvent?
It must not react with the material being recrystallized.
For the following image of a developed TLC plate, assume the conditions were the same as ours in lab. Which lane contains the most polar compound?
Lane 3
After removing the flask with the solution of dissolved organic compound from the hot plate, the flask was cooled to room temperature before being chilled in an ice bath. Why is it necessary to cool down the flask before putting it in an ice bath?
Letting it cool down slowly allows the pure crystals to form slowly and not get any impurities mixed in with the crystals. If it is put immediately into an ice bath, then it will form impure crystals.
Gas chromatography movement
Material in column that boils closer to temp of oven doesn't move as quickly.
You ran a distillation to separate a mixture of ethanol and propanol. You took samples from various points in the distillation to determine how the distillation was doing. The data below corresponds to one of the fractions you collected. Use the following data to answer the following question. What is your best guess for when during the distillation the above fraction was taken?
Near the end
What occurs in an E2 mechanism?
No carbocation intermediate and no rearrangment
How is optical rotation identified?
One side is darker than the other
In a chiral resolution, how is a compound identified as pure?
Optical rotation
What are the mobile and stationary phases in TLC?
Polar (silica) substances will not move far but nonpolar (hexane) tends to be mobile
What is the orbital geometry of the indicated carbon (starred) in the figure below?
Tetrahedral
Why do we use UV light during the TLC lab?
The UV light helps reassure that the drops are there. It is so that we can see it better.
In the Diels-Alder Lab, how did we differentiate between the endo and exo products?
The endo and exo products have melting points that differ by about 20 degrees celcius.
In the Diels-Alder Lab, if the reaction's melting point was closer to 164-165 degrees celsius then which product was mostly formed?
The endo product because that's its melting point
In the Diels-Alder Lab, if the reaction's melting point was closer to 142 degrees celcius then which product was mostly formed?
The exo product because that's its melting point
In the distillation lab, how do we distinguish which chemical came first on the graphs?
The first peak on the graph is going to be the chemical with the lowest boiling point material (ethyl acetate) and the higher boiling point material would be the second peak.
In the Diels-Alder Lab, what was the limiting reagent?
The maleic anhydride
Why does the melting point of a mixture drop below that of a pure compound?
The melting point drops below because of the presence of impurities. These impurities get in the way of the most ideal stacking of the molecules by taking up big gaps in between the molecules.
Why is only pencil used instead of pen in the TLC lab?
The pen's ink would dissolve because it is nonpolar and it would travel with the solvent messing up the other solvents' path moving upward and interfering with the data. The pencil lead is graphite so it will not dissolve.
What is resolution?
The process of separating a mixture into it's components
What would be the effect (be specific) on the separation of the two compounds in this experiment if the sample was not concentrated before loading it onto the column?
The samples wouldn't separate as efficiently and they would mix together. They must be concentrated and completely separate in order to get accurate results.
In the thin layer chromatography lab, which elements were polar or nonpolar?
The silica gel was more polar and the cyclohexane and acetane is nonpolar
What is a FALSE statement about taking a melting point?
The starting temperature should be above the melting point of the solid.
Which part of the TLC lab was the stationary phase and the mobile phase?
The stationary phase was the TLC plate and the mobile phase was the solvents moving upwards
Why is the wick needed in the TLC lab?
The wick is used to pull the solvent up to the top part of the paper, because without it the solvents would look like they stopped traveling.
In the distillation lab, what were the axis on the graph which gave us the peaks needed?
The y-axis had absorbance units and the x-axis had the retention time on the column
What is the relationship between absorbance and concentration of a sample?
They are directly proportional to each other
What are some characteristics of enantiomers?
They have identical BP and solubility that make them difficult to separate. They also have different effects on polarized light
What effect do racemic mixtures have on polarized light?
They have no effect on polarized light because they have equal amounts of each enatiomer
What occurs when alcohols are treated with a strong acid?
They lose water to form a carbon-carbon double bond (alkene)
What is the purpose of a fractionating column in distillation?
This extra column increases the surface area of the distillation apparatus and results in a greater separation of components in a mixture
Why is it important to add the solvent to the solid during recrystallization and not the other way around?
This is important because you want the minimum amount of solvent mixed into the solution to make a very concentrated solution so that the maximum output of pure crystals will be reached.
In the Diels-Alder Lab, why did we let it heat up slowly?
To allow it to cool to room temperature so crystals can form slowly because if we force it too fast then it would trap impurities and the crystal would not form right
How to find equivalents?
To calculate molar equivalents for each reagent, divide the moles of that reagent by the moles of the limiting reagent
What is the purpose of the Diels-Alder lab?
To form 6 membered rings through a cycloaddition reaction aka Diels-Alder reaction
What is the use of a UV/Vis spectrometry?
To see which frequencies are absorbed and to quantify how much pigment is present
Name the most stable compound:
Trans 1,2-dibromocyclohexane
You repeat the Diels-Alder reaction from lab using 3.4 g of maleic anhydride (98.06 g/mol) and 2.5 g of cyclopentadiene (66.1 g/mol) to form the final solid product (164.16 g/mol). All compounds are in a 1:1 ratio in the balanced equation. Calculate mmol of the limiting reagent and identify the limiting reagent. Make sure to keep the correct number of sig figs.
Your Answer: limiting reagent = Maleic Anhydride mmol = 35
In a column chromatography experiment which uses a silica-filled column and a mixture of hexane and ethyl acetate as a solvent (identical to the conditions we have used in this week's experiment),
a relatively nonpolar compound will be collected first due to the weaker attraction between the compound and the stationary phase.
Which solvents were used in the TLC lab?
acetaminophen, aspirin, and caffeine. These were the chemicals that were dotted at the bottom where the line was drawn.
Percent Yield
actual yield/theoretical yield x 100
What occurs in a hydroboration-oxidation reaction?
adds water in an anti-Markovnikov fashion and there is no carbocation intermediate.
Specific rotation
alpha/ (c*l) alpha= optical rotation c= concentration of sample g/mL l= length of sample in decimeters (1 or 2)
Which solvent moves the furthest upward from the line in the TLC lab?
aspirin
In the mobile phase of the TLC lab, the molecules moved upwards with... ?
capillary action
In the Diels-Alder Lab, what was the name of the endo and exo products formed?
cis-4-Norbornene-endo-dicarboxylic-anhydride and cis-4-Norbornene-exo-dicarboxylic-anhydride
how to determine limiting reagent?
compound with lowest mmol
what are enantiomers?
compunds that have at least 1 stereocenter and are non-superimposible
How to calculate theoretical yield?
convert moles of compound into grams using the molar ratio and MW of product.
Fractions during distillation taken near the beginning or end correspond to bp how?
fractions at beginning = low bp material fractions at end = high bp material
Explain the effect on the observed temperature reading if the bulb of the thermometer in a distillation apparatus is positioned two inches below the opening to the condenser.
if the thermometer was below, the temperature readings would be too high (higher), disturbing the data.
What occurs in an E1 mechanism?
involves a carbocation intermediate, which can rearrange
Why are we not concerned with losing our product material during the steam bath step, which removes the solvents?
it's not volatile and the solute will turn back into liquid before they are able to leave the beaker. The solvent is also reactive with water.
Column chromatography separates compounds based on their ________; this is similar to ________, which also uses the same characteristics of the compounds for their separation.
polarity; TLC
In the Diels-Alder Lab, why does cyclopentadiene have to be distilled?
so the cyclopentadiene does not react with itself
What is the hybridization of the carbon which is double-bonded to the oxygen in the figure below?
sp2
enantiomeric excess (ee)
specific rotation of sample/ specific rotation of pure sample *100
Regioselectivity
the preference of one direction of chemical bond making or breaking over all other possible directions
If you calculate that the ee of your solution is 93%, what is the remaining 7%?
the remaining 7% is the R-amine.
fractionating column
the vertical column which is used to bring about the separation of liquids in fractional distillation Advantage: vapors of two liquids get further separated from each other, but the distillation takes a lot longer. Better choice.
In the distillation lab, where do we place the thermometer and why?
we placed the thermometer a little below the opening to get an accurate temperature reading of the vapor that is going over
% Recovery
weight of S-amine/ weight of racemic mixture * 100