Chem midterm
Could we separate the pigments of spinach using GC?
No, they are not volatile liquids and the pigments will degrade
Boiling/normal boiling point
Occurs when the vapor pressure of the liquid being heated equal to the external pressure
Aryl
On ring
How many theoretical plates does simple distillation have?
One
Types of fractionating columns
Packing condenser with glass beads or metal mesh, metal mesh provides more theoretical plates, glass beads can have differing diameters, needs to be insulated
Analyzing chromatograms
Pay attention to solvent used, usually the largest peak
Molar ratio is equal to the...
Peak area ratio
Does the distribution of elimination products depend solely on the strength of the base used?
No
When could simple distillation be ideal for separation?
1. If the boiling point difference is 100 deg C or greater 2. When the mixture contains 90% of one compound and 10% of the other 3. If the mixture contains pure liquid and a non-volatile solid
What steps compose this mechanism?
1. Initiation 2. Propagation 3. Termination
What are the steps of a GC?
1. The sample is put into the injection port 2. Volatized 3. Enters the column 4. Pushes through columns with different retention times 5. Enters FID and is burned 6. Current is measured
First peak is...
2-methyl-1-butene (KOtBu, Hofmann)
Sample analyzed composed of
2-methyl-1-butene and 2-methyl-2-butene
Second peak is...
2-methyl-2-butene (KOH, Zaitsev)
How much sample does GC require?
A minuscule amount
How to we determine the amount of sample?
A potential of a few hundred volts is applied across a collector electrode the ions creased in the combustion produce a current which is proportional to the number of ions which is in turn proportional to the amount of sample - translated into peaks on chromatogram
Distillation
A process in which a liquid is heated to its boiling point and the resulting vapor phase is conducted to and condensed in a different container
Electrolyte
A solution that contains ions and conducts electricity
Bayer test
Alkene is oxidized to change color
Reflux
Allows us to run the reaction at the solvent's boiling point without boiling off, recycling the solvent by attaching a water-cooled condenser to the reaction flask
Amines are the bases in organic chemistry. You may recall from lecture that pyridine is often used as a base in various reactions you have learned about. Why an amine such as diethylamine ((Et)2NH) is not typically used in elimination reaction?
Amines have the potential to be bases or nucleophiles, diethylamine does not have resonance structures, little steric hindrance
Hold-up volume
Amount of liquid that remains inside the column after the distillation is complete
Gas chromatography (GC)
Analytical technique of separation and analysis
In an E2 reaction, the leaving group must be ____________ to the hydrogen being removed
Anti-periplanar
The H and X need to be ______________ in E2
Anti-periplanar
Consider the molecule given below. How many hydrogens on this molecule could theoretically be substituted by Br in a free radical bromination - circle them. Put an asterisk to mark the one most likely to be substituted first.
Benzylic<allylic<3<2<1<aryl
What is the basis of separation for GC?
Boiling point/volatility
Free radical Brormination is a slower reaction than chlorination but it is a synthetic route often used to make alky bromides. Alkyl fluorides are not made by Free Radical Halongenation. Explain why free radical Fluorination is not a viable route of synthesis for these compounds (show your work to support your claim with all math calculations - values are available in the Brown textbook used in lecture). What is the major concern with performing free radical fluorination?
Bonds broken-bonds formed, fluorine is highly exothermic and unstable
Stereo-inversion
Both
Is the vapor composition of simple distillate all A, B, or both?
Both - it is often impractical to completely separate a mixture by means of simple distillation
The purpose of free radical bromination is to substitute a ___ for a ____
Br, H
Bromine in dichloromethane
Bromine decolorization, rapidly decolorized when added to an alkene, distinguish alkanes from alkenes
Why does bromine react faster?
Bromine is a better leaving group and has a lower activation energy
Based on the experimental results (group A reactions), what conclusion can be drawn regarding the rate of reaction of the alkyl bromide vs. alkyl chloride? Is this conclusion in accord with theory discussed in lecture regarding the SN2 mechanism? Explain
Bromine is more reactive (as explained above)
How can we control the time and resolution in GC?
By changing the flow rate of the mobile phase gas and the temperature in the column
Allylic
C attached to a carbon atom that is double bonded
Benzylic
C attached to a ring
Why do we not cap the apparatus?
Capping could lead to a fire/explosion
Stationary phase
Coated on the column walls, very thin coating of 95% silicone oil and 5% phenyl methyl silicone oil, very high bp, relatively non-polar, can change polarity
What is a variable that influences retention time?
Column length
Objective of this lab
Compare the relative rate of free radical halogenation of six hydrocarbons under three different conditions
Is GC destructive or constructive?
Destructive
Is the composition of the vapor phase different or the same as that of the liquid phase?
Different
The separation of two liquids on the GC is very similar to the distillation process. Compare the two methods describing the process while pointing out to the similarities and differences between the two methods.
Distillation: compounds are evaporated then condensed, doesn't have a mobile or stationary phase, set-ups are different, theoretical plates, not destructive, requires more sample Both: uses boiling points to separate components, separation techniques, involve purification of samples GC: has a mobile and stationary phase, destructive, analytical technique, do not need a large sample, considers retention time
Intermediate
E1
Concerted process
E2
In this experiment you were asked to collect two fractions and early fraction and a late fraction. Which of the two fractions would be richer in cyclohexane? Would there be a difference between the percent of cyclohexane in the fractions collected from fractional vs. simple distillation.
Early fraction is richer in cyclohexane, percent of cyclohexane is higher in fractional distillation
What are all the possible products of the reaction in group B? Which product would you expect as the major one for this reaction? Which mechanistic pathway would be predominant for the reaction? Explain
Elimination and substitution products
Why do we not see substitution products?
Elimination is favored, stopped distillation at 60 deg C
What do substitution reactions compete with?
Elimination reactions
Response run
Equalize the chromatograms that contain a mixture of two compounds, equals peak areas divided by one another (one equals 1 while the other does not, the non-1 is the response factor)
Response factor
Equalizer, enables us to refer to the area under the peak as moles regardless of the compounds structure
Objective of this lab
Explore several aspects of the substitution reaction and compare factors that affect the mechanism by which the reaction proceeds as well as the relative rate at which the reaction proceeds
Objective
Explore the effect of the base structure on the distribution of elimination products
T/F. Elimination reactions are often used to create sigma bonds between carbons
F
What can GC be used for?
Find the amount of a compound or how many separate compounds a sample contains and the purity of a sample, determine the structure of an unknown sample, identification tool by comparison of a compound with known compounds
Group A
Finkelstein reaction, SN2, precipitate is formed, CL vs Br
Did the result from your GC analysis match the theory of distillation? Which fraction had more cyclohexane then toluene? Was the simple distillation method better than the fractional method? Compare the theory and the actual results based on the GC results.
First fractions contained cyclohexane, fractional had more moles of desired compound
FID
Flame ionization detector, sample is burned in hydrogen/air flame, produce ions that conduct electricity
What is the difference between the simple and fractional distillation apparatus?
Fractionating column
Good leaving groups
H2O, Br, I, CH3OH, CH3SH
Does retention time solely depend on the FID?
No
What factors favor elimination over substitution?
High heat, tertiary
What needs to happen to the temperature to distill liquid B?
Increased
When a liquid mixture is distilled, the temperature...
Increases
How to decrease resolution
Increasing the temperature and increasing the flow rate, vary the temperature
Mobile phase
Inert gas (helium gas in our lab)
How does the sample travel in the column?
Interacts with the stationary phase while being pushed by the mobile phase, the more volatile a compound the more time it is in the gas phase and the smaller the retention time
Why do we not need the response factor?
Isomers
In this experiment we used two different bases to perform an elimination reaction on 2-bromo-2-methylbutane. What was the product distribution from both reactions? Was that the expected result? Explain the product distribution obtained from both procedure.
KOH - Zaitsev, KOtBu - Hofmann, both are good bases but KOtBu is bulky
Bases used
KOH, KOtBu
Strong bases
KOH, KOtBu, NaNH2
What are good nucleophiles?
KOH, NaNH2, NaCN, NaI
How do we determine compounds on a chromatogram?
Know boiling points and retention times corresponding to them
What would happen if the external pressure was lowered?
Less energy is needed to boil
How are the initial free radicals formed?
Light/heat exposure
Simple distillation
Liquids in the mixture have about a 50 deg C difference in their boiling point
For the GC analysis of this reaction (group B), we do not have standard products, however, comparing the starting material's retention time and new products appearing in the GC can be done based on the boiling point of the compounds. Given your knowledge of boiling point dependency, which product/starting material should have a higher boiling point? Explain. Perform a literature search to verify your assumption regarding the boiling point of the products and of the starting material ((3-bromopropyl)benzene. You may need to use the nomograph in the lab manual (see distillation theory) to make sure you are comparing the boiling points at the same pressure. Based on the boiling points, which compound should have the highest retention time? Which would have the lowest retention time?
Look at the amount of bonds, molecular weights, and interactions between the atoms
Based on the GC results, what is the major product for the reaction? What was the effect of temperature on the reaction outcome? Is this result in agreement with theory?
Major product was the substitution product as seen by the GC results
Azeotropic mixtures
Many liquids when combined exhibit different behavior, namely a constant boiling point
Conductivity detector
Measures the strength of the electrolyte, red is a weak electrolyte, red-green is a strong electrolyte, can be utilized to indicate that a reaction is taking place
Fractional distillation
Multiple simple distillations
How to use a nomograph
Pressure then boiling point at usual atmospheric pressure, connect lines and draw to observed boiling point
What factors favor substitution over elimination?
Primary, aqueous, low heat
What types of runs do we do when determining retention time of pure compounds?
Pure compound run
What indicates a reaction has occurred?
Red/orange to clear color change
Boiling point affects...
Retention time
Different conditions
Room temp, elevated temp, irradiation
Carbocation intermediate
SN1
What types of mechanisms exist for substitution?
SN1 and SN2
Concerted mechanism
SN2
What is distillation used for?
Separation and purification technique for mixture of liquids that have different boiling points
When toluene is used in free radical bromination, a very small amount of product is formed that contains only carbons and hydrogens and no bromine. Show the structure of that product and the arrow curved mechanism of how it is formed starting from the alkyl radical intermediate of the reaction.
Similar to picture shown above
Types of distillation
Simple, fractional, steam, vacuum
In this distillation experiment we start the experiment with 10 mL of liquid mixture of cyclohexane and toluene. However, it is not possible to obtain all 10 mL back under the current procedure particularly for fractional distillation. Explain why that is the case for distillation and in particular for fractional distillation.
Some material is lost to evaporation and some is left behind in the apparatus as "hold-up", a small amount of material is purposefully left in the flask
A researcher at a chemical company was asked to prepare several binary mixtures of liquids for a distillation demo but he forgot to label the composition. He then took 10 mL of the mixture he prepared and ran a fractional distillation on the mixture. Given below is the data he obtained from the distillation. Plot the data as volume (X) vs. Temperature (Y) and determine the composition and identity of the two liquids in the mixture. Explain why he did not need to collect more than 5 mL of distillate. The following liquids are the possible liquids used for making the mixture: water, aetone, dichloromethane, diethyl ether, etheyl acetate, methanol and ethanol.
Spike indicates change in compound being distilled, can tell which compounds by boiling points that correspond to the data
T/F. Elimination is favored at higher temperatures because the reaction is exothermic
T
T/F. Hofmann's rule states that sterics will effect the regioselectivity of elimination reactions
T
T/F. The E2 mechanism is stereoselective
T
T/F. The reaction of KOtBu produces a higher proportion of the Hofmann while KOH produces Zaitsev
T
Why is it important to maintain a temperature gradient?
Temperature decreases toward the top allowing the compound to condense and drip back down the column
What does the fractionating column provide?
Temperature gradient, surface area for repeated vaporizations and condensation to take place before collecting the final product - at the top it is nearly pure compounds
Group B
Temperature, room temp - substitution, high temp - substitution and elimination
Group C
Temperature, tertiary vs secondary, Cl vs Br
Based on the experimental results (group C reactions), what conclusion can be drawn regarding the rate of reaction of the tertiary alkyl bromide vs. tertiary alkyl chloride? Is this conclusion in agreement with the conclusion drawn from the group A results? What conclusion can be drawn regarding the rate of reaction of a tertiary alkyl halide vs. secondary alkyl halide? Is the conclusion in agreement with theory discussed in lecture regarding the SN1 mechanism? Explain
Tertiary alkyl halides react faster since SN1 goes through a carbocation intermediate
Resolution
The degree of separation of the compounds in the mixture being analyzed
Ratio of the peak areas on a gas chromatogram reflects
The mole ratio
Zaitsev's rule
The most substituted alkene will be the major product
What function does the probe serve?
The probe measures the formation of Br/Cl anions that are byproducts of the reaction
How can we measure how "good" a column is for separation?
Theoretical plates, the more theoretical plates the better the separation
Ethylbenzene and o-xylene can be separated and analyzed using GC without injecting an equimolar standard to a high accuracy. Explain why that is the case for these two compounds while we had to inject an equimolar solution for our experiment.
They are constitutional isomers, will ionize in the FID similarly, similar molecule weights
How do we find the boiling point at a different pressure?
Use a temperature-pressure nomograph
What are good leaving groups?
Weak bases, H2O, Br, I, HSO4-
When do we use a response factor?
When comparing compounds that do not have the same molecular formula
Does GC have a mobile and stationary phase?
Yes
Is GC precise enough to distinguish between constitutional isomers?
Yes
Does FID burning vary?
Yes, depending on its functional group and composition
Reactivity of hydrocarbons
t-butylbenzene<cyclohexane<methylcyclohexane<toluene<ethylbenzene<isopropylbenzene
Plot a theoretical distillation curve of temperature (y-axis) vs. volume in mL (x-axis) for a 15 mL of a mixture containing 60% 1-propanol and 40% 2-propanol. Are these two compounds easier to separate by distillation than cyclohexane and toluene? Explain your answer.
take 60% and 40% of 15mL to determine the volume of each compound, create graph T vs V, the larger boiling point difference is preferred
Hydrocarbons used:
toluene, ethylbenzene, isopropylbenzene, t-butylbenzene, cyclohexane, methylcyclohexane
