OChem Final
Dehydration of cyclohexanol At what pressure does distillation occur at?
760 mmhg (atmospheric pressure)
Melting point experiment How many mm's of substance do you want at the end of your capillary tube?
2-4mm of solid.
Oxidative cleavage of alkenes What are the products formed is a alkene is reacted with Kmno4 under cold and neutral pH conditions?
A diol is formed. (a alcohol with 2 hydroxyl groups)
Resolution of 1-Phenylamine experiment what is a racemic mixture?
A equimolar mixture of 2 enantomers.
Acid -Base extraction What is acid-base extraction?
A form of liquid liquid extraction. it is used to seperate a mixture that has components with solubilities to similar to be seperated by traditional liquid liquid extraction. However, they can still be seperated if one the components exhibits a acid functional group. The acid can be converted to a water-soluble salt by treatment with a acid (HCL). The salt can be seperated form the solution. Neutralization of the solution can be performed to convert the salt back into its origional form.
Recrystallization experiment What is Recrystalization
A method of purifying a crystalline solid. This allows u to seperate the impure parts of a solid out.
Chromatography
A method used to separate a mixture based on its polarity.
TLC experiment What does a small or large RF value mean?
A smaller RF value means that sample is more polar becuase it has a higher affinity for the gel (stationary phase. The stationary phase is very polar as well.) A larger RF value means that the sample is less polar and was more attracted to the mobile phase (which is less polar that the gel)
TLC experiment What is the stationary phase?
A solid Silica gel This is very polar
Acid- Base extraction What are drying agents? How do they work?
Drying agents are anhydrous salts that combine with water to form "hydrate complexes".
Acid- Base extraction If an extraction was 100% effective, what value for K would we have?
Either zero of infinity.
Resolution of 1-Phenylamine experiment Enantomers vs diasteromers
Enantomers- All chiral centers are opposite configuration Diasteromers- At least one chiral center is the same and at least one chiral center is opposite configuration.
Resolution of 1-Phenylamine experiment What is special abut the physcial properties of Enantomers and diasteromers?
enatomers- Same physical properties Diasteromers- Different physcial properties (physical properties include melting and boiling point and solubility)
Acid- Base extraction How much drying agent is necessary to add?
enough to cover the bottom of the flask. Too much can cause some product to be lost becuase some of it can adhere to the agent.
Recrystallization experiment Why would you not use folded filter paper in a suction funnel
folds in the paper would prevent it from sitting properly.
Recrystallization experiment What does it mean if a compound is a ether?
Ether means the substance is volitile. Volitile means how easily a substance vaporizes (turns from liquid to gas). More volitile - vaporizes at lower temperature.
Recrystallization experiment what is a filtrate?
Filtrate is the liquid part that the solid was removed from during suction filtration. (its the liquid that goes into the beaker after suction filtration)
Dehydration of cyclohexanol Which is a better leavign group? OH or H2O?
H2O
SN1 substution Why is it desirable to keep the concentration of HBr low?
HBr is volitile We want to keep the concentration of it low so that we dont have a lot of HBr escaping from the reaction mixture. despite this, some loss of HBr does occur and we will make a gas trap to prevent it from escaping into the atmostphere
Recrystallization experiment What is the heat source used during hot gravity filtration?
Hot plate
Melting point experiment Why does a sample need to be finely powdered?
If sample is in chunks, sample will not pack well, causing air pockets that slow heat transfer. Air pockets are not good head conductors
Melting point experiment How can a melting point tell you if a compound is pure?
If the range is between 0.5 to 2.0 degrees celcius, then the compound is considered pure. the mp should be within + or - 2 °C of the literature values.
Dehydration of cyclohexanol Why is Le Chateliers principal utilized in this experiement?
We want to convert the alcohol as completely possible to the alkene.
Melting point experiment What is not considered melting?
shrinking, sagging, color change, texture changes and sweating (sweating is where droplets appear on the outside surface of sample due to inefficient drying)
TLC What is the dryign agent?
sodium sulfate
Melting point experiment Why does the sample shrink before melting?
* Ionic molecules often decompose before melting as do compounds having strong hydrogen bonds.* Due to crystal structure changes
Melting point experiment lets say you are heating up your sample at a voltage of 5V. When you sample is within 25 degrees celsius of the mp, you turn the dial back to what: If you mp is: - 100-120 °C - 120-140°C - 140-160 °C
- 100-120 °C -- 35V (2.9V) - 120-140°C -- 40V (3V) - 140-160 °C --- 45V (3.2V) All around 3V This heats the sample at a rate of 2-3 °C per minute
Oxidative cleavage of alkenes List the products if the following reacted with potassium permanganate (KMno4) under warm basic conditions (NaOH): - Tetrasubstuited double bond - Trisubstuited double bond - Disubstuited double bond (vicinal) - Disubstuited double bond (geminal) - Monosubstuited double bond
- 4R's = 2 ketones - 3 R's = Ketone + Carboxcyllic acid - 2 R's vicinal = 2 carboxcyllic acids -2 R's geminal = Ketone + carbon dioxide - 1R = Carboxcyllic acid + carbon dioxide. OH replaces H's. If a aldehyde is formed it is automatically converted to a carboxcyllic acid If a carbon has no R groups and just hydrogens, it turns into carbon dioxide.
Acid- Base extraction Sources of error (4)
- Adding to much drying agent can decrease yield. - Not shaking the seperatory funnel vigirously enough can lead to decreased yield. - Adding to much diethyl ether can decrease your yield. - Drawing off the top layer instead of decanting it can lead to impurities.
Acid -Base extraction What organic solvents are immesible in water? (6)
- Benzene - Chloroform -Carbon tetrachloride -Dichloromethane - Diethyl ether -Ethyl acetate BCCDDE
Rank the pigments in order from increasing to decreasing Rf value: - Carotenes - Chlorophyll a - xanophylls - chlorophyll b
- Carotenes (1. Largest RF= less polar) - Chlorophyll a (2. intermediate RF = less polar) - chlorophyll b (3. intermediate RF = less polar) - xanophylls (4. Smallest Rf = most polar)
TLC What is the color associated with each of the following pigments? - Carotenes - Chlorophyll a - xanophylls - chlorophyll b
- Carotenes (orange) - Chlorophyll a (blue-green) - xanophylls (yellow) - chlorophyll b (green)
Melting point experiment List 5 precautions that you must take while using the melting point apparatus?
- Dont leave mel-Temp on if you arent using it. - Dont heat it to rapidly. - Dont touch heating element - Dont overpack the sample in the capillary tube. - Make sure the sample is pulverized before adding it.
Recrystallization experiment What is the point of gravity filtration?
- Gravity filtration of a hot solution removes impurities that are insoluble in the solvent (solid impurities like lab debris). - You isolate the solid and end up with the liquid portion. Use the liquid portion.
Dehydration of cyclohexanol What are 2 ways to encourage maximum yield of a product?
- Large excess of one of the reactants - To remove the product as quickly as it is formed.
Acid- Base extraction What are some things you dont want to do when using a seperatory funnel?
- Make sure stopcock is closed before adding solution to it. - Never add hot solutions to it (this can be dangerous with high vapor pressures) - You should add material slowley (safety)
Melting point experiment How can impurities effect the melting point of the solid? How does this happen?
- Melting points are depressed (lower than the pure substance) - Melting point range is broadened Impurities disrupt crystal lattice, so requires less energy to break intermolecular forces.
Oxidative cleavage of alkenes Sources of error (2)
- Not vigorously stirring the mixture - Adding to much sodium hydrogen sulfite can cause precipitate of the suberic acid out of solution.
TLC experiment Describe each of the following compounds as polar or non polar: - Petroleum ether - hexane - Ethyl acetate - Acetone
- Petroleum ether = nonpolar - hexane = nonpolar - Ethyl acetate = less polar - Acetone = more polar
Recrystallization experiment How do you do recrystalization using 2 solvents? (5 steps)
- Prepare a hot solution of the compound with the solvent that has a higher solubility (ethanol is more soluble) - Do hot gravity filtraiton - Add the second solvent (water) that the compound has lower solubility in until cloudieness occurs. - Add a few drops of the first solvent to get the cloudieness to go away. - isolate crystals by suction filtration.
Recrystallization experiment What happends if you cool the hot solution to rapidly after adjustment of the saturation point?
- Rapid cooling can result in trapping of the solvent or other impurities inside the crystals. - Oiling out (when the compound seperates from the solution to rapidly for crystals to form properly.) You end up with the material seperate as a viscious liquid or oil which can trap impurities. The solid may end up as large clumps.
Recrystallization experiment what is the purpose of suction filtration?
- Suction filtration of crystalized solid washed in cold solvent removes impurities that are soluble in the solvent. (you end up with solid)
Recrystallization experiment Common errors (4)
- The hot solution should be added gradually to the gravity filtration and kept hot between additions. If you dont do this crystals can form and lead to material losses. - using to much hot solvent (super saturation) can lead to loss of material in the final cold filtrate. - If you remove the gravity filtration from the hot plate, then crystals can form in the funnel - Cooling the solution to rapidly after u adjust the saturation point. This can lead to trapping of the solvent or other impurities in the crystals.
SN1 substution What is reactivity of an alkyl halide based on?
- The type of halogen (how good of a leaving group it is) - The level of substuition/ stability of the carbocation intermediate - The solvent
Acid- Base extraction What is the top layer and bottom layer in the seperatory funnel?
- Top layer = organic layer= diethyl ether (becuase less dense than water) + neutral compound - bottom layer = aqeous layer= water (becuase more dense than ether) + carboxylic acid
Dehydration of cyclohexanol Bromine Test
- Used to indicate the presence of multiple bonds (alkene or alkyne) - It adds bromine substituents to alkenes. - Add sample + dichloromethane in test tube . Add drops of bromine in - Positive result= Rapid disappearance of red without any clouding over the mouth of the tube. - It turns clear because bromine is naturally a brownish/red and alkenes are naturally colorless. When bromine reacts with the alkene and turns into dibromoalkene which is colorless.
Dehydration of cyclohexanol Bayer Test
- Used to indicate the presence of multiple bonds (alkene or alkyne) - It oxidizes the alkene present. - Add sample + ethanol in test tube - Negative control= ethanol alone - Add drops of KMNo4 to tubes. KMNO4 is natural a purple color. If it dissapears and turns brown/clear you know your product formed. - Positive result: Brown/clear solution + brown participate at the bottom. - Brown precipitate is MnO2.
Oxidative cleavage of alkenes What does vicinal and geminal mean?
- Vicinal = 2 hydroxyal groups added to adjacent carbons - geminal = 2 hydroxyal groups aded to the same carbon (in the pic, the OH's would be where the H's are)
Dehydration of cyclohexanol How do we determine an ideal temperature to carry out a dehydration reaction?
- We want a temperature higher than the boiling point of the alkene or water, but lower than the boiling point of alcohol. This allows the alkene and water to be distilled from the reaciton as fast as they are formed. This helps us to shift the reaction towards completion. This is accomplished nby a fractional distillation apparatus.
Recrystallization experiment When using a mixed solvent combination, how do you choose two solvents?
- You want to 2 solvents to be soluble in each-other, but have different polarities. (miscible)
Recrystallization experiment What are good mixed solvent pairs? (6)
- acetone-water - ethanol-water - ether-petroleum ether - benzene-hexane - chlorform-hexane - ethanol-ether
Resolution of 1-Phenylamine experiment - What is the difference between specific rotation and optical rotation - when do these change?
- optical rotation- the rotation of plane-polarized light when a light beam is directed through certain materials. (changes based on the sample) - specific rotation- A characteristic property of a certain substance. It is the standard measurement for optical rotation for that substance. (changes when wavelength changes) (it is usually a constant). You can calculate specific rotation from optical rotation.
Oxidative cleavage of alkenes - What are the layers of in the seperatory funnel?
- organic layer- top layer= hexane + cyclooctene - aqueous layer- bottom layer= potassium suberate salt + H2O
Acid- Base extraction What would we need to add to convert the following to water-soluble salts? - Phenols - Amines
- phenols + NaOH = salt - Amines + HCL = salt
What is the overall procedure for recrystalization? (5)
- sample is heated and dissolved in a solvent. - Hot solution is filtered under gravity filtration (removes insoluble impurities) - Hot solution is concentration with a co-solvent to put it near the saturation point for the purified compound - Let hot solution cool to room temp --> ice bath --> crystalized - Solid is collected by suction filtration, washed with cold solvent and dried.
Recrystallization experiment How can you encourage crystal formation? (3)
- slowley cool the solution - scratch the inner surface of the flask with a glass rod - seeding (adding a few crystals of the compound of intrest)
Resolution of 1-Phenylamine experiment Compare the solubility in methanol of the following: - (R)-1-phenylethylammonium (2R,3R)-tartate - (S)-1-phenylethylamine (23,3R)-tartate
- the R version is soluble in methanol - The S version is not soluble in methanol and crystalizes from solution
Recrystallization experiment How do you choose an appropriate solvent? (4)
- the solvent should be chemically inert towards the compound being purified. - The desired compound should be soluble in hot solvent and insoluble in cold solvent. -Impurities should be either insoluble in the hot solvent (and removed in hot gravity filtration) or soluble in cold solvent (removed during seperation filtraiton). (opposite of the compound) - Solvent should be volatile so that it will evaporate from the crystals. It should not dissolve in room temp. It should dissolve when heated. When you put it in an ice bath, crystals should form.
Recrystallization experiment Why is only a small amount of solvent used?
- using to much hot solvent can lead to loss of material in the final cold filtrate.
Recrystallization experiment Rank the following solvents in order of highest to lowest polarity? - acetone - benzene - chloroform - diethyl ether - ethanol - ethyl acetate - hexane - petroleum ether - water
- water - ethanol - acetone - ethyl acetate - chloroform - diethyl ether - benzene - hexane - petroleum ether Wow elephants are eating cans dropping below healthy ponds
Acid -Base extraction What type of mixtures do you want to place in a seperatory funnel?
- water - organic solvent with a low boiling point that is immisible in water.
Melting point experiment What is the melting point of water?
0 degrees celcius.
TLC Do not let the spot on the TLC plate diffuse more than ___mm ?
1-2 mm
TLC experiment What was the mobile phase?
A solvent (liquid) A ratio of non-polar solvent and a polar solvent.
Acid -Base extraction What is liquid-liquid extraction?
A technique used to seperate a mixture that has components in it with different solubilities. having different solubilities causes them to exist in different phases which can be exploited to remove one compound from the other.
SN1 substution Concentration of what reactant determines the rate of the reaction?
Alcohol becuase its the slow step Rate = k [ROH]
Dehydration of cyclohexanol Which has a higher boiling point? An alcohol or alkene?
Alcohol becuase of stronger intermolecualr forces.
SN1 substution What donates a proton in the SN1 reaction?
Ammonium bromide in aqueous sulfuric acid will provide a hydrogen halide. That means bromide acts as an acid
Acid- Base extraction Why can suction filtration be used on the ether and acid solution?
Because carboxylic acid is a solid so it can be isolated via suction filtration. This removes the ether solution from it.
Dehydration of cyclohexanol Why is only a small amount of acid required during dehydration of alcohol?
Becuase the acid isnt actually consumed in the reaction so we dont need a lot of it.
Oxidative cleavage of alkenes What are the 2 ways that oxidative clevage of alkenes can occur?
Both break a c-c bond and form a c-o bond Ozonolysis (addition of O3) - alkene + O3 --> intermediate ozonide --> Aldehyde + ketone Using potassium permanganate - alkene + Kmno4 --> ketone or carboxcyllic acid
SN1 substution What is the nucleophile in this reaction?
Bromide ion
In the TLC lab, what problem might be encountered with the following experimental procedures: A single micropipet was used to stop four analgesic samples of a TLC plate
Cross contamination of substances
dehydration of cyclohexanol What is the limiting reagent?
Cyclohexanol
Oxidative cleavage of alkenes What is the limiting reagent?
Cyclooectene
Dehydration of cyclohexanol What does dehydration of an alcohol produce?
Dehydration of an alcohol is when the alcohol is heated in the presence of an acid catalyst to remove water from the alcohol. This results in the formation of a carbon carbon double bond.
Melting point experiment What happends if you heat your sample to fast? why?
If your voltage is too high then you can get a broadening and depression of the melting point observed. To give enough time for heat to be transferred equally from heating block to sample to the thermometer. If you go too fast, temperature at sample is very different than temperature at thermometer. And sample appears to have a wide range.
Dehydration of cyclohexanol Why are acids required during dehydration of an alcohol?
It acts as a catalyst and lowers the energy required to cleave the oxygen carbon bond. This helps facillitate the dissociation of the carbon-oxygen bond. No acid is consumed in the reaction. This is becuase it provides a proton, but the proton is regenerated at the end.
Oxidative cleavage of alkenes Why is Aliquat 336 used?
It acts as a spectator ion and is not actually involved in the reaction, although it helps it occur. Aliquat 336 is a phase transfer catalyst that is used to bring cyclooctene and potassium permanganate into the same phase. It has a positive nitrogen atom, but large nonpolar hydrocarbon chain. It is a simple salt (which usually are soluble in polar solvents, but since it is lipophillic it is soluble in polar and nonpolar solvents) It passes back and fourth between the 2 layers. Aliquatt 336 hangs onto permanganate ions as it passes between the hexane and the water layer because its lipophilic. When it enters the hexane layer (where cyclooctane likes to be) the permanganate reacts with the cyclooctane to produce suberate ions and hydroxide ions. The ions produced as a result of the reaction are carried back into the aqueous layer and exchanged for more potassium permanagate.
Recrystallization experiment What is the use of activated carbon in this experiment?
It can be added to the hot solution before gravity filtration. Activated carbon can remove some of the insoluble impurities. Carbon absorbs polar compounds. When hot filtration is carried hot, the carbon particles stay behind.
Resolution of 1-Phenylamine experiment Is (S)-1-phenylethylamine a solid or liquid? WHere does it end up?
It is a clear liquid. It ends up in the round bottom flask after simple distillaiton. The dichloromethane boiles out to the other side.
Recrystallization experiment What is petroleum ether?
It is a solvent consisting of mixtures of saturated hydrocarbons isolated as distillate fractions from petroleum. It has a solvent property similar to saturated hydrocarbons. it is volitile.
Acid- Base extraction Why do extra precautions need to be taken when using diethyl ether?
It is flammable and can accumulate on the floor. it should be far away from flames.
Oxidative cleavage of alkenes What do you have to be careful with potassium permanganate?
It is posinious and an irritant. It causes brown stains on ur skin.
Oxidative cleavage of alkenes Describe the solubility of cyclooctene?
It is soluble in solvents of low polarity (hexane) Insoluble in water
Resolution of 1-Phenylamine experiment What is the point of adding sodium hydroxide to (s)-amine, (R,R) tartaric acid complex?
It reacts with it to convert the salt to a free amine (S) amine.
Resolution of 1-Phenylamine experiment What does it mean if a sample is optically active?
It rotates plane polarized light in a polarimeter
Acid- Base extraction How do you remove the top layer out of a seperatory funnel?
It should be decanted out the top of the funnel. If it is allowed to go through the stopcock, then it can become contaimed with the bottom layer as it goes through.
SN1 substution What is the rate determining step of the SN1 reaction?
Loss of water to product an alkyl carbocation.
Oxidative cleavage of alkenes Why is filter aid added to the reaction mixture before it is filtered by suction filtration?
Magnesium dioxide is a solid that tends to clog filter paper Filter aid is highly porous and absobrs and traps magnesium dioxide making it easier to filter.
Melting point experiemnt Why does a sample need to be dry before filling the mp capillary
Moisture from solvents like water and other solvents will depress the melting point like an impurity.
SN1 substution List the order of reactivity for the following compounds. GO from most reactive to least: - 2-bromo-2methlpropane - 2-bromobutane - 1-bromobutane
Most reactive - 2-bromo-2methlpropane - 2-bromobutane - 1-bromobutane Least reactive
SN1 substution Rank the following in order from most reactive to least reactive: - chlorine - Iodine - Bromine
Most reactive Iodine Bromine chlorine least reactive Goes from largest to smallest (up the periodic table) This is becuase the larger they are, they easier they are to break (the ones further down on the period table are larger)
SN1 substution Rank the following based on stability from most stable to least stable in a SN1 reaction? - primary alkyl halide - secondary alkyl halide - tertiary alkyl halide - methanol
Most stable and most reactive - tertiary alkyl halide - secondary alkyl halide - primary alkyl halide - methanol
SN1 substution Do all nucleophillis substuition reaction exhibit first order kinetics?
No SN2 has second order kinetics.
Resolution of 1-Phenylamine experiment Do racemic mixtures rotate plane polarized light?
No becuase they rotate light in equal but opposite directions. Rotation of place poalrized light only occurs when there is more of one enantomer than the other.
Sn1 reaction Sources of error:
Not properly cleaning and dryign your ismple distillation apparatus can lead to your product codistilling with water. If you dont connect your reciever to the adaptor then 2-bromobutane can evaporate.
Melting point experiment How do you convert voltage from the old model to the new model? - old model = 36V - new model = 4V
Old model has a range from 0 to 120 V new model has a range from 0 to 10 V Old model = voltage x 12 - old model = 36V --> 3V - new model = 4V --> 48V
Acid- Base extraction What would be a bad solvent to use in this experiment?
One that is miscible in water. - acetone -ethanol - methanol - acetic acid - pyridine If they are added, they will form a homogenous solution and so no seperation of layers occurs.
TLC experiment What can u do to see spots on ur plate that are not immately visible?
Place ur plate in a sealed jar containing a few crystals of iodine. The iodine will be observed at points where organic material is found and will turn spots brown. You could also expose the plate to UV light.
SN1 substution Why do secondary and tertiary alcohols usually proceed via SN1 mechanisms?
SN1 proceeds via a carbocation intermediate. Sn2 does not have a intermediate. Its eassier for secondary and tertiary alcohols to form a carbocation intermediate and so they proceed via SN1 reaction.
SN1 substution What type of substuiton reaction do primary alcohols and methanol usually undergo?
SN2 reaction becuase they have less steric hinderance so they dont need to form a carbocation intermediate.
Recrystallization experiment If you decide to obtain a second crop, why is it important to keep ti seperate from your first one?
Second crops are more likley to not be as pure as the first crop becuase they were mixed with impurities found in the filtrate.
Oxidative cleavage of alkenes Whats the difference between a sigma (σ) bond and a pi bond?
Sigma bond = stronger pi bond = Weaker and polarizable.
Acid-base extraciton Why does turning a acid into a salt change its solubility? How does it change it.
Simple salts are limited solubility in solvents of low polarity, but high solubility in water. Soluble in aqueous Insoluble in organic (ether)
Sn1 reaction Why is a reflux procedure used in this reaction?
Since we were heating the mixture at around 90-100 degrees celcius which is above the boiling point of the 2-Bromobutane (our product), this helps prevent the product from boiling out and instead it falls back in. This prevents loss of product or change in concentration of your reagents. This does allow evolution of HBR (which turns into a gas) to leave.
Oxidative cleavage of alkenes Why are double bonds easier to break than single bonds?
Single bond = sigma bond Doubel bond = sigma + pi bond A double bond is easier to break because the pi bond is weaker than a sigma bond. As a result double bonds are more reactive.
TLc experiment Do smaller or larger molecuels travel faster on a TLC plate?
Smaller molecuels travel faster than larger ones.
Acid -Base extraction Why do you want the organic sovlent to have a low boiling point?
So that once extraction is completed the compounds dissolve in the organic layer can be isolated by evaporating the solvent.
In the TLC lab, why did you cover the TLC chamber with a watch glass or glass cover?
So the solvent couldn't evaporate
Oxidative cleavage of alkenes Describe the solubility of potassium permanganate?
Soluble in water Insoluble in solvents of low polarity (hexane).
Recrystallization experiment What happends if you dont keep the solution cold during suction filtration?
Some of the crystals will go back into the solution.
TLC How far up and to the side shoudl the spots be made on the TLC plate?
Spots should be 5-10mm up the plate. (1cm=10mm) Spots should not be to close to the side of the plate becuase it doesnt run up uniformly on the edges.
Sn1 substuiton reaction What precautions should be taken when handeling sulfuric acid?
Sulfuric acid is corrosive and so spilled material needs to be cleaned up quickly.
Resolution of 1-Phenylamine experiment What is the purpose of adding a single enantomer of tartaric acid?
The enantomer reacts with the different enatomers of 1-phenylethalamine to produce 2 diasteromeric salts. The enantomers naturally have the same solubility in methanol, but as diasteromers, the R version is more soluble in methanol.
In the TLC lab, what problem might be encountered with the following experimental procedures: Using a TLC plate which was marked with ink
The ink would contaminiate the plate and could cause different results A pencil should be used to avoid this.
TLC experiment what is an elutant?
The mobile phase of the experiment. It moves up the TLC plate. It is the solvent that you put in the mason jar.
Oxidative cleavage of alkenes What happends if you add too much sodium hydrogen sulfite? how do you fix this?
The pH may become so low that suberic acid begins to precipitate. if this happends, add sodium hydroxide to redissolve it.
Acid- Base extraction What is the purpose of Congo red indicator paper?
The paper is naturally orange. This is used to tellus when the solution is acidic. When it turns red, we know it is acidic.
Dehydration of cyclohexanol Why was phosphoric acid used in this experiemnt?
The phosphate counter ion is non-nucleophillic which minimizes substuition pathways. It also has a high boiling point so it remains behind in the reaction vessel. This is good because the acid can catalyse the reverse reaction. however, if it doesnt ever come in contact with the product, that prevents it from doing so.
In the TLC lab, what problem might be encountered with the following experimental procedures: The depth of the solvent is above the levol of the spots
The results would be wrong b/c the spots wouldn't travel up the plate, but instead stay at the bottom
TLC experiment Rf
The retardation factor distance traveled by a compound/ distance traveled by the mobile phase.
Melting point experiment Why can you not use a sample that you had already melted to run another melting point determination?
The sample may decompose, so second mp will be lower than the first
TLC expeirment if you use a mobile phase/ elute mixture that has more polar components, what happens to the TLC plate and Rf values?
The seperation of the pigments decreases. More nonpolar= more seperation of compounds.
Recrystallization experiment Why is a long-stemmed funnel not used for hot gravity filtration?
The solution cools as it passes through the stem so it if is longer, this is more likley to occur.
In the TLC lab, what problem might be encountered with the following experimental procedures: A TLC plate touching the filter paper liner of the development chamber
The spots of solution could rub off onto the filter paper
TLC experiemnt Why do you not want the spots on the TLC plate to be to large
The spots run in all directions. If the spots are to large, they could run into eachother
SN1 substution how can the stability of carbocation intermediates be evaluated?
The stability is directly realted to reactivity. We can determine its stability based on the rate of ionization of alkyl halides. We can see when ionizaiton occurs by looking at how quickly the silver halide precipitate forms.
Melting point experiment What is the melting point of a solid
The temperature at which a phase transition from solid to liquid occurs. When this temperature is reached, this means there is enough energy to break down the crystal lattic of the solid.
Dehydration of cyclohexanol what happends if the rate of heating is too rapid in the fractional distillation?
The vapor produced may exceed the capacity of the columb and seperation efficency will suffer.
Oxidative cleavage of alkenes Why do we let the reaction sit on vigirous stir for an hour?
The way aliquat 336 works is through the interface of the 2 solvents. In order to ensure adequate mixing, viqirous stirring it required. This increases the surface area of contact.
Dehydration of cyclohexanol Why was fractional distillation used in this experiemnt?
There are 4 compounds that need to be seperated from one another all with different boiling points. When this is the case, it is best to use fracitonal distillation becuase seperation is more efficient.
Recrystallization experiment What does it mean if 2 liquids are miscible?
They are completely soluble in eachother giving a homogenous solution.
Recrystallization experiment What type of solvent properties do petroleum ether and ligroin have?
They are similar to pure saturated hydrocarbons like hexane.
Melting point experiment What happends if you overfill your capillary tube?
This can lead to broadening and depression of the melting point due to increased surface area. It wont heat as evenly.
TLC experiment What is the purpose of addign water to the pigment solution (organic layer)?
This helps remove ethanol and any water soluble materials in teh leaves.
Recrystallization experiment Why should you place a watch glass on top of the funnel during gravity filtration?
This helps to retain ehat and prevent evaporation of the solvent and prevent crystal formaiton in the funnel.
TLC experiment What occured when you increased the concentration of petroleum ether (the nonpolar aspect)?
This lead to greater seperation.
Acid- Base extraction Where does partitioning of components of the mixture occur in a separatory funnel?
This only occurs at the interface, which is why you need to shake vigirously
SN1 reaction Why do we rinse the adaptor and condensor with acetone and dry it before using it in the second distillation?
To prevent any contamination If you dont do this, a significant amount of your material may co-distill with water before 86 degrees, This will seriously decrease your yield.
Acid- Base extraction Why is the water and acid solution washed with water during suction filtration?
To remove NaCl This works becuase NaCl is soluble in water
Recrystallization experiment What do you observe during supersaturation?
Too much sovlent was added. When this occurs, you may not see clouding when adding your solvent. The clouding occurs when you reach a visible saturation point. Crystals dont form when you cool your solution. Crystal formation does not form upon cooling if you super saturate.
TLC experiment How does the solvent move up the plate?
Via capillary action
SN1 substution Are intermediates involved in the nucleophillic substuition reaction?
Yes Carbocation intermediates are formed.
Oxidative cleavage of alkenes What can you do to remove any unreacted potassium permanganate?
You can add sodium hydrogen sulfite. It removes unreacted potassium permanganate and helps turn the color of the filtrate form pink to white.
Recrystallization experiment If oiling out occurs, how can you fix it?
You can reheat the solution until the oil dissolves and a a bit more solvent. Cool again more gradually.
Recrystallization experiment How do you concentrate the solution after gravity filtration is complete? Why do you do this?
You concentrate the solution to get close to the saturation point. You concentrate the solution by boiling away some of it. This continues until clouding or the onset of crystla formation occurs. Once this occurs, you passed the saturation point. Add a small amount of solvent until the solution is once again homogenous. Crystals will not form if there is a large excess of solvent. Heating the beaker allows some of the solvent to evaporate so crystals are more likley to form.
Melting point experiment What is a rapid determination mp? Whats the purose?
You keep the voltage at 5V the whole time. The purpose of this is to determine a range so you have an idea of where to stop when you do the accurate determination.
Acid- Base extraction How are drying agents selected?
You must use one that is chemically inert towards the compounds in the solution. Sodium sulfate is usually a safe choice, but its not as powerful as others.
TLC experiment Why does the use of filter paper in the developing chamber improve the consistancy of results?
You place filter paper inside of the developing chamber to keep the atmosphere within the glass saturated with solvent vapors all the time. This increases reliability.
Melting point experiment If your mp is greater than + or - 2°C of literature values, what could you have done wrong?
You probably heated the sample too fast or it is contaminated.
TLC experiemtn If the pigment solution isnt deeply colored what can u do?
concentrate it using a gentle stream of air to remove some of the solvent.
SN1 substution What is the rate order of the reaction? Why?
it is first order All SN1 reaction use first order kinetics. This is becuase the reactants (the alcohol) are the only ones that contribute to the rate since its the slow step.
Acid- Base extraction what is K?
partition coefficent This tells us how efficent the seperation is. The efficiency of seperation is based on how soluble the compound is in in the 2 solvents being used. It is a constant
Acid- Base extraction Why is the aqeous solution heated after it is removed from the seperation flask.
the seperation flask had some ether in it. Dissolved ether can be removed by heating.
Recrystallization experiment Why are saturated hydrocarbons good solvents?
their bonds are easier to break and so they are more reactive. Single bonds are the easiest to break. Tripple bonds are the hardest to break.
TLC experiement What are some sources of error (5)
· If too dark/ doesn't separate, then you grinded it too much · Use pencil to mark the dot, not pen. · Don't put spot to low down because it can mix with the solution. · Allow dot to air dry before adding another one. - Touching the plate with your hands can get oils on it.
