Final Exam
What are the products of the 2-methylcyclohexanol (E1) reaction?
1) 1-methylcyclohexene 2) 3-methylcyclohexene 3) Methylenecyclohexane (HS)
What occurred in the gas chromatography experiment?
1) A liquid mixture is vaporized in the injection port and carried to the column by the carrier gas 3) The components of the mixture partition themselves between the stationary liquid phase and the mobile gas phase 3) Compounds are generally separated based on polarity and volatility 4) However, if the compounds are similar in polarity as in the alkene experiment, the compounds separate based on their volatility (boiling point). 5) The lower the boiling point, the faster the compound travels through the column.
What occurs in gas chromatography?
1) A liquid mixture is vaporized in the injection port and carried to the column by the carrier gas. 2) Then the components of the mixture partition themselves between the stationary liquid phase and the mobile gas phase.
Describe the mechanism.
1) Cl leaves and the carbocation is formed 2) Water attacks the carbocation and becomes positively charged 3) Another water molecule grabs the hydrogen from the positively charged water molecule and forms OH and H3O+
Quantitative Analysis: know what the area under each peak represents
1) The area under each peak in the chromatogram is directly proportional to the number of moles of that compound present in the mixture injected onto the column 2) . The AREA UNDER THE CURVE is the mole % of compound in mixture Mole % = (area under curve / total area under all curves ) x 100
If a given solvent can hold 0.5g of solid at room temperature and 6.0g of solid at its boiling point, how much solid can be recovered by crystallization?
6.0g (boiling point mass) - 0.5g (room temperature mass) = 5.5g of solid via crystallization leaving 0.5g of solid in solution
During filtration, what instruments can be used to collect the desired solid?
A Hirsh or a Buchner funnel can be used
What moves faster, a compound with a short retention time or a compound with a long retention time?
A compound with a short retention time moves through the column faster than a compound with a large retention time which indicates it has a lower boiling point (volatility)
What is the difference between the mother liquor and slurry, and where does the impure substances remain after crystallization?
After the crystallization process, a flask will contain the desired crystallized solid and solvent. The liquid in contact with the solid is called the mother liquor and the solid + liquid mixture is known as the slurry. The impure substances remain in the mother liquor.
When distilling, how was the liquid kept from overheating and bumping in the flask?
Boiling chips were added to the liquid
If compounds are similar in polarity as in the alkene experiment what must the compounds separate based on?
Boiling points (volatility)
What is the difference between crystallization and precipitation?
Both are phase changes in which a solid is formed; however, crystallization produces crystalline solids while precipitation produces amorphous solids.
Why can you not distill to dryness?
By leaving a small residue of liquid in the boiling flask, you will not overheat the flask and break it nor will you char the last drops of residue or cause an empty/dried flask to increase in temperature and explode...boom!
Compounds that are not similar seperate based on what?
Compounds are generally separated based on polarity.
What does the process of distillation do and how does it occur?
Distillation separates the components of a liquid mixture based on the difference in their boiling points. The differences in boiling point of the two components allowed for only one of the components to vaporize while the other remained in liquid form.
Why do you NOT perform distillation in a sealed apparatus?
Distillation should never be carried out in a sealed apparatus. If the apparatus is sealed, pressure could build up and cause an explosion.
How many times were the trials completed and why?
Each experiment was carried out three times. The average time of reaction was then calculated and used in the experimental data. Running each experiment three times allowed for a realization of consistency or error in the procedure.
What are some properties of a bad solvent for recrystallization?
For an unknown compound, a solvent that allows a solid to be soluble at high and low temperatures is unwanted. Likewise, a solvent that allows a solid to be insoluble at high and low temperatures is also unwanted.
What are the properties of a good solvent for recrystallization?
For an unknown compound, the solvent is selected by experimentation. An ideal solvent is one that allows a solid to be very soluble at higher temperatures (boiling) and insoluble at lower temperatures (room temperature).
What is the rate determining step of the Hydrolysis of T-Butyl Chloride?
Formation of the carbocation intermediate
How do impurities impact a pure compound's melting point?
Impurities cause the range of a compound's melting point to become wider. Also, the actual process of melting begins to occur at lower temperatures than that of the pure compound.
In fractional distillation, what is the theoretical plate?
In fractional distillation, a fractioning column is used which is packed with some material that increases the surface area of the column. The packing allows for a series of condensations and evaporations to occur so that the vapor that finally is condensed and collected in enriched in the lower boiling point component. This is like doing a series of simple distillations. Each of these simple distillations is called a theoretical plate.
What occurred in the overall reaction of alkenes from alcohols in this experiment?
In this experiment, an alcohol was dehydrated with the aid of phosphoric acid. Water was eliminated from the molecule and a C=C double bond was formed (E1)
Be able to determine the major and minor products on a chromatogram using the peak area
Major product has the larger peak
What can the melting point be used for?
Melting points are used to determine a compound's identity and purity.
How do you calculate the percent yield if you are given the experimental yield?
Percent Yield = (Experimental Yield/Theoretical Yield) x 100 de
What is the recrystallization?
Recrystallization is a process in which an impure solid is dissolved in a solvent at an elevated temperature. As the solution slowly cools, the purified form of the originally tained compound will crystallize out of the solution.
What are ways to induce crystallization if a compound fails to crystallize out of solution?
Scratch the inner wall of the Erlenmeyer flask with a glass stirring rod to release small particles of glass which act as nuclei for crystal growth. Addition of a small crystal of desired compound (a seed crystal) to serve as a template for crystallization of the rest of the compound.
What is the purpose of gas chromatography?
To separate and identify volatile substances in a mixture.
Where was, the thermometer placed to receive accurate results?
The bulb of the thermometer should be place just below the opening of the sidearm to the condenser to ensure the bulb is bathed in vapor before the vapor reaches the condenser. If the thermometer is not properly placed, inaccurate readings could occur.
What are the 4 characteristics of a good recrystallization?
The compound to be recrystallized should be soluble in a hot solvent, insoluble in a cold solvent, impurities should be soluble in the solvent at room temperature//insoluble in the hot solvent, the solvent must be unreactive with the compound being recrystallized, and the solvent should be volatile enough to be removed from the compound during drying.
Why must the filter paper be fluted and porous?
The fluted filter paper provides more surface area for the solution, making it more efficient than an unfluted filter, and it must be porous; otherwise, the solid crystallizes in the filter paper.
What kind of reaction is the Hydrolysis of T-Butyl Chloride?
The hydrolysis of t-butyl chloride is a first order nucleophilic substitution (SN1).
Define melting point?
The melting point is the temperature at which a solid state is in equilibrium with the liquid state (a phase transition from solid to liquid). It can range from the point the solid begins to melt to the point the solid has completely melted and has become a clear liquid.
What is the purpose of the Hydrolysis of T-Butyl Chloride Experiment?
The purpose of this experiment was the study of the rate of reaction and the effects on the rate that concentration and temperature had.
To obtain accurate results, how must the solid be heated for it to melt?
The solid must be heated slowly to obtain accurate results because solids melt at a temperature range and not one single temperature.
What are the two processes involved in distillation?
Vaporization and condensation. The liquid is heated to its boiling point, the vapors rise into the distillation head where they enter the condenser, the vapors then cool, return to the liquid state, and the distillate is collected in a flask
What is bumping and why should it be avoided?
When a superheated solution boils, it frequently results in a violent action called bumping; some of the solution might be lost.
What is the meaning of azeotrope?
1) A mixture of miscible liquids with a definite composition that distills at a constant temperature (meaning they cannot be completely separated). 2) These properties cause azeotropes to behave as single pure compounds even though they are actually mixtures. 3) Therefore, the two components of an azeotrope cannot be separated using fractional distillation, regardless of the number of theoretical plates in the column. Example The mixture of 95.57% ethanol and 4.43% water ("95%0020 ethanol") which boils at 78.15 C, just below the boiling point of pure ethanol (78.3 C)
What were the effects on concentration and temperature?
1) Concentration Effect: The overall solvent concentration was kept the same, but the volume of the solvent was doubled. The average time for the reaction was slightly higher in this procedure than it was in the normal. 2) Temperature (Heat): The addition of heat clearly sped up the reaction greatly. Rate increased proportionally with temperature increase. 3) Temperature (Ice): Running the experiment after cooling the reactants caused a significant decrease in reaction time. The data collected was used to calculate values using two major formulas. The value k was calculated using
Be able to assign the peaks on a chromatogram if given the boiling point of the compounds
1) If molecules have very similar structures, they will normally elute from a gas chromatograph in order of increasing boiling point 2) In regards to this experiment: the two possible products, 1-methylcyclohexene and 3-methylcyclohexene, have boiling points of 110°C and 104°C, respectively 3) Thus, in this experiment, one can also deduce that the product represented by Peak 1 is 2-methylcyclohexene (lower boiling point—elutes faster) and the one represented by Peak 2 is 1-methylcyclohexene (higher boiling point—elutes slower) by an analysis of the boiling points of the two possible products.
What are the steps in recrystallization (S.D.D.G.C.F.D.)?
1) Solvent Selection An ideal solvent is one the has a steep solubility vs temperature curve with respect to the solid. The solid must be soluble in the solvent when the temperature of the solution is high (boiling), but when the temperature is lowered (room temperature) the solid must be insoluble to the solvent. 2) Dissolution The solid being crystalized is placed inside of a test tube. Then the chosen solvent is added to the test tube one drop at a time at high temperatures. Enough of the solvent will be added to dissolve the solid. The insoluble material will not be dissolved. 3) Decolorization This step is needed when a colored impurity must be removed from the solution. A small amount of powered charcoal is added to the hot solution because the charcoal attracts to colored impurities so these impurities can be removed when the solution is filtered. 4) Gravity Filtration Gravity filtration via filter paper is used to collect the unwanted solid impurities (these impurities did not dissolve in solution when the solvent was heated). This includes the charcoal that was added in the decolorization step. A piece of filter paper is fluted in one direction only. It is then placed into a stemless funnel (stemless so the solid does not crystallize in the stem during this filtration step). The filtrate is collected in a flask. 5) Crystallization Because the cold solution cannot hold as much solvent as the hot solution, the excess solid will crystallize out of solution. Using an Erlenmeyer flask, there will be both solid and liquid present once the solution slowly cools down. 6) Filtration The Erlenmeyer flask now contains the desired solid and solvent (with a very small amount of dissolved solid contained in the solvent). The slurry mixture is poured through a filter allowing the filter to catch the desired solid while the solvent passes through. The solid is then obtained and washed with a small amount of cold liquid over the solid. 7) Dry the Solid The filtered and washed solid is placed in a safe place to dry. Any solvent that adheres to the desired solid will evaporate leaving the pure solid.
Know the mechanism for alkenes from alcohols.
1) The OH group on 2-methylcyclohexanol grabs a hydrogen from the water molecule to form water because water is a better leaving group 2) The newly formed water molecule on the 2-methylcyclohexanol leaves to form a carbocation 3) There are 2 beta carbons with beta hydrogens next two the carbocation so two products are initially formed 4) A beta hydrogen on the left of the carbocation (where the methyl group is) can be extracted to form a double bond (1-methylcyclohexene) 5) A beta hydrogen on the right of the carbocation can be extracted to form a double bond (3-methylcyclohexene) 6) A HS can occur to form the 3rd product Methylenecyclohexane (least stable) where the beta hydrogen from the methyl group is extracted
What instrument is used in fractional distillation?
1) The fractional distillation apparatus has an additional fractionating column between the distillation flask and distillation head 2) This additional fractionating column is needed when a mixture is composed of two liquids with very similar boiling points 3) Copper threads are placed in the column in order to increase the surface area. Because the vapor from the distillation flask must pass through the fractionating column before reaching the condenser, additional distillation occurs. 4) This additional distillation occurs due the constant condensing and revaporization of the component on the copper threading. 5) Because of these repeated condensations and vaporizations, the vapor is repeatedly undergoing a series of simple distillation as it travels up the column to the condenser.
What is the retention time?
1) The time of the sample (mixture) to the detection of the component 2) The distance on the chromatogram from the point of injection to the maximum height of a peak gives the corresponding compound's retention time
Know the four factors that can affect the separation on a gas chromatograph
1) The type of stationary liquid phase used 2) The column's length 3) The column temperature 4) The flow rate of the carrier gas
Describe the reaction of alkenes from alcohols
1) This reaction is an acid-catalyzed dehydration of an alcohol 2) It is a first order elimination reaction (E1)
Be able to draw the energy diagram for the reaction of Hydrolysis of T-Butyl Chloride.
1) Two steps 2) Two transition states 3) Carbocation intermediate
What are sources of error in crystallization?
1) Use of too much solvent will result in a lower percent of recovery as more of the solid you are trying to recrystallize remains in solution. 2) Faster cooling results in smaller crystals of lower purity. 3) Plunging hot liquid into an ice bath forces rapid cooling and traps impurities within the crystals.
What is a mixed melting point and how is it used?
A mixed melting point can be used to confirm the identify and purity of a compound. In this technique, an authentic sample of the unknown compound is obtained. The unknown and known samples are mixed together in equal amounts. Then the melting point of the mixture is obtained. If there is no lowering of the melting point then the two samples are identical, but if the melting point of the unknown compound is broadened or lowered then the two samples are not identical.
What are the characteristics of a pure compound's melting point?
A pure compound's melting point will illustrate a narrow range which indicates high purity of the compound.
Why do you use a rheostat with the sand bath?
A rheostat acts as a resistor in circuit. By decreasing the power settings on the rheostat, the resistance of rheostat is increased and therefore less voltage is delivered to the sand bath, causing it to operate at a lower voltage and lower heat. It is important to use the rheostat, as the adjustment of the resistances allows for the controlling of the amount of heat delivered by the sand bath
Along with the solid being crystalized, what is added to the test tube in the dissolution step and why?
A wooden boiling stick or a boiling chip is placed in the test tube to keep the solvent from overheating. Any air can be trapped within the wooden stick or boiling chip and then bubble to the top to allow the liquid to boil evenly rather than all at once, and keep solution from bumping.
Why is charcoal used in the decolorization step?
Charcoal is used because it attracts colored impurities out of the solid, and it presents a large surface area on which the impurity can adhere.
In which step are the colored impurities removed?
Charcoal is used in the decolorization. If there are no colored impurities to remove, this step can be eliminated.
What is the difference between simple distillation and fractional distillation?
Simple and fractional distillation both separate and purify miscible liquids. However, simple distillation is used to separate miscible liquids whose boiling points differ by more than 75 degrees' Celsius while fractional distillation is used to separate miscible liquids whose boiling points differ by less than 75 degrees' Celsius.
Explain how crystallization works in terms of saturation and the heating/cooling process?
When a liquid holds the maximum amount of solid in a solution, at a given temperature, the solution is known as a saturated solution. A solvent can hold more solute in a solution at its boiling point than at room temperature. Thus, crystallization depends on the solubility of the solid at the boiling point temperature and room temperature. In crystallization, first a hot saturated solution is heated to its boiling point to obtain the maximum amount of solid at higher temperatures then the solvent can hold (if too much solvent has been added the solution will not be saturated). The hot solution is slowly cooled to room temperature. Since the cooler solution cannot hold the amount of solute it could hold while boiling, the solid will crystalize out of solution.
