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OBJECTIVES OF CHE 234 SECOND YEAR PRACTICAL ORGANIC CHEMISTRY
By the end of the semester you should, a) Be able to assemble apparatus in a neat, correct and efficient way, and to work in a tidy fashion; b) Be able to write a neat, concise, and accurate account of your lab work; c) Be able to apply the theory of simple extractions and use a separatory funnel for separating pairs of immiscible liquids, washing solutions and performing extractions; d) Be able to apply the theory of distillation and carry out a distillation, both to remove a solvent and to get a pure liquid; e) Be able to carry out a reflux; f) To understand different steps in recrystallization and be able to purify a solid by recrystallization; g) Know how and when to use a Buchner funnel and when not to; h) Know how to determine the melting point of a solid, and hence to assess its purity; i) Be able to carry out simple test tube reactions to identify common classes of organic compounds; j) Be able to follow understand, apply and follow practical instructions appropriately. CHE 234 ASSESSMENT
This lab course will be assessed by lab reports, lab quiz (1 or 2) and/ or practical test(s)!!!
c. Handle all chemicals with care. Remember to stopper containers after use and put these back in an appropriate place. Clean spill on all surfaces, especially balances. d. Heating by using a Bunsen burner or a flame should be avoided as much as possible, and only do so when instructed accordingly. Make sure organic solvents and any other flammable compounds are far away from naked flames. Remember to turn off the gas immediately after you have finished using the burner. e. Make sure your working space is in order to avoid unnecessary breakages and spillages. Do not sit on benches since on these surfaces have been spilled an unknown variety of chemicals that may be corrosive to your clothing and skin. f. Prepare adequately and efficiently for the laboratory class by reading the instructions. Write your own summary of the instructions before you start the experiment. You will save a lot of time this way, and you will also find the experimental sessions more interesting and rewarding. Carrying instruction manuals to every corner of the laboratory is cumbersome and may be hazardous. g. Minimise talking to others once the laboratory session has started. It is very easy to distract and disturb serious minds by talking to them in the midst of an experiment.
EXPERIMENTAL ACCOUNT FOR ORGANIC CHEMISTRY EXPERIMENTS
The following sections contain suggestions as to how you should record your experimental work. (i) Your Report The report sheets for all experiments are included at the end of this manual. ALWAYS use a pen for writing observations, measurements, descriptions, drawing organic structures etc. Pencil is to be used only for diagrams. DO NOT USE correcting fluid / rubbers / paste-on paper to correct errors. Simply use a couple of neat parallel lines ruled through the offending words or data, which should still be legible, and explain why you have made the deletion or alteration. (ii) The Format Generally this should be as follows: I Aim PRE-LAB PREPARATION II Principle PRE-LAB PREPARATION III Answers to Questions/Mechanism
PRE-LAB PREPARATION
IV Action Plan PRE-LAB PREPARATION V Procedure AFTER THE EXPERIMENT VI Observations and Results DURING THE EXPERIMENT VII Inferences/Calculations AFTER THE EXPERIMENT VIII Conclusion(s) AFTER THE EXPERIMENT IX References AFTER THE EXPERIMENT The format may vary from one experiment to another. Thus a "mechanism" may not be relevant in certain experiments, but calculations might be.
Please complete sections I to IV before you come to
the practical as part of pre-laboratory preparation!
(iii) Aim A brief statement indicating the purpose of the experiment.
the action plan. It is important to report on any variations from the instructions in the manual. The time taken for different processes must also be noted and reported. This section must be written in past tense and passive voice! (ix) Observations and Results It is essential that these are written directly into your report sheets. Use of rough notes, scraps of paper, edges of instruction sheets, etc. is not acceptable. Laboratory staff reserves the right to destroy these if they catch you using them. It is important to note and report: Exact masses used, including those of flasks and volumes of solvents. Appearance of reactants as well as products. Smell of compounds and shapes of crystals. Changes occurring during a reaction, or even the fact there appeared to be no change. Physical properties, like melting or boiling points for every compound prepared. The section on Procedure and Observations/Results should enable yourself or any other reasonably intelligent person both to repeat exactly what you have done, and to be able to interpret your results. (x) Yield and Inferences/Calculations Remember you need to work out which reactant is present in least molar amount (is limiting), and also to write a stoichiometric equation for each step. Calculate the mole quantities for each reactant and then determine which is present in least molar amount. This can be used in calculating theoretical yield. See section (xv). (xi) Conclusion
This is a brief summary of the findings in the experiment. You have to conclude on why these results were obtained. (xii) References You may need to consult various books for the introduction, mechanism and questions. If you do, you should note the source you referred to using this pattern: (a). For a journal article: Author, Journal, Year of issue , Volume number, page. e. g. Story, P. R., J. Org. Chem., 1961 , 26, 287. (b). For a book article: Author, Title of the book, edition, publisher, city of publication, year of publication, and location of information (volume, chapter, page). e. g. Heftmann, E., Chromatography, 2 nd^ ed., Reinhold Publishing Corporation, New York, 1967, p 52. (c) For an internet article : You should write the complete website address and include the date the website was created and the date you accessed it. e.g. http://wine1.sb.fsu.edu/chm1045/notes; ©2000 Dr Michael Blaber; 25/10/ (xiii) Failure Every chemist will one day attempt an experiment, which fails to give the desired product or even any product at all; you are unlikely to be an exception to this. Before repeating the experiment, the correct action is to note the appearance of whatever material produced, if possible identify it, and then try to explain why the experiment failed. This is more than a vague reference to experimental error. Was the failure the result of careless calculation (wrong quantities), careless weighing, careless addition (it said ”slow addition”), or careless heating ("gentle reflux").
We can then calculate the number of moles of reactants, and hence from the reaction equation, work out how many moles of product could be produced. The limiting reactant is the reagent that limits the amount of product which can be formed in the reaction. All other reagents are present in excess of the amount needed to react with the limiting reactant, which gets used up completely. In accord with the balanced equation for a reaction, the limiting reactant will determine the theoretical yield (maximum yield possible). The actual yield of the product is unlikely to match the theoretical yield. The lower actual yield is due to factors such as experimental errors and side reactions yielding other organic products. We usually find the actual yield by measuring the mass of product on a balance. The percent yield is the ratio of actual yield to theoretical yield multiplied by 100: Actual yield 100% = % yield Theoretical yield
EXPERIMENT 1
MELTING POINTS AND MIXED MELTING POINTS
(i). Introduction Most pure organic solids have sharp (narrow range), invariable melting points that are characteristic for each compound and may readily be determined. This constitutes a valuable technique in helping to identify unknown compounds. Determination of the melting point can also be used to ascertain the purity of a compound of which the identity is already known. The presence of impurities will always lower the melting point of a solid, and thus if the melting point of a particular substance is found to be below that recorded in the literature, it is probably impure. Further, the melting point will have lost its sharpness, and the substance may melt over a range of several degrees. Eutectic mixtures (a mixture at such proportions that the melting point is as low as possible, and furthermore all the constituents crystallize simultaneously at this temperature from molten liquid solution) however, do have sharp melting points. The lowering of the melting point of a solid by impurities also affords a useful method for confirming the identity of a substance whose nature is suspected but not yet proven. If the substance in question were to be mixed with another compound of similar melting point, the melting point of the mixture will be lower than the melting point of either constituent since each is in effect an impurity of the other. If however, the substance were to be mixed with another sample of its kind, the melting point of the mixture would obviously be unchanged and the identity of the original sample would be confirmed. This is called the method of mixed melting points. In this experiment, the melting points of a series of compounds A, B and C are taken first. (When carrying out a series of melting point determination it is always better to start with the lowest melting substance (if this is known), so that time need not be wasted in waiting for the melting point apparatus to
Figure 1.2: Loading a melting point / capillary tube
- Insert an appropriate thermometer into the melting point apparatus. This will normally be the thermometer with a range to 360oC.
- Insert the capillary tube with the closed end into the melting point apparatus. Make sure that you can clearly see the solid in the tube through the eye view.
- Switch on the melting point apparatus, and select an appropriate rate of heating. It is essential that as you approach the melting point, the temperature rise must not be more than 2°C per minute. If it does you will get false readings and your compound may appear less pure than it really is.
- Record the melting point by noting the temperature at which there are the first obvious signs of liquid forming, and then the temperature at which the all solid has melted (or decomposed). The melting point should be reported as a range and not as one temperature, i.e. 237- 238°C and not as 238°C. (iii) Experimental Procedure (WORK IN PAIRS)
- Grind a small quantity of dry material on a clean watch glass.
- Push a little of the powdered solid into the open end of a melting point tube and lightly tap the tube until the powder has settled to the bottom. The length of the column of powder should be no more than 2 mm. See figure 1.2.
- Place the melting point tube in the melting point apparatus and select the rate of heating by turning the control dial.
- For greater accuracy care should be taken to raise the temperature very slowly.
- Determine the melting point of each member in any one set allocated to you – (AI, BI, and CI), (AII, BII, and CII), and (AIII, BIII, and CIII).
- Record the temperature at which each substance begins to melt, and the temperature at which it has completely melted (range).
- Mix thoroughly small samples of each unknown solid with an approximately equal quantity of organic acid using a spatula and determine the melting point of each mixture and record. Find below the organic acid to be used for each set: Set I BENZOIC ACID Set II ADIPIC ACID Set III trans-CINNAMIC ACID
- From your results, deduce which of the substances AI, BI, and CI is benzoic acid ; which of AII, BII, and CII is adipic acid ; and which of AIII, BIII, and CIII is trans-cinnamic acid. **LAST NAME: INITIALS: I.D.NO.: LAB DAY & DATE: PARTNER’S NAME: PARTNER’S I.D.NO.: MARKS: % EXPERIMENT 1 MELTING POINTS AND MIXED MELTING POINTS Please complete sections I – IV before you come to the practical I AIM
II PRINCIPLE
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**V (i) PROCEDURE
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VII CONCLUSION
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VIII REFERENCES
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