Organic Compound Analysis

 A marsh downstream from an old land-fill is leaching out organic compounds. Organic compounds (specifically carboxylic acids) contaminating the site will be identified by physical/chemical properties and spectroscopically.

Structure elucidation, or the identification of a chemical compound, is one of the more challenging tasks a chemist encounters. There are many techniques used to facilitate this, for example: 

-              spectroscopy (IR, NMR, UV-Vis, etc)

-              chromatography (GC, HPLC, LC, TLC, etc)

-              evaluation of physical properties (MP, BP, density, solubility, etc)

-              flame test evaluation

-              determination of molecular weight

 In this experiment, NMR (nuclear magnetic resonance), melting point, solubility, flame test, and molecular weight evaluation will be used to determine the identity of an unknown mono- or diprotic carboxylic acid.

You will work as individuals in this experiment. Keep full, legible records of your work, data, and observations in you Laboratory Notebook. You will hand in the copies of your lab work with your typed Analysis/Discussion and Conclusions.

Overview

This experiment is a two week project to determine the identity of an organic acid [R(CO2H)n], where n=1 for monoprotic acids and n=2 for diprotic acids. During the first week, you will be given an “unknown” carboxylic acid. Data collection in this week will focus on titrating your unknown acid with a standardized 0.1 M NaOH solution in order to determine the accurate molecular weight of the acid. Before the second week, using your determined molecular weight and a given nuclear magnetic resonance (NMR) spectrum, you will need to figure out possible Lewis structures for your unknown. After arriving at one or more possible formulas/structures, look up these possibilities in the formula index of an appropriate reference book, such as the Merck Index, Aldrich catalog, and CRC Handbook of Chemistry and Physics (several copies will be available in the lab). When you are looking up structures, remember to consider all possible isomers. For any carboxylic acids that you find as possibilities, record physical properties such as melting point, solubility, etc. in your notebook. If the carboxylic acid formula that you consider can exist in isomeric forms, you will need to distinguish among the isomers by measuring physical properties as well as interpreting the NMR spectrum of your unknown. Before leaving the lab at the end of the first week, consult with your instructor about the molecular weight that you have determined.

The second week of analysis will sort out which of the possible structures that you are considering is the correct one. A melting point determination along with qualitative analyses for halogens and unsaturation (double bonds from alkenes or arenes) are the key pieces of information to be obtained in the second week of the project. If you are still unsure of your accurate molecular weight, you may need to repeat some titrations.

In addition to the data you obtain in the lab, NMR spectra (1H and 13C NMR) of your unknown will be provided during the first week. Interpretation of NMR spectra will be discussed in pre-lab lecture. You have already been introduced to 1H NMR during the Reaction Mechanisms lab earlier this semester.


Lecture Connections

Review the following topics:

-              acid/base reactions

-              neutralization reactions

-              acid/base indicators

-              monoprotic and diprotic acids

 

Pre-lab Assignment

Sample Problems

The following problems use calculations similar to those required in this experiment. Learn how to work these problems, showing your calculations with units.

 

1.) A student titrates a solution containing 0.254 g of a monoprotic acid in 35 mL of water. The student reaches the endpoint after adding 18.35 mL of 0.107 M NaOH. How many moles of the acid are present in the sample? ANSWER: 0.00196 moles of acid.

 

2.) What is the molecular weight of the acid? ANSWER: 129 g/mol.

 

3.) Titration of 10.00 mL of a solution of a diprotic acid requires 14.35 mL of 0.107 M NaOH to reach the endpoint. What is the molarity of the acid solution? ANSWER: 0.0765 M.


Notebook Preparation

In your lab notebook, prepare the following information:

-             your name, the date and a title of the experiment.

-            a brief objective of the lab (2-3 grammatically correct sentences).

-              several “bullet points” summarizing the tasks involved in the procedure.

-            a table of glassware, equipment, and chemicals to be used. Include relevant properties and safety information for each chemical. Use this helpful link for online MSDS http://siri.org/msds/ to find out the hazards associated with substances you use and make in this experiment.

- a list of chemical and mathematical equations that you will using to complete the lab.


Background Information and Procedure: Week 1

Determination of the Molecular Weight of a Carboxylic Acid by Titration

Titration is a common technique used to determine the concentration of a solution. A solution containing a known concentration of a chemical is added to a solution of unknown concentration. The solutions undergo a chemical reaction until the known solution is completely consumed. At this point, equal molar amounts of the known and unknown solutions exist. This point is called the equivalence point, and a change in color of an indicator dye signals the equal molar amounts of unknown and known solutions. From the chemical reaction and its stoichiometry, the concentration of the unknown solution can be calculated.

A very accurately determined concentration, or standardized solution, of NaOH has been prepared for you. The molecular weight of an “unknown” acid can be determined by titrating a solution that contains a known mass of the acid with the standard base solution. The calculation of the molecular weight follows directly, since at the equivalence point in the titration, the moles of base added equals the moles of acidic protons present in the solution.

Procedure

Record the “unknown” acid number in your notebook. Check its solubility in water by adding a small amount of the solid to 1-2 mL of deionized water. If the sample dissolves, water will be the appropriate solvent to use for your titration. If the sample does not dissolve in water, use methanol as the solvent. All of the solid must dissolve before you begin each titration.

Rinse the inside of a clean buret three times with small quantities of the 0.1 M NaOH solution (this is called “rinsing in” with the solution to be used in the buret). Drain the rinses through the stopcock and tip. Don’t forget to rinse liquid through the tip, to replace water there. Fill the buret with the standardized NaOH solution, and record the initial volume. In your notebook, record the actual concentration of the NaOH written on the bottle, it should be approximately 0.1 M.

Note: Do not waste the time it takes to set the starting level of NaOH in the buret to exactly 0.0 mL. It is more efficient and more accurate to set the level between 1 and 2 mL and read the starting level precisely.

Accurately weigh between 250 and 350 mg (to the nearest 0.1 mg) of the “unknown” acid. Record the mass of the sample in your notebook. In a clean 125-mL Erlenmeyer flask, add the acid and 20-30 mL  of the appropriate solvent, and swirl until the solid is completely dissolved. Add two drops of the phenolphthalein indicator to the flask, mix well, and titrate with the standardized NaOH solution until a faint pink end-point persists for 30 seconds (see figure below). Record the final buret volume to 0.01- mL precision. Repeat this procedure until the calculated molecular weights of three successive titrations are within 2% of each other. Your final volume of NaOH added should be no less than 3-4 mL and no more than 12-15 mL. If the volume added is outside of these ranges, either increase or decrease the mass of the unknown, respectively.


Phenolphthalein color, slightly past the endpoint (pink color should be fainter)

Tips on Technique

- to read the buret accurately, hold a white card with a black stripe behind the buret, with the black stripe below the meniscus, and the meniscus itself in front of the white region above the black stripe (see illustration). The meniscus will appear black against the white card. Keep your eye level with the meniscus, and read the buret.

- estimate one more digit than those marked on the scale.

- the buret scale reads increasing volume downward, not upward. For example, this buret reading is 0.55 mL (NOT 1.45 mL).

 - Mix the solution in the titration flask thoroughly after each addition of titrant, to ensure complete reaction before adding more.

- As you near the endpoint, wash the sides of the flask with distilled water to make sure that all delivered titrant is insolution.

- When you see that you are within a drip or two of the endpoint, split drops to avoid overshooting the endpoint. To split drops, open the stopcock very slowly until a drop is suspended from the tip of the buret. Touch the side of the flask to the tip and wash the drop down into the solution with a stream of distilled water from your squeeze bottle. When splitting drops gives you a permanent color change (persists for 30 seconds), you have reached the endpoint. If you are not sure whether or not you have reached the endpoint (equivalence point,), add another split drop.

Go here http://www.youtube.com/watch?v=g8jdCWC10vQ to see a video on how to perform a titration (here’s a more detailed video http://www.youtube.com/watch?v=sFpFCPTDv2w&feature=related and calculations http://www.youtube.com/watch?v=2z4mlE6MK0U&feature=endscreen&NR=1).


Calculations

To calculate the molecular weight of your acid, first determine the moles of NaOH added to reach the equivalence point. At the equivalence point, the moles of OH- equal the moles of acid. The molecular weight is calculated by dividing the grams of acid used by the number of moles of acid. The molecular weight you determine is equal to, or is a fraction of, the true molecular weight of the acid. If the acid is monoprotic, the determined weight equals the molecular weight; if the acid is diprotic, meaning that you are getting two moles of protons for each mole of the acid, the determined weight is half the molecular weight. Keep this in mind when figuring out the formula/structure of your acid.


Disposal – ALL TITRATED SOLUTIONS MAY BE DISCARDED IN THE SINK


Background Information and Procedure: Week 2 

Qualitative Analysis:

As an introduction to qualitative organic analysis and as an aid in the identification of an unknown carboxylic acid, the following “flame test” procedures are suggested as tests for the presence of unsaturation (double bonds - alkenes and arenes) and halogens. Bunsen burners will be set up as your flame source. Samples of carboxylic acids that contain a halogen and unsaturation will be provided so that you can observe what a positive test looks like. Melting point apparatuses will be available to determine the melting point of your unknown.

Procedure

Flame Test: Unsaturation

Place a small portion of your unknown acid on a spatula. Burn the acid in a very hot flame. The emission of a very gray or black smoke (soot) indicates the presence of a double bond.

 

Flame Test: Halogen

Obtain a copper wire loop and heat it with a flame. Dip the hot wire in a little of your unknown that has been placed on a watch glass. Burn the sample in the flame: a green color indicates that a halogen is present. Be careful – it is very easy to obtain a false positive result since many sources of halogen contamination are present in the chemistry lab (simply touching the wire with your fingertips will produce a positive test – and a burn if the wire is hot).

This video http://www.youtube.com/watch?v=WmWUqOFGXsY shows you the color of the flame for  a positive test (it’s a little hard to hear). In general, the green color will not persist for long.


Melting Point Determination: simple capillary melting point

After placing a small amount of sample in a capillary tube, turn on the power to the melting point apparatus and set the heating dial to 4 or 5 (you may need to adjust this depending on the MP of your sample). Look through the “eye piece” and record the melting point range for your unknown acid. The lower end of the melting point range is the temperature when the sample starts to deform; the upper end

of the melting point range is the temperature when the sample has completely melted and looks like a liquid. Since your sample is pure, the melting point range should be fairly narrow. You may need to do several trials until you get the hang of the instrument. Compare this melting point range to the literature values of the possible acids you are considering.

Nuclear Magnetic Resonance Interpretation Resources

A brief introduction to 1H NMR spectra interpretation is available as either a PowerPoint or PDF format.

I have posted five YouTube videos (mp4 format) by Daniel Allwood on the basics of NMR spectroscopy that is an excellent video introduction to NMR spectroscopy.

Experimental Videos

           (a) Preparation for titration of unknown, 4.5 min

            (b) Titration of unknown, trial 1 & 2, 23.8 min

            (c) Titration trial 3, 7.0 min

            (d) Flame Tests, 5.5 min 


After lab – WEEK 1

1.) Determine the molecular weight of your acid unknown before you leave lab.

2.) After obtaining the correct molecular weight from your instructor, calculate the accuracy of your results [(average MW – actual MW)/actual MW x 100%] (note: use the absolute value of average MW- actual MW).

3.) Obtain NMR spectra of your unknown acid from your instructor.

4.) Before next week, figure out possible Lewis structures for your unknown.

After lab – WEEK 2

1.) Use your results from the flame tests to determine if a halogen or unsaturation (alkene or arene) is present in your unknown. Correlate this with your molecular weight.

2.) Use your results from the melting point test to narrow down your choices of carboxylic acids.

 

Post Experiment Analysis

 Include the following information in your analysis/discussion and conclusions to be turned in the week after you have completed Week 2 analysis along with your laboratory notebook pages for this experiment. The analysis/discussion and conclusions should be typed, using grammatically correct sentences, and a copy taped into your notebook. All tables should be labeled and referred to in your discussion. Do not just list the information – incorporate it into your discussion.


1.) A short purpose including experimental techniques used.

2.) Titration results – include mg acid used, concentration of standardized base, volume of base used, and molecular weight of the unknown acid (include average and standard deviation) (calculations should be in your notebook).

3.) Melting point and flame test results.

*4.) Explanations and rationale for Lewis Structure determination from the experimental molecular weight.

*5.) Interpretation of the NMR spectra.

*6.) Interpretation of all experimental results leading to your final carboxylic acid selection. 

7.) Comparison of your results with the known values for your selected carboxylic acid.

8.) References (CRC handbook, Merck Index, online references, etc).

 

*most important to interpret your data.

References

Adapted from: Gettysburg College, General Chemistry laboratory – Identification of an Organic Acid.

 HINTS

- In looking up the physical constants of your unknown, avoid any compounds with the word “ester” in the name. An “ester” is not a carboxylic acid.

- Hydroxyl groups (OH) attached directly to arene rings (benzene rings) should not be considered. Hydroxyl groups of this type are acidic and would react with NaOH in your titrations. Likewise, do not consider compounds that have nitrogen atoms with lone pairs such as amines or amides. A nitrogen atom with a lone pair is basic and would interfere with the titration.

- Don’t choose exotic elements for your unknown. Work with the more common elements. 

- Remember that the arene ring (benzene ring) weighs 78 grams (mono-substituted is 77 grams, di-substituted is 76 grams, etc.).

 Chart of NMR chemical shifts

     (pdf)

  Sample Post Experiment Analysis

     (pdf)

 Grading Rubric 

     (pdf)