ACID-CATALYZED DEHYDRATION OF AN ALCOHOL WITH REARRANGEMENT Experiment 10: ACID-CATALYZED DEHYDRATION OF AN ALCOHOL WITH REARRANGEMENT

Objectives To perform a dehydration of 2-methylcyclohexanol to form isomeric alkenes under E1 conditions. To purify the product using simple distillation. To analyze the product using GC analysis in order to identify and quantify products. To characterize the reactant and products using IR spectroscopy.

Before coming to lab… Please review: Simple distillation E1 elimination reactions GC Analysis

more substituted = lower energy = more stable = more predominate! CHEMICAL EQUATION Three different ISOMERIC alkenes can be produced. Because this reaction is performed near equilibrium conditions, the relative amount of each product reflects its stability. more substituted = lower energy = more stable = more predominate!

RATE LIMITING STEP The rate of elimination of water depends on the stability of the carbocation formed. Formation of the carbocation is the most energetically unfavorable, and therefore the slowest, step in dehydration reactions.

E1 MECHANISM 2o Carbocation rearrangement 3o 1-methyl-1-cyclohexene 2. …which forms a new O-H bond, where oxygen bears a positive charge (oxonium ion). Water is eliminated-forms 2o carbocation. 1. The hydroxyl oxygen attacks and removes a proton from sulfuric acid… 3. Products may form from the 2o carbocation, but it is more likely that the 2o C+ will rearrange to a 3o C+. 4. At the carbocation stage, water will remove a proton from the carbon ADJACENT to the carbocation. The electrons form the pi bond of the alkene. 2o Carbocation rearrangement 3o 1-methyl-1-cyclohexene 1-methyl-1-cyclohexene methylenecyclohexane 3-methyl-1-cyclohexene

Always end up in units of grams of product!!! THEORETICAL YIELD The only reactant is 2-methylcyclohexanol. The H2SO4 is simply a catalyst, since it is regenerated in the end. Theoretical yield is calculated assuming that the major product formed is one that results from the most stable carbocation intermediate. Theoretical yield (g) = # g reactant 1 mol of reactant 1 mol product # g # g 1 mol reactant 1 mol product Always end up in units of grams of product!!! Amount you started with Molecular weight of reactant Stoichiometric ratio Molecular weight of product

OVERVIEW Set up and perform simple distillation to collect products. Obtain final product mass and calculate percent yield. Prepare and submit GC sample for analysis. Pick up GC results and record standard retention times. Identify components in sample chromatogram by comparing to standard chromatogram. Quantify alkenes by calculating adjusted area percent. Characterize reactant and products using provided IR spectra.

EXPERIMENTAL PROCEDURE: (Simple distillation) Clamp flask to ring stand here! Place 2-methylcyclohexanol, sulfuric acid and boiling chips in 50 mL round bottom flask. Clamp flask to ring stand. Weigh 10 mL flask. Clamp to other ring stand. Attach clear hoses to condenser. Run water in at the bottom, out at the top! Build rest of distillation apparatus, using blue Keck clips to secure top and bottom joints around condenser. Blue Keck clips here! Clamp flask to ring stand here! 50 mL 10 mL

EXPERIMENTAL PROCEDURE: (Simple distillation) Begin water flow, and apply heat (VR@30) to boil solution. Record temperature when distillate begins to collect in 10 mL flask (Ti). Collect ~ 5 mL distillate. Record temperature right before you drop the heating mantle (Tf). Allow the solution to cool. Reweigh 10 mL flask to obtain actual product yield. Prepare GC sample and submit! Don’t forget!!! Keck clips!

Table 10.1: Experimental Results Theoretical Yield (g) must calculate the amount of product that can be formed based on the amount of 2-methylcyclohexanol used! Actual Yield (g) This mass will be obtained by weighing the 10 mL round bottom flask before and after the distillation. The difference in the mass is the actual product yield. % yield Actual yield (g) X 100 Theoretical yield (g) Product Appearance physical state and color of distillate.

Table 10.2: GC Analysis Results Compound GC Retention time (min) Area Percent Adjusted Area Percent Standard Sample methanol Never calculate adjusted area % based on the solvent! 2-methylcyclohexanol No need to calculate adjusted area % on the reactant, either! 1-methyl-1-cyclohexene Area % THIS alkene X 100 Sum area% all alkenes 3-methyl-1-cyclohexene methylenecyclohexane

Infrared Spectroscopy (IR) Q: What is it? Vibrational energy of bonds Certain types of polar bonds absorb IR radiation and vibrate (excited state) Q: Why is it useful? Certain functional groups absorb at characteristic frequencies. By looking at what frequencies are absorbed, we can identify the presence or absence of certain types of bonds!

Infrared Spectroscopy (IR) Q: How does it work? This molecule is represented with a potential energy diagram. Each horizontal line represents a vibrational state of a C=O bond. If we add IR light energy at the correct wavelength, we get excitation to the next vibronic energy level.

Infrared Spectroscopy (IR) Q: What is an IR spectrum? % transmittance of IR radiation Frequency of vibration (in wavenumbers)

EXPERIMENTAL PROCEDURE: IR Analysis THINGS TO CONSIDER… What kinds of bonds do I have? Ex. C-O, C=C, CH3, etc. If they appeared in the IR spectrum, where would they be? Use a correlation table to determine the approximate frequency for that type of bond. Now, look at the spectrum. Are they there?

EXPERIMENTAL PROCEDURE: IR Analysis Full IR Absorption Correlation Table in Appendix J Base values for Absorptions of Bonds (cm-1) OH ~3400 C-O ~1100 C-H (sp2) ~3100-3000 C-H (sp3) ~3000-2850 C=C ~1630

Table 10.3: IR Spectral Analysis Results IR spectra are on page 87 in lab manual! Functional Group Base Values (cm-1) 2-methyl- cyclohexanol 1-methyl-1- cyclohexene 3-methyl-1- Methylene- cyclohexane Frequency (cm-1) OH stretch 3200-3500 N/A C-O stretch 1000-1200 sp3 CH stretch 2850-3000 sp2 CH stretch 3000-3100 C=C stretch 1600-1680

Infrared Spectroscopy (IR) (How to answer the questions…) Your goal is to explain clearly how you were able to use IR spectroscopy to DIFFERENTIATE between reactant and product. Always discuss the appearance of certain types of absorptions, or the disappearance of others, which indicate that functional groups have changed. Always answer like this: (fill in the blanks) In the IR spectrum of the product, the appearance of the _____ (type of bond) absorption at _____ (actual frequency) indicates the conversion of the reactant to the product. The typical frequency for this type of absorption is _____ (base value frequency).

SAFETY CONCERNS The alcohol and resulting alkenes are extremely flammable. Be very cautious when applying heat. Concentrated sulfuric acid is VERY CORROSIVE and will burn skin on contact. Please use gloves and goggles at all times when in laboratory.

WASTE MANAGEMENT Place all liquid waste in the container labeled “LIQUID WASTE”. Be careful when disposing of acidic waste remaining in 50 mL round bottom flask! It is extremely corrosive. Use a small amount of water to rinse it into the waste container before cleaning it thoroughly using directions on next slide…

CLEANING After disposing of the liquid waste, clean the 50 mL round bottom flask with soap, water, brush, and a final rinse with wash acetone. All other ground glass joint ware can simply be rinsed with wash acetone into a waste container. Be sure all ground glass joint ware is completely dry before returning to plastic container in fume hood. Be sure all other glassware used is completely dry before returning to lab drawer.

IN LAB QUESTIONS (The following questions should be answered in laboratory notebook.) Predict the products and draw a complete mechanism for their formation from acid-catalyzed dehydration of 2-methyl-2-pentanol. Be sure to show all steps and intermediates. Circle the major product.

IN LAB QUESTIONS (The following questions should be answered in laboratory notebook.) Calculate the theoretical yield for the reaction above based on 4.0 g of the starting alcohol and a catalytic amount of sulfuric acid. The molecular weight of the alcohol is given, but the molecular weight of the product must be determined based on the structure. Be sure to include units.