Synthesize t-butyl (or t-pentyl) Chloride

Transcription

1 Synthesize t-butyl (or t-pentyl) Chloride Note: This experiment may utilize either t-butyl Alcohol (m.p o C) or t-pentyl Alcohol (m.p o C) as one of the starting reactants Text References Slayden - pp Pavia - Exp # 21 - pp Pavia - Tech 12 - pp /17/2015 1

2 Today s Experiment Reaction of t-butyl Alcohol (or t-pentyl Alcohol) with conc. HCL to form t-butyl Chloride (or t-pentyl Chloride) Three-step Sn1 Nucleophilic Substitution Reaction This is a First Order Rate Reaction where the Rate of Formation of t-butyl Chloride (t-pentyl Chloride) is dependent only on the concentration of the Alcohol, i.e., it is independent of the amount of acid (HCL) used The strong acid (HCL) protonates the electron rich hydroxyl group (nucleophile) allowing it to leave as a molecule of water This leaves a highly electrophilic carbon atom (positively charged carbocation) that can be attacked by the negatively charged chloride anion, forming the final product 1/17/2015 2

3 Today s Experiment (Con t) NOTE: Rate of Formation and Limiting Reagent are independent of each other. Thus, the Limiting Reagent must be computed Determine the limiting reagent and theoretical yield from the masses & moles of the two reactants (t-butyl or t-pentyl Alcohol & Conc HCl) and the stoichiometric molar ratios This experiment will require the student to separate and wash two immiscible liquids using a separatory funnel (liquid/liquid Extraction) In this experiment, Liquid/Liquid Extraction generates gases requiring care in using the separatory funnel and its stopcock 1/17/2015 3

4 t-butyl Alcohol (2-Methyl-2-Propanol) B.P o C M.P o C Density g/ml Refractive Index Mol Wgt g/mole Water Solubility - Soluble t-butyl Chloride (2-Chloro-2-Methyl Propane) ( ) B.P o C M.P o C Density g/ml Refractive Index Mol Wgt g/mole Water Solubility - Sl Soluble Conc HCl Molecular Wgt g/mole Molarity moles/l Density g/ml % Acid /17/2015 4

5 t-pentyl Alcohol (2-Methyl-2-Butanol B.P o C M.P o C Density g/ml Refractive Index Mol Wgt g/mole Water Solubility - Soluble t-pentyl Chloride (2-Chloro-2-Methyl Butane) ( ) B.P o C M.P o C Density g/ml Refractive Index Mol Wgt g/mole Water Solubility - Slightly Sol Conc HCl Molecular Wgt g/mole Molarity moles/l Density g/ml % Acid /17/2015 5

6 Stoichiometric Reaction The Mechanism 1/17/2015 6

7 The Stoichiometric tert-pentyl Reaction The Mechanism 1/17/2015 7

8 Limiting Reagent Calculations The yield (mass or moles) of the washed and dried t-butyl (t-pentyl) Chloride product is compared to the theoretical amount of product expected, which is computed from a Limiting Reagent calculation using the Stoichiometric Molar Ratio The Limiting Reagent is that reactant whose mass (on a molar equivalent basis) is totally consumed in the reaction leaving an excess of the other reactant The Limiting Reagent, thus, determines the maximum amount of product that can be expected 1/17/2015 8

9 Limiting Reagent Calculations (Con t) Limiting Reagent Steps Determine the mass of the alcohol to the nearest gram. Measure the volume of conc HCL solution to be used to the nearest 0.1 ml Compute the mass of the HCL from the volume, density and % composition (see table) From the amounts (mass) of reactants used, calculate the number of moles of each: moles = mass / mol wgt Moles of concentrated HCl can also be computed directly from its Volume and Molarity Conc HCl 12 moles / Liter If this approach is used, then back calculate the mass of HCL from the moles 1/17/2015 9

10 From the balanced stoichiometric equation determine the molar ratio among the reactants and products i.e., how many moles of Alcohol react with how many moles of HCL to give how many moles of t-butyl (t-pentyl) Chloride. The ratio here is 1:1 If the ratio of moles of Alcohol to moles of HCl actually used is greater than the stoichiometric molar ratio, then the Alcohol is in Excess and HCl is Limiting If, however, the ratio of actual moles of Alcohol to moles of HCl is less than the reaction molar ratio, then HCl is in excess and t-butyl (t-pentyl) Alcohol is Limiting 1/17/

11 Examples A + B C Molar ratio A:B = 1 : 1 = 1.0 Moles actually used: A = 0.05; B = 0.12 Molar ratio A:B actually used: 0.05 / 0.12 = 0.42 The ratio of A:B is less than 1.00; thus A is limiting Only 0.05 moles of the 0.12 moles of B would be required to react with the 0.05 moles of A available Since 0.05 < 0.12; then B is in excess, A is limiting 1/17/

12 Examples (Con t) A + 2B C Molar ratio A:B = 1 : 2 = 0.5 Moles actually used A = ; B = Molar ratio A:B actually used = / = 0.30 The ratio A:B is less than 0.5, thus, A is limiting Only = moles of B are required to react with moles of A. Since < 0.023: B is in excess, A is limiting. Any actual molar ratio less than the reaction molar ratio indicates B is in Excess and A is Limiting. Any actual molar ratio greater than the reaction molar ratio indicates A is in Excess and B is Limiting 1/17/

13 Examples (Con t) In the Friedel-Crafts alkylation of Biphenyl with t-butyl Chloride to form 4,4 -Di-tert-Butyl Biphenyl, g of Biphenyl is reacted with g of t-butyl Chloride. The stoichiometric equation indicates that 2 moles of t-butyl Chloride react with 1 mole of Biphenyl Determine the Limiting Reagent and the Theoretical Yield Amounts Actually Used Maximum Amounts Expected 1 Biphenyl + 2 t-butyl Chloride 1 4,4 di-tert-butylbiphenyl + 2 HCl Mol Wgt Mass (g) Moles In the above example, Biphenyl is the limiting reagent because moles is less than / 2 = moles. Thus, a maximum of moles (1.838 g) 4,4 ditert-butyl Biphenyl can be expected 1/17/

14 Theoretical Yield The limiting reagent sets the maximum amount of product that can be expected The actual number of moles of product is the product of the moles of Limiting reagent and the molar ratio of product to Limiting reagent To get the mass of product simply multiply the expected moles of product by the molar ratio of limiting reagent to product (1:1) and the molecular weight of the product Mass = Moles limiting Reagent X molar ratio X Mol Wgt Product 1/17/

15 Macro Scale Procedure: Obtain vial of t-butyl (or t-pentyl) Alcohol from instructor s desk Note: Melting point of t-butyl Alcohol is near room temperature and could be solid if lab is cold. Warm vial with hands to melt Determine the mass of the Alcohol Obtain 25 ml, measured to nearest 0.1 ml, of concentrated HCl from the hood Determine the mass of acid from its volume, density, and % composition Using a ring stand, setup an iron ring and clay triangle (if necessary) to support a 125 ml separatory funnel 1/17/

16 Macro Scale Procedure (con t): Using about 20 ml of distilled water, check the integrity of the stopcock to insure it does not leak while vigorously shaking the funnel Drain the water from the funnel Transfer the alcohol to the Separatory Funnel using a long stem glass funnel Add the acid to the alcohol in the separatory funnel Stopper the funnel, firmly holding the stopper with your finger, and gently swirl the mixture for approximately one (1) minute Invert the funnel and slowly open the stopcock to vent pressure 1/17/

17 Macro Scale Procedure (con t) Close the stopcock and swirl the mixture again, repeating the swirling and gas release process Repeat this process for 3-4 times until no more gas is released Two layers will have formed in the funnel Note: Based on the densities of the organic layer and the aqueous layer (H 2 0, HCl, etc.) determine which layer is on top Drain the aqueous layer into a large waste beaker In the following steps the organic layer will be extracted once with Water, two (2) times with Sodium Bicarbonate (NaHCO 3 ), and again with water 1/17/

18 Macro Scale Procedure (con t) The Extraction procedure must be done in an expeditious manner as t-butyl (t-pentyl) Chloride is unstable in Water and Sodium Bicarbonate Add 10 ml of Distilled Water to the crude product in the separatory funnel Swirl the mixture for 30 seconds Carefully invert the funnel and slowly open the stopcock to release any excess gas Close the stopcock and repeat the mixing/venting process for about 60 seconds Drain the aqueous phase into the waste beaker Retain the organic phase (top layer) in the separatory funnel 1/17/

19 Macro Scale Procedure (con t) Add 10 ml of 5% aqueous Sodium Bicarbonate (NaHCO 3 ) to the funnel containing the organic layer Note: The Sodium Bicarbonate reacts with any aqueous acid (HCL) in the organic layer releasing Carbon Dioxide gas This gas can be under considerable pressure and care must be taken to secure the glass stopper with your finger and very slowly open the stopcock to release the gas Repeat the mixing and venting process several times until gas is no longer being vented Allow the layers to separate and drain the aqueous layer again into the waste beaker 1/17/

20 Macro Scale Procedure (con t) Repeat the washing process with a second 10 ml portion of 5% NaHCO 3 Wash the organic layer again with 10 ml Distilled Water After removing the aqueous layer to the waste beaker, drain the organic layer into a 100 ml, clean, dry beaker With instructors help, add Anhydrous Sodium Sulfate to the crude product, swirling the mixture until it is clear Note: See p in Pavia for techniques on determining dryness of sample 1/17/

21 Macro Scale Procedure (con t) Transfer (by decanting) the clear product into a clean, dry, pre-weighed 50 ml Erlenmeyer flask Weigh the flask and contents Determine the mass of product by difference Calculate the percentage yield Determine the Refractive Index; Correct for Temperature Obtain IR Spectrum 1/17/

22 Semi-Micro Scale Procedure (Do not use this procedure unless specifically instructed to do so by Instructor) Obtain vial of t-butyl (t-pentyl) Alcohol ( 4 ml) from instructor s desk Note: Melting point of t-butyl Alcohol is near room temperature and could be solid if lab is cold. Warm vial with hands to melt Weigh the vial and contents to nearest g; record in notebook Transfer sample to Centrifuge Tube using a long stem glass funnel Reweigh the empty vial Calculate Mass of t-butyl Alcohol Calculate Moles of t-butyl Alcohol 1/17/

23 Semi-Micro Scale Procedure (Con t): Add 8 ml, measured to nearest 0.1 ml, of concentrated HCl to the Centrifuge tube In your report calculate Mass of HCL from the Volume, Density, % composition Note: This calculation is different from Alcohol mass, therefore, it is a separate procedure Compute Moles of HCl Note: As an alternative, the Moles of HCl can be computed directly from the Volume and the Molarity of Conc. HCl In the report, setup the Stoichiometric balanced equation Determine the Limiting Reagent 1/17/

24 Semi-Micro Scale Procedure (Con t): Calculate the Theoretical Yield Note: Each computation in the Limiting Reagent/ Theoretical Yield determination must be set up and all calculations shown Screw the sealing cap onto the Centrifuge Tube and shake the tube gently for about 10 minutes. Be sure to unscrew the cap carefully every minute or so to vent any gases that may form Two layers will form in the funnel Note: Based on the densities of the organic layer (t-butyl Chloride) and the aqueous layer (H 2 0, HCl, etc.) determine which layer is on top Remove the Aqueous layer using a Pasteur Pipet Place the aqueous waste in a waste beaker 1/17/

25 Semi-Micro Scale Procedure (Con t): The Extraction procedure that follows must be done in an expeditious manner as t-butyl Chloride is unstable in Water and Sodium Bicarbonate Extract (wash) the organic product, once with 10 ml Distilled Water, twice with 10 ml 5% Sodium Bicarbonate (NaHCO 3 ) and once again with water Be sure to vent gases carefully, especially with NaHCO 3 Note: This is one Procedure Each time, remove the Aqueous layer using a Pasteur Pipet Place the aqueous waste in the waste beaker 1/17/

26 Semi-Micro Scale Procedure (Con t): Add Anhydrous Sodium Sulfate to the crude product, swirling the mixture until it is clear Note: See p in Pavia for techniques on determining dryness of sample Decant the clear material into clean, dry, pre-weighed Erlenmeyer Flask Weigh the flask and contents Compute mass of product by difference Compute the % yield Determine the Refractive Index; Correct for Temperature Obtain IR Spectrum 1/17/

27 The Report The Purpose should reflect the type of reaction and principle reactants involved. It should also reflect introduction of any new techniques that you are to become familiar The Approach is a sequential step by step overview of the principle procedures to be used The first 6 procedure should reflect the theoretical yield calculations, such as mass, moles, molar ration, limiting reagent, and theoretical yield determinations It should also reflect how the results will be quantified, such as yield and percent yield The Procedures should be stated in the student s own words, using short, concise statements in paragraph form 1/17/

28 The Report (Con t) In the Summary section summarize the Results, i.e. an overview in paragraph form of the experimental results obtained In the Conclusion section consider the following questions: What was the Molar ratio of HCl to t-butyl Alcohol and what was the impact of this ratio on the selection of the Limiting Reagent and the amount of product expected? Provide details of the Refractive Index and the IR Spectrum analysis using the arguments to confirm the product obtained was indeed t-pentyl Chloride 1/17/