Organic Chemistry Lab Report #7 PDF

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Chemistry 2612 Organic Chemistry Lab

Lab Report #7

SN1 Reaction: Synthesis of 2-Chloro-2-Methylbutane from 2-Methyl-2Butanol

04-10-17

Introduction The purpose of this experiment is to synthesize 2-Chloro-2-Methylbutane from 2-Methyl2-Butanol and HCl via SN1 reaction mechanism. SN1 is a substitution reaction mechanism that exhibits 1st order kinetics. The rate for an SN1 is rate = k[E+]. This means that the rate determining step is unimolecular where the rate equation is shown having 1st order dependence on the electrophile [E+] and zero-order dependence on the nucleophile. SN1 is a stepwise process which may include two or more transition states and formation of a carbocation intermediate that is sp2 hybridized trigonal planar. This allows two different avenues of approach for the nucleophilic attack in the last step of the overall mechanism. In the case in our experiment the nucleophile can attack from the top or the bottom side forming equal amounts of racemization and two +enantiomers, because neither avenue is preferentially favored. The formation of the carbocation intermediate is the rate determining step for SN1. SN1 mechanism favors tertiary carbons the most meaning the carbon that the leaving group is attached to is also attached to three other groups. Secondary carbons also undergo SN1, but also can for E1 products as well. Primary & 1° carbons do not undergo SN1 mechanism. During this experiment the reaction mechanism occurs in three steps. First is the protonation of the OH group on 2-Methyl-2-Butanol. This occurs when the hydrogen from HCl is cleaved off due to the electronegativity of the Chlorine atom within HCl. The protonation of the OH group forms H2O+. The second step consists of the leaving group leaving. In this case the leaving group is H2O+, and is cleaved off of the carbon chain which now forms the carbocation intermediate that is sp2 hybridized trigonal planar. This is the rate determining step within the overall reaction. The third step is a nucleophilic attack from Cl- forming 2-Chloro-2Methylbutane.

Reaction Mechanism Step 1: Protonation

Step 2: Leaving group leaves, Formation of Carbocation Intermediate **Rate determining step

Step 3: Nucleophilic attack by Cl-

Properties & Safety of Chemicals Compound

Boiling Point (°C)

D (g/mL)

2-Chloro-2Methylbutane

85- 86

0.86

Hazards

-

2-Methyl-2Butanol

102

Hydrochloric Acid

108.58

0.805

1.2

-

Irritant of skin, eye, ingestion & inhalation Flammable

-

Irritant of skin, eye, ingestion & inhalation Flammable

-

Irritant corrosive

Experimental Procedure 10 mL of 2-Methyl-2-Butanol and 25 mL of HCl were added in a separatory funnel, then shaken and vented for 10 minutes. The product was on the top layer. The product was extracted and washed by the following steps. The bottom layer was drained and then 5-10 mL of cold (NaHCO3) was added and shaken and vented. The bottom layer was drained again and then 10 mL of NaCl was added and shaken and vented. The bottom layer was drained again and the top layer was poured into a clean beaker and dried with anhydrous sodium sulfate. The product was transferred to a vial and weight of product was obtained for % yield. An IR was ran and a silver nitrate & sodium iodide test was also ran to confirm the identity of the product as well as reaction mechanism.

Data & Observations 2-Chloro-2-methylbutane: 1.065g

Calculations % Yield = (

actual ) *100 theoretical

Mols 2-Methyl-2-Butanol = (10 mL)(.805g/mL) = (8.05g)(1mol/88.15g) = 0.091 mols Mols HCl = (25mL)(1.2g/mL) = (30g)(1mol/36.5g) = 0.822 mols Mols 2-Chloro-2-Methylbutane = (1.065g)(1mol/106.56g) = 0.00999 mols % Yield =

(

0.00999mol ) *100 = 10.98 % 0.091mol

Conclusion Mixing 2-Methyl-2-Butanol with HCl in the separatory funnel initiated the SN 1 reaction mechanism by the protonation of the OH group forming a good leaving group. The H2O is then cleaved off forming the carbocation intermediate. The positively charged sp2 trigonal planar carbocation is attacked by the negatively charged nucleophile Cl- in the last step of the reaction mechanism forming 2-Chloro-2-Methylbutane as the product. A silver nitrate and sodium iodide confirmatory iodide tests were done on the product. The sodium iodide test was negative and the silver nitrate formed a white precipitate confirming the reaction mechanism was in fact an SN 1 mechanism. In comparing the IR’s of 2-Chloro-2-Methylbutane and 2-Methyl-2-Butanol; 2-Methyl-2Butanol contains the OH peak at 3369.02 cm-1 and 2-Chloro-2-Methylbutane does not. This occurs, because during the 1st step of the reaction mechanism the OH is protonated forming a good leaving group which is H2O, and then is cleaved off in the second stop forming the

carbocation intermediate. So, in the product the OH has been substituted for the nucleophile which is Cl-. The IR of 2-Chloro-2-Methylbutane also has sharper and higher peaks which indicate the presence of an electro negative atom such as Chlorine. Discussion The product yielded 10.98%. Considering this is a low percent yield, there were some human errors that occurred during the process of synthesis. Some errors include spilling of chemicals and error in measuring the solvents correctly. In addition, my TA added some water to the product, hence, the low percent yield of the product....