Dehydrohalogenation of an Alkyl Halide

Transcription

1 Dehydrohalogenation of an Alkyl alide Preparation and Characterization of an Alkene Mixture DID SOMEONE SAY VICTORY? OW ABOUT TAT GAME???? 1

2 Dehydrohalogenation Alkenes are most often prepared by dehydration of alcohols or by dehydrohalogenation of alkyl halides O -Z E1 X :Y E2 Dehydrohalogenation of The alkyl halide we are using today is a 3 o alkyl chloride, so S N 2 substitution is very unlikely. 3 O X: Cl S N 2 2

3 Dehydrohalogenation of In an E2 elimination, the transition state requires the C- bond to be anti-periplanar (180 o dihedral angle) to the C-X bond. X :Y E2 + Y + X: Reaction is faster when X is a better leaving group (I > Br > Cl ) Dehydrohalogenation of In 3-chloro-3,7-dimethyloctane, we have 3 different sets of C- bond that can react in the E2 reaction. b b c c a a a Cl 3

4 Dehydrohalogenation of Elimination involving any of the three a protons will give the same product. b b c c a Cl C 2 a a 2-ethyl-6-methylheptane Dehydrohalogenation of owever, elimination of an b proton can give either of two diastereomeric products. b b c c (E)-3,7-dimethyloct-2-ene a Cl a a (Z)-3,7-dimethyloct-2-ene 4

5 Dehydrohalogenation of b2 C 3 Cl C 2 R Y: E2 b1 C 3 C 3 (Z) C 2 R C 3 + Y + Cl Y: 3 C b1 b2 Cl C 3 C 2 R E2 3 C (E) C 2 R C 3 + Y + Cl Dehydrohalogenation of Likewise, elimination of one of the c protons can give two different diastereomeric products. b b c c (E)-3,7-dimethyloct-3-ene a Cl a a (Z)-3,7-dimethyloct-3-ene 5

6 Procedure Weigh 1.32g of your stored 3- chloro-3,7-dimethyloctane into a micro round-bottomed flask. Add 6mL of 95% ethanol, 1.0 ml of 45% KO, and 3-4 boiling chips. Connect a Claisen head and air condenser. Be sure to grease all joints! Procedure Use the heating mantle to heat the mixture vigorously at reflux for 20 min. Note: Reflux means to heat so that the vapors formed condense and return to be heated again. 6

7 Procedure You need to start counting toward 20 minutes ONLY after the solution is refluxing vigorously. NOT BEFORE. Use heat shield (porcelain) on your heating mantles. Turn the heat up to 35 on the Variacs to start reflux and reduce to about 30 after reflux starts. EXTRACTION - IMPORTANT Reread the section on pages 67 and 68 in the lab manual about extraction in test tubes. In your extractions today, the organic layer (the layer to save) will ALWAYS be the upper layer. Do NOT use a lower layer as your sample layer. If you use an aqueous layer as your product layer, you will have NO alkene and your qualitative tests will all be negative. 7

8 EXTRACTION After the 20 minutes of reflux, pour the mixture into a test tube containing 15 ml of water. Let the layers separate, and transfer all of the upper organic layer to another test tube containing 15 ml of water. Mix and let the layers separate. Procedure (Cont.) EMULSION! If the layers don t separate well, obtain 1 2 ml of concentrated sodium chloride solution from your TA, add to the tube, and shake again. Remove the lower aqueous layer and wash the organic layer 3 times with 3-mL portions of water. Remove as much water as possible in the last wash. 8

9 Procedure (Cont.) Transfer the organic product to a sample vial and add 0.5 g of calcium chloride to dry the product. There must be a single layer with no visible water droplets. IMPORTANT NOTE: Do NOT leave the cap off of the CaCl 2 bottle! Keep it capped; it is very hygroscopic! Procedure (Cont.) Filter the product through a small plug of cotton in a pipet into a preweighed vial. Weigh the vial to obtain a yield. Perform a bromine test and a permanganate test on small amounts of the product. 9

10 PROCEDURES FOR ANALYSIS You MUST perform the qualitative tests for alkenes and take an IR spectrum BEFORE you will be allowed to do a GC analysis on your product! You will be allowed to do a GC analysis ONLY IF your qualitative tests are positive and your IR does not show excessive O peaks. If the tests are negative you have NO alkene to analyze. Your IR will likely confirm the lack of organic peaks and presence of water peaks. SAFETY Be very careful with the ethanolic solution of KO. Allow glassware to cool before disassembling. Potassium permanganate, bromine and bromine vapor are hazardous. 10