Nucleophilic Substitution Synthesis of 1-Iodobutane.

Transcription

1 Nucleophilic Substitution Synthesis of 1-Iodobutane

2 Nucleophilic Substitution of Alkyl alides: Synthesis of 1-Iodobutane C 4 9 Br MW d 1.28 g/ml BP o C Br + NaI I + NaBr 1-Bromobutane is a primary alkyl halide (not sterically hindered). Acetone is a polar aprotic solvent. It poorly solvates the iodide nucleophile, promoting the S N 2 mechanism. Despite having an unfavourable equilibrium (the products are less stable than the starting materials), the reaction still proceeds essentially to completion. Why is this? O MW C 4 9 I MW d 1.62 g/ml BP o C MW sodium bromide is insoluble in acetone

3 eaction Work-Up Filter the reaction mixture to remove the precipitated sodium bromide. Wash with water to remove remaining traces of sodium iodide. The primary purpose, however, is to remove the bulk of the acetone solvent from the reaction mixture. The lower layer is the organic layer (containing our product). Wash with aqueous sodium bisulfite (NaSO 3 ) to remove any colouration due to a small amount of iodine that is often formed as a by-product in reactions of this type: I 2 + SO O 2I + SO O + Dry the organic product by standing over sodium sulfate (Na 2 SO 4 ).

4 Product Purification The 1-iodobutane product is purified by distillation. Before performing the distillation, you should pre-calculate the expected boiling point of the 1-iodobutane product: B.P. 760mmg = B.P. observed 0.05 (A.P. mmg 760) You know the literature boiling point (for 760 mmg) You know the current atmospheric pressure You can thus predict the boiling point in today s lab (at the current atmospheric pressure)

5 Qualitative Tests Perform these test-tube tests once reflux is underway! NaI in Acetone test for S N 2 reactivity Iodide ion (I ) is a good nucleophile and a poor base. Acetone is a polar aprotic solvent. A positive result is indicated by formation of a white precipitate, since NaBr and Na are insoluble in acetone. AgNO 3 in Ethanol test for S N 1 reactivity Nitrate ion (NO 3 ) is a poor nucleophile and a poor base. Ethanol is a polar protic solvent. Silver ion (Ag + ) promotes cation formation (and thus S N 1 reactivity): Ag Ag + Ag A positive result is indicated by formation of Ag precipitate

6 Qualitative Tests Substrate structure is very important in determining the mechanism of reactivity. You ll test a range of substrate structural types. Br 1-bromobutane 1-chlorobutane 2-chlorobutane 2-chloro-2- methylpropane benzyl chloride chlorobenzene 1 o 2 o 3 o benzylic aryl Note that the only difference between 1-bromobutane and 1-chlorobutane is the identity of the leaving group. We can compare these two substrates to determine whether bromide or chloride is a better leaving group.

7 Qualitative Tests Note that the only difference between 1-bromobutane and 1-chlorobutane is the identity of the leaving group. We can compare these two substrates to determine whether bromide or chloride is a better leaving group. Br 1-bromobutane 1-chlorobutane The observed reactivity (S N 1 and S N 2) sequence for halides is: 1 o I > Br > > F C- bond strength as well as the stability of the ion control this order.

8 Qualitative Tests Owing to steric effects, the order of reactivity in S N 2 chemistry is: 3 C > C > C > C ~ ~ aryl and vinyl substrates do not readily undergo substitution In S N 1 chemistry, the order of reactivity reflects the ease of cation formation: C > C ~ > C > 3 C ~ ~ aryl and vinyl substrates do not readily undergo substitution

9 Tips and eminders Today s experiment will be conducted in pairs Note that most alkyl halides, particularly iodides, are very toxic and are cancer suspect agents During the work-up procedure, the precipitate may be washed with an additional (small) quantity of fresh acetone in order to maximize the percent yield During liquid-liquid extraction, the lower layer is the organic layer (halogenated compounds are almost always more dense than water) Before performing the distillation, pre-calculate the expected boiling point Atmospheric pressure today: Don t forget that a lab report is required for this experiment!