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Transcript
Table of Physical Properties for Investigation of SN1 and SN2 Reactions M.W. M.P. B.P. DEN. SOL. HAZARD 1-butanol 74.12 -89.5 117.6 0.81 Ac, Bz,EtOH,Et2O 1 2-butanol 74.12 -114.7 99.5 0.808 W,Al,eth 1 t-butylalcohol 74.12 25.6 82.41 0.781 W,Al t-amylalcohol 88.15 -12 102 0.805 W,Al 1 Benzylalcohol 108.14 -15.19 204.7 1.042 W,eth Al,eth,petr eth 2 Phenol 99.11 40.85 182 1.071 W,Bz,Al,Chlor,eth,petr 2 1-chlorobutane 92.57 -123 78.5 0.8865 Al,eth 2 2-chlorobutane 92.57 -140 68 0.871 W,Al,eth 2 t-amylchloride 106.59 -74 83 0.865 W,Al,eth 2 3 0 3 0 3 0 1 3 0 1 0 3 0 3 0 3 0 3 0 3 t-butylchloride 92.57 -26.5 51.0 0.847 W,Al,eth 2 Benzylchloride 126.59 -43 179 1.10 Al, chl,eth 2 Chlorobenzene -45 131 1.107 Al,Bz,Chl,eth 2 1-bromobutane 137.02 -112 101.3 1.269 Al, Eth 2 2-bromobutane 137.02 -112 91.2 1.253 Al,eth 2 t-butylbromide -16.3 73.3 1.2125 Org.solvent Benzylbromide 171.04 -3.9 198 1.4380 Et2O,eth Bromobenzene -31 156 1.491 Chl,Bz,Al,eth,petr carbons 0 2 0 3 112.56 0 3 0 3 0 3 137.02 0 2 3 157.01 0 3 2 2 0 Investigation of SN1 and SN2 Reactions If a positive result does not occur in 1 min, continue timing but start a test on a new compound (i. e. do experiments in parallel). PART A: REACTIVITY OF BROMIDES TOWARD SODIUM IODIDE. Sodium Iodide in acetone. In this part of the experiment you will test the reactivity of several alkyl halides under SN2 conditions. In acetone solution, other alkyl halides can be converted to alkyl iodides. Although we might expect such a reaction to be reversible, it can be driven forward by using anhydrous acetone as the solvent. Sodium iodide is soluble in this solvent, whereas sodium chloride and sodium bromide are not. If a reaction occurs, a precipitate of sodium bromide or sodium chloride will form. Directions: Use scrupulously dry test tubes for this experiment. Add 2 drops of the liquid to be tested to about 2 mL of a 15% solution of sodium iodide in anhydrous acetone. Mix the contents and note the time required for a precipitate to form up to 15 minutes. Test each of the following organic bromides: 1-bromobutane, 2-bromobutane, t-butyl bromide, benzyl bromide, and bromobenzene. Report your results in tabular form. I- Br I * Br I Br- PART B: REACTION OF ALCOHOLS WITH LUCAS REAGENT Lucas's Test: Of the various methods available for preparing alkyl halides, the most common is replacement of the hydroxyl group of an alcohol. The ease with which this can be done depends on the structure of the alkyl group and on the mechanism of the reaction. In this experiment you will test the relative ease of conversions of several alcohols into alkyl chlorides with Lucas reagent (HCI + ZnCl2). The reaction may occur by one of two mechanisms, SN1 or SN2, depending on the structure of the R group. In either mechanism, the first step is a rapid protonation of the alcohol to form an oxonium ion. What happens next depends on the nature of R. If R is a group that readily forms a carbocation, then the slow, rate-determining step will be the loss of a water molecule from the oxonium ion. The carbocation then reacts rapidly with a halide ion to form the alkyl halide. This mechanism, called SN1 ZnCl2 (substitution, nuceleophilic, H2O R + ZnCl3 HO R + H Cl unimolecular in the rate-determining step), is followed when R is a tertiary H2O R H2O + R alkyl group and may also be involved when R is a secondary group or when R can form a resonance-stabilized ZnCl3- + R Cl R + ZnCl2 carbocation, such as an allyl or benzyl ion. If R is a primary alkyl group, then substitution occurs by an SN2 mechanism and is usually not observed with Lucas reagent at room temperature, but may be seen if the solution is heated. Directions: Place about 2 mL of Lucas reagent in each of five test tubes (Note: Lucas reagent is a contact and vapor hazard. Avoid contact and do not breathe it’s vapors). Test each alcohol by adding about 5 drops of the alcohol to the reagent. “Thump” the tube and note the length of time it takes for the mixture to become cloudy or separate into two layers (a positive test). Test 1-butanol, 2-butanol, t-butyl alcohol, t-amyl alcohol, benzyl alcohol, and phenol. If no reaction is observed after one hour, warm the test tube in bath of boiling water for a few minutes and allow to cool to room temperature. Report the results in tabular form. PART C: REACTIVITY OF HALIDES TOWARD SILVER NITRATE. Silver nitrate in ethanol. In this part of the experiment you will test the reactivity of several alkyl halides under SN1 reaction conditions. Organic halides may react with ethanol to form ethyl ethers. If the ethanol contains silver ion, the reaction is speeded up because the silver ion acts as an electrophile toward the halogen and helps to break the carbon-halogen bond. Alkyl chlorides yield a discernible silver chloride precipitate, which is insoluble in ethanol and thus provides a signal that a reaction has occurred. C Cl + AgNO3 R + AgCl(s) + NO3 R R + HOCH2CH3 R C OCH2CH3 R H C OCH2CH3 C OCH2CH3 + H H Directions: Add 2 drops of the organic chloride to be tested to 2 mL of 2% ethanolic silver nitrate solution (Avoid skin contact with the silver nitrate solution, as it can form a dark, hard to remove stain.), “Thump” to mix and record the time required for a silver chloride precipitate to form. Test each of the following chlorides: 1-chlorobutane, 2-chlorobutane, t-butyl chloride, t-amyl chloride, benzyl chloride, and chlorobenzene. Report your results in tabular form. Use the following type of table for reporting your observations in parts A, B, and C. R group Benzyl n-butyl 2-butyl t-butyl t-amyl phenyl OBSERVED REACTION TIMES Time for reaction to be observed HCl/ZnCl2/water AgNO3/ethanol NaI/acetone 1. Arrange the alcohols tested in order of decreasing experimental reactivity toward Lucas reagent. Consider the aliphatic alcohols first and then consider the aromatic compounds separately. What structural change correlates with reactivity? Assuming this to be an SN1 reaction (see scheme in part B), and that the reaction is faster for more stable cation intermediates, indicate the order of stability of all the carbon cations formed from each alcohol. Examine the cations and explain what makes some cations more stable than others. Hint: Draw structures of all carbon cations. 2. Arrange the chlorides tested in order of decreasing experimental reactivity toward silver ion. Consider the aromatic compounds separately. Assuming this to be an SN1 reaction, how do these results compare with that observed in the Lucas test. 3. Arrange the bromides tested in order of decreasing experimental reactivity toward iodide ion. Consider the aromatic compounds separately. Does the reactivity follow the same order as before? What structural change correlates with reactivity? Assuming this to be an SN2 reaction (see scheme in Part A), the lower the energy of the transition state the faster the reaction. Draw the transition state for each compound and suggest a reason for the observed order of reactivity. Dr. Mohammad A. R. Ismaiel / College of Science for Women Babylon University