| Demonstration of Microscale Projection Experiments -
Chemistry en miniature Dehalogenation of 1,2-Dibromopropane Objective: Elimination - E2 Mechanism Peter Keusch |
Safety precautions:
Safety glasses, protective gloves, good ventilation. The solution of KI in DMF is prepared in a fume hood! Experimental procedure: Two test tubes (T1, T2) are each filled with 1 mL of KI / DMF solution. 1 mL of starch solution is pipetted into test tube T3. When 10 drops of 1,2-dibromopropane are added to T2 the KI / DMF solution turns yellowish. Within one minute the color becomes more intense (Photo 1). Finally, two drops of the yellow orange solution in T2 are added to the starch solution in T3. Results: Intensely blue colored 'schlieren' are formed in T3 (Photo 2).
Discussion: · Iodide ions induce the elimination of bromine from 1.2-dibromopropane. Iodide is oxidized to iodine (1). Iodine reacts with starch to form a blue complex. 
|  · An elimination reaction is a type of organic reaction in which two substituents (atoms, atom groups) are removed from a molecule in either a one or two-step mechanism. The reaction involves the formation of a double bond. The major product is the one in which the maximum number of substituents is attached to the doubly bonded carbons (Zaitsev's rule). There are two types of elimination reactions: E1 and E2. The difference between the two is that an E2 reaction proceeds by second order kinetics and involves a single step. Bond making and bond breaking occur simultaneously in a concerted mechanism. The E1 elimination on the other hand is not concerted and proceeds by first order kinetics. It occurs by formation of a carbocation which successively eliminates the leaving group. Important classes of eliminations are dehalogenation and dehydrohalogenation reactions. They involve alkyl halides with good leaving groups, reacting with a Lewis base. The reaction of 1.2-alkyl dihalides with iodide is second order: R = k [ I - ] [ alkyl dihalide ]. E2 reactions are mostly , as in the present case, trans eliminations. The following mechanism (2) is in agreement with both the reaction rate relation and the stereochemistry.  General experimental instructions and index of experiments 
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