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Q UALITATIVE O RGANIC A NALYSIS Introduction Organic molecules have numerous types of functional groups. Qualitative test are used to help identify various functional groups. These “wet” chemical tests cannot determine the complete structure of a compound, only the functional groups. Not one of these tests proves the functional group alone, for example, 2,4-dinitrophenylhydrazine gives a positive test for both ketones and aldehydes. Results from the qualitative tests can be combined with other data such as melting point, boiling point, IR, UV, NMR, GC-MS, HPLC, X-ray crystallography, etc…, in an attempt to identify an unknown organic compound The functional groups that this experiment will identify are shown below: X R R R Alkyl halides OH R R R Alcohols R R R R Alkenes H O Aldehydes R O R R Ketones OH Phenols R OH R Enols R R O O H Stable enols References shown at the beginning of each functional group correspond to the 6 th edition of Solomons (S) and the 2 nd edition of Bruice (B). Theory ALKYL HAIDES Alkyl halides can undergo nucleophilic substitutions through two different mechanisms, S N 1 and S N 2. Remember, S N 1 and S N 2 reactions were discussed in the synthesis of t-butyl chloride. S N 1 test – AgNO 3 /Ethanol (AgNO 3 turns skin brown and does not come off until skin dies) When 2 o and 3 o alkyl halides, R-X are treated with a very weak nucleophile, such as water or an alcohol, the halide undergoes a unimolecular nucleophilic substitution, S N 1. In S N 1 reactions the rate-determining step is the formation of the carbocation. Therefore, the more stable the carbocation that is formed the faster the reaction occurs. Primary alkyl halides do not undergo S N 1 reactions. Table 1: Carbocation Stability 3 o benzylic 3 o allylic > 2 o benzylic 2 o allylic ~ C R R R > CH 2 ~ CH 2 CH CH 2 Benzylic Allylic H 3 C CH H 3 C Tertiary Secondary > CH 3 CH 2 Primary > CH 3 CH 2 CH Methyl Vinyl ~ ~ Why are tertiary carbocations more stable then secondary, and secondary more stable than primary? Tertiary carbocations are stabilized by the presence of nearby alkyl groups. Alkyl groups are electron-donating and donate electrons to help stabilize the positive charge. Why are benzylic and allylic carbocations stable enough to undergo S N 1 reactions? Resonance stabilized carbocations - practice drawing the different resonance structures.
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An alcoholic solution of silver nitrate promotes S N 1 reactions. The silver helps pull the halogen off promoting the formation of the carbocation. This carbocation can then undergo nucleophilic attack by the alcohol. Silver chloride, AgCl, is formed which is a white solid and also indicates a positive test. White precipitate = a positive test.
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This note was uploaded on 03/31/2009 for the course CHEM 2364 taught by Professor Davila during the Spring '08 term at LSU.

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