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Week 8 - Preparation of Tertiary Alkyl Chlorides by...

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Preparation of Tertiary Alkyl Chlorides by Nucleophilic Aliphatic Substitution. Sodium Iodide and Silver Nitrate Tests for Alkyl Halides Mark Gumapas November 8, 2007 Methods and Background The purpose of this lab is to successfully convert a tertiary alcohol into a tertiary alkyl chloride through nucleophilic aliphatic substitution with hydrochloric acid, and to verify the results with sodium iodide and silver nitrate tests. Nucleophilic aliphatic substitution is the process where the substitution of one group for another at a sp 3 -hybridized carbon atom occurs. This reaction can be considered as a lewis acid-base reacton since the carbon atom with an electronegative leaving group displays lewis acid characteristics. In nucleophilic aliphatic substitution, a nucleophile, a neutral molecule or anion that has at least one nonbonding pair of electrons, attacks an electrophilic carbon atom, donating its nonbonding pair of electrons to form a new covalent bond. The leaving group in the process, which may be neutral or negatively charged, accepts the pair of bonding electrons from the carbon atom as the bond between it and the carbon atom breaks. The order that these groups leave have been experimentally determined by different rates of reaction and it has been found that it correlates with the strength of the leaving groups conjugate acid. Nucleophilic substitution is a general reaction for aliphatic compounds which has two different mechanistic pathways. One mechanism is unimolecular nucleophilic substitution (S N 1), where the reaction occurs in two successive steps. The first step involves heterolytic cleavage, or ionization, of the bond between the carbon atom and the leaving group, using the assistance of the polar interactions between solvent molecules and the incipient cationic and anionic centers. This yields a carbocation, where the carbon atom that was bound to the leaving group now has a positive charge. This carbocation then combines with a nucleophile to form the substitution product. In the second step, nucleophilic attack of the carbocation to yield the product, is much faster because it is an exothermic process. The first step of the reaction is much slower than the first due to the breaking of the C-L (L=Leaving Group) bond, thus it is the rate- determining step of the reaction, and because it depends only on the concentration of the substrate, it is labeled unimolecular and expressed through the first-order rate constant( k 1) . Unimolecular Nucleophilic Substitution (S N 1) The other mechanistic pathway is bimolecular nucleophilic substitution (S N 2). In this mechanism, the nucleophile drectly attacks the substrate, with the angle of approach
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being 180º to the C-L bond. This is known as “backside attack,” where inversion of stereochemistry is observed. Unlike unimolecular substitution, there is no carbocation intermediate, but everything occurs at the same time. The C-L bond is broken in concert with the formation of the C-Nu bond. Since both parts happen at the same time, the
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