Orgo6 - Organic Chemistry Lab I Experiment title:...

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Organic Chemistry Lab I Experiment title: Preparation of 1-Bromobutane Introduction/Purpose of experiment: The main purpose of this experiment was to demonstrate the conversion of a primary alcohol to an alkyl halide by treatment of the alcohol with a hydrogen halide (HBr in this case). This reaction, commonly known as a nucleophilic alipathic substitution, is the substitution of one group for another at a sp 3 hybridized carbon. The replaced group is commonly known as the leaving group (L), while the group replacing it is known as the nucleophile. The reaction may be considered a Lewis acid-base reaction because the initial carbon attached to the leaving group has Lewis acidic character (electron acceptor); this is due to the electronegative leaving group which polarizes the C-L bond producing a partial positive charge on the carbon atom. Commonly, the exit of the leaving group produces an empty 2p orbital for the nucleophile to occupy and donate its electrons to form a new covalent bond. In the experiment, 1-bromobutane is formed by way of S N 2 (bimolecular) mechanism. In this case, the nucleophile directly attacks the substrate from the “backside” which leads to an inversion of stereochemistry. Unlike the S N 1 reaction, which occurs in two successive steps, the S N 2 reaction is a 1-step process. Breaking of the C-L bond occurs during formation of the C-Nu bond, so that the substrate and nucleophile are both involved in the rate-determining step (transition step). In this experiment, we generate HBr in situ from NaBr and H 2 SO 4 and react and heat it with 1-butanol to conclusively obtain 1- bromobutane. The alcohol is first protonated to give an oxonium ion which undergoes displacement of the bromide ion to form 1-bromobutane and water. Addition of the H 2 SO 4 has two important purposes: 1) it serves as the dehydrating agent that reduces the activity of water and shifts the position of the equilibrium to the right and 2) it adds a source of hydrogen ions to increase the concentration of the oxonium ion. The process is much more complicated than simply heating the chemicals, involving procedures such as simple distillation, extraction with a separatory funnel, and short path distillation. It is
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This note was uploaded on 02/16/2010 for the course CHEMISTRY V25.0246 taught by Professor Callahan during the Spring '08 term at NYU.

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Orgo6 - Organic Chemistry Lab I Experiment title:...

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