Lab5 - Lab #5 1-Bromobutane from 1-Butanol Chen Zhou...

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Lab #5 1-Bromobutane from 1-Butanol Chen Zhou February 27 th , 2008 Purpose: To carry out S N 2 displacement of 1-butanol to produce 1-bromobutane. Introduction: - S N 2 reaction is a 2 nd order nucleophile displacement of an electrophile, producing a product and a leaving group. - The nucleophile is an electron donor and the electrophile, also known as a substrate, is an electron acceptor. - One of the factors that control the S N 2 reaction are the nucleophile, the leaving group, the solvent and the accessibility of the-alpha carbon. - A good nucleophile is effective at removing the leaving group from the alpha-carbon, which results in a faster and more efficient reaction. A base is a better nucleophile than its own conjugate acid. The nucleophilicity decreases moving left to right in the periodic table due to more electronegativity. The nucleophilicity increases as one move down the periodic table, as larger elements hold less tightly to electrons. - Another factor that controls the S N 2 reaction is the leaving group as it polarizes the C-x bond, making the alpha carbon positive. A good leaving group hold on tightly to its electrons after being displaced, which results in stability. Weak bases and Chlorine, Bromine and Iodine ions are good leaving groups. - This makes strong bases such as in this lab, the hydroxyl group, a poor leaving group. In this case, the sulphuric acid is used as a catalyst that modifies the hydroxyl group into a weak base. - The solvent affects the S N 2 reaction. The protic solvents can form H-bonds with small, charged nucleophiles, making it to have a less electron density to donate to the electrophile. S N 2 work best in dipolar aprotic solvents, as these dissolve salts but do not form H-bond. - In the accessibility of the alpha carbon, large molecules, which has a more crowed alpha carbon, does not allow the nucleophile to come into bonding distance, which S N 2 reaction will not occur. - In this experiment, the 1-bromobutane can be produced from 1-butanol reacting with potassium bromide in the presence of the sulphuric acid. The balanced equation is shown below.
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CH 3 (CH 2 ) 3 OH + KBr + H 2 SO 4 CH 3 (CH 2 ) 3 Br + KHSO 4 + H 2 O - Dibutyl ether, a side product can be formed due to the competition between two nucelophiles in the solution. As not every molecule of 1-butanol is protonated by the sulphuric acid, it can compete with the bromide ion to be the nucleophile. - The dibutyl ether can be removed by two methods. By washing the product with concentrated sulphuric acid, as the dibutyl ether would be protonated and dissolves in the sulphuric acid. - It can also be removed by simple distillation as dibutyl ether has a very different boiling point from 1-bromobutane. -
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This note was uploaded on 01/30/2011 for the course CHEM 1321 taught by Professor Rashmi during the Spring '10 term at Carleton CA.

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Lab5 - Lab #5 1-Bromobutane from 1-Butanol Chen Zhou...

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