Experiment 7

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Experiment 7: The Sn2 reaction: factors affecting Sn2 reaction Chase Morris CHM2210L-29 Heng Liu

Introduction: An Sn2 reaction or a substitution nucleophilic bimolecular reaction that involves the replacement of a leaving group on a carbon atom with a nucleophile. Sn2 is a concerted reaction in that the leaving group leaving and being replaced is done in one step without an intermediate. An Sn2 reactions begins with the nucleophile approaching the carbon with the leaving group attached. The nucleophile then attacks the carbon causing the bond between the carbon and leaving group to weaken and the leaving group to leave, as it leaves the nucleophile continues to approach and replace the leaving group. Sn2 reactions are affected by the substrate structure, primary and secondary are typically more reactive than tertiary substrates due to the steric hindrance. A good leaving group is important to an Sn2 reaction it is usually a weak base that is used to stabilize the negative charge that forms on it as the leaving groups leave. The nature and strength of the nucleophile are also important, a good nucleophile is typically a strong base that donate an electron pair the carbon and displace the leaving group from the carbon as well. The solvent is important in regard to an Sn2 reaction, polar aprotic solvents are often used in Sn2 reactions to dissolve the reactants and stabilize the nucleophile without interfering with the reaction. Temperature is also important for Sn2 reactions as a higher temperature can promote a faster reaction but can also cause unwanted side reactions to occur as well. Finally steric hindrance is very important to Sn2 reactions, bulky groups may inhibit the nucleophile from approaching the carbon atom, this is especially important when using secondary and tertiary substrates. An example of an Sn2 reaction that is rate determined by the nature of the nucleophile would be 1-bromo-3-chloropropane reacting with OH- and CN- under identical conditions, the rate would depend on the strength of the nucleophile OH- is a stronger nucleophile because it can donate protons easier than CN- therefore OH- and 1-bromo-3- chloropropane would react faster than CN-. A reaction of methyl bromide and hydroxide

because methyl bromide is a primary alkyl halide it will react relatively quickly with OH-, however with a substrate like t-butyl bromide reacting with OH- would take much longer due to the steric hindrance of the bulky carbon. A potential side reaction between a primary alkyl halide and an amine is the production of an N-alkylated amine, this can occur if the condition of the reaction is not carefully controlled the alkyl group is attached to the nitrogen atom of the amine and not the carbon atom creating a N-alkylated amine. Procedure: Test Tubes 5mL 1:5:4 acetone, diethyl ether, pentane 20 drops of 1: triethylamine 2: tripropylamine 3: ethyldiisopropylamine 10 drops iodomethane New tubes 20 drops triethylamine 15 drops 1: iodomethane 2: 1-bromopropane 3: 2- bromopropane New Test tube 40 drops of unknown 20 drops iodomethane 2mL solvent let stand 1-15 minutes Collect with vacuum filtration Dry and measure MP

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Chemicals used:  Triethylamine o MP: -114.7  c o BP: 89.28  c o MM:101.19 g/mol o Formula: C6H15N o IUPAC name:Triethylamine  Tripropylamine o MP: -93.5  c o BP: 156  c o MM: 143.27g/mol o Formula: C9H21N o IUPAC name: Tripropylamine  Ethyldiisopropylamine o MP: -46  c o BP: 126-127  c o MM: 129.24 g/mol o Formula: [(CH3)2CH]2NCH2CH o IUPAC name: Ethyldiisopropylamine  Iodomethane o MP: -108  c o BP: 72  c

o MM: 141.94 g/mol o Formula: CH3I o IUPAC name: Iodomethane  1-bromopropane o MP: -110.5  c o BP: 71  c o MM: 122.9 g/mol o Formula: C3H7Br o IUPAC name: 1-bromopropane  2-bromopropane o MP: -89  c o BP: 59  c o MM: 122.9 g/mol o Formula: C3H7Br o IUPAC name: 2-bromopropane

Results: Reacted with iodomethane Molecule Triethylamine Tripropylamine ethyldiisopropylamine Speed of precipitation Fastest Medium Slowest Reacted with triethylamine Molecule Iodomethane 1-bromopropane 2-bromopropane Speed of precipitation Fastest Medium Slowest Unknown Reaction speed Precipitate formed Melting point Weight Fast White powder 180.8-183  c .74g Discussion: The unknown amine that was reacted with iodomethane was triethyl amine, this could be determined by the speed of the reaction and type of precipitate that was formed from the reaction. The ammonium salt that this reaction forms is tetramethylammonium iodide which also has a melting point between 180-183  c which confirms it. Steric hindrance is a major limiting factor in Sn2 reactions, in order for the nucleophile to get close enough to attack and cause the leaving group to leave the three needs to be enough room for the nucleophile to get close. Steric hindrance of larger or bulky molecules limits the ability for the nucleophiles to get close enough to attack. Nucleophilicity is the ability of a nucleophile to displace the leaving group, a strong nucleophile is important to a successful Sn2 reaction

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because if the nucleophile is not strong enough to displace the leaving group, then the reaction cannot proceed. From this experiment it is very clear which leaving groups are more favorable for Sn2 when reacting with iodomethane, triethylamine had the best leaving group, tripropylamine had the second best and ethyldiiospropylamine had the worst, there is a trend of how bulky these molecules are and how good the leaving groups are, the bulkier the molecule is the worse the leaving group and the slower the reaction. The melting point of 180-183  c along with knowing that the unknown reacts quickly, and the precipitate is a white crystalline makes it very likely that the unknown was triethylamine due to the similarities in the reaction. After seeing what a reaction between iodomethane and triethylamine produces it was determined that the product was tetramethylammonium which also has a melting point between 180-183  c. Conclusion: The theoretical background is very connected to the information later in this lab. To have a good understand of how an Sn2 reaction happens step by step it is easy to identify why certain molecules are reacting better or worse, it also makes it easy to see patterns when determining the identity of the unknown amine. The experimental data reveals exactly what the background information had said, more bulky molecules with more steric hindrance react slower or not at all. In regard to the unknown after watching the first reactions and knowing the background made it very easy to know that the unknown was very reactive and was likely a smaller molecule with little steric hindrance. Sn2 reactions are a key part of nature and of science, Sn2 reactions are used in organic synthesis to make new carbon atoms, pharmaceutical industry to introduce new functional groups to molecules. The experiment accomplished what was set out, we were able to make predictions on which reactions would be favored and watch

what would happen, using these observations we were also able to determine the identity of the unknown amine. References: Weldegirma, S. Experimental Organic Chemistry , 11th ed.; Pro-Copy: Tampa, FL, 2023. Triethylamine. https://pubchem.ncbi.nlm.nih.gov/compound/Triethylamine (accessed Mar 24, 2023). Tripropylamine. https://pubchem.ncbi.nlm.nih.gov/compound/Tripropylamine (accessed Mar 24, 2023). Diisopropylethylamine. https://pubchem.ncbi.nlm.nih.gov/compound/Diisopropylethylamine (accessed Mar 24, 2023). Iodomethane. https://pubchem.ncbi.nlm.nih.gov/compound/Iodomethane (accessed Mar 24, 2023). 1-bromopropane. https://pubchem.ncbi.nlm.nih.gov/compound/1-Bromopropane (accessed Mar 24, 2023). 2-bromopropane. https://pubchem.ncbi.nlm.nih.gov/compound/2-Bromopropane (accessed Mar 24, 2023).