SN2

Group Experiment 2 Individual Lab Report (Save as pdf and submit, due by 12:00 NOON one week after experiment)

Last Name: Hossain

First Name: Ishtiaque

Lab Partner(s): James Cui

TA Name: Ryan Barrett

Date Lab Performed: 11/04/2015

Date Lab Submitted: 11/11/2015

Group A, B, or C: Group C

Comments for Grading TA: (Please indicate if you performed the lab on a day other than your regularly scheduled day and/or with a TA other than your regular TA). Page Limit: report must not exceed FOUR pages (including this page) LIMIT DOES NOT INCLUDE ANY GRAPHS, SPECTRA, OR REFERENCES (Please see General Lab Report Guidelines for detailed descriptions of all other requirements)

DEDUCTIONS FOR HANDWRITTEN REPORT/STRUCTURES/GRAPHS AND EXCEEDING PAGE LIMIT

LATE PENALTY IS 2 MARKS PER DAY LATE (NOT ACCEPTED AFTER 7 DAYS)

Chemistry 212 FALL 2015

* written component check General Report Guidelines for grammar requirements 2 of 4

Group Experiment 2: SN2 Report Sheet Objective: (3 marks)* (What is the purpose of this experiment?) The purpose of this experiment is to perform a substitution reaction between 1-methylimidazole and 1-bromobutane via an SN2 reaction to produce an ionic liquid, which is then reacted with KPF6 to produce another ionic liquid [C4-mim]PF6, the target product. Since the activation energy of the reaction is high, the reactants would boil off at the temperatures at which this reaction was carried out. Hence, the process of heat under reflux is also learned. Furthermore, we have learned how to carry out solvent extraction where to immiscible liquids the aqueous and the organic - were separated from each other, using dichloromethane (DCM). Finally, we also learned about the process of fractional distillation to separate the DCM form the ionic liquid we were trying to obtain. Introduction: (3 marks)* (Brief description of the concept/reaction studied SN2 reactions and why it is important) SN2 reactions are substitution reactions which are bimolecular in the rate determining step, that is, the slowest step of the reaction, the overall order of the reaction is 2. This means that the rate of the reaction is dependent on the concentration of both the nucleophile and the substrate. Thus, this is a concerted mechanism, which results in the inversion of the configuration of the carbon atom being attacked by the nucleophile. As a result, the stereochemistry of the chiral carbon may change from an R- to an S- configuration and vice versa, if the priority of the leaving group and new substituent remain the same. Furthermore, this type of reaction is stereospecific, which means the carbon atom being attacked will end up with only one of the two possible stereoisomers. This is because the nucleophile can only attack from the backside of the carbon atom, where the antibonding orbital present. This is of particular value to pharmacies which produce metabolic drugs as the cells biochemical reactions are stereospecific the right stereoisomer has the desired effect whereas the wrong stereoisomer may have no or a deleterious effect. Thus, if the pharmacists starting material has only stereoisomer present, then his product will also be composed of only one stereoisomer, reducing the costs of purification. Reaction Scheme: (3 marks) (Balanced chemical equation and data [structures, molar mass, concentration, density, volume, mass, moles, etc as appropriate including theoretical yield] for reactants and products)

Chemistry 212 FALL 2015

* written component check General Report Guidelines for grammar requirements 3 of 4

Property 1-methylimidazole 1-bromobutane [C4-mim]Br KPF6 [C4-mim]PF6

Volume (ml) 0.20 0.27 - - - Molar mass

(gmol-1) 82.10 137.02 219.12 184.06 283.96

Density (gcm-3)

1.03 1.27 - - -

Amount (mol.)

2.50*10-3 2.50*10-3 2.50*10-3 2.50*10-3 2.50*10-3

Mass (g) 0.21 0.34 0.55 0.46 0.71

Table 1. Results Yield and Physical Properties of Ionic Liquid (3 marks)

Ionic Liquid

Yield (g) 0.52

Yield (%) 73

Appearance Viscous yellow liquid

Note: There is a discrepancy between the percentage yield quoted here and that I had provided to the TA since I calculated the molar mass of [C4-mim]PF6 wrong and hence the theoretical came out to be different. Discussion Questions: (18 marks)* 1. Suggest one chemical reason, specific to this experiment, why your percent yield could

be less than 100%. The reaction may have taken place via an E2 mechanism since the nucleophile is bulky and the reaction to produce the initial ionic liquid was carried out at a very high temperature of about 100C. 2. What is an ionic liquid? Why are they useful/important? A liquid whose constituents are made of oppositely charged ions, held to each other by electrostatic force of attractions is an ionic liquid. An ionic liquid is basically a salt. Traditionally, the term ionic liquid is associated with salts in which the electrostatic force of attraction between the anion and the cation is weak enough for its melting temperature to be below 100C. Ionic liquids can be very useful as solvents as they have extremely low vapour pressure (boils at very high temperatures), they do not breakdown, unless at very high temperatures and they lack combustibility. So ionic liquids are safer and less wasteful options than traditional solvents like hexane and ether, which have very high vapour pressures and can produce harmful fumes.

Chemistry 212 FALL 2015

* written component check General Report Guidelines for grammar requirements 4 of 4

3. The melting point of [C-4min]PF6 is 6-7 C where as that of [C-4min]Br is 65-75 C. Explain the observed difference in melting temperature.

PF6- is a very bulky anion, since it has a very large ionic radius, so it has a very low charge density, compared to Br- which is comparatively smaller and has a smaller ionic radius. So according to Coulombs Law, the electrostatic forces of attraction between between [C4-mim]+ and Br- is stronger than that between [C4-mim]+ and PF6-. Hence, it takes less energy (and thus a lower temperature) to melt a sample of [C-4mim]PF6 than a sample of [C-4mim]Br. 4. What is the main difference between an SN2 and an SN1 reaction? In this experiment, why is the SN2 reaction favoured? In an SN1 reaction, only the substrate takes part in the rate determining step, whereas, in an SN2 reaction, both the nucleophile and the substrate take part in the rate determining step. SN2 is favored since the substrate is a1 alkyl halide. Since a hypothetical 1 carbocation is very unstable, the activation energy of the reaction would be very high. However, since it is a 1 alkyl halide, it is sterically unhindered, hence the nucleophile can attack and the leaving group

can break away from the substrate in a concerted manner. 5. What would be the E2 product (if it could form)? Suggest two analysis techniques/chemical tests that you could use to determine that you do indeed have the SN2 product (your ionic liquid) and NOT the E2 product. The E2 product would be but-1-ene. One analysis technique is IR spectroscopy, which would show the lack of an absorption peak in the C-C double bond region and the presence of that in the C-N single bond, if the SN2 product is formed. A chemcical test would be to add bromine liquid in limited quantity to the sample, in the dark. If alkene would be formed, the the sample would turn from orange-brown to colourless, but if the SN2 product is formed the sample would remain brown. 6. Draw a detailed mechanism for the formation of your ionic liquid.

Chemistry 212 FALL 2015

* written component check General Report Guidelines for grammar requirements 5 of 4

References: 1) http://www.chemspider.com/Search.aspx?q=PF6 2) http://www.chemspider.com/Chemical-Structure.7711.html?rid=b34839c7-3045-

421e-8ffc-d81a95eb0e7d&page_num=0 3) http://www.chemspider.com/Chemical-Structure.1348.html?rid=ed385618-f1e3-

4056-89c0-ef4f5720dd83 4) http://www.chemspider.com/Chemical-Structure.2015930.html?rid=9eaca17a-

fd5d-4028-a2c5-543acdedbb6d