EXP 8 The SN2 reaction Facros affecting SN2 reaction PDF

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The SN2 reaction: Factors affecting SN2 reaction Dennis Rossi and Roger Mendez CHM 2210L TA: Suzeeta Bhandari March 16 , 2019

Intro A SN2 reaction is where the nucleophile replaces the leaving group. The nucleophile using its own lone pair to form a new bond with the carbon atom (Solomon, 2016). A SN2 reaction is second order reaction and occurs in one step. As the same time the nucleophile attacks, the leaving group leaves. This process changes the stereochemistry of the molecule because it has a “back attack”. The “back attack” occurs because there is no room for the nucleophile to attach, therefore the strong nucleophile attaches to the electrophile carbon from the opposite side of the leaving group , inverting the leaving group. (Solomon, 2016) This reaction is a second order reaction and the rate law is k[Nu][substrate]. Therefore a factor that will affect the reactions are the concentration of the nucleophile and the concentration of the substrate. Other factors that may affect the reaction are the steric hindrance, the nucleophilicity of the amine, the nature of the leaving group, and the type of solvent used. An SN2 reaction requires strong nucleophile so using a weak nucleophile such as H2O. SN2 favors aprotic solvents because they are not able to hydrogen bond and not protonate. This allows for no hindrance of the reaction (Solomon, 2016) For example, the concertation of the nucleophile affects the rate of the reaction. If the reaction has a lower concentration of the nucleophile, than the reaction will have a lower rate and vice versa. A 1M Nu and 1M substrate will have a higher rate than a 0.5M Nu and 1M substrate. Also, another factor affecting the rate is the concentration of the substrate. If the substrate is more concentrated than the rate would increase and vice versa. A 1M substrate and 1M Nu will have a higher rate than 0.5M substrate and 1M Nu.

Reaction

Side reaction

Experimental Section 1) Nature of nucleophile Test Tube 1 (triethylamine)  





5mL of solv. mixture was added 20 drops of triethylamine was added 10 drops of Iodomethane was added Observations and time was recorded

Test tube 2 (tripropylamine)   

5mL of solv. mixture was added 10 drops of iodomethane was added Observations and time was recorded

Testtube 3 (ethyldiisopropylamine)  

 

5 mL of solv. mixture was added 20 drops of ethyldiisopropylamine was added 10 drops of iodomethane was added Observations and time was recorded

2) Nature of substrate and leaving group

Test Tube 1 (Iodoethane) 





20 drops of triethylamine was added 15 drops of Iodoethane was added Observations and time was recorded

Test tube 2 (1-bromopentane)   

20 drops of triethylamine was added 15 drops of iodomethane was added Observations and time was recorded

Test tube 3 (2-bromopentane)   

20 drops of triethylamine was added 15 drops of 2-bromopentane Observations and time was recorded

3) Unknown amine

Test Tube    

20 drops of unknown was added 10 drops of Iodoethane was added while slowly mixing Approx. 2 mL of solv. mixture was added It was let stand for 10-15 minutes

The mass was identified by the melting point of the precipitate

The precipitate was collected into a vacuum flask

The precipitate was dried, and the melting point was taken

The precipitate was washed with solv. mixture

eTable of Chemicals Densit

Molecula

Molecula

Melting

Boilin

r formula

r weight

Point

g point y

(g/mol)

(oC)

(oC)

(g/cm3

Triethylamine

C6H15N

101.19

-144.70

89

) 0.726

Ethyldiisopropyla

C8H19N

129.24

-46

126.5

0.766

mine

Structure

Acetone

C3H6O

58.08

-95

56

0.784

Tripropylamine

C9H21N

143.27

-93

156

0.753

Iodomethane

CH3I

141.94

-66.5

2.28

2.28

Diethyl ether

C7H10O

74.12

-116.3

34.6

0.713

Pentane

C5H12

72.15

-129.8

36.1

0.626

1-bromopentane

C5H11Br

154.04

-95

130

1.22

2-bromopentane

C5H11Br

151.047

-95.6

116-

1.233

117

iodoethane

C2H5I

155.97

Triethylamine

-111.10

72

1.94

Tripropylamine



Extremely flammable



Causes eye burns



May be harmful if swallowed



Causes skin burns



May cause respiratory irritation



Harmful if inhaled



Harmful if absorbed through the skin



Keep container tightly closed in well-



Keep container tightly closed in wellventilated area

Ethyldiisopropylamin

ventilated area Iodomethane 

Causes eye/skin irritation



Highly flammable



Harmful if swallowed



May be corrosive to metals



May cause central nervous system



Harmful if swallowed



Causes eye irritation



Toxic if inhaled



Keep container tightly closed in wellventilated area

effects 

Keep container tightly closed in wellventilated area

Diethyl ether 

Extremely flammable

Acetone



Harmful if swallowed



Extremely flammable



May cause respiratory irritation



Breathing vapors may cause



May cause drowsiness and dizziness

drowsiness and dizziness



May cause damage to organs



May cause eye irritation



Keep container tightly closed in well-



May cause damage to organs



Keep container tightly closed in well-

ventilated area Iodoethane

ventilated area Pentane 

Extremely flammable



Causes eye/skin irritation



Harmful if inhaled



Ay cause drowsiness and dizziness



Keep container tightly closed in well-



Highly flammable



Causes skin/eye irritation



May damage organs



Keep container tightly closed in wellventilated area

ventilated area

Results Observations

Time

Melting Point (oC)

Test tube 1: Triethylamine

Milk color and cloudy

46.1 seconds

with Iodomethane Test tube 2:

A little cloudy

4:08 minutes

Tripropylamine with Iodomethane Test tube 3:

No reaction

>15 minutes

with Iodomethane Test tube 4: Iodoethane

Cloudy

2.0 - 3.0 seconds

with Trimethylamines Test tube 5: 1-

N/A

>15 minutes

N/A

>15 minutes

Yellow tint

~5:00 minutes

Ethyldiisopropylamine

bromopentane with Triethylamine Test tube 6: 2bromopentane with Triethylamine Unknown with

174

Iodomethane

The unknown amine was identified as being as (C6H5)CH2N(CH3)3+ IDiscussion Steric hindrance plays a vital role in the reaction, since the nucleophile attacks and the leaving group leaves simultaneously there is no racemic mixture, just an inversion of configuration. In test tube 1 through 3, the substrate (Iodomethane) was constant while the nucleophiles were changed for each test tube. Test tube 1 (Triethylamine) occurred the fastest, this is because the nucleophile was the smallest and allowed for a faster rate because there is less steric hindrance. There was white precipitate that was formed and mixture became white cloudy. Test tube 2 took more time than test tube 1 because the amine is bigger than Triethylamine.

However, it still produced a white cloudy precipitate just not as profound as test tube 1. The biggest amine (Ethyldiisopropylamine) with Iodomethane did not have any reaction occur. This may because the amine has two isopropyl groups which will hinder that’s why the time was long. A reaction my have occur but after 15 minutes there was no reaction. For test tube 4-6, the nucleophile of Triethylamine was kept constant while the substrate was changed. The only reaction that occurred was in test tube 4 with Triethylamine with iodoethane. It became cloudy and it happened instantly within 2-3 seconds. The other two test tubes had no reaction in 15 minutes. This may be due to human error because the reactions may take longer to occur but due to time constraints the reactions were not allowed to be kept going. The first occurred the quickest because iodine is a better leaving group then bromine therefore 1bromopentane and 2-bromopentane should have occurred at a slower rate. The unknown amine was identified by reacting it with iodomethane. A precipitate did form and then it was vacuum filtrated to remove the moisture out of the precipitate. After it was dried, the melting point was taken and it was 174 degrees Celsius. After comparing it to the literature boiling points of several amines that was provided to us. We concluded that the unknown Amine was (C6H5)CH2N(CH3)3+ I- because the boiling point recorded was closes to this one. The literature value was 178 degrees Celsius and 174 degrees Celsius was record. The difference in boiling point could be that the precipitate was not completely dried when the boiling point was taken causing the temperature than the literature recorded. The actual unknown compound was Dimethylbenzylamine, there our conclusion of the unknown was accurate. Conclusion

The theoretical background and the results that were gathered, do correlate to an extent with each other. As SN2 was discussed, one was able to learn what solvents and nucleophile will work the best. The reactions in the test tubes put this to the test. The experiment although did correlate with the background information of a SN2 mechanism and the factors that affect the reaction. The only part of the experiment that do not correlate with the background are the test tubes that were recorded to have no reaction. The goal of this experiment was to observe the affect that the nucleophile and the leaving group on the reaction rate and that was successful through this experiment. The techniques performed can be applied to other situations such the reaction for Sadenosyl nethionine that occur. Some biological reactions occur through SN2 reactions. (Chemistry: LibreTexts,2015) References Libretexts. 7.12: Application: Useful SN2 Reactions. https://chem.libretexts.org/Courses/University_of_Illinois,_Springfield/UIS:_CHE_267__Organic_Chemistry_I_(Morsch)/Chapters/Chapter_07:_Alkyl_Halides_and_Nucleophilic_Subs titution/7.12:_Application:_Useful_SN2_Reactions (accessed Mar 2019). Solomons, T. W. G.; Fryhle, C. B.; Snyder, S. A. Organic chemistry; John Wiley et Sons, Inc.: Hoboken, NJ, 2016. Weldegirma, Solomon. “Experiment 8: Simple and Fraction Distillation of a Binary Mixture.” Experimental Organic Chemistry. 8 th Edition, Tampa, Pro-Copy Inc. 2018. 46-49.’...