330771819-experiment-2.docx

Experiment 2 STRUCTURAL EFFECTS OF REACTIVITY

Discussion

In the first part of the experiment, the two known samples, benzyl chloride and chlorobenzene was reacted with 1 ml of alcoholic AgNO3.

Benzyl chloride undergoes

Nucleophilic substitution reaction with alcoholic silver nitrate that results to the formation of cloudy solution. Since benzyl chloride is a primary benzylic halide it reacts via S N2 pathway. The reaction that happened is:

However, there was no reaction that takes place upon the addition of alcoholic silver nitrate to chlorobenzene. The three major reasons why an aryl halide sample did not undergo Nucleophilic substitution reaction are (1) it is resonance stabilized thus the energy of activation for displacement of chlorine is much greater than benzyl chloride, a benzylic halide;

(2) the carbon atom to which the chlorine is attached is sp2 hybridized thus it is difficult to replace it by nucleophile, OH;

and (3) chlorobenzene is electron rich due to presence of double bonds thus it repel the attacking nucleophile. On the other hand, chlorobenzene and other aryl halides are reactive towards Electrophilic aromatic substitution.

In the second part of the experiment, the relative reactivities of alkyl halides are observed under two sets of conditions, the SN1 and the SN2 mechanism. Four samples, n-butyl bromide, sec-butyl chloride, t-butyl bromide and benzyl chloride underwent a series of tests, (1) the addition of 15% NaI in acetone solution and (2) the addition of 1% ethanolic AgNO3.

The SN2 reaction is observed by the displacement of the chloride or bromide by an iodide ion (I-) in acetone solution. The iodide ion is a good nucleophile for the SN2 reaction, whereas acetone is a polar aprotic solvent, a relatively poor ionizing solvent that minimized the dissociation of the SN1 reaction. Sodium iodide is easily dissolved in acetone, but alkyl chloride and alkyl bromide have very low solubilities hence, the course of the reactions can be followed by the formation of NaCl or NaBr. The general reaction is shown in the following equation.

As a result of the test for SN2 reaction, the n-butyl bromide, t-butyl bromide and benzyl chloride yields the formation of precipitate upon addition of the reagent and all the samples appeared to be light-yellow. Since there was no precipitate formed when the reagent is added to sec-butyl chloride, it was heated via hot water bath. Upon heating, except for the evaporation of the solution, there was still no observable formation of precipitate. Following the rate of reaction of alkyl halides for the SN2 mechanism, 1˚ > 2˚ > 3˚ and the fact that benzylic halides reacts with nucleophile via either SN2 or via SN1 processes, the result for the n-butyl bromide and benzyl chloride is valid; also, the presence of precipitate shows that NaBr and NaCl were formed, respectively. However, inaccuracy of the results can be seen for the sec-butyl chloride and tertbutyl bromide. Based on the structures, sec-butyl chloride must be more likely to react with the reagent added than the tert-butyl bromide. Errors are may be due to personal judgment during the observation.

On the other hand, the SN1 reaction is observed by treating the alkyl halide samples with a solution of silver nitrate in aqueous ethanol. Since nitrate ion (NO3-) is a weak nucleophile, SN2 displacement is not likely to occur. Dissociation of the alkyl chloride and alkyl bromide by the SN1 process is followed by the precipitation of the insoluble AgCl and AgBr, respectively

then the carbocation is captured by alcohol or water. The following general equation shows the occurrence of the reaction.

For the SN1 reaction’s test result, the time required for n-butyl bromide, sec-butyl chloride, t-butyl bromide and benzyl chloride to become turbid was 21, 50, 2, and 80 seconds, respectively. The SN1’s rate of reaction for alkyl halides is 3˚ > 2˚> 1˚ which implies that the result for the reaction of t- butyl bromide and benzyl chloride with the reagent is valid. However, inaccuracy can be observed between n-butyl bromide and sec-butyl chloride. Sec-butyl-chloride should have faster reaction rate than n-butyl bromide based on their structures. The inaccuracy is caused by the personal judgment of the observer in recording the time of occurrence of turbidity.

In the third part of the experiment, Lucas test was performed. The test is used to distinguish the rate of reaction of primary, secondary, and tertiary alcohols with the Lucas reagent, HCl-ZnCl2 wherein ZnCl2 acts as catalyst that speeds up the reaction. The samples of primary, secondary, and tertiary alcohols used in the experiment are ethyl alcohol, sec-butyl alcohol, and tert-butyl alcohol, respectively. Two milliliters of Lucas reagent was added in three separate test tubes containing few drops of the samples. As a result, visible cloudiness and formation of precipitate was first observed in tert-butyl alcohol thirty-two seconds after the addition of HCl-ZnCl. After one minute and thirty-four seconds, formation of precipitate was observed in sec-butyl alcohol. However, even after several minutes, there was no observable change in the ethyl alcohol implying that no reaction takes place. The observation of the group who conducted the experiment is valid since the positive indicator of Lucas test for alcohols is the formation of a water insoluble alkyl chloride (RCl) precipitate. The reaction involves replacing the –OH group of the alcohol with a chloride ion (Cl-) from hydrochloric acid (HCl), forming an alkyl chloride, as shown in the general equation below.

The reaction of ethyl alcohol, sec-butyl alcohol, and tert-butyl alcohol with Lucas reagent is shown in the next three equations, respectively.

The test for the reactivity of benzene and naphthalene with dilute permanganate solution was done in the last part of the experiment. Naphthalene reacted with dilute KMnO4 turning from colorless to red-wine liquid with precipitate. The oxidation reaction is shown below.

However, the formation of two immiscible layers upon the addition of the oxidizing agent into the test tube containing benzene implied that there was no reaction occurred. Stabilization due to aromaticity makes benzene significantly less reactive.