CLASSIFICATION TESTS FOR ORGANIC HALIDES Lorraine M. Ramos, Sharmaine Michelle M. Reyes, Rhone Arevyn E. Roque Joseph T. Sabido and Jan Armelynn S. Santos Group 7 2A Medical Technology Organic Chemistry Laboratory
ABSTRACT
The experiment is to understand and classify organic halides as organic compounds containing a halogen atom bonded to a carbon atom. Most organic halides are synthetic and not flammable. To attain the objectives of this experiment, different samples of organic halides; primary, secondary and tertiary were classified according to S N reactivity and also to differentiate the SN1 and SN2 mechanisms. Sample compounds of chlorobenzene, n-butyl chloride, sec-butyl chloride, tert-butyl chloride were analyzed with Beilstein test, alcoholic AgNO3 and NaI in acetone. All the sample compounds had produced a positive result with Beilstein test, affirming the presence of Chloride ion. When the compounds were reacted with 2% ethanoic AgNO3 to observe SN1 reactivity, all have attained a white precipitate in different time required except for chlorobenzene which had observed to exhibit no reaction. While in the S N2 phase reactivity using NaI in Acetone, it was observed that all have exhibit the presence of white precipitate except for chlorobenzene; noted that the time required for each compound was different from that of SN1. Therefore, it was observed that there are differences of the results attained from each S N reactivity or mechanisms and that these differences can be used to classify organic halides.
INTRODUCTION
Chlorine, fluorine, bromine and iodine are halogen atoms. A halogen atom that have been substituted in one or more hydrogen atoms are organic compounds commonly known as organic halides. These compounds are considered to be flame retardant and synthetic. [1] The reactivity of organic halides depends on the halogen atom that is bonded to the carbon atom in the particular compound and can be classified depending on the halogen atom position on the chain of carbon atoms. The carbon which is attached with the halogen atom is linked up with only one other alkyl group in primary halogenoalkanes, whereas directly linked up with two and three other alkyl groups in secondary halogenoalkanes and tertiary halogenoalkanes respectively. [2] An organic halide can be named as primary, secondary and tertiary depending on the degree of substitution of the alpha hydrogen atom by an R group on an sp3 hybridized alpha carbon atom. [3] By the use of substitution reaction with a nucleophile, the reaction of an electron pair donor (the nucleophile, Nu) with an electron pair (the electrophile). A sp3-hybridized electrophile must have a leaving group (X) in order for the reaction to take place. [4] The reaction is SN1 or unimolecular is a multi-step process and happens if the rate of reaction is dependent on the organic halide and it is SN2 or bimolecular is a single step process and happens if the rate of reaction is dependent on both the organic halide and the nucleophile. [3] For the compounds to be classified, it must be subjected to some tests. Beilstein test is a simple chemical test used in chemistry as a qualitative test which uses color of the flame to indicate the
presence halides and was developed by Friedrich Konrad Beilstein. [5] To test for SN1 reactivity the samples were reacted with alcoholic AgNO3 and to test for SN2 reactivity it is reacted with NaI in acetone. [3] The experiment must be able to attain the following objectives: (1) to differentiate primary, secondary, and tertiary organic halides based on their SN reactivity and (2) to differentiate SN1 and SN2 mechanisms with organic halides.
EXPERIMENTAL
A. Samples used n-butyl chloride sec-butyl chloride tert-butyl chloride chlorobenzene 2% ethanoic AgNO3 15% NaI in anhydrous acetone B. Procedure 1. Beiltein Test: Copper Halide Test
Figure 1. Organic Halide Samples
A small loop was made in one end of the copper wire and was heated in the oxidizing zone of a non-luminous flame until the green color imparted to the flame disappeared. The loop was cooled and dipped into the solid or liquid sample. And the loop was heated together with the sample in the inner zone to the outer zone of a non luminous flame. A blue-green (or green) colored flame was observed to indicate the presence of chlorine, bromine or iodine. 2. SN1 Reactivity: Reaction with Alcoholic AgNO3 Five drops of the sample was added to 20 drops of 2% ethanoic AgNO3. The sample was shaked and the time in seconds or minutes for a silver halide precipitate to form was observed and recorded. The color of the precipitate was then observed and described.
The Beiltein test was performed as a quick preliminary check for the presence of halogens, wherein all samples was observed with a blue green colored flame, exhibiting the presence of halide. It was also the simplest method for establishing the presence of such, but does not positively differentiate between Chloride, Bromide and Iodide ion. The blue-green color was observed due to the emission of light from excited states of copper halide that has vaporized in the burner flame. To remove the traces of sodium chloride that may be present on the wire from handling it with fingers was carried out by heating the copper wire before the test. Cu + O2
CuO (black solid)
RX + CuO
CuX2+ CO2+H2O
CuX2 is volatile and imparts a blue-green flame. Table 2. Data obtained using SN1 reactivity to organic halides
Figure 2. SN reactions or mechanisms 3. SN2 Reactivity: Reaction with NAI in Acetone 5 drops of each sample was added to 2 drops of 15% sodium iodide in anhydrous acetone. The contents were mixed and the time required for a precipitate to form and the color of the precipitate was recorded and noted.
RESULTS AND DISCUSSION
On the experiment performed using different kinds of classification tests for organic halides, certain results were obtained. Table 1. Data obtained from Beilstein Test Compound used
Beilstein test
n-butyl chloride
Green flame
Sec-butyl chloride
Green flame
Tert-butyl chloride
Green flame
Chlorobenzene
Green flame
Compound used
Reaction with 2% ethanolic silver nitrate
n-butyl chloride
5 mins White ppt.
Sec-butyl chloride
2 mins. White ppt.
Tert-butyl chloride
18 secs White ppt.
Chlorobenzene
No reaction No ppt.
In the test for SN1 Reactivity by reaction with alcoholic AgNO3 tert- butyl chloride was the first to produce a white precipitate followed by secbutyl chloride, n-butyl chloride and no reaction with chlorobenzene. The kinetics of the reaction is dependent on the alkyl halide. The more stable the carbocation intermediate the faster the rate of SN1 reaction. A good leaving group was important for the reaction to occur. The solvent used should be a protic, polar solvent that stabilizes the carbocation intermediate by solvation that increases the reaction rate. In terms of stereochemistry, this involves inversion and retention. The SN1 reaction happens when substrate dissociate to a carbocation in a slow rate-limiting step, followed by a rapid attack of a nucleophile. This reaction shows first order
kinetics and take place with racemization of configuration at the carbon atom. Table 3. Reactions obtained using SN2 reactivity to organic halides Compound used
Reaction with 15% sodium iodide in anhydrous acetone
n-butyl chloride
immediately White ppt.
Sec-butyl chloride
immediately White ppt.
Tert-butyl chloride
immediately White ppt.
Chlorobenzene
No reaction No ppt.
In the test for SN2 reactivity by reaction of NaI in acetone, n-butyl chloride is the first to show a white precipitate followed by sec-butyl chloride, tert- butyl chloride and no reaction in chlorobenzene. The kinetics of this reaction was dependent on the alkyl halide and nucleophile. Due to steric effect, the substrate effect in SN1 reaction was the opposite in SN2 reactions. It favors primary and secondary substrates. A strong nucleophile was needed for reaction to occur because it parallels basicity. The solvent should be a polar, aprotic solvent. In terms of stereochemistry, it involves Walden inversion of configuration. The reaction occurs as the entering nucleophile attacks the halide 180 away from the leaving group. The reaction exhibits secondorder kinetics and it was affected by steric effect caused by the bulky group of the reagents. The SN2 reaction favors primary and secondary substrates compared to SN2 reaction that favors tertiary substrates.
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Chemistry
Portal.
Nucleophilic
http://www.organicchemistry.org/namedreaction s/nucleophilic-substitution-sn1-sn2.shtm 9/12/11 [3] Chemical Land. Butyl Chlorides http://chemicalland21.com/industrialchem/organi c/TERT-BUTYL%20CHLORIDE.htm 9/12/11
[4] Chemistry education. Beilstein Test http://ull.chemistry.uakron.edu/organic_lab/beil/ 9/12/11 [5] Paar, L. et al. (2008). Organic Chemistry Laboratory Experiments for Organic Chemistry Laboratory. http://webcache.googleusercontent.com/search? q=cache:2LqimCkaFBYJ:cns.uni.edu/~manfredi/8 60-121/ORG%2520LAB%2520MAN%2520S08. pdf+sublimation+formal+report&hl=tl&gl=ph 9/12/11.
