Classification Tests for Organic Halides Based on S N Reactivity and Identification of Organic Halides Using Beilstein Test Authors: Authors: *Corpuz, Khrisna Dasha S.; S .; Chua, Holiday O.; Coning, Jumel D.; Elmubarak, Fatima S.; Encabo, Jana Q.; Flores, Lyka J. Group No. 3
Abstract: Organic halides are organic compounds containing a halogen atom bonded to a carbon atom. Most organic halides are synthetic and not flammable. This experiment was conducted to differentiate primary, secondary, secondary , and tertiary organic halides based on their SN reactivity and to differentiate S N1 and SN2 mechanisms with organic halides. The samples used were n-butyl chloride, sec-butyl chloride, tert-butyl chloride, and chlorobenzene. For detecting the presence of halides in the sample, the Beilstein test, also known as the Copper halide test, was conducted. This test resulted to all the samples producing a blue-green flame upon heating the copper wire containing the samples. In determining the S N1 reactivity of the samples, the samples were made to react with 2% ethanolic silver nitrate. As observed, all the samples produced a white precipitate except for chlorobenzene. This was because chlorobenzene is already stabilized by resonance. Among all the samples that produced a precipitate with the addition of the 2% ethanolic silver nitrate, tert-butyl chloride was the fastest while nbutyl chloride was the slowest, showing that during S N1 reactions, tertiary halides are most favored. In determining the S N2 reactivity of the samples, the samples were made to react with 15% sodium iodide in anhydrous acetone. As a result, all the samples produced a white precipitate except for chlorobenzene due to the strong resonance within its structure. Among all the samples that produced a precipitate, n-butyl chloride was the fastest while tert-butyl chloride was the slowest, showing that in S N2 reactions, primary substrates are most favored. Introduction: Organic halides are organic compounds containing a halogen atom bonded to a carbon atom, known as the alpha carbon. The alpha carbon can be classified as primary, secondary, or tertiary depending on the number of alkyl group attached to it. The organic halide is primary if an alkyl group is attached to the alpha carbon. The
compound is secondary if two alkyl groups are attached to the alpha carbon, and tertiary if three groups are attached to the alpha carbon [1]. Organic halides can easily be prepared from other compounds, such as alcohols and alkenes. In turn, they are the starting materials for the synthesis of a large number of new
functional groups [1]. The syntheses are often carried out by nucleophilic substitution reactions in which the halide is replaced by another group [1]. Nucleophilic substitution is any reaction in which one nucleophile is substituted for another [2]. A carbocation is often formed as a reactive intermediate in these reactions. The structure of a carbocation is trigonal-planar with a vacant p-orbital and is sp2 hybridized [1]. The alkyl groups stabilize the positive charge of the carbocation by displacing or releasing electrons toward the positive charge. The stabilization increases as the number of alkyl groups attached to the carbocation increases [1] . Substitution reactions can occur in one smooth step or it can occur in two discrete steps, depending primarily on the structure of the R group [1]. In a smooth one-step reaction, the nucleophile must collide with the alkyl halide. This reaction is dependent on the concentration of both the nucleophile and the halide. Such reaction is said to be a bimolecular nucleophilic substitution reaction, S N2 [1] . This type of substitution reaction is classified as bimolecular because both the haloalkane and the nucleophile are involved in the rate-determining step [2]. SN2 reaction occurs with Walden inversion of configuration to give a product of the opposite chirality from the starting material [1].
Figure 1: This figure shows inversion of a compound in an S N2 reaction, which inverts the R-S configuration of the compound. In a two-step process, the rate of the slow step depends only on the concentration of the halide and is said to be unimolecular nucleophilic [1] substitution reaction, SN1 . In this reaction bond breaking between carbon and the leaving group is completed before bond forming with the [2] nucleophile begins . SN1 reactions proceed through a planar carbocation and occurs with racemization of configuration. Racemization is the process by which the product is a 50:50 mixture of enantiomers because the planar intermediate can form a bond with the nucleophile on either face [1].
Figure 2: The figure shows the formation of two enantiomers from the chiral alkyl halide, known as racemization. There are different factors concerning SN reactivity. SN1 reactions are governed mainly by electronic factors [2]. This means that the primary factor in the order of reactivity in S N1 reactions is the relative stability of the carbocation that is formed [1]. Tertiary halides are most favored by this reaction due to the stability of their carbocation.
SN2 reactions, by contrast, are governed mainly by steric factors [2]. This means that primary and secondary halides are most favored in this reaction because of the ease by which the nucleophile can come within bonding distance of the alkyl halide; tertiary halides are usually not reactive due to the steric hindrance [1] . Relative nucleophilicity also plays an important role in S N reactions. It is defined as the relative rates at which a nucleophile reacts in a reference nucleophilic substitution reaction [2]. The strength of a nucleophile, an atom that donates a pair of electrons, can easily differentiate between S N1 and SN2 reactions. SN2 reactions occur in smooth one-step reactions, therefore, a strong nucleophile is needed to attack the halogen. The better or stronger the nucleophile, the more likely it is that the reaction will occur by an S N2 reaction [2]. Thus, an SN1 reaction can, in principle, occur at approximately the same rate with any of the common nucleophiles, regardless of their relative [2] nucleophilicities . Solvents provide the medium in which reactants are dissolved and in which nucleophilic substitution reactions take place [2]. The solvents for these reactions are divided into protic and aprotic. If the solvent is polar and protic (a hydrogen bond donor solvent), S N1 reaction will react much faster because the solvent stabilizes the carbocation intermediate by solvation thereby increasing the reaction rate [2]. Since the hydrogen atom in a polar protic solvent
is highly positively charged, it can interact with the anionic nucleophile which would negatively affect an S N2 reaction, so a solvent that is polar and aprotic (solvent that cannot serve as a hydrogen bond donor) is needed for S N2 reactions to occur much faster [3]. The rate of S N1 and SN2 reactions also depend on the leaving group. The ability of a group to function as a leaving group is related to how stable it is as an anion [2]. Weak bases are usually the most stable anions and the best leaving groups because they can hold the charge [3]. Among the halogens, the iodide ion is the best leaving group, followed by bromide and lastly chloride; fluorine’s electronegativity affects its ability of becoming a good leaving group. The hydroxide ion, methoxide ion, and amide ion are such poor leaving groups that they are rarely displace in nucleophilic aliphatic substitution [2] reactions . SN2 reactions are usually associated with the best leaving groups since they occur in one step reactions. In contrast, SN1 reactions are more associated with the weaker leaving groups since they occur in a slower twostep reaction. Included in the experiment is a qualitative test called the “Beilstein Test.” It tests organic compounds that contain halogens namely chlorine, bromine or iodine [4]. Hydrogen decomposes on ignition in the presence of copper oxide to yield the corresponding hydrogen halides. These hydrogen halides react to form the
voltaic cupric halides that impart a green or blue-green color to a nonluminous flame.
Figure 3: The figure above shows the general reactions involved in the Beilstein Test in which CuX 2 reacts with the flame to produce a green or blue- green color. Objectives: The objectives of this experiment includes differentiating between primary, secondary, and tertiary organic halides based on their S N reactivity, and also to differentiate between S N1 and SN2 as reactive mechanisms with organic halides. Materials and Methodology The materials that were needed in this experiment were test tubes, matches, copper wire, burner, calibrated droppers and test tube holder and rack. Copper oxide was also used in the Beilstein test. The reagents that were used were 2% ethanolic AgNO3 and 15% NaI in anhydrous acetone while the sample compounds used were nbutyl chloride, sec-butyl chloride, tertbutyl chloride, and chlorobenzene. In the Beilstein test, copper wires with a small loop with one end were needed. The loop of each copper wire was heated directly in the oxidizing zone of a non-luminous flame until the green color imparted to the flame disappears. While the loop was still hot it was
dipped in some copper oxide powder and was reheated until the oxide adhered to the loop (this step is optional). Then the loop of the copper wire was cooled slightly and was dipped into the solid or liquid sample. The loop with the sample was heated in a nonluminous flame: first in the inner zone, then in the outer zone near the edge of the flame. If the flame was blue to green in color, it indicated the presence of a halide. In the SN1 reactivity, 20 drops of 2% ethanolic AgNO3 was placed in a test tube. Then 5 drops of the sample was added. The test tube was shaken and the time (in seconds or minutes) for a silver halide precipitate to form was recorded. This process was repeated using the remaining sample compounds. In SN2 reactivity, 2 drops of 15% NaI in anhydrous acetone was placed in a dry test tube. Then 5 drops of the sample was added. The contents of the test tube were mixed and the time (in seconds or in minutes) required for a precipitate to form and also the color of the precipitate was noted. This process was repeated using the remaining sample compounds. Results: Table 1. This table presents the condensed structural formula of the given sample compounds. Compounds Studied:
Condensed Structural Formula:
n -butyl chloride
CH3CH2CH2CH2Cl
Sol’n sec -butyl chloride
2n : 20 secs Cloudy Sol’n
2nd: 2 secs White ppt.
tert -butyl chloride
1st: 2 secs Cloudy Susp.
3rd: 2.5 secs White ppt.
Chlorobenzene
2.63 secs White ppt.
2.25 secs White ppt.
sec -butyl chloride
tert -butyl chloride
Chlorobenzene
Table 2. This table represents the results of the Beilstein Test. Compounds Studied:
Beilstein Test:
n -butyl chloride
Blue-Green Flame
sec -butyl chloride
Blue-Green Flame
tert -butyl chloride
Blue-Green Flame
Chlorobenzene
Blue-Green Flame
Figure 4: The picture shows a blue- green flame, a positive result of the Beilstein Test. Discussion:
Table 3. This table presents the results of the S N 1 and S N 2 reactivity Tests.
Compounds Studied:
n -butyl chloride
Rxn w/ Rxn w/ 15% NaI 2% in Ethanolic Anhydrous AgNO3: Acetone: 3rd: 50 secs Cloudy
1st: 1 sec White ppt.
The Beilstein test, reaction with ethanolic silver nitrate and reaction with sodium iodide in anhydrous acetone were performed for the organic compounds to be classified accordingly. In the Beilstein test, all of the samples, namely n-butyl chloride, secbutyl chloride, tert-butyl chloride and chlorobenzene produced a blue-green flame. The Beilstein test checks for the presence of a halogen and all of the
compounds contain a chloride resulting in a positive result [4].
ion,
Organic halides that undergo S N1 substitution reaction react with alcoholic silver nitrate to form a white precipitate of the corresponding silver halide [3]. The solvent used is polar and protic, which stabilizes the transition state more than it does the reactants, lowering the energy of activation for the reaction and thus, increasing the reaction rate [1]. All the sample reacted with the alcoholic silver nitrate except chlorobenzene. The formation of a white precipitate was not due to nucleophilic substitution reaction but it is because chlorobenzene is insoluble in the given solvent [4]. Chlorobenzene is not reactive towards nucleophilic substitution reaction because of its aromacity; its resonance gives stability to the compound, which would not react unless used with a catalyst. Tertiary chloride reacted with the reagent immediately. The primary factor in the order of reactivity in S N1 reaction is the relative stability of the carbocation that is formed. The carbocation formed by tertiary chloride is the most stable out of all of the compounds, therefore it reacted the fastest. Sec-butyl chloride and n-butyl chloride reacted very slowly, only proving further that the more stable the carbocation intermediate, the faster the rate of S N1 reaction. Primary alkyl halides can be distinguished from aryl and alkenyl halides by reaction with sodium iodide in acetone [3]. SN2 substitution reaction reacts with solvents that are polar and
aprotic such as acetone used in the sodium iodide anhydrous acetone reaction, increasing the reaction rate. Amongst the four compounds, n-butyl chloride reacted the fastest. The structure of primary halides like n-butyl chloride allows for an easy attack of the nucleophile. In SN2 reactivity, the primary factor in the order of reactivity is steric hindrance, therefore tert-butyl chloride resulted with the least reactivity with sodium iodide in acetone due to its structure, which hinders the nucleophilic attack of the alpha carbon from the backside. Chlorobenzene showed a false positive result due to its insolubility in the given solvent. Chlorobenzene does not react in SN2 reactions due to its bulky ring, which hinders the attack of the nucleophile. In conducting the experiment, there were a few sources of error. First, the glassware that were used during the experiment could have contained other substances like contaminants, contributing to wrong results. Also, the reagents that were used needed to be completely clear liquids, otherwise, turbidity of the solution could have given false positive results in testing S N1 and SN2 reactions. Lastly, the droppers could have contained water, a polar and protic solvent, which could have intervened in the conduction of S N2 reactions, a reaction favoring polar and aprotic solvents. References:
