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Relative Rates of Nucleophilic Substitution Reactions - Lab Report Example

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This experiment is aimed at determining the effects of the structure of the alkyl halides on the rate of SN1 and SN2 reactions. It also sought to find…
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Relative Rates of Nucleophilic Substitution Reactions
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Relative Rates of Nucleophilic Substitution Reactions Chemistry Lab Report 17 November Nucleophilic substitution reactions are type of reactions that involves step-by-step substitution of one nucleophile by another. This experiment is aimed at determining the effects of the structure of the alkyl halides on the rate of SN1 and SN2 reactions. It also sought to find the effects of the nature of the leaving group, the size of the nucleophile as well as the polarity of the solvent on the rate of SN1 and SN2 reactions. Primary, secondary and tertiary alkyl bromides were reacted with NaI in presence of acetone and ethanol solvents to determine their effects on the reaction rates. In SN2 mechanisms, primary alkyl halides reacted faster than tertiary ones while in SN1 mechanism the tertiary alkyl halides reacted faster than primary ones. It was concluded that aprotic, polar solvents favoured SN2 reactions while SN1 reactions were favoured by protic, polar solvents.
Introduction
Nucleophilic substitution reactions are reactions that involve a systematic substitution of one nucleophile by another (McMurry 228).
Nucleophilic substitution reactions occur by two major pathways namely SN1 and the SN2 reactions (McMurry 228). In all nucleophilic substitution reactions, the nucleophile (Nu:-) reacts with the substrate (R-X) and substitutes it for a leaving group (X:-) yielding the product R-Nu. For a neutral nucleophile (Nu:), the product is positively charged for charge conservation while for a negatively charged nucleophile (Nu:-), the product is neutral (McMurry 228).
In SN2, which stands for substitution nucleophilic bimolecular, the alkyl halide and the nucleophile are involved at the transition state (Carey 306). Bond formation between carbon and the nucleophile aids in cleavage of the bond between carbon and the leaving group. In the changeover position, the carbon atom is partially bonded to the leaving group and the incoming nucleophile (Carey 307). Since the nucleophile attacks the substrate from the side that is opposite the bond to the leaving group, the mechanism leads to the inversion of configuration in the resultant product.
Different rates are observed when methyl, primary, secondary and tertiary alkyl halides undergo nucleophilic substitution in SN2 (Carey 310). The rate is faster in methyl halides than in tertiary halides due to steric hindrance offered to the nucleophilic attack by the tertiary halides. In most SN2 reactions, the leaving group is expelled with a negative charge. Therefore, the best leaving groups are those that produce the most stables anions (McMurry 233). Among the halides, I- ion is the most reactive while F- ion is the least reactive. Most aprotic polar solvents cause the solvation of the metal counterion that is in reaction with the nucleophile thereby destabilizing the nucleophile making it react faster.
In SN1, which stands for substitution nucleophilic unimolecular, the bond between carbon and the halide breaks before the bond between carbon and the nucleophile forms making it a two-step process (Smith 254). The carbon-halide bond breaks hence leading to the formation of an intermediate carbocation, which then forms a bond with the nucleophile. The rate of reaction is faster in tertiary alkyl halides than in methyl halides due to the stability of the tertiary carbocation formed. A 50:50 (racemic) mixture of the enantiomers is obtained in SN1 since the carbocation formed is planar, sp2 hybridized and achiral (McMurry 236). Just as in SN2 reactions, the leaving groups that give the most stable anions are the best. Protic, polar solvent greatly increases the rate of carbocation formation of an alkyl halide because of its ability to solvate cations and anions effectively (Solomons and Fryhle 261). A stable transition state is achieved by solvation, which leads to the intermediate carbocation and halide ion more than it does the reactants lowering the free energy of activation.
Questions
1. Steric accessibility to the halide-bearing carbon determines whether 2-bromobutane undergoes SN1and/ or SN2 reactions.
2. The "cage" structure in adamantyl cation is constrained making it incapable of reaching the same "flat" sp2 structure of the tert-butyl cation.
3.
4.
5. Benzyl bromide is a primary alkyl halide hence undergoes SN2 reaction easily. It also ionizes readily because the carbocation formed is stabilized by resonance. Bromobenzene, on the other hand, does not undergo SN2 reaction due to the hindrance caused by the bulky ring, and SN1 reaction cannot occur due to the presence of sp2 hybridized carbon orbitals.
6. Bromine is a good leaving group and ethanol being a protic polar solvent greatly increases the rate of carbocation formation of an allyl halide because of its ability to solvate cations and anions effectively.
7. Alkyl fluorides are poor substrates since fluorides are poor leaving groups. The poor the leaving groups make it difficult to accept the electron pair in the C–X bond thereby leading to a slower reaction.
8. Most good leaving groups are weak bases, have large sizes and have resonance stabilized structures.
9. The three oxygen atoms pull electron density past the nitrogen, and there is an amount of resonance, which lessens the nucleophilicity hence making the nitrate a poor nucleophile.
Works Cited
Carey, A. Francis. Organic Chemistry. 4th. McGraw-Hill, 2000. Print.
McMurry, John. Fundamentals of Organic Chemistry. 6th. Belmont: Brooks, 2011. Print.
Smith, G. Janice. Organic Chemistry. 2nd. New York: McGraw-Hill, 2008. Print.
Solomons, T. W. Graham and Fryhle, B. Craig. Organic Chemistry. 10th. John Wiley & Sons, Inc, 2011. Print. Read More
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