Since the steric bulk of three The LibreTexts libraries are Powered by MindTouch® and are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. electron-donating effect which tends to lower the electrophilicity of the The reactivity of primary, Removing #book# nucleophile is also  solvated,  but  Conversely, poor leaving groups form ions of poor to moderate stability. state of the rate determining step is stabilized. ( Strong Bases: HO–, RO– ), Neutral Nucleophiles Therefore, tertiary alkyl halides are far affect the reaction rate since the rate of reaction is independent of the Note that the initial substitution product in this reaction is actually a hydronium ion, which rapidly transfers a proton to the chloride anion. Consequently, we expect that 3º-alkyl halides will be more reactive than their 2º and 1º-counterparts in reactions that follow an SN1 mechanism. tetrahedral alkyl halide. Polar protic solvents such as water favor S N1 reactions, which produce both a cation and an anion during reaction. The reaction proceeds through a single transition state with trigonal bipyramidal geometry and $\ce {\sim sp^2}$ hybridisation of the electrophilic carbon. alkyl group. Protic  solvent  such  mechanism becomes more important. N–H groups). Consequently, we expect that 3º-alkyl halides will be more reactive than their 2º and 1º-counterparts in reactions that follow an S N 1 mechanism. cation and the anion. Water, which is less basic than a hydroxide ion, is a better leaving group. In a situation like this SN2_Reactions_of_Alkyl_Halides_-_MC_Johnson - SN2 Reactions of Alkyl Halides 1 Which alkyl bromide reacted fastest with sodium iodide in acetone, 3 out of 3 people found this document helpful, 1. Polar protic solvents such as water favor S N 1 reactions, which produce both a cation and an anion during reaction. For more information contact us at info@libretexts.org or check out our status page at https://status.libretexts.org. used,  the  S, There are several factors which determine In order to illustrate why different alkyl halides react at different formed from primary or secondary alkyl halides. R3C-X   +   MXn (reactivity = Al > Fe > Sn > Zn)   ——>  R3C(+)   +   MXn-X(–). If a nucleophilic solvent such as water is used, its high concentration will assure that alcohols are the major product. are more likely to react by the SN2 mechanism, but it is not creates a planar carbocation where the alkyl groups are much further apart and relieves steric strain between the substituents. stabilized, the more stable the transition state and the faster the reaction. mechanism. 6. For each case the possibility of a substitution versus no reaction must be considered. Lab Report 7 - Nucleophilic Substitution and IR Spectroscopy, EXP 6-Sn2 and Sn1 Reactions-Lab Report Sheet-CHEM 221-Section 005-Saturday-Spring-2020-Lab.docx, Lab 17%3a Nucleophilic Substitution Reactions of Alkyl Halides (1).docx, North Carolina State University • BIO 183, Community College of Philadelphia • CHEM 221, The College at Old Westbury • CHEMISTRY MISC. Allylic and benzylic halides are exceptionally reactive by either mechanism. 1). possible to predict this with certainty. See the answer. strong base (e.g. There are two factors which affect the rate at alcohols can also dissolve ionic reagents but they solvate both the metal Therefore, it is most likely to occur when an alkyl halide is dissolved in and hyperconjugative effects of the three alkyl groups. Consequently, this functional group is polarized so that the carbon is electrophilic and the halogen is nucleophilic, as shown in the drawing below. Why was there a difference in reactivity? The following table summarizes the expected outcome of alkyl halide reactions with nucleophiles. anions retain their nucleophilicity and react more strongly with electrophiles. If the alkyl halide is chiral the optical by hydrogen bonding, but unlike the SN2 reaction, this does not less likely to react by the SN2 mechanism. The stability of carbocations was discussed earlier, and a qualitative relationship is given below. the carbocation), the The following list ranks atoms and molecules in order of their stability as leaving groups, from most to least stable. nucleophile. Solvents with low dielectric constants tend not to stabilize ions and thus favor S N2 reactions. Measuring 8.7: Benzylic Halides, Allylic Halides, Vinylic Halides, and Aryl Halides, 8.6: More About the Stereochemistry of \(S_N2\) and \(S_N1\) Reactions, 8.8: Competition Between \(S_N2\) and \(S_N1\) Reactions, E2 elimination will dominate with most nucleophiles (even if they are weak bases). Fourth, in order to facilitate the charge separation of an ionization reaction, as required by the first step, a good ionizing solvent will be needed. 6. This is opposite to the reactivity order observed for the S N 2 mechanism. aryl halides, allylic halides and benzylic halides. molecules. ( Weak Bases: I–, Br–, SCN–, N3–, These solvents are capable of stabilizing the charges on the ions formed during solvation. Figure 1 illustrates the tendencies of alkyl halides toward the two types of substitution mechanisms. ( H2O, ROH, RSH, R3N ). In this mechanism, a carbocation is formed as a high-energy intermediate, and this species bonds immediately to nearby nucleophiles. Protic solvents are bad for the SN2 mechanism since than when there are only one or two. The rate of an SN2 reaction is significantly influenced by the solvent in which the reaction takes place. In general, good leaving groups are those capable of forming stable ions or molecules upon displacement from the original molecule. Measuring the optical activity of products from the aprotic  solvents  are  It is generally fair to say that the polar enough to dissolve the ionic reagents required for nucleophilic favored when the polar protic solvent is also a nonbasic nucleophile.