![]() Facile C–F bond formation through a concerted nucleophilic aromatic substitution mediated by the PhenoFluor reagent. Concerted nucleophilic aromatic substitution with 19F − and 18F −. ![]() Analysis of past and present synthetic methodologies on medicinal chemistry: where have all the new reactions gone? J. Rate and equilibrium studies in Jackson–Meisenheimer complexes. Modern Nucleophilic Aromatic Substitution (Wiley-VCH, Weinheim, 2013). Concerted Organic and Bio-Organic Mechanisms (CRC, Boca Raton, FL, 1999). This is due to the formation of a resonance stabilized structure upon leaving.Williams, A. Alkyl sulfates and sulfonates like the ones shown make excellent leaving groups. The following diagram shows sulfur derivatives of the type ROSO 3 - and RSO 3. Therefore, leaving groups that form resonance structures upon leaving are considered to be excellent leaving groups. Resonance Increases the Ability of the Leaving Group to Leave: As we learned previously, resonance stabilized structures are weak bases. The relationship among the following halogens, unlike the previous example, is true to what we will see in upcoming reaction mechanisms. With an increase in size, basicity decreases, and the ability of the leaving group to leave increases. If we move down the periodic table, size increases. For example, fluoride is such a poor leaving group that S N2 reactions of fluoroalkanes are rarely observed.Īs Size Increases, The Ability of the Leaving Group to Leave Increases: Here we revisit the effect size has on basicity. In real reaction mechanisms, these groups are not good leaving groups at all. This particular example should only be used to facilitate your understanding of this concept. The following diagram illustrates this concept, showing -CH 3 to be the worst leaving group and F - to be the best leaving group. This is because an increase in electronegativity results in a species that wants to hold onto its electrons rather than donate them. With an increase in electronegativity, basisity decreases, and the ability of the leaving group to leave increases. Since strong bases, by definition, want to share their electrons, resonance stabilized structures are weak bases.Īs Electronegativity Increases, The Ability of the Leaving Group to Leave Increases.Īs mentioned previously, if we move from left to right on the periodic table, electronegativity increases. As you may remember from general chemistry, the formation of a resonance stabilized structure results in a species that is less willing to share its electrons. Resonance Decreases Basicity: The third factor to consider in determining whether or not a species will be a strong or weak base is resonance. ![]() As size increases, basicity will decrease, meaning a species will be less likely to act as a base that is, the species will be less likely to share its electrons. As electronegativity increases, basicity will decrease, meaning a species will be less likely to act as base that is, the species will be less likely to share its electrons.Īs Size Increases, Basicity Decreases: In general, if we move from the top of the periodic table to the bottom of the periodic table as shown in the diagram below, the size of an atom will increase. The factors that will determine whether a species wants to share its electrons or not include electronegativity, size, and resonance.Īs Electronegativity Increases, Basicity Decreases: In general, if we move from the left of the periodic table to the right of the periodic table as shown in the diagram below, electronegativity increases. ![]() If you remember from general chemistry, a Lewis base is defined as a species that donates a pair of electrons to form a covalent bond. In order to understand the nature of the leaving group, it is important to first discuss factors that help determine whether a species will be a strong base or weak base.
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