# Lecture 8: Nucleophilic Substitution: SN2 Mechanism ## Organic Chemistry I --- ## Learning Objectives By the end of this lecture, students will be able to: 1. Define nucleophilic substitution and identify the components of the reaction 2. Describe the SN2 mechanism including the concerted, one-step process 3. Draw the energy diagram for an SN2 reaction 4. Explain the stereochemical outcome of SN2 reactions (inversion of configuration) 5. Predict the effect of substrate structure on SN2 reaction rate 6. Evaluate nucleophile strength and its effect on SN2 reactions 7. Describe the role of the leaving group in SN2 reactions 8. Explain solvent effects on SN2 reactions --- ## Lecture Content ### I. Overview of Nucleophilic Substitution In a nucleophilic substitution reaction, a nucleophile replaces a leaving group on an electrophilic carbon. The general reaction is Nu:- + R-LG yielding R-Nu + LG:-, where the nucleophile is an electron-rich species that donates an electron pair to the electrophilic carbon, the substrate contains the electrophilic carbon and its leaving group, and the leaving group is the group that departs with the bonding electrons. Two major mechanisms exist for nucleophilic substitution: SN2 (bimolecular nucleophilic substitution, the subject of this lecture) and SN1 (unimolecular nucleophilic substitution, covered in Lecture 9). The pathway a reaction follows depends on the interplay of substrate structure, nucleophile strength, leaving group quality, and solvent. ### II. The SN2 Mechanism The designation SN2 stands for Substitution, Nucleophilic, Bimolecular. This is a one-step, concerted mechanism in which bond making and bond breaking occur simultaneously, with no intermediate formed. The nucleophile attacks the electrophilic carbon from the side opposite the leaving group, an approach termed backside attack. The transition state features a pentacoordinate carbon with trigonal bipyramidal geometry, in which the nucleophile is partially bonded and the leaving group is partially departed, represented as [Nu---C---LG] with a double-dagger symbol. The rate law for an SN2 reaction is rate = k[nucleophile][substrate], making it second-order overall and first-order in each reactant. Both the nucleophile and the substrate are involved in the single rate-determining step, which is why the reaction is termed bimolecular. Doubling the concentration of either reactant doubles the reaction rate. Panel A: The SN2 mechanism shown step-by-step: the nucleophile (Nu:-) approaches from the back side of the substrate (R-LG), forming a transition state [Nu---C---LG]‡ with pentacoordinate carbon and trigonal bipyramidal geometry, then yielding the product (R-Nu) with the leaving group departed. Curved arrows show electron flow from nucleophile to carbon and from C-LG bond to LG. Panel B: A reaction coordinate diagram (energy vs. reaction progress) showing a single energy maximum (transition state) between reactants and products, with activation energy (Ea) and overall energy change (delta G) labeled. No intermediate is present. ### III. Stereochemistry of SN2: Walden Inversion The SN2 reaction proceeds with complete inversion of configuration at the stereocenter, a phenomenon known as Walden inversion after its discoverer Paul Walden, who first reported it in 1896. The effect is analogous to an umbrella inverting in the wind. Inversion occurs because the nucleophile must attack from the backside, causing the three remaining substituents to flip to the opposite side as the new bond forms and the leaving group departs. Frontside attack does not occur because the leaving group's electron density blocks approach from that direction. The stereochemical outcome is that an (R)-substrate produces an (S)-product, or vice versa, assuming the priorities of the groups remain unchanged. However, the R-to-S change is not guaranteed because the nucleophile may have a different CIP priority than the leaving group; what is always true is that the spatial arrangement is inverted. The SN2 reaction is stereospecific, meaning the stereochemistry of the product is fully determined by the stereochemistry of the starting material. ### IV. Substrate Structure Effects on SN2 Steric effects dominate SN2 reactivity because the nucleophile must access the electrophilic carbon from the backside, and bulky groups surrounding that carbon hinder its approach. The relative rates by substrate class clearly illustrate this: methyl substrates (CH3-X) react fastest, at roughly 30 times the reference rate, followed by primary substrates (RCH2-X) at a relative rate of 1, secondary substrates (R2CH-X) at about 0.03, and tertiary substrates (R3C-X), which essentially do not undergo SN2 at all. Neopentyl substrates are an instructive case: despite being primary, they react very slowly because branching at the beta carbon blocks the nucleophile's approach. The trend reflects the progressive increase in steric crowding as alkyl groups are added. Methyl substrates offer maximum accessibility because nothing blocks the backside. Each additional substituent adds steric bulk, and by the time three alkyl groups surround the electrophilic carbon in a tertiary substrate, the nucleophile simply cannot approach closely enough for reaction. SN2 reactions also do not occur at sp2 carbons in vinyl and aryl halides, because the p orbital of the pi bond blocks backside attack. ### V. Nucleophile Effects on SN2 Nucleophilicity is the kinetic ability of a species to attack an electrophilic carbon, and a stronger nucleophile produces a faster SN2 reaction. Because the nucleophile appears directly in the rate law, its strength has a proportional effect on the reaction rate. Several factors determine nucleophilicity. Charge is important: anionic nucleophiles such as HO- and RO- are invariably stronger than their neutral counterparts, H2O and ROH. Within the same row of the periodic table, nucleophilicity parallels basicity, so NH2- is stronger than RO-, which is stronger than F-. When comparing atoms down a column in polar protic solvents, however, the trend reverses because larger, more polarizable atoms are better nucleophiles despite being weaker bases: I- > Br- > Cl- > F-. This trend reverses again in polar aprotic solvents, where the smaller, more basic halides become more reactive. Steric effects also matter: the unhindered methoxide ion (CH3O-) is a better nucleophile than the bulky tert-butoxide ion ((CH3)3CO-), even though both are alkoxides. Common strong nucleophiles include I-, RS-, CN-, N3-, RO-, and HO-, while H2O and ROH are common weak nucleophiles. Panel A: A ranking of common nucleophiles from strongest to weakest for SN2 reactions in protic solvents: RS- > I- > CN- > HO- > N3- > Br- > CH3COO- > Cl- > F- > H2O > ROH. Each species is shown with its structure. Panel B: A diagram comparing nucleophilicity trends. Within a row: nucleophilicity parallels basicity (NH2- > RO- > F-). Down a column in protic solvents: nucleophilicity increases with polarizability (I- > Br- > Cl- > F-). This is displayed on a miniature periodic table highlighting N, O, F, S, Cl, Br, I with arrows showing the trends. ### VI. Leaving Group Effects on SN2 Better leaving groups lead to faster SN2 reactions because the C-LG bond is breaking in the transition state, and a good leaving group stabilizes the developing negative charge. Leaving group ability correlates inversely with basicity (weaker bases are better leaving groups), directly with the stability of the departing anion, and inversely with C-LG bond strength (weaker bonds break more easily). The ranking from best to worst leaving groups is: excellent (triflate, tosylate, mesylate, iodide), good (bromide, chloride), fair (fluoride, water from protonated alcohols), and poor or never (hydroxide, alkoxide, amide, hydride). Since the hydroxyl group is a poor leaving group, it must be activated before an alcohol can participate in substitution reactions. Protonation converts -OH to -OH2+, making water an acceptable leaving group. Sulfonylation converts -OH to -OTs or -OMs, which are excellent leaving groups. ### VII. Solvent Effects on SN2 The choice of solvent has a critical impact on SN2 reaction rates. Polar protic solvents such as water, methanol, ethanol, and acetic acid contain O-H or N-H bonds capable of hydrogen bonding. They solvate anions strongly by surrounding them with a shell of hydrogen bonds, which stabilizes the nucleophile in its ground state and thereby reduces its reactivity. As a result, SN2 reactions are generally slower in polar protic solvents. Polar aprotic solvents such as DMSO, DMF, acetone, acetonitrile, and HMPA are polar enough to dissolve ionic compounds but lack O-H or N-H bonds. They solvate cations effectively but leave anions relatively "naked" and highly reactive. Consequently, SN2 reactions are generally much faster in polar aprotic solvents, with rate increases of 100-fold to 1000-fold commonly observed when switching from a protic to an aprotic solvent. For example, the reaction of CH3Br with Cl- proceeds very rapidly in DMSO but much more slowly in methanol, where the chloride is stabilized by hydrogen bonding. Solvent selection is therefore a practical tool for optimizing SN2 reactions. ### VIII. Summary: Ideal Conditions for SN2 The ideal SN2 reaction uses a methyl or primary substrate that is unhindered, a strong and unhindered nucleophile such as I-, CN-, RS-, or RO-, a good leaving group that is a weak base and stable anion (such as I-, TsO-, or TfO-), and a polar aprotic solvent like DMSO, DMF, or acetone. Under these conditions, the reaction proceeds with inversion of configuration and follows second-order kinetics with rate = k[Nu][substrate]. Several synthetically important reactions proceed through the SN2 mechanism. The Williamson ether synthesis combines an alkoxide with a primary alkyl halide to form an ether. Alkylation with cyanide converts primary alkyl halides into nitriles, extending the carbon chain. The Gabriel synthesis uses the phthalimide anion to prepare primary amines. The Finkelstein reaction exchanges chloride for iodide by exploiting the insolubility of sodium chloride in acetone. A summary table with four columns showing the key factors for SN2 reactions. Column 1 "Substrate": methyl (fastest) through tertiary (no reaction) with molecular models showing increasing steric bulk. Column 2 "Nucleophile": list of strong to weak nucleophiles. Column 3 "Leaving Group": list from best (TfO-) to worst (HO-). Column 4 "Solvent": polar aprotic (fast, examples listed) vs. polar protic (slow, examples listed). At the bottom, a summary box states: "SN2 favored by: unhindered substrate + strong nucleophile + good leaving group + polar aprotic solvent." ---