Before the reaction takes place, the electrophile contains an atom or group called the Leaving Group, because it ultimately detaches from the electrophile. Nucleophilic Substitution (S N 1 S N 2) Nucleophilic substitution is the reaction of an electron pair donor (the nucleophile, Nu) with an electron pair acceptor (the electrophile). If you want the mechanisms explained to you in detail, there is a link at the bottom of the page. (Reproduced, with permission, from Carey FA, Giuliano RM.
Both of these compounds can be formed through substitution reactions. Nucleophilicity is also affected by the presence of electron withdrawing or donating substituents that interact with the nucleophilic atom via inductive or resonance effects. If the electrophilic carbon happens to be chiral, an SN2 reaction will invert the original configuration. Electrophilic sites are those sites possessing partial positive character, shown in blue. Addition reactions of this type are covered in Section 6.9. The focus of this chapter is on saturated carbon electrophiles, electrophilic C–C double bonds, and aromatic systems. S N i or Substitution Nucleophilic intramolecular stands for a specific but not often encountered nucleophilic aliphatic substitution reaction mechanism. The solvents which are capable of donating hydrogen bond will slow down the SN2 reaction because of association with the reactants. When a nucleophile reacts with a C=O bond, a tetrahedral intermediate is formed in which the negative charge is borne and stabilized by the electronegative oxygen atom. The halide anions iodide, bromide, chloride, and fluoride are nucleophilic anions from the same column or “group” of the periodic table. It is this carbon that is attacked by the Nucleophile. In HSAB theory a “hard” acid or base is a species with very little polarizability—think of a proton (a very hard acid) or fluoride anion (a very hard base). When describing nucleophilic substitution reactions, the term nucleophilicity is often used to describe the relative strength of a nucleophile—its ability to donate electrons. Each anion possesses a full octet of valence electrons and a formal –1 charge, while the number of protons in the nucleus increases in the order C (6), N (7), O (8), F (9). The leaving group leaves as a neutral molecule or anion. The relationship between basicity and nucleophilicity may be further refined by a brief introduction to hard-soft acid base (HSAB) theory. The most general form of the reaction may be given as the following: The electron pair (:) from the nucleophile (Nuc) attacks the substrate (R-LG) forming a new bond, while the leaving group (LG) departs with an electron pair. (Reproduced, … Now it's time to examine it in detail. Figure 6.11 Potential energy diagram describing the one-step biomolecular SN2 reaction of hydroxide anion with methyl bromide. An example of such a molecule is methyl bromide (CH3–Br), with its relatively weak C–Br bond to a good leaving group (Br). (Reproduced, with permission, from Carey FA, Giuliano RM. Resonance and inductive effects combine to make the triflate anion an exceptionally good leaving group.
We've studied a few reactions which proceed by this mechanism. Thus, proton-abundant and electronegative fluorine holds its electrons very tightly, making fluoride the least nucleophilic anion in the series. As SN2 reactions were affected by sterics, SN1 reactions are determined by bulky groups attached to the carbocation. NUCLEOPHILIC SUBSTITUTION AND ELIMINATION REACTIONS s ubstitution reactions involve the replacement of one atom or group (X) by another (Y): We already have described one very important type of substitution reaction, the halogenation of alkanes (Section 4-4), in which a hydrogen atom is re- placed by a halogen atom (X = H, Y = halogen). We'll study these reactions next time. The thiol containing tripeptide glutathione is sometimes referred to as the “guardian of the cell,” on account of its various roles as an antioxidant and a nucleophilic scavenger of potentially harmful electrophilic species. A more general description of acids and bases is that first proposed by the chemist Gilbert N. Lewis, who described a covalent bond as the sharing of an electron pair between two atoms. The nucleophilicity of different nucleophiles can be compared by using the following factors. called a transition state, the energy is at a maximum. Two examples of SN2 processes are shown in Figure 6.9. Another way to understand the backside trajectory of attack in SN2 reactions is to inspect the molecular orbitals (MOs) involved. The carbocations formed in SN1 reactions are susceptible to rearrangement. That is, it approaches from the side opposite the Leaving Group, not the same side as the leaving group. Considering the acetate and trifluoroacetate anions, we might draw two resonance forms to show that the negative charge is shared equally by the two oxygen atoms, thus stabilizing the negative charge. The topic of addition reactions is introduced here and expanded upon in the following chapter on carbonyl chemistry. Conversely, if we determine that a nucleophilic substitution reaction proceeds with inversion of configuration, we conclude that its mechanism is SN2. By contrast the SN1 reaction involves two steps. The bond between nucleophile and carbon forms at the exact same time that the bond between carbon and Leaving Group breaks. DNA cross-linking prevents cell division and ultimately leads to cell death.
Smallest in size and with its valence electrons held close to the nucleus, the electron cloud of fluoride is not at all polarizable and thus least reactive with carbon electrophiles. The reactions of electrophilic C=O double bonds (carbonyl species) are the topic of Chapter 7. Figure 6.9 Examples of nucleophilic substitution reactions. Unless otherwise noted, LibreTexts content is licensed by CC BY-NC-SA 3.0.
Table 6.1 compares the relative reactivity of a variety of common nucleophiles. Watch the recordings here on Youtube! Missed the LibreFest? Electrophile: An electron-deficient species that acts as a target for a Nucleophile. Ironically, these life-extending drugs trace their chemical provenance back to the earliest chemical weapons, in particular mustard gas. Electrophiles are Lewis acids—species that accept an electron pair in the formation of a new covalent bond. You should also be able to write reasonable mechanisms for your reactions, making the proper use of curly arrows to show the movement of electrons as chemical bonds are formed and broken.
This page gives you the facts and simple, uncluttered mechanisms for the nucleophilic substitution reactions between halogenoalkanes and hydroxide ions (from, for example, sodium hydroxide). That is, if the electrophile was originally S, then the product will now be exclusively R, and vice versa. This means that Hydrogens or Methyl Groups might migrate to other parts of the carbocation in order to form a more stable carbocation. Out of the four acids, the order of acidic strength is as follows, HI > HBr > Hcl > HF (Bond enthalpy). The necessary inversion of configuration for an SN2 process makes this reaction a stereospecific one (i.e., it affords a single stereoisomeric product that can be predicted based on the reaction mechanism). Figure 6.7 Leaving group ability is correlated with the ability to stabilize a negative charge and thus is related to basicity. For example, the SN2 mechanism is: Should you need it, the two stages of the SN1 mechanism are: Help! This rate law tells us that the nucleophile and electrophile are involved in a bimol… Compared to the reactant’s product have both inverted and retained configuration. Keep in mind that high pKa numbers mean weak acids which have strong conjugate bases. Remember that in the reactions of carboxylic acid derivatives there was first an addition to a the carbonyl group in which the carbon-oxygen pi bond was broken. It should be evident that the best overlap of MOs will occur along the axis of the C–Br bond. When this is the case the reaction is said to be first order in alkyl halide. This is illustrated for the alkyl halide electrophiles shown below. If we assign the electrophile an S configuration, starting with the Leaving Group and moving counterclockwise, then the product has the R configuration, starting with the Nucleophile and moving clockwise. Be aware, though that although the groups are always flipped in the SN2 mechanism, priority may cause the product to have the same R or S designation as the starting molecule.
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