12.3 reactionsNucleophilic is a reaction in which a

12.3 – Reactions of nucleophilic substitution of haloalkanes You will:know what nucleophilic reactions arebe able to formulate equations for the reactions of haloalkanes with nucleophilic reagentsbe able to explain the mechanism of reactions of nucleophilic substitution of haloalkanes Nucleophilic reactionsNucleophilic is a reaction in which a molecule of an organic substance is exposed to the action of a nucleophilic reagent.Nucleophilic (“nucleus-loving”) reagents, or nucleophiles – are particles (anions or molecules) that have an unshared pair of electrons at an external electronic level.Examples of nucleophilic particles: OH-, Cl-, Br-, CN-, H2O, CH3OH, NH3.Structures of some nucleophilic reagents:????? ???????????? ??? ?????????? Structures of some nucleophilic reagents Due to the mobility of p-electrons, nucleophilic properties are also possessed by molecules containing p-bonds: CH2=CH2, CH2=CH-CH=CH2, C6H6, and etc. Example of nucleophilic reaction: , where OH- is nucleophilic reagent. Chemical properties of haloalkanes The reactivity of haloalkanes mainly depends on the polarizability of the C-X bond decreasing in the row: C-I> C-Br> C-Cl > C-F.The chemical activity of haloalkanes can vary widely, depending on their structure. Nucleophilic substitution of haloalkanesFor haloalkanes, nucleophilic substitution reactions (SN) are characteristic. This is due to the distribution of the electron density in the molecule.  Chlorine shifts the electron density to itself due to higher electronegativity. A partial positive charge arises on the carbon atom. This electron-deficient carbon is an electrophilic center and can be attacked by some nucleophile.With the help of nucleophilic substitution reactions, it is possible to convert haloalkanes to alcohols, thiols, amines, and ethers.The general scheme of the nucleophilic substitution reaction can be represented as follows: Under the action of a nucleophilic reagent (electron pair donor) OH-, a heterolytic disruption of the C-Cl polar bond occurs. The electron pair of this bond goes to the more electronegative atom Cl, which turns into the Cl- ion.A new bond is formed due to the electron pair of the nucleophile (the nucleophile replaces the halogen in the halogenoalkane molecule). The particle X- leaves, carrying electrons of the former ?-bond. This particle is called a “leaving group”. Reactions of zero-oleophilic substitution are often reversible, since any leaving group is also a nucleophile. To shift the equilibrium of the reaction to the right, a “good” nucleophile and a “good” outgoing group are needed. A “good” nucleophile is an active nucleophile. How can we compare the strength of different nucleophiles?1) Anions are more active nucleophiles than the corresponding neutral molecules, because the anions are easier to give off electrons. For example, the OH- anion is a stronger nucleophile than the neutral H2O water molecule. Alcoholate anion R-O- is more active than molecule of alcohol R-OH, thiolate anion R-S- is more active than molecule of thiol R-SH.2) The lower the electronegativity of the atom of the nucleophilic center, the more active the nucleophile, such an atom more readily transfers the electron pair to the formation of a new ?-bond. Therefore, the ammonia molecule NH3 is a stronger nucleophile than the H2O water molecule (the electronegativity of nitrogen is lower than that of oxygen).A “good” outgoing group, on the contrary, should be a weak nucleophile. For example, a neutral water molecule is a better leaving group than a hydroxide anion. The halogen anions are “good” leaving groups due to the high electronegativity of the halogen atoms. Nucleophilic substitution in general form:  Y: + RX ?  RY + X: