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LA.2.1 Reaction Mechanisms

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LA.2.1 Reaction Mechanisms LA.2.1 Reaction Mechanisms Curly Arrows In a chemical reaction bonds are broken and new bonds made. A reaction mechanism describes the steps involved and makes it clear how these bonds are formed and broken. The slowest step in the reaction is known as the RATE DETERMINING STEP. Curly arrows represent the movement of electrons. A FULL arrow represents an electron pair (see Figure 1) whereas a HALF arrow represents a single electron (see Figure 2) which is traditionally reserved for radical chemistry. H+ + Cl- 2Cl Figure 1 The heterolysis of HCl Figure 2 The homolysis of Cl2 When a bond is broken and both electrons go to the same atom (represented by a full arrow), this is called HETEROLYTIC FISSION. When a bond is broken and the bonding electrons are evenly split (represented by a half arrow), this is called HOMOLYTIC FISSION. Nucleophiles Electrophiles Definition: Electron pair DONOR. Nucleophiles Definition: Electron pair ACCEPTOR. are also known as Lewis bases. Electrophiles are also known as Lewis acids. Compounds which act as nucleophiles have a Compounds which act as electrophiles are high electron density electron deficient Common nucleophiles: Common electrophiles: - - - + + + Anions CN, OH, Cl Cations H , CH3 , NO2 Π Bonds Alkenes, Alkynes, Polar molecules Haloalkanes, Ketones, (electrophilic site at δ+) Aromatic compounds e.g benzene Alcohols Atoms with Lone Amines, Ketones, Pairs Electron movement always goes from the nucleophile to the electrophile: Water Electrophilic Addition to Alkenes (Bromination) A classic test for alkenes is the addition of bromine (Br2) as the presence of an alkene such as ethene, changes the brown bromine to colourless. Ethene (ETHLEN10) has a high electron density π bond so it’s natural to assume it would act as the nucleophile in this reaction. Bromine will have an induced dipole (from Van de Waal’s forces) and consequently there will be a δ+ present giving it the ability to act as an electrophile. The reaction mechanism is presented in Figure 3. Figure 3 The mechanism for the bromination of ethene. Other compounds which happen in this way include other halogens and acids such as HBr, HCl and H2SO4. THE MECHANISM IN FURTHER DETAIL The alkene’s filled π orbital (the HOMO) attacks bromine’s empty σ* orbital (the LUMO). The highest electron density is in the middle of the bond between the 2 carbon atoms. In order to achieve the best orbital overlap the bromine σ* must approach ethene end on as illustrated in Figure 4. As a result, a 3 centred bromonium intermediate is formed. Being as 2 new bonds are formed, it is critical to represent this with the use of curly arrows which is why a second arrow is introduced. The bromonium ion isn’t the final product. The final step of the reaction is the attack of the bromonium ion (electrophile) by the bromide ion (nucleophile). Figure 4 The true mechanism for the bromination of ethene showing the key intermediate. Notice the stereochemistry has been included in the mechanism presented in Figure 4. The step where the bromide ion attacks the intermediate is an SN2 substitution (the C-Br bond is being broken at the same time as the new C-Br bond is being formed). In order for the Br- 0 (nucleophile) to attack the LUMO (C-Brσ*), it must attack at 180 . This is why the bromine atoms have opposite stereochemistry. PROOF BROMONIUM IONS EXIST Very hindered alkenes form bromonium ions resistant to nucleophilic attack. In fact, they are stable enough to be characterized by X- Ray crystallography (see Figure 5). Figure 5 The crystal structure of an extermemly hindered bromonium Produced by Leticia Bonita Prince Newcastle University MChem student. ion, stable to nucleophilic attack. .
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