chapter 5 clayden organics 大学有机化学.pdfVIP

Organic reactions Based on Clayden’s Organic Chemistry, Chapter 5 The aims of this chapter are to learn ….. • Why molecules generally don’t react with each other • Why sometimes molecules do react with each other • Understand reactivity in terms of charges and orbitals and the movement of electrons • How molecular shape and structure determine reactivity • Representing the movement of electrons in reactions by curly arrows Chemical reactions To understand organic chemistry you must be familiar with two languages – the structure and representation of molecules – the description of the reaction mechanism in terms of curly arrows - charge–charge repulsion between these electrons ensures that all molecules repel each other. - reaction will occur only if molecules are given enough energy (the activation energy for the reaction) for the molecules to pass the repulsion and get close enough to each other. Charge attraction brings molecules together - repulsive force - attractive force Cations (+) and anions (–) attract each other electrostatically and this may be enough for reaction to occur. A common cause of organic reactions is attraction between a charged reagent (cation or anion) and an organic compound that has a dipole. It is not necessary for the reagent to be charged — a lone pair of electrons is attracted to the positive end of the carbonyl dipole. Bond polarity Polarity can arise from σ bonds too. An electronegative element bonded to an electropositive element causes bond polarization. The negative end of the dipole is attracted to the electropositive end ► the presence of a dipole in a molecule represents an imbalance in the distribution of the bonding electrons due to polarization of a σ bond or a π bond or to a pair of electrons or an empty orbital localized on one atom. ► when two molecules with complementary dipoles collide with sufficient energ