Chapter 5 Clayden Organics 大学有机化学

Nucleophiles and Electrophiles
- best interactions is between the electrons in the orbital highest in energy
(highest occupied molecular orbital (HOMO)) and the unfilled orbital of lowest energy (lowest unoccupied molecular orbital (LUMO))
reaction occurs when electrons are transferred from a lone pair to an empty orbital
curly arrow shows the movement of a pair of electrons from nitrogen into boron to form a new σ bond between those two atoms
Nucleophiles
Nucleophiles are either (i) negatively charged, (ii) neutral species with lone pair(s) of electrons, (iii) carbon with lone pairs of electrons
Correct orientation → effective orbital overlap
incorrect orientation: no reaction
- filled sp3 orbital on N interacts with empty p orbital on B to give s bond orbital and s* antibonding orbital - electrons dropped from the non-bonding sp3 orbital to a lower energy s orbital
- 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.
Most organic reactions involve interactions between full and empty orbitals. Many also involve charge interactions. The electron donor is called a nucleophile (nucleus-loving) while the electron acceptor is called the electrophile (electron-loving).
(v) σ bonds too can act as nucleophiles
(vi) compounds with carbon-metal bonds
Electrophiles
Electrophiles are neutral or positively charged species with an empty atomic orbital (the opposite of a lone pair) or a low-energy antibonding orbital
Electrophile accept electrons into empty low-energy orbitals represented by one of the following:
Curly arrows represent reaction mechanisms
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
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
filled p MO of C=C
empty s* MO of Br2
Reaction occurs because of attractive interaction between a full orbital (the p bond) and an empty orbital (the s* orbital of the Br–Br bond) which leads to bonding
when atomic orbitals interact, their energies split to produce two new molecular orbitals, one above and one below the old orbitals. in each case there is a loss in energy when the electrons from the old lone pair drop down into the new stable bonding molecular orbital. the energy difference is greatest when the two orbitals are the same and least when they are very far apart in energy. the best reactions are those where the energies of the interacting orbitals are similar in energy
Orbital overlap controls angle of successful attack
- electrostatic forces provide a generalized attraction between molecules in chemical reactions (eg Na+ and Cl-) - the orbitals of the nucleophile and electrophile are directional and so the molecular orbitals of the reacting molecules exert important control ► for a new bond to form, orbitals of the two species must be correctly aligned
► 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 energy to overcome the general electronic repulsion, chemical change or reaction can occur.
Orbitals overlaห้องสมุดไป่ตู้ brings molecules together
- organic reactions take place between completely uncharged molecules with no dipole moments
- molecules are attracted to each other because this interaction is between an empty and a full orbital and leads to bonding
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.
Charge attraction brings molecules together