IMPORTANT CONCEPTS
1. Organic chemistry is the chemistry of carbon and its compounds.
2. Coulomb’s law relates the attractive force between particles of opposite electrical charge to the distance between them.
3. Ionic bonds result from coulombic attraction of oppositely charged ions. These ions are formed by the complete transfer of electrons from one atom to another, typically to achieve a noble-gas configuration.
4. Covalent bonds result from electron sharing between two atoms. Electrons are shared to allow the atoms to attain noble-gas configurations.
5. Bond length is the average distance between two covalently bonded atoms. Bond formation releases energy; bond breaking requires energy.
6. Polar bonds are formed between atoms of differing electronegativity (a measure of an atom’s ability to attract electrons).
7. The shape of molecules is strongly influenced by electron repulsion.
8. Lewis structures describe bonding by the use of valence electron dots. They are drawn so as to give hydrogen an electron duet and the other atoms electron octets (octet rule). Formal charge separation should be minimized but may be enforced by the octet rule.
9. When two or more Lewis structures differing only in the positions of the electrons are needed to describe a molecule, they are called resonance forms. None correctly describes the molecule, its true representation being an average (hybrid) of all its Lewis structures. If the resonance forms of a molecule are unequal, those which best satisfy the rules for writing Lewis structures and the electronegativity requirements of the atoms are more important.
10. The motion of electrons around the nucleus can be described by wave equations. The solutions to these equations are atomic orbitals,which roughly delineate regions in space in which there is a high probability of finding electrons.
11. An s orbital is spherical; a p orbital looks like two touching teardrops or a “spherical figure eight.” The mathematical sign of the orbital at any point can be positive, negative, or zero (node). With increasing energy, the number of nodes increases. Each orbital can be occupied by a maximum of two electrons of opposite spin (Pauli exclusion principle, Hund’s rule).
12. The process of adding electrons one by one to the atomic orbitals, starting with those of lowest energy, is called the Aufbau principle.
13. A molecular orbital is formed when two atomic orbitals overlap to generate a bond. Atomic orbitals of the same sign overlap to give a bonding molecular orbital of lower energy. Atomic orbitals of opposite sign give rise to an antibonding molecular orbitalof higher energy and containing a node. The number of molecular orbitals equals the number of atomic orbitals from which they derive.
14. Bonds made by overlap along the internuclear axis are called σ bonds; those made by overlap of p orbitals perpendicular to the internuclear axis are called π bonds.
15. The mixing of orbitals on the same atom results in new hybrid orbitals of different shape. One s and one p orbital mix to give two linear sp hybrids, used, for example, in the bonding of BeH2. One s and two p orbitals result in three trigonal sp2 hybrids, used, for example, in BH3. One s and three p orbitals furnish four tetrahedral sp3 hybrids, used, for example, in CH4. The orbitals that are not hybridized stay unchanged. Hybrid orbitals may overlap with each other. Overlapping sp3 hybrid orbitals on different carbon atoms form the carbon – carbon bonds in ethane and other organic molecules. Hybrid orbitals may also be occupied by lone electron pairs, as in NH3.
16. The composition (i.e., ratios of types of atoms) of organic molecules is revealed by elemental analysis. The molecular formula gives the number of atoms of each kind.
17. Molecules that have the same molecular formula but different connectivity order of their atoms are called constitutional or structural isomers.They have different properties.
18. Condensed and bond-line formulas are abbreviated representations of molecules. Dashed-wedged line drawings illustrate molecular structures in three dimensions.
From "Organic Chemistry" Textbook of VOLLHARDT & SCHORE
No comments:
Post a Comment