Carbon-based molecules are held together by covalent bonds between atoms

Transcription

1 hapter 1: hemical bonding and structure in organic compounds arbon-based molecules are held together by covalent bonds between atoms omposition: Mainly nonmetals; especially,, O, N, S, P and the halogens Representations: Lewis electron-dot structures Kekule or line-bond structures Space-filling models Bonding theories: Valence bond theory ovalent bonds form from overlap of atomic orbitals Electron pair occupies space where orbitals overlap Bonding electrons are attracted to both nuclei (Bonds may be polar or nonpolar depending on the electronegativities of the bonding atoms) Bond strength increases with amount of orbital overlap σ bonds result from head-on overlap; π bonds from sideways overlap Molecular orbital (MO) theory Atomic orbitals combine to form larger molecular orbitals Orbitals combine in an additive way (bonding MO) to increase stability or in a way that cancels each other to decrease stability (antibonding MO) MOs cover much of the molecule Bonding MOs are lower energy and fill up first ybridization: ybrids: A combination of atomic orbitals within an atom that results in a new set of orbitals with geometry determined by VSEPR and equalized energies for the bonding electrons sp 3 : set of 4 orbitals, tetrahedral, bond angles of o sp 2 : set of 3 orbitals, trigonal planar, bond angles of 120 o sp: set of 2 orbitals, linear, bond angles of 180 o Any unhybridized p orbitals may form pi bonds

2 ommon bonding patterns in organic chemistry and their geometries Tetrahedral carbon (109.5 o ) Trigonal planar (120 o ) Linear (180 o ) Tetrahedral N (109.5 o ) Pyramidal N (107 o ) Bent N (120 o ) Linear N (180 o ) Bent O (105 o ) Terminal O Terminal Terminal F, l, Br, I The top five things you NEVER see in organic compounds 1) A arbon atom with more than 4 bonds. Super arbon just doesn t exist! 2) A ydrogen atom in between two other atoms. makes one bond PERIOD. 3) Ditto for halogens in organic chemistry. They go on the end. 4) A dangling Oxygen all neutral oxygens have two bonds (or they d better be charged!) 5) Noble gas elements. Just pretend they don t exist. But pretty much any other element could be bonded to carbon in some way.

3 Step-by-step method for drawing ORRET Lewis structures 1. Draw the skeletal structure of the compound by arranging the atoms of the molecular formula in the correct order. If you don t know the arrangement, start by trying to make the molecule symmetrical 2. Determine how many valence electrons each atom in the formula has. Add all the valence electrons together; this is the total available to distribute.* 3. First, assign bonding electron pairs so that one bond connects each pair of atoms in the structure. Subtract the number of electrons used from the total. 4. Next, fill in nonbonding electron pairs around the outer atoms in the structure so that each atom has a complete octet. Subtract this number of electrons from the total. 5. Place any remaining electrons around the central atom. 6. heck to see if each atom has a complete octet. If any atoms have less than an octet, move pairs of nonbonding electrons to make double or triple bonds until each atom has an octet. Do not add any extra electrons. *If there s a charge on the molecule, adjust the total number of electrons to account for the charge. For example, if the charge is -2, add 2 e- to the total.

4 1. Double bonds: two examples Multiple bonds and molecular geometry Ethylene ( 2 4 ) Formaldehyde ( 2 O) O ommon features: sp 2 hybridized Resulting arrangement of atoms around is trigonal planar Remaining p orbital on is available to form a pi bond with neighboring atom Double bonds consist of one sigma plus one pi bond Molecules are flat 2. Triple bonds: Acetylene ( 2 2 ) Acetonitrile ( 3 N) N ommon features: sp hybridized Arrangement of atoms around these is linear 2 p orbitals on each available to form pi bonds Triple bonds consist of one sigma + two pi bonds

5 Fig. 1.9 Energetics of bond formation: two theories

6 Orbitals and bonding: single σ bonds

7 Orbitals and bond formation: double bonds = σ + π

8 orbitals and bond formation: triple bond = σ + 2 π bonds energy of π bond formation

9 Skeletal or line-bond structures do not show the atoms Representing structure of organic compounds