Summary of Functional Groups ether alcohol (can be 1°, 2°, or 3°) alkene haloalkane or alkyl halide (can be 1°, 2°, or 3°) alkyne 1° amine
Summary of Functional Groups aldehyde carboxylic acid 2° amide ketone ester
Hydrocarbons • Compounds that contain only H and C atoms. • alkanes • alkenes • alkynes • aromatics • Why does C form four bonds and how can we tell its geometry?
Now is the Time to Review Hybrid Orbitals • If you do not remember the basis of the sp, sp2 and sp3 hybrid orbitals and the octet rule, please study the PowerPoint presentation (on the 2114 website) Review: Hybrid Orbitals
Valence Bond Theory and Double Bonds • In ethylene H2C=CH2, there are three electron domains around each C atom. VSEPR theory says the geometry is trigonal planar and the bond angles are 120°. • The hybrid orbitals present are formed from the 2s orbital and two of the 2p orbitals.
sp2 Hybrid Orbitals If we mix 3 orbitals (2s, 2px, 2py), we will get 3 sp2 hybrid orbitals oriented at 120° to each other. NOTE: The 2pz orbital is still present.
Valence Bond Theory and Double Bonds H H H H Ethylene C = C σ bonds are formed from the end-to-end overlap of orbitals. • Each C – H bond is a σ bond formed from the overlap of a C sp2 orbital with the H 1s orbital. • One of the C – C bonds is a σ bond formed from the overlap of C sp2 orbitals.
Valence Bond Theory and Double Bonds H H H H Ethylene C = C 3. The second C – C bond is a π bond formed from the overlap of the unhybridized C 2pz orbitals.
π Bonds H H H H Ethylene C = C The overlap in a π bond is side-to-side (NOT end-to-end like in σ bonds). This puts electron density above and below the line connecting the atoms.
Valence Bond Theory and Triple Bonds • In acetylene HC≡CH, there are two electron domains around each C atom. VSEPR theory says the geometry is linear and the bond angles are 180°. • A third set of hybrid orbitals must be present. They are formed from the 2s orbital and one of the 2p orbitals.
sp Hybrid Orbitals If we mix 2 orbitals (2s, 2px), we will get 2 sp hybrid orbitals oriented at 180° to each other. NOTE: The 2py and 2pz orbitals are still present.
Valence Bond Theory and Triple Bonds Acetylene H-C≡C-H Two bonds are π bonds formed from the overlap of the C 2py and C 2pz orbitals.
Hybrid Orbitals and Geometry • Identify the hybridization and bond angles around each atom in the following molecule.
Resonance Structures • Sometimes more than one valid Lewis structure can be drawn. Each of the Lewis structures is then called a resonance structure. • The real structure is a blend of the resonance structures.
Resonance Structures • A species with resonance structures is, in general, more stable than one without. • The presence of resonance structures is one criterion you will use to determine the strength of a base or a nucleophile.
Using Curved Arrows to Generate Resonance Structures • We will use curved arrows a LOT in this course. • These let you shift pairs of electrons to generate a new resonance structure. • The shift of the electron pair is from the tail to the head of the arrow.
Rules for Drawing Resonance Structures • Never break a single bond. • Period 2 elements may NEVER have more than an octet.
Rules for Determining Which Resonance Structures Are Significant • Although several resonance structures may be drawn for a species, some are more significant than others. • Rule 1 - A structure should have as few charges as possible. • Rule 2 - A structure with two formal charges is okay so long as the negative charge is on the more electronegative atom. • Rule 3 - The exception to #2 is when a positive charge on an electronegative atom will give all of the atoms an octet.
Rules for Determining Which Resonance Structures Are Significant • Draw resonance structures for acetic acid and assess the significance of each.
Formal Charges • Draw the Lewis structure, being careful to show nonbonding electrons (lone pairs). • formal charge = # of valence e- - # of bonds - # of nonbonding e-