Structure and Reactivity: Acids and Bases, Polar and Nonpolar Molecules

2-1          Kinetics and Thermodynamics of Simple Chemical Processes

Reactions controlled by ΔG = ΔH – TΔS

Organic reactions not affected dramatically by ΔS

Equilibrium positioned controlled by ΔG

2-2          Acids and Bases; Electrophiles and Nucleophiles

pKa is a measure of acidity and equilibrium in acid-base reactions

pKa exaggerates acidic strength

2-3          Functional Groups: Centers of Reactivity

Functional groups are collections of atoms that undergo characteristic reactions. Examples:

C–H Alkanes; rx with Cl2 and light yields C–Cl

C–O–H Alcohols; rx with HCl yields C–Cl

C–Cl Alkyl chlorides; rx with NaOH yields C–OH

2-4       Straight-Chain and Branched Alkanes

            Carbons can be connected in a straight chain where there are only methyl groups (primary, at the ends) and methylene (secondary) groups in the middle

            Carbon chains can be branched where some carbon atoms have 3 (tertiary, methyne)  and 4 other carbons

2-5       Naming the Alkanes

            Nomenclature rules are straight forward and will be covered only when needed in lecture. Students should learn how to draw a structure from a name.

2-6       Structural and Physical Properties of Alkanes

            Physical properties of organic compounds are controlled by structure which impacts intermolecular forces. As molecular weight increases, so does boiling point (in general) and as greater intermolecular attraction increases, so does boing point.

2-7       Rotation about Single Bonds: Conformations

            Conformational isomers differ by rotation about sigma bonds

            In ethane, the two extremes of rotation differ by 2.9 kcal/mol

2-8          Rotation in Substituted Ethanes

Energy difference between eclipsed and staggered conformations of propane is 3.2 kcal/mol and represents the activation energy for the rotational process.

Increase in energy due to rotation is called torsional energy or torsional strain.

For rotation about the central C–C bond of butane there are two unique staggered conformations differing in the relative position of the two methyl groups. The anti conformation has the methyl groups farthest apart and is 0.9 kcal/mol more stable than is the quache conformation where the methyl groups are close to each other.

There are also two difference eclipsed conformations. That with the two methyl groups closest to each other is 4.9 kcal/mol less stable than is the anti conformation. The other eclipsed conformation where the methyl groups are 120o apart is 3.6 kcal/mol less stable than is the anti.

In conformations where atoms (usually HÕs) would otherwise be closer than their van der Waals radii, torsional angles and bond angles change so as to avoid the overlap.

 

            The Big Picture