IMPORTANT CONCEPTS
1. Chemical reactions can be described as equilibria controlled by thermodynamic and kinetic parameters. The
change in the Gibbs free energy, ΔGo, is related to the equilibrium
constant by ΔGo = - RT ln K = - 1.36 log K
(at 25oC). The free energy has
contributions from changes in enthalpy, ΔHO, and entropy, ΔSo: ΔGo = ΔHo – TΔSo. Changes
in enthalpy are due mainly to differences between the strengths of the bonds
made and those of the bonds broken. A reaction is exothermic when the former
is larger than the latter. It is endothermic when there is a
net loss in combined bond strengths. Changes in entropy are controlled by the
relative degree of energy dispersal in starting materials compared with that in
products. The greater the increase in energy dispersal, the larger a positive ΔSo.
2.
The rate of a chemical reaction depends mainly on the concentrations of
starting material(s), the activation energy, and temperature. These correlations
are expressed in the Arrhenius equation: rate constant k = Ae-Ea/RT.
3.
If the rate depends on the concentration of only one starting material, the
reaction is said to be of first order. If
the rate depends on the concentrations of two reagents, the reaction is of second
order.
4.
Brønsted acids are proton donors; bases are proton acceptors. Acid strength is
measured by the acidity constant Ka; pKa = - log
Ka. Acids and their deprotonated forms have a conjugate relation.
Lewis acids and bases are electron pair acceptors and donors, respectively.
5. Electron-deficient
atoms attack electron rich atoms and are called electrophiles. Conversely, electron-rich
atoms attack electron-poor atoms and are called nucleophiles. When a
nucleophile, which may be either negatively charged or neutral, attacks an
electrophile, it donates a lone electron pair to
form a new bond with the electrophile.
6. An organic
molecule may be viewed as being composed of a carbon skeleton with attached functional
groups.
7. Hydrocarbons are
made up of carbon and hydrogen only. Hydrocarbons possessing only single bonds
are also called alkanes. They do not contain functional groups. An alkane may
exist as a single continuous chain or it may be branched or cyclic. The
empirical formula for the straight-chain and branched alkanes is CnH2n+2.
8. Molecules that
differ only in the number of methylene groups, CH2, in the chain are
called homologs and are said to belong to a homologous series.
9. An sp3carbon
attached directly to only one other carbon is labeled primary. A secondary
carbon is attached to two and a tertiary to three other carbon atoms. The
hydrogen atoms bound to such carbon atoms are likewise designated primary,
secondary, or tertiary.
10. The IUPAC rules for naming saturated hydrocarbons are (a) find the longest continuous
chain in the molecule and name it; (b) name all groups attached to the longest
chain as alkyl substituents; (c) number the carbon atoms of the longest chain;
(d) write the name of the alkane, citing all substituents
as prefixes arranged in alphabetical order and preceded by numbers designating their
positions.
11. Alkanes
attract each other through weak London
12. Rotation about
carbon – carbon single bonds is relatively easy and gives rise to conformations
(conformers). Substituents on adjacent carbon atoms may be staggered or
eclipsed. The eclipsed conformation is a
transition state between staggered conformers. The energy required to reach the
eclipsed state is called the activation energy for rotation. When both carbons
bear alkyl or other groups, there may be additional conformers: Those in which
the groups are in close proximity (60o) are gauche; those in which
the groups are directly opposite (180o) each other are anti. Molecules
tend to adopt conformations in which steric hindrance, as in
gaucheconformations, is minimized.
From "Organic Chemistry" Textbook of VOLLHARDT & SCHORE
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