2. Chemical Reaction: A transformation resulting in a change
of composition, constitution and/or configuration of a
compound.
Organic reactions occur between organic molecules
(molecules containing carbon and hydrogen). Since there is a
virtually unlimited number of organic molecules, the scope of
organic reactions is very large. However, many of the
characteristics of organic molecules are determined
by functional groups—small groups of atoms that react in
predictable ways.
TYPES OF REACTIONS:
A. By Structural Change
Addition- the number of σ-bonds in the substrate molecule
increases, usually at the expense of one or more π-bonds
3. Elimination- the number of σ-bonds in the substrate
decreases, and new π-bonds are often formed.
Substitution-characterized by replacement of an atom or
group (Y) by another atom or group (Z). Aside from these
groups, the number of bonds does not change.
4. Rearrangement- generates an isomer, and again the
number of bonds normally does not change.
EXAMPLE: addition and elimination reactions
5. B. Classification by Reaction Type
Acid- Base Reactions- also called neutralization reactions.
Uses the Bronsted Theory and the Lewis Theory
. NH4
(+) + H2O H3O(+) + NH3
Redox Reactions-the change in oxidation state of those
carbons involved in a chemical transformation is taken into
account. To determine whether a carbon atom has undergone
a redox change during a reaction we simply note any changes
in the number of bonds to hydrogen and the number of bonds
to more electronegative atoms such as O, N, F, Cl, Br, I, & S
that has occurred
6.
7. INTERMEDIATES- The products of bond breaking, shown
above, are not stable in the usual sense, and cannot be
isolated for prolonged study.
Carbocations are compounds in which carbon bears a
positive charge. Classical carbocations or carbenium ions
are trivalent, and have only six valence electrons.
Nonclassical carbocations or carbonium ions are tetra-
or pentavalent, and have eight valence electrons. Examples
are the methyl cation (classical) CH3
+, and the methonium ion
(nonclassical) CH5
8. Carbanions are compounds in which carbon bears a
negative charge. A carbanion will always have a
positive counterion in association with it; depending upon the
particular cation and the stability of the carbanion, the
association may be ionic, covalent, or some intermediate
combination of ionic and covalent bonding, as shown below (M
= metal). Carbanions are trivalent, with eight valence
electrons.
Free radicals are neutral compounds having an odd number
of electrons and therefore one unpaired electron. Carbon free
radicals are trivalent, with seven valence electrons, and
typically assume a planar structure.
9. Free radicals are primarily electron-deficient species and are
stabilized by structural features which donate electron density
or delocalize the odd electron by resonance
Radical ions - charged compounds with an unpaired
electron, and are either radical cations or radical anions. They
are derived from a stable neutral molecule by addition of one
electron or removal of one electron
Carbenes - compounds which have a divalent carbon. The
divalent carbon also has two nonbonded electrons, for a total
of six valence electrons. The two nonbonded electrons may
have either the same spin quantum number, which is a triplet
state, or an opposite spin quantum number, which is
a singlet state. Generation of carbenes is most commonly
by photolysis or thermolysis of diazo compounds or ketenes,
or by alpha-elimination reactions
10. REAGENTS- common partner of the reactant in many
chemical reactions. It may be organic or inorganic; small or
large; gas, liquid or solid. The portion of a reagent that ends
up being incorporated in the product may range from all to
very little or none.
TYPES:
•Electrophiles- Reagents that seek electrons so as to achieve
a stable shell of electrons like that of a noble gas.
•Nucleophiles- On the other hand, these are reagents that
seek a positive centre other than a proton.
11. Factors that Influence Reactions
A. Energetics: The potential energy of a reacting system
changes as the reaction progresses. The overall change may
be exothermic ( energy is released ) or endothermic (
energy must be added ), and there is usually an activation
energy requirement as well. As a rule, compounds
constructed of strong covalent bonds are more stable than
compounds incorporating one or more relatively weak bonds.
B. Electronic Effects: The distribution of electrons at sites of
reaction (functional groups) is a particularly important factor.
Electron deficient species or groups, which may or may not be
positively charged, are attracted to electron rich species or
groups, which may or may not be negatively charged.
12. We refer to these species as electrophiles &
nucleophiles respectively. In general, opposites attract and
like repel.
The charge distribution in a molecule is usually discussed with
respect to two interacting effects: An inductive effect, which is
a function of the electronegativity differences that exist
between atoms (and groups); and a resonance effect, in
which electrons move in a discontinuous fashion between
parts of a molecule.
C. Steric Effects: Atoms occupy space. When they are
crowded together, van der Waals repulsions produce an
unfavorable steric hindrance. Steric hindrance may
influence conformational equilibria, as well as
destabilizing transition states of reactions.
13. D. Stereoelectronic Effects: In many reactions atomic or
molecular orbitals interact in a manner that has an optimal
configurational or geometrical alignment. Departure from this
alignment inhibits the reaction.
E. Solvent Effects: Most reactions are conducted in
solution, not in a gaseous state. The solvent selected for a
given reaction may exert a strong influence on its course.
Remember, solvents are chemicals, and most undergo
chemical reaction under the right conditions.
.
14. ACTIVATION ENERGY
Every reaction in which bonds are broken will have a high
energy transition state that must be reached before products can form.
In order for the reactants to reach this transition state, energy must be
supplied and reactant molecules must orient themselves in a suitable
fashion. The energy needed to raise the reactants to the transition state
energy level is called the activation energy, ΔE‡.
Exothermic Endothermic Exothermic
Single Step Reaction Single Step Reaction Two Step Reaction
15. What is the source of the activation energy that enables
a chemical reaction to occur?
Often it is heat, as noted above in reference to the flame or
spark that initiates methane combustion. At room
temperature, indeed at any temperature above absolute
zero, the molecules of a compound have a total energy that
is a combination of translational (kinetic) energy, internal
vibrational and rotational energies, as well as electronic and
nuclear energies. The temperature of a system is a measure
of the average kinetic energy of all the atoms and molecules
present in the system.
