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---------------------------------------------------------------------------------------------------------Chemistry 211: Organic Chemistry 1
Syllabus
Kinetics and Energetics of Reactions; Hydrocarbons, Multiple Bonds; Aromatic Compounds;
Chemistry of C-Halogens Bond; Chemistry of Hydroxyl group; Ethers, Thiols and Thioethers
.
Chapter 1: Kinetics and Energetics of Reactions
1.1 Reaction
A chemical reaction takes place when a molecule (starting material or a reactant) is
transformed into another molecule (product) under a specified condition such as temperature,
pressure, concentration, state of the reacting species etc. When a reaction takes place, bonds
are broken and new ones formed which indicates that there is a need for energy input or
output for a reaction to occur.
A reaction may be a one-step (no intermediate step) or a multi-step process (involves
formation of intermediates) and is affected by factors such as: physical state of participating
species, steric hindrance, reaction conditions, type of reactants (electron rich or deficient) etc.
The step involved in chemical reactions is called the reaction mechanism for that process.
The reaction mechanism describes how a chemical change occurs and is described as a
theoretical construct or model for a chemical change. The mechanism can then be proven
experimentally to establish the mechanism. Each step of a reaction mechanism is called an
elementary step
The elementary step is a process whereby the reactant is converted to the product by passing
through one transition stage with no intermediate stage. The step always occurs by simple
nuclear motions that occurs simultaneously with no intermediate stage and is characterized
by forward and reverse microscopic rate constants.
1.2 Types of Reaction
There are four major types of reactions:
(1) Displacement: Involves removal of an atom or group of atoms from the central atom
(usually carbon). The displacement could be electrophilic (E): Ph + NO2+ Ph-NO2 +
H+,
Nucleophilic (N) e.g. NC- + R-Cl NC-R + Cl- or
Radical mechanism (R) (halogenations of alkanes)
Usually the displaced atom is hydrogen, group of atoms or any other atom
1
, (2) Addition: Adding an atom, molecule or ion to another molecule. It could be initiated by
electrophilic, nucleophilic or radical mechanism. The attack is usually on C-C multiple
bonds (E, R) e.g. H2 C=CH2 + HF H3C-CH2-F
or by nucleophilic attack on C-O bonds
(3) Elimination: Opposite reaction of addition reaction. Involves loss of hydrogen and a loss
of another atom or group of atoms to from an unsaturated bond
CH3CH2F CH2=CH2 + HF
(4) Rearrangement: this involves rearrangement of the skeleton of a compound (carbon
compounds). It may sometimes involve rearrangement through an intermediate in a
chemical reaction. The rearrangement process may involve a cation, anion, radicals,
carbonium ions or other electron deficient species. Rearrangement is often accompanied
by addition or elimination reaction.
H+
Me2C(OH)-C(Me2)OH Me3C-C=O(Me)
1.3. Factors that Affects a chemical Reaction
- There are many factors that affect a chemical reaction: type of bonds (single, multiple,
conjugated, polarized etc)
- Others include availability of electrons (electron density), resonance, steric effect, reagent type
(electron donating or withdrawing, nucleophilic, electrophilic, carbonium etc)
Chapter 2 Energetics, Kinetics and Mechanism of a Chemical Reaction
2.1 Chemical Reaction and Energy Change
- A chemical reaction involves converting a starting material (reactants) to a product:
A↔ B
- There is always an equilibrium between the reactant and the product
- The direction of a reaction depends on the stability of the reactant versus the product at
equilibrium
- Whichever of the two is the more stable determines the direction of the reaction
- Reactant
-
Increasing Stability ∆stability
Product
2
, - Conversion of reactants to products is always accompanied by an energy change which
can be measured simply as the heat of reaction, ∆H (enthalpy, difference between bond
energies of the product and the reactants)
- However, ∆H is not a sufficient measure of the difference in stability of the reactant vs.
product
- Other factors like the entropy ∆S contributes to the measure of stability of the reactant vs.
product (second law of thermodynamics i.e. every ordered system always wants to be
disordered)
- The most stable condition tends to have the minimum energy or enthalpy and maximum
entropy
- Hence change in energy in going from a reactant to a product is expressed by the
equation:
∆G ∆H= ∆H -T∆S (T= absolute temperature)
- ∆G is also related to the equilibrium constant (K) by the equation :
-∆G = 2.303RTlog K
- As the value of -∆G becomes larger, value of K also becomes larger and the formation of
the product is more favored.
2.2 Entropy of a Chemical Reaction
- Consider and equilibrium reaction: A ↔ B + C, the entropy of this reaction can be explained
in terms of (i) the number of molecules or new substances formed (ii) the total # of ways in
which its total energy can be distributed among all possible molecules or substances arising
because of the reaction and (iii) the number of ways in which each molecules quantum energy
will be shared between its translational, rotational, vibrational energies. Oftentimes, it is the
translational energy that will take the largest share of each molecule’s quantum energy.
- For example, when calcium carbonate is heated, it decomposes to carbon dioxide and water
CaCO3s CO2g + H2Og
- There is an increase in the number of molecules (2 on product side vs. one on reactant
side), which indicates an increase in translational degrees of freedom: results into
increase in entropy.
- The term -T∆S in the equation ∆G ∆H= ∆H -T∆S becomes larger,
- If -T∆S >∆H endothermic or exothermic , then ∆G will be largely negative which favors
formation of the products
- The converse is also very true: if the reaction is reversed, there will be a decrease in the
number of molecules form (decrease in degrees of translational energy, e.g. cyclisation
reactions),
- hence, decrease in the value of entropy and the term -T∆S <∆H may be sufficiently small
such that
3
, - ∆G may be largely positive, which then favors formation of the reactants
- However, if ∆H is sufficiently negative (exothermic), the reaction may favor formation
of the product.
2.3 Kinetics of a Chemical Reaction
- The value of ∆G only tells us how far a reaction can go (in the direction of the product or
reactant)
- it will not tell us how fast a reaction can go in forming a product
- For example, cellulose can be oxidized thus: (C6H10O5)n + 6nO2 ↔ 6nCO2 + 5nH2O
- ∆G is largely negative, hence the reaction is totally converted to carbon dioxide and water
- Paper is also cellulose but it can stay in room full of oxygen for a very long time before it starts
to be oxidized
- the first reaction is fast while the second reaction (paper oxidation) is very slow even thou both
has a very large negative value of ∆G
- In any reaction, there is an energy barrier that needs to be surmounted in converting a reactant
to a product as shown in the energy profile below. This energy barrier is called the activation
energy
∆G y
∆G#
Reactant
-∆G-
Product
Figure 2.1 Energy Profile
2.4 Reaction Rate and free Energy of Activation
- The position y in the energy profile shown above is the least stable position in the process of
forming the product
- This position is called the activated complex or activated transitional state (AC)
4
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