CHM 102 (General Chemistry II) Lecture: Prof. Olowu R.A
Topic: Organic Reaction Mechanism Session: 2022/2023
REACTION MECHANISM: A reaction mechanism is a sequence of two or more steps, each one
represented by an equation that shows how a reaction takes place. A mechanism tries to explain
the actual changes that occur in a reactions that have been carefully studied, and for which there
are explanation of exactly how they occur.
BOND BREAKING IN ORGANIC REACTION
Organic compounds contain covalent bond. For a bond to break, the electrons in the bonding
molecular orbital (MO) must receive energy from their surroundings. Specifically, the surrounding
molecules must transfer some of their kinetic energy to the system (the bond being broken). The
term enthalpy is used to measure this exchange of energy:
∆H = q (at constant pressure)
The change in enthalpy (∆H) for any process is defined as the exchange of kinetic energy, also
called heat (q), between a system and its surroundings under conditions of constant pressure. For
a bond-breaking reaction, ∆H is primarily determined by the amount of energy necessary to break
the bond homolytically.
There are two ways for the covalent bond to break. These are hemolytic fission and heterolytic
fission.
Homolytic Fission: This type of bond breaking generates two uncharged species, called radicals,
each of which bears an unpaired electron. In contrast, heterolytic fission is a type of bond breaking
that generates charged species, called ions. Heterolytic fission usually occurs when the two atoms
bonded together have different electronegativities. The atom with the higher electronegativity is
the one that keeps both electrons from the bond and bears the negative charge, while the other
atom
becomes
positive
charge.
Figure 1: Homolytic fission produce two radicals
Figure 2: Heterolytic fission
produces ions
The energy required to break a covalent bond via homolytic bond cleavage is called the bond
dissociation energy. The term ∆H° (with the “naught” symbol, or small circle, next to the H)
refers to the bond dissociation energy when measured under standard conditions (i.e., where the
pressure is 1 atm and the compound is in its standard state: a gas, a pure liquid, or a solid). Table
1 gives ∆H° values for a variety of bonds commonly encountered in organic reaction.
Table
1:
Bond
dissociation
energies
(∆H°)
of
common
bonds
Most reactions involve the breaking and forming of several bonds. In such cases, we must take
into account each bond being broken or formed. The total change in enthalpy (∆H°) for the reaction
is referred to as the heat of reaction. The sign of DH° for a reaction (whether it is positive or
negative) indicates the direction in which the energy is exchanged and is determined from the
perspective of the system. The direction of energy exchange is described by the terms endothermic
and exothermic. In an exothermic process, the system gives energy to the surroundings (∆H° is
negative). In an endothermic process, the system receives energy from the surroundings (∆H° is
positive).
Figure 3: Energy profile for exothermic and endothermic reactions
Class activity: Predict the sign and magnitude of ∆H° for the following reaction. Give your answer
in units of kilojoules per mole, and identify whether the reaction is expected to be endothermic or
exothermic.
Solution:
Identify
all
bonds
that
are
either
broken
or
formed:
Now we must decide what sign (+ or -) to place in front of each value. For each bond broken, the
system must receive energy in order for the bond to break, so ∆H° must be positive. In contrast,
for each bond formed, the electrons are going to a lower energy state, and the system releases
energy to the surroundings, so ∆H° must be negative.
Total bond breaking =
+ 624 kJ/mol
Total bond formed = - 762 Kj/mol
∆H° = Bond formed + bond breaking
-138 kJ/mol.
For this reaction ∆H° is negative, which means that the system is losing energy. It is giving off
energy to the environment, so the reaction is exothermic..
Practice questions: Using the data in Table 1, predict the sign and magnitude of DH° for each of
the following reactions. In each case, identify whether the reaction is expected to be endothermic
or exothermic:
ENTROPY OF ORGANIC REACTION
Entropy is informally defined as the measure of disorder associated with a system, although this
definition is overly simplistic. Entropy is more accurately described in terms of probabilities.
Entropy is formally defined as a property of matter associated with the degree of disorder or degree
of randomness, of particles (i.e. atoms, molecules or ions) and also with the distribution of quanta
of energy between the particles. A process that involves an increase in entropy is said to be
spontaneous. A spontaneous process the one that takes place without continuous intervention by
us. Chemical reactions are no exception, although the considerations are slightly more complex
than in a simple free expansion. In the case of free expansion, we only had to consider the change
in entropy of the system (of the gas particles). The surroundings were unaffected by the free
expansion. However, in a chemical reaction, the surroundings are affected. We must take into
account not only the change in entropy of the system but also the change in entropy of the
surroundings:
∆Stot = ∆Ssys + ∆Ssurr
Where ∆Stot is the total change in entropy associated with the reaction. In order for a process to
be spontaneous, the total entropy must be positive.
Entropy Change of System:
∆Ssys is calculated using the expression:
Entropy Change of Surrounding:
Gibbs Free Energy of Organic reaction
Gibbs free energy is a quantity that is used to measure the maximum amount of work done in a
thermodynamic system when the temperature and pressure are kept constant. Gibbs free energy is
denoted by the symbol ∆G.
In other words, ∆G can be calculated by using the expression:
In order for a process to be spontaneous, ∆G for that process must be negative. Any process with
a negative DG will be spontaneous. Such processes are called exergonic. Any process with a
positive ∆G will not be spontaneous. Such processes are called endergonic.
Figure 4: Energy diagrams for an exergonic and endergonic process
Reaction Equilibria in Organic Reaction
Consider the energy diagram in Figure 6.9, showing a reaction in which the reactants, A and B,
are converted into products, C and D. The reaction exhibits a negative DG and therefore will be
spontaneous. Accordingly, we might expect a mixture of A and B to be converted completely into
C and D. But this is not the case. Rather, an equilibrium is established in which all four compounds
are present. Why should this be the case? If C and D aretruly lower in free energy than A and B,
then why is there any amount of A and B present when the reaction is complete?
Figure 6: The energy diagram of an exergonic reaction in which reactants (A and B) are
converted into products (C and D).
When the reaction begins, only A and B are present. As the reaction proceeds, the concentrations
of A and B decrease, and the concentrations of C and D increase. As the reaction proceeds, the
free energy decreases until it reaches a minimum value at very particular concentrations of
reactants and products. If the reaction were to proceed further in either direction, the result would
be an increase in free energy, which is not spontaneous. At this point, no further change is
observed, and the system is said to have reached equilibrium. The exact position of equilibrium
for any reaction is described by the equilibrium constant, Keq,
Figure 7: An energy diagram illustrating the relationship between ∆G and
concentration. A minimum in free energy is achieved at particular concentrations
(equilibrium).
If the concentration of products is greater than the concentration of reactants, then Keq will be
greater than 1. On the other hand, if the concentration of products is less than the concentration of
reactants, then Keq will be less than 1. The term Keq indicates the exact position of the equilibrium,
and it is related to DG in the following way, where R is the gas constant (8.314 J/mol · K) and T
is the temperature measured in Kelvin:
∆G = -RT ln Keq
For any process, whether it is a reaction or a conformational change, the relationship between Keq
and G is defined by the equation above. If ∆G is negative, the products will be favored (Keq > 1).
If DG is positive, then the reactants will be favored (Keq < 1). In order for a reaction to be useful
(in order for products to dominate over reactants), ∆G must be negative; that is, Keq must be greater
than 1.
Practice Questions:
In each of the following cases, use the data given to determine whether the reaction favors reactants
or products:
(a) A reaction for which ∆G = +1.52 kJ/mol (b) A reaction for which Keq = 0.5
(c) A reaction carried out at 298 K, for which DH = +33 kJ/mol and DS = +150 J/mol # K
(d) An exothermic reaction with a positive value for ∆Ssys (e) An endothermic reaction with a
negative value for ∆Ssys
Types of Reactions in Organic Chemistry
There are five basic types of reaction in organic chemistry:
1. Addition reaction
2. Substitution reaction
3. Oxidation reaction
4. Reduction reaction
5. Polymerization reaction
1. Addition Reaction: In addition reaction, two reactant species combine together to form a
single species. All the species involved are usually molecules. One example is the reaction
between ethane and bromine.
C2H4 + Br2 → C2H4Br2
2. Substitution Reaction: In a substitution reaction, two reactants species combine together to
form two products.
C2H5Br + OH- → C2H5OH + BrIn the reaction above, OH has taken the place of, or substituted the Br atom.
3. Oxidation Reaction: This is a reaction in which a substance gains oxygen or loses
hydrogen. The oxygen atom is usually from on oxidizing agent (eg. K2CrO7 or KMnO4)
represented by [O].
C2H5OH + [O] → CH3CHO + H2O
4. Reduction Reaction: This is a reaction in which a substance gains hydrogen or loses
oxygen. The hydrogen is usually from a reducing agent (eg. NaBH4)
CH3CHO + 2[H] → CH3CH2OH
5. Polymerization Reaction: This is a reaction in which two or more smaller molecules
combine together to form a larger one. The smaller molecules are called monomer while
the larger one is called polymer. A general equation for this reaction is
NUCLEOPHILE AND ELECTROPHILE
Nucleophile: This a chemical species that forms bonds by donating an electron pair. All molecules
and ions with a free pair of electrons or at least one pi bond can act as nucleophiles.
Examples of nucleophiles are anions such as Cl− OH- carbanion (C-), or a compound with a lone
pair of electrons such as H2O, NH3 (ammonia) and PR3
Electrophile: This is a species that is attracted to a region of high electron density. They are either
positively charged or neutral species.
Examples include carbocations (C+) and carbonyl
compounds.
QUESTIONS:
SIMPLE MECHANISM OF CHLORINATION OF METHANE
In the free radical chlorination of methane, the chain initiation step involves the formation of
chlorine radicals. Here, the homolytic cleavage of the Cl2 molecule takes place in the presence of
heat or light because the Cl−Cl bond is weaker than C−C and C−H. Hence, Cl−Cl is easier to break
and
Chlorination of methane:
forms
chlorine
radicals.