REACTION-MECHANISM Handout

REACTION MECHANISM Steps in Free Radical Substitution
What is reaction mechanism? 1. Initiation = making radicals.
 In chemical reactions, the detailed 2. Propagation = we’re getting rid of one

processes by which chemical radical species but then making
substances are transformed into another radical.
other substances.
 The reactions themselves may 3. Termination = stopping the reaction of
involve the interactions
radicals
of atoms, molecules, ions, electrons,
and free radicals, and they may take NUCLEOPHILIC SUBSTITUTION
place in gases, liquids, or solids—or
at interfaces between any of these.  Nucleophile, in chemistry, an atom
RADICAL or molecule that in chemical
reaction seeks a positive centre,
 Moses Gomberg, discovered an such as the nucleus of an atom,
organic free radical. because the nucleophile contains an
 Radical, also called Free Radical, electron pair available for bonding.
in chemistry, molecule that contains
at least one unpaired electron.  Substitution reactions are divided
 Chemically reactive and highly into three general classes,
unstable.
depending on the type of atom or
2 Types of Bond Cleavages group that acts as the substituent. In
one, the substituent is electron-rich
 HETEROLYTIC CLEAVAGES and provides the electron pair for
- Two different atoms attached by bonding with the substrate (the
means of a bond. When this bond breaks molecule being transformed). This
the electrons will go towards to the atom type of reaction is known
that has a high electronegativity. as nucleophilic substitution.
 HOMOLYTIC CLEAVAGES  Examples of nucleophilic reagents

are the halogen anions (Cl-, Br-, I-),
- when the bond breaks the electron
will distributed equally ammonia (NH3), the hydroxyl group,
the alkoxy group (RO−), the cyano
Halogenation of Alkane group, and the hydrosulfide group.
● Alkanes are notoriously unreactive Sn1 and Sn2 Reaction

compounds because they are non-
polar and lack functional groups at  In 1935, Edward D. Hughes and Sir
which reactions can take place. Christopher Ingold studied
nucleophilic substitution reactions
● The reaction of a halogen with an of alkyl halides and related
alkane in the presence of ultraviolet compounds. They proposed that
or heat leads to the formation of there were two main mechanisms at
haloalkane. work, both of them competing with
each other. The two main
mechanisms were the SN1
reaction and the SN2 reaction,
where S stands for Sn1 Reaction Mechanism
substitution, N stands for  SN1 reaction involves two steps. SN1
nucleophilic, and the number reactions tend to be important when
represents the kinetic order of the the central carbon atom of the
reaction. substrate is surrounded by bulky
Sn2 Reaction Mechanism groups, both because such groups
interfere sterically with the SN2
 In the SN2 reaction, the addition of reaction (discussed above) and
the nucleophile and the elimination because a highly substituted carbon
of leaving group take place forms a stable carbocation.
simultaneously (i.e. a concerted
reaction). SN2 occurs when the
central carbon atom is easily
accessible to the nucleophile.
Few factors that affect the reaction

rate of SN1 reactions
In SN2 reactions, there are a few conditions  Instead of having two concentrations
that affect the rate of the reaction. that affect the reaction rate, there is
only one, substrate. The rate
 First of all, the 2 in S N2 implies that equation for this would be
there are two concentrations of Rate=k[Sub]. Since the rate of a
substances that affect the rate of reaction is only determined by its
reaction: substrate (Sub) and slowest step, the rate at which the
nucleophile. The rate equation for leaving group "leaves" determines
this reaction would be Rate=k[Sub] the speed of the reaction. This
[Nuc]. means that the better the leaving
group, the faster the reaction rate.
- For a SN2 reaction, an aprotic
solvent is best, such as acetone,  As SN2 reactions were affected by
DMF, or DMSO. sterics, SN1 reactions are
determined by bulky groups
 A final factor that affects reaction attached to the carbocation. Since
rate is nucleophilicity; the there is an intermediate that actually
nucleophile must attack an atom contains a positive charge, bulky
other than a hydrogen. groups attached are going to help
stabilize the charge on the
carbocation through resonance and
distribution of charge.
Take note that:
 The nucleophile can attack from the

top or the bottom and therefore
create a racemic product. It is
important to use a protic solvent,
water and alcohols, since an aprotic
solvent could attack the intermediate
and cause unwanted product. It
does not matter if the hydrogens
from the protic solvent react with the
nucleophile since the nucleophile is
not involved in the rate determining
step.
ELIMINATION
 elimination reaction, any of a class

of organic chemical reactions in
which a pair of atoms or groups of
atoms are removed from a molecule,
usually through the action of acids,
bases, or metals and, in some
cases, by heating to a
high temperature.
ELECTROPHILIC ADDITION
 addition reaction, any of a class of

chemical reactions in which
an atom or group of atoms is added
to a molecule.
 A typical addition reaction may be

illustrated by the hydrochlorination of
propene (an alkene), for which the
equation is
CH3CH = CH2 + HCl → CH3C+HCH3 + Cl− →

CH3CHClCH3

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REACTION MECHANISM Steps in Free Radical Substitution

What is reaction mechanism? 1. Initiation = making radicals.

 In chemical reactions, the detailed 2. Propagation = we’re getting rid of one

 HOMOLYTIC CLEAVAGES  Examples of nucleophilic reagents

● Alkanes are notoriously unreactive Sn1 and Sn2 Reaction

Few factors that affect the reaction

Take note that:

 The nucleophile can attack from the

 elimination reaction, any of a class

 addition reaction, any of a class of

 A typical addition reaction may be

CH3CH = CH2 + HCl → CH3C+HCH3 + Cl− →

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