CH221 CLASS 9
CHAPTER 5: AN OVERVIEW OF ORGANIC REACTIONS
Synopsis. This class presents an elementary account of reaction mechanism,
starting with free radical reactions and moving on to polar (ionic) reactions.
Mechanisms are discussed in terms of bond polarity and the terms electrophile
and nucleophile are introduced. Kinetic and thermodynamic aspects of reaction
mechanism are dealt with in the next class.
Kinds of Organic Reactions
Although millions of organic compounds, with dozens of different functional
groups, appear to undergo a daunting number of seemingly different and
unrelated reactions, even a brief analysis of these reactions soon shows that
they nearly all fall into four distinct general reaction types:
ORGANIC REACTIONS
ADDITION
ELIMINATION
SUBSTITUTION
REARRANGEMENT
Even seemingly more complex reactions, such as the condensation reactions of
aldehydes and ketones and polymerization of alkenes, fall into these categories –
for example, the condensation reaction is seen to be an addition reaction,
followed by an elimination reaction.
Additions
These reactions are characteristic of unsaturated systems (C=C, CC, C=O, etc)
and occur when a small molecule (H2, H2O, HCl, HBr, etc) adds across the
unsaturated group to give a saturated (or less unsaturated) product (the only
product).
+
Br
H
Br
H
Most additions are 1,2-additions (as above), but there are other types (e.g. 1,4additions to conjugated unsaturated systems).
Eliminations
These are the opposite of additions: a small molecule is removed from a
molecule to give an unsaturated product. The small molecule is often generated
by elimination of substituents from adjacent atoms (often carbon atoms, as
below), in which case they are called 1,2- or –eliminations.
Br
Br
H
H
+
base
CH3
CH
CH3
CH2
CH
CH2
Substitutions
Here, two reactants exchange parts to give two products. Commonly, substitution
occurs at carbon (as below), but can also occur at other atoms, like phosphorus
and sulfur.
light (h)
CH3
H
+
Cl
CH3
Cl
+
OH
CH3
Cl
-
CH3 OH
Cl
+
+
H
Cl
Cl -
Rearrangements
These reactions occur when a single reactant undergoes a re-organization of
bonds and atoms to yield (usually) an isomeric product. Often, rearrangements
involve a profound change of carbon skeleton, but some (e.g. sigmatropic
rearrangements) are more subtle (see textbook, chapter 30).
CH3
O
OH
C
acid
N
CH3
C
NH
Ph
The Beckmann
rearrangement
Ph
acetophenone oxime
acetanilide (an amide)
acid
CH3
CH2
1-butene
CH
CH2
CH3
CH
2-butene
CH
CH3
Alkene
isomerization
Mechanisms: How Organic Reactions Occur
The particular route an organic reaction takes between reactants and products is
called the reaction mechanism. A full mechanism includes details of the order of
bond breaking and bond making, information on intermediates (if any), as well as
kinetic and thermodynamic information. The most fundamental aspect of
mechanism is bond breaking and bond formation. Bonds can be broken evenly or
unevenly – the first type is called homolysis and the second kind is known as
heterolysis.
Homolysis: A----B
A.
B. (radicals)
+
Electronegativity of A ≈ B (A-B is a non-polar bond)
Heterolysis: A----B
A+
+
:B- (ions)
Electronegativity of A ≠ B , here B > A (A-B is a polar bond)
Bonds can be made in a similar fashion.
Homogenic bond making:
Heterogenic bond making:
A. + B. (radicals)
A+ + :B- (ions)
A----B
A----B
Reactions involving homolysis are called free radical or radical reactions,
because of the odd-electron species (called radicals) that are generated by such
a process. Reactions involving heterolysis are called polar (or ionic) reactions,
because of the polarity of the bonds that are broken in such a process. In
addition to radical and polar reactions, there is a third, less common type, known
as electrocyclic (or pericyclic) reactions (see textbook, chapter 30).
Radical Reactions
Although these are less common than polar reactions in organic chemistry, they
are nonetheless important, especially in some biological pathways. Radicals are
odd-electron species and because they are one electron short of a complete
outer octet, they tend to be reactive. They commonly undergo substitution and
additions reactions, as shown in the examples overleaf.
Note that electron redistribution occurs with single electrons, indicated by a half
arrow or “fish hook”. In general, radical reactions occur in three steps: initiation,
propagation and termination, as illustrated by the chlorination of methane, below.
light
CH4
+
Cl2
CH3Cl
+
HCl
Initiation
Radicals are first of all produced in initiation steps, often by the homolysis of
weaker covalent bonds, as in this example.
..
: Cl
..
..
Cl :
..
h
..
.
:
2 Cl
..
Propagation
Once a small population of chlorine atoms (radicals) has built up, a series of
radical-molecule reactions occur, in which radicals are continually regenerated.
Because the reactivity of radicals is generally high, their total concentration is low
throughout the reaction and hence radical-molecule reactions are statistically
much more likely than radical-radical reactions (see next section, termination).
Cl
.
.
CH3
H
+
+
Cl
Cl
CH3
Cl
CH3
Cl
.
CH3
+
H
+
Cl
Termination
When two radicals react to produce even-electron molecules, the reaction cycle
above is broken and the reaction is terminated. Normally, there is only a small
chance of this happening (see above), but if the initiation is stopped, then the
reaction will gradually stop as radical eventually meets radical and all radicals are
removed.
Cl
.
+
.
CH3
+
.
.
Cl
Cl
Cl
etc
Cl
CH3
Cl
Polar Reactions
Polar reactions, the most common types in organic chemistry, occur when a
polar reagent (or polarizable reagent) reacts with a molecule that contains a polar
.
bond (or polarizable bond). It has been shown in class 2 that many covalent
bonds in organic molecules have a significant degree of permanent polarity:
+
CH3
Cl
CH3
+
Li
H
+  C O
H
+
CH3 MgI
Other bonds, although of relatively low polarity (or even non-polar), can be
temporarily polarized by the approach of a polar reagent, which occurs during the
course of a reaction,
E.g.
 + C C
H+
and
HO
-
+
CH3
I
Because unlike charges attract, an electron rich center (called a nucleophile)
can donate an electron pair to an electron poor center (called an electrophile).
The resulting redistribution of electron pairs is indicated by the use of curved (or
curly) arrows: this is the fundamental nature of polar reactions. Curved arrows
should be used only to indicate change in electron pair distribution and only in
the sense (direction) shown below.
Electrophiles and Nucleophiles
Electrophiles
An electrophile is any species that is electron loving: it is electron deficient and
accepts an electron pair. Examples of electrophiles are given below.
Nucleophiles
A nucleophile is any species that is positive loving. It is electron rich and can
donate an electron pair. Examples are given below.
Examples of Polar (Electrophile-Nucleophile) Reactions, Using Curved
Arrows
The examples are chosen to illustrate three of the most important reaction types
in organic chemistry.
Electrophilic Addition to Alkenes
The term electrophilic in the title refers to the reagent needed, here in the first
(slow or rate-determining) step.
H
H
Step 1
H
C
C
H
+
H
H
slow
Br
H
+
C
C
H
Br-
+
H
H
nucleophile
electrophile
H
Br-
Step 2
+
H
+
C
Br
H
C
C
H
H
fast
C
H
H
H
H
H
On approach of the polar HBr molecule, via its electrophilic hydrogen atom, the
C=C bond becomes more and more polarized (double bonds are more easily
polarized than single bonds) until it eventually breaks to form the charged
carbocation intermediate.
Nucleophilic Substitution in Alkyl Halides and Alcohols
Alkyl halides need no activation, but the OH group of alcohols needs to be
activated (e.g. by protonation) before substitution occurs.
electrophile
nucleophile
:N
C:
Cl:
-
+
+
CH3
I
CH3
+
O
..
N
C
CH3
H
Cl
CH3
+
: I-
+
..
O
..
H
H
H
Only one particular mechanism, the single step SN2 mechanism, is shown here.
Note the flow of electron pairs and note the synchronous bond breaking and
bond making. Note also the balance of charge and the following of the octet rule.
Nucleophilic Addition to the Carbonyl Group
Note again the balance of charge and the following of the octet rule in these two
examples. Nucleophilic additions to carbonyl are often helped by activation of the
carbonyl group, such as by protonation, as shown here.
electrophile
nucleophile
:N
C:
:N
C:
-
+
+
C
:
O
..
C
+
O
..
N
C
C
.. O
.. :
C
..
O
..
H
N
C
H
Class Questions
1. Which of the following species is likely to be an electrophile, and which a
nucleophile?
(a) HCl (b) (CH3)3N (c) (CH3)2S (d) CH3CHO
Cl
Nucleophilic
+
H
Electrophilic
CH3
N:
CH3
CH3
Nucleophilic
..
S:
CH3 CH3
Nucleophilic
..  O:
Nucleophilic
CH3
+
C
H
Electrophilic
2. Draw a Lewis diagram for boron trifluoride (BF3) and explain whether it will
be electrophilic or nucleophilic in its chemical behavior.
..
: F:
B
: ..
F:
: F:
..
B is electron deficient (only 6
electrons in outer shell), hence
BF3 is electrophilic in character
: