Bond Breaking and Bond Making in Organic Compounds
Bond Breaking and Bond Making
** Having now
learned how to write and identify some common kinds of organic reactions, we can
turn to a discussion of reaction mechanism.
** A reaction mechanism is a detailed description
of how bonds are broken and formed as a starting material is converted to a
product.
** A reaction
mechanism describes the relative order and rate of bond cleavage and formation.
It explains all the known facts about a reaction and accounts for all products
formed, and it is subject to modification or refinement as new details are
discovered.
** A reaction
can occur either in one step or in a series of steps:
(1) A
one-step reaction is called a concerted reaction. No matter how many bonds are
broken
or formed, a
starting material is converted directly to a product.
(2) A
stepwise reaction involves more than one step. A starting material is fi rst
converted to
an unstable
intermediate, called a reactive intermediate, which then goes on to form the
product.
Bond Cleavage
** Bonds are
broken and formed in all chemical reactions. No matter how many steps there are
in the reaction, however, there are only two ways to break (cleave) a bond: the
electrons in the bond can be divided equally or unequally between the two atoms
of the bond.
(A) Breaking a bond by equally dividing the
electrons between the two atoms in the bond is called homolysis or homolytic
cleavage.
(B) Breaking a
bond by unequally dividing the electrons between the two atoms in the bond is called
heterolysis or heterolytic cleavage. Heterolysis of a bond between A and B can
give either A or B the two electrons in the bond. When A and B have different electronegativities,
the electrons normally end up on the more electronegative atom.
** Homolysis
and heterolysis require energy. Both processes generate reactive intermediates,
but the products are different in each case.
(A)
Homolysis generates uncharged reactive intermediates with unpaired electrons.
(B) Heterolysis
generates charged intermediates.
** Each of
these reactive intermediates has a very short lifetime, and each reacts quickly
to form a stable organic product.
Radicals, Carbocations, and Carbanions
** The curved
arrow notation (is a convention that shows how electron position differs
between the two resonance forms) works fine for heterolytic bond cleavage
because it illustrates the movement of an electron pair. For homolytic
cleavage, however, one electron moves to one atom in the bond and one electron
moves to the other, so a different kind of curved arrow is needed.
** To illustrate the movement of a single
electron, use a half-headed curved arrow,
sometimes
called a fishhook.
** The
following figure illustrates homolysis and two different heterolysis reactions for a
carbon compound using curved arrows. Three different reactive intermediates are
formed. Homolysis of the C – Z bond generates two uncharged products with
unpaired electrons.
** A reactive intermediate with a single unpaired
electron is called a Radical.
** Most
radicals are highly unstable because they contain an atom that does not have an
octet of electrons. Radicals typically have no charge. They are intermediates
in a group of reactions called radical reactions, which are discussed in detail
in Chapter 15. Heterolysis of the C – Z bond can generate a carbocation or a
carbanion.
** Giving two
electrons to Z and none to carbon generates a positively charged carbon intermediate
called a carbocation.
** Giving two
electrons to C and none to Z generates a negatively charged carbon species
called a carbanion.
** Both
carbocations and carbanions are unstable reactive intermediates:
(1) A
carbocation contains a carbon atom surrounded by only six electrons.
(2) A
carbanion has a negative charge on carbon, which is not a very electronegative
atom.
(3) Carbocations
(electrophiles) and carbanions (nucleophiles) can be intermediates in polar
reactions—reactions in which a nucleophile reacts with an electrophile.
** Thus,
homolysis and heterolysis generate radicals, carbocations, and carbanions, the
three most common reactive intermediates in organic chemistry.
** Radicals and
carbocations are electrophiles because they contain an electron-deficient carbon.
** Carbanions
are nucleophiles because they contain a carbon with a lone pair.
Conclusion
(1)
Breaking bonds generates reactive intermediates.
(2)
Homolysis generates radicals with unpaired electrons.
(3) Heterolysis
generates ions.
Bond Formation
** Like bond
cleavage, bond formation occurs in two different ways.
(a) Two
radicals can each donate one electron to form a two-electron bond.
(b) Alternatively,
two ions with unlike charges can come together, with the negatively charged ion
donating both electrons to form the resulting two electron bond.
** Bond
formation always releases energy.
All Kinds of Arrows
** The
following Table summarizes the many kinds of arrows used in describing organic reactions.
** Curved arrows
are especially important because they explicitly show what electrons are involved
in a reaction, how these electrons move in forming and breaking bonds, and if a
reaction proceeds via a radical or polar pathway
Solved problem
Use full-headed
or half-headed curved arrows to show the movement of electrons in each equation:
Solution:
(a) In
this reaction, the C – O bond is broken heterolytically. Because only one
electron pair is involved, one full-headed curved arrow is needed.
The electron pair in the C-O bond ends up on O
(b)
This reaction involves radicals, so half-headed curved arrows are needed to
show the movement of single electrons. One new two-electron bond is formed
between H and Cl, and an unpaired electron is left on C. Because a total of
three electrons are involved, three halfheaded curved arrows are needed.
Reference: Organic chemistry / Janice Gorzynski Smith , University of Hawai’i at Manoa / (Third edition) , 2011 . USA
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