Chemical Reactivity
Organic chemistry encompasses a very large number of compounds (
many millions ), and our previous discussion and illustrations have
focused on their structural characteristics. Now that we can recognize
these actors ( compounds ), we turn to the roles they are inclined to
play in the scientific drama staged by the multitude of chemical
reactions that define organic chemistry. We begin by defining some basic terms that will be used frequently as this subject is elaborated.
Chemical Reaction: A
transformation resulting in a change of composition, constitution
and/or configuration of a compound ( referred to as the reactant or
substrate ).
Reactant or Substrate: The organic
compound undergoing change in a chemical reaction. Other compounds may
also be involved, and common reactive partners ( reagents ) may be
identified. The reactant is often ( but not always ) the larger and
more complex molecule in the reacting system. Most ( or all ) of the
reactant molecule is normally incorporated as part of the product
molecule.
Reagent: A common partner of the
reactant in many chemical reactions. It may be organic or inorganic;
small or large; gas, liquid or solid. The portion of a reagent that
ends up being incorporated in the product may range from all to very
little or none.
Product(s) The final form taken by the major reactant(s) of a reaction.
Reaction Conditions The
environmental conditions, such as temperature, pressure, catalysts
& solvent, under which a reaction progresses optimally. Catalysts
are substances that accelerate the rate ( velocity ) of a chemical
reaction without themselves being consumed or appearing as part of the
reaction product. Catalysts do not change equilibria positions.
Chemical reactions are commonly written as equations: |
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Classifying Organic Chemical Reactions
If you scan any organic textbook you will encounter what appears to
be a very large, often intimidating, number of reactions. These are the
"tools" of a chemist, and to use these tools effectively, we must
organize them in a sensible manner and look for patterns of reactivity
that permit us make plausible predictions. Most of these reactions
occur at special sites of reactivity known as functional groups, and these constitute one organizational scheme that helps us catalog and remember reactions. Ultimately,
the best way to achieve proficiency in organic chemistry is to
understand how reactions take place, and to recognize the various
factors that influence their course.
This is best accomplished by perceiving the reaction pathway or mechanism of a reaction.
1. Classification by Structural Change
First, we identify four broad classes of reactions based solely on the structural change
occurring in the reactant molecules. This classification does not
require knowledge or speculation concerning reaction paths or
mechanisms.
The letter R in the following illustrations
is widely used as a symbol for a generic group. It may stand for simple
substituents such as H– or CH3–, or for complex groups composed of many atoms of carbon and other elements.
Four Reaction Classes |
Addition |
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Elimination
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Substitution |
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Rearrangement
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In an addition reaction the number of σ-bonds
in the substrate molecule increases, usually at the expense of
one or more π-bonds. The reverse is
true of elimination reactions, i.e.the number of
σ-bonds in the substrate decreases,
and new π-bonds are often formed.
Substitution reactions, as the name implies, are characterized
by replacement of an atom or group (Y) by another atom or group
(Z). Aside from these groups, the number of bonds does not change.
A rearrangement reaction generates an isomer, and again
the number of bonds normally does not change.
The examples illustrated above involve simple alkyl and alkene
systems, but these reaction types are general for most functional
groups, including those incorporating carbon-oxygen double bonds
and carbon-nitrogen double and triple bonds. Some common reactions
may actually be a combination of reaction types. The reaction
of an ester with ammonia to give an amide, as shown below, appears
to be a substitution reaction ( Y = CH3O &
Z = NH2 ); however, it is actually two reactions,
an addition followed by an elimination.
The addition of water to a nitrile does not seem to fit any of
the above reaction types, but it is simply a slow addition reaction
followed by a rapid rearrangement, as shown in the following equation.
Rapid rearrangements of this kind are called tautomerizations.
2. Classification by Reaction Type
At the beginning, it is helpful to identify some common reaction
types that will surface repeatedly as the chemical behavior of
different compounds is examined. This is not intended to be a complete
and comprehensive list, but should set the stage for future
elaborations.
Acidity and Basicity
It is useful to begin a discussion of organic chemical reactions
with a review of acid-base chemistry and terminology for several
reasons. First, acid-base reactions are among the simplest to recognize
and understand. Second, some classes of organic compounds have
distinctly acidic properties, and some other classes behave as bases,
so we need to identify these aspects of their chemistry. Finally, many
organic reactions are catalyzed by acids and/or bases, and although
such transformations may seem complex, our understanding of how they
occur often begins with the functioning of the catalyst.
Organic chemists use two acid-base theories for interpreting and planning their work: the Brønsted theory and the Lewis theory.
Brønsted Theory
According to the Brønsted theory, an acid is a proton donor, and a base is a proton acceptor.
In an acid-base reaction, each side of the equilibrium has an acid and
a base reactant or product, and these may be neutral species or ions.
H-A + B:(–) |
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A:(–) + B-H |
(acid1) (base1) |
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(base2) (acid2) |
Structurally related acid-base pairs, such as {H-A and A:(–)} or {B:(–) and B-H} are called conjugate pairs. Substances that can serve as both acids and bases, such as water, are termed amphoteric.
H-Cl + H2O |
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Cl:(–) + H3O(+) |
(acid) (base) |
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(base) (acid) |
H3N: + H2O |
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NH4(+) + HO(–) |
(base) (acid) |
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(acid) (base) |
The relative strength of a group of acids (or bases) may be
evaluated by measuring the extent of reaction that each group member
undergoes with a common base (or acid). Water serves nicely as the
common base or acid for such determinations. Thus, for an acid H-A, its
strength is proportional to the extent of its reaction with the base
water, which is given by the equilibrium constant Keq.
H-A + H2O
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H3O(+) + A:(–)
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Since these studies are generally extrapolated to high dilution, the
molar concentration of water (55.5) is constant and may be eliminated
from the denominator. The resulting K value is called the acidity constant, Ka. Clearly, strong acids have larger Ka's than do weaker acids. Because of the very large range of acid strengths (greater than 1040), a logarithmic scale of acidity (pKa) is normally employed. Stronger acids have smaller or more negative pKa values than do weaker acids.
Some useful principles of acid-base reactions are:
• Strong acids have weak conjugate bases, and weak acids have strong conjugate bases.
• Acid-base equilibria always favor the weakest acid and the weakest base.
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Examples of Brønsted Acid-Base Equilibria
Acid-Base Reaction |
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Conjugate Acids |
Conjugate Bases |
Ka |
pKa |
HBr + H2O |
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H3O(+) + Br(–) |
HBr H3O(+) |
Br(–) H2O |
105 |
-5 |
CH3CO2H + H2O |
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H3O(+) + CH3CO2(–) |
CH3CO2H H3O(+) |
CH3CO2(–) H2O |
1.77*10-5 |
4.75 |
C2H5OH + H2O |
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H3O(+) + C2H5O(–) |
C2H5OH H3O(+) |
C2H5O(–) H2O |
10-16 |
16 |
NH3 + H2O |
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H3O(+) + NH2(–) |
NH3 H3O(+) |
NH2(–) H2O |
10-34 |
34 |
In all the above examples water acts as a common base. The last example ( NH3
) cannot be measured directly in water, since the strongest base that
can exist in this solvent is hydroxide ion. Consequently, the value
reported here is extrapolated from measurements in much less acidic
solvents, such as acetonitrile.
Since many organic reactions either take place in aqueous
environments ( living cells ), or are quenched or worked-up in water,
it is important to consider how a conjugate acid-base equilibrium
mixture changes with pH. A simple relationship known as the Henderson-Hasselbach equation provides this information.
When the pH of an aqueous solution or mixture is equal to the pKa
of an acidic component, the concentrations of the acid and base
conjugate forms must be equal ( the log of 1 is 0 ). If the pH is
lowered by two or more units relative to the pKa, the acid concentration will be greater than 99%. On the other hand, if the pH ( relative to pKa
) is raised by two or more units the conjugate base concentration will
be over 99%. Consequently, mixtures of acidic and non-acidic compounds
are easily separated by adjusting the pH of the water component in a
two phase solvent extraction.
For example, if a solution of benzoic acid ( pKa = 4.2 ) in benzyl alcohol ( pKa
= 15 ) is dissolved in ether and shaken with an excess of 0.1 N sodium
hydroxide ( pH = 13 ), the acid is completely converted to its water
soluble ( ether insoluble ) sodium salt, while the alcohol is
unaffected. The ether solution of the alcohol may then be separated
from the water layer, and pure alcohol recovered by distillation of the
volatile ether solvent. The pH of the water solution of sodium benzoate
may then be lowered to 1.0 by addition of hydrochloric acid, at which
point pure benzoic acid crystallizes, and may be isolated by filtration.
Basicity
The basicity of oxygen, nitrogen, sulfur and phosphorus compounds or
ions may be treated in an analogous fashion. Thus, we may write
base-acid equilibria, which define a Kb and a corresponding pKb.
However, a more common procedure is to report the acidities of the
conjugate acids of the bases ( these conjugate acids are often "onium"
cations ). The pKa's reported for bases in this system are proportional to the base strength of the base. A useful rule here is: pKa + pKb = 14.
We see this relationship in the following two equilibria:
Acid-Base Reaction |
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Conjugate Acids |
Conjugate Bases |
K |
pK |
NH3 + H2O |
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NH4(+) + OH(–) |
NH4(+) H2O |
NH3 OH(–) |
Kb = 1.8*10-5 |
pKb = 4.74 |
NH4(+) + H2O |
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H3O(+) + NH3 |
NH4(+) H3O(+) |
NH3 H2O |
Ka = 5.5*10-10 |
pKa = 9.25 |
Although it is convenient and informative to express pKa
values for a common solvent system (usually water), there are serious
limitations for very strong and very weak acids. Thus acids that are
stronger than the hydronium cation, H3O(+), and weak acids having conjugate bases stronger than hydroxide anion, OH(–),
cannot be measured directly in water solution. Solvents such as acetic
acid, acetonitrile and nitromethane are often used for studying very
strong acids. Relative acidity measurements in these solvents may be
extrapolated to water. Likewise, very weakly acidic solvents such as
DMSO, acetonitrile, toluene, amines and ammonia may be used to study
the acidities of very weak acids. For both these groups, the reported pKa values extrapolated to water are approximate, and many have large uncertainties.
Lewis Theory
According to the Lewis theory, an acid is an electron pair acceptor, and a base is an electron pair donor. Lewis bases are also Brønsted bases; however, many Lewis acids, such as BF3, AlCl3 and Mg2+,
are not Brønsted acids. The product of a Lewis acid-base reaction, is a
neutral, dipolar or charged complex, which may be a stable covalent
molecule. Two examples of Lewis acid-base equilibria are shown in
equations 1 & 2 below.
In the first example, an electron deficient aluminum atom bonds to a
covalent chlorine atom be sharing one of its non-bonding valence
electron pairs, and thus achieves an argon-like valence shell octet.
Because this sharing is unilateral (chlorine contributes both
electrons), both the aluminum and the chlorine have formal charges, as
shown. If the carbon chlorine bond in this complex breaks with both the
bonding electrons remaining with the more electronegative atom
(chlorine), the carbon assumes a positive charge. We refer to such
carbon species as carbocations. Carbocations are also Lewis acids, as the reverse reaction demonstrates. Many
carbocations (but not all) may also function as Brønsted acids.
Equation 3 illustrates this dual behavior; the Lewis acidic site is
colored red and three of the nine acidic hydrogen atoms are colored
orange. In its Brønsted acid role the carbocation donates a proton to
the base (hydroxide anion), and is converted to a stable neutral
molecule having a carbon-carbon double bond.
A terminology related to the Lewis acid-base nomenclature is often used by organic chemists. Here the term electrophile corresponds to a Lewis acid, and nucleophile corresponds to a Lewis base.
Electrophile:
An electron deficient atom, ion or molecule that has an affinity
for an electron pair, and will bond to a base or nucleophile.
Nucleophile: An atom, ion or molecule that has an electron pair that may be donated in bonding to an electrophile (or Lewis acid).
Oxidation and Reduction Reactions
A parallel and independent method of characterizing organic reactions is by oxidation-reduction terminology. Carbon atoms may have any oxidation state from –4 (e.g. CH4 ) to +4 (e.g. CO2
), depending upon their substituents. Fortunately, we need not
determine the absolute oxidation state of each carbon atom in a
molecule, but only the change in oxidation state of those
carbons involved in a chemical transformation. To determine whether a
carbon atom has undergone a redox change during a reaction we simply
note any changes in the number of bonds to hydrogen and the number of
bonds to more electronegative atoms such as O, N, F, Cl, Br, I, & S
that has occurred. Bonds to other carbon atoms are ignored. This count
should be conducted for each carbon atom undergoing any change during a
reaction.
If the number of hydrogen atoms bonded to a carbon increases,
and/or if the number of bonds to more electronegative atoms decreases,
the carbon in question has been reduced (i.e. it is in
a lower oxidation state).
If the number of hydrogen atoms bonded to a carbon decreases,
and/or if the number of bonds to more electronegative atoms increases,
the carbon in question has been oxidized (i.e. it is in
a higher oxidation state).
If there has been no change in the number of such bonds,
then the carbon in question has not changed its oxidation state.
In the hydrolysis reaction of a nitrile shown above, the blue
colored carbon has not changed its oxidation state.
These rules are illustrated by the following four addition reactions involving the same starting material, cyclohexene. Carbon atoms colored blue are reduced, and those colored
red are oxidized. In the addition of hydrogen both carbon atoms
are reduced, and the overall reaction is termed a reduction.
Peracid epoxidation and addition of bromine oxidize both carbon
atoms, so these are termed oxidation reactions. Addition of HBr
reduces one of the double bond carbon atoms and oxidizes the other;
consequently, there is no overall redox change in the substrate
molecule.
Since metals such as lithium and magnesium are less electronegative
than hydrogen, their covalent bonds to carbon are polarized so that the
carbon is negative (reduced) and the metal is positive (oxidized).
Thus, Grignard reagent formation from an alkyl halide reduces the
substituted carbon atom. In the following equation and half-reactions
the carbon atom (blue) is reduced and the magnesium (magenta) is
oxidized.
3. Classification by Functional Group
Functional groups are atoms
or small groups of atoms (usually two to four) that exhibit a characteristic
reactivity when treated with certain reagents.
A particular functional group will almost always display its
characteristic chemical behavior when it is present in a compound.
Because of this, the discussion of organic reactions is often organized
according to functional groups. The following table summarizes the
general chemical behavior of the common functional groups. For
reference, the alkanes provide a background of behavior in the absence
of more localized functional groups.
Functional Class |
Formula |
Characteristic Reactions |
Alkanes |
C–C, C–H |
Substitution (of H, commonly by Cl or Br)
Combustion (conversion to CO2 & H2O) |
Alkenes |
C=C–C–H |
Addition Substitution (of H) |
Alkynes |
C≡C–H |
Addition Substitution (of H) |
Alkyl Halides |
H–C–C–X |
Substitution (of X) Elimination (of HX) |
Alcohols |
H–C–C–O–H |
Substitution (of H); Substitution (of OH) Elimination (of HOH); Oxidation (elimination of 2H) |
Ethers |
(α)C–O–R |
Substitution (of OR); Substitution (of α–H) |
Amines |
C–NRH |
Substitution (of H); Addition (to N); Oxidation (of N) |
Benzene Ring |
C6H6 |
Substitution (of H) |
Aldehydes |
(α)C–CH=O |
Addition Substitution (of H or α–H) |
Ketones |
(α)C–CR=O |
Addition Substitution (of α–H) |
Carboxylic Acids |
(α)C–CO2H |
Substitution (of H); Substitution (of OH) Substitution (of α–H); Addition (to C=O) |
Carboxylic Derivatives |
(α)C–CZ=O (Z = OR, Cl, NHR, etc.) |
Substitution (of Z); Substitution (of α–H) Addition (to C=O) |
This table does not include any reference to rearrangement, due to
the fact that such reactions are found in all functional classes, and
are highly dependent on the structure of the reactant. Furthermore, a
review of the overall reaction patterns presented in this table
discloses only a broad and rather non-specific set of reactivity
trends. This is not surprising, since the three remaining categories
provide only a coarse discrimination (comparable to identifying an
object as animal, vegetable or mineral). Consequently, apparent
similarities may fail to reflect important differences. For example,
addition reactions to C=C are significantly different from additions to
C=O, and substitution reactions of C-X proceed in very different ways,
depending on the hybridization state of carbon.
The Variables of Organic Reactions
In an effort to understand how and why reactions of functional
groups take place in the way they do, chemists try to discover just how
different molecules and ions interact with each other as they come
together. To this end, it is important to consider the various properties and characteristics of a reaction that may be observed and/or measured as the reaction proceeds . The most common and useful of these are listed below:
1. Reactants and Reagents
| A. Reactant Structure:
Variations in the structure of the reactant may have a marked influence
on the course of a reaction, even though the functional group is
unchanged. Thus, reaction of 1-bromopropane with sodium cyanide
proceeds smoothly to yield butanenitrile, whereas
1-bromo-2,2-dimethylpropane fails to give any product and is recovered
unchanged. In contrast, both alkyl bromides form Grignard reagents
(RMgBr) on reaction with magnesium.
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B. Reagent Characteristics: Apparently minor changes
in a reagent may lead to a significant change in the course of a
reaction. For example, 2-bromopropane gives a substitution reaction
with sodium methylthiolate but undergoes predominant elimination on
treatment with sodium methoxide.
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2. Product Selectivity
| A. Regioselectivity:
It is often the case that addition and elimination reactions may, in
principle, proceed to more than one product. Thus 1-butene might add
HBr to give either 1-bromobutane or 2-bromobutane, depending on which
carbon of the double bond receives the hydrogen and which the bromine.
If one possible product out of two or more is formed preferentially,
the reaction is said to be regioselective.
Simple
substitution reactions are not normally considered regioselective,
since by definition only one constitutional product is possible.
However, rearrangements are known to occur during some reactions.
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B. Stereoselectivity: If the reaction products are
such that stereoisomers may be formed, a reaction that yields one
stereoisomer preferentially is said to be stereoselective. In the
addition of bromine to cyclohexene, for example, cis and trans-1,2-dibromocyclohexane
are both possible products of the addition. Since the trans-isomer is
the only isolated product, this reaction is stereoselective.
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C. Stereospecificity: This term is applied to cases in which stereoisomeric reactants behave differently in a given reaction. Examples include:
(i) Formation of different stereoisomeric products, as
in the reaction of enantiomeric 2-bromobutane isomers with sodium
methylthiolate, shown in the following diagram.
Here, the (R)-reactant gives the
configurationally inverted (S)-product, and (S)-reactant produces (R)-product.
(ii) Different rates of reaction, as in the base-induced
eleimination of cis & trans-4-tert-butylcyclohexyl bromide
(equation 1 below).
(iii) Different reaction paths leading to different products,
as in the base-induced eleimination of cis &
trans-2-methylcyclohexyl bromide (equation 2 below).
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3. Reaction Characteristics
| A. Reaction Rates:
Some reactions proceed very rapidly, and some so slowly that they are
not normally observed. Among the variables that influence reaction
rates are temperature (reactions are usually faster at a higher
temperature), solvent, and reactant / reagent concentrations. Useful
information about reaction mechanisms may be obtained by studying the
manner in which the rate of a reaction changes as the concentrations of
the reactant and reagents are varied. This field of study is called kinetics.
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B. Intermediates: Many reactions
proceed in a stepwise fashion. This can be convincingly demonstrated if
an intermediate species can be isolated and shown to proceed to the
same products under the reaction conditions. Some intermediates are
stable compounds in their own right; however, some are so reactive that
isolation is not possible. Nevertheless, evidence for their existence
may be obtained by other means, including spectroscopic observation or
inference from kinetic results. |
4. Factors that Influence Reactions
It is helpful to identify some general features of a reaction that
have a significant influence on its facility. Some of the most
important of these are:
| A. Energetics: The potential energy of a reacting system changes as the reaction progresses. The overall change may be exothermic ( energy is released ) or endothermic ( energy must be added ), and there is usually an activation energy requirement as well. Tables of Standard Bond Energies
are widely used by chemists for estimating the energy change in a
proposed reaction. As a rule, compounds constructed of strong covalent
bonds are more stable than compounds incorporating one or more
relatively weak bonds. |
B. Electronic Effects:
The distribution of electrons at sites of reaction (functional groups)
is a particularly important factor. Electron deficient species or
groups, which may or may not be positively charged, are attracted to
electron rich species or groups, which may or may not be negatively
charged. We refer to these species as electrophiles & nucleophiles respectively. In general, opposites attract and like repel.
The charge distribution in a molecule is usually discussed with respect to two interacting effects: An inductive effect, which is a function of the electronegativity differences that exist between atoms (and groups); and a resonance effect, in which electrons move in a discontinuous fashion between parts of a molecule. |
C. Steric Effects: Atoms occupy space. When they are crowded together, van der Waals repulsions produce an unfavorable steric hindrance. Steric hindrance may influence conformational equilibria, as well as destabilizing transition states of reactions. |
D. Stereoelectronic Effects:
In many reactions atomic or molecular orbitals interact in a manner
that has an optimal configurational or geometrical alignment. Departure
from this alignment inhibits the reaction. |
E. Solvent Effects:
Most reactions are conducted in solution, not in a gaseous state. The
solvent selected for a given reaction may exzert a strong influence on
its course. Remember, solvents are chemicals, and most undergo chemical
reaction under the right conditions. |
Mechanisms of Organic Reactions
A detailed description of the changes in structure and bonding that
take place in the course of a reaction, and the sequence of such events
is called the reaction mechanism. A reaction mechanism should
include a representation of plausible electron reorganization, as well
as the identification of any intermediate species that may be formed as
the reaction progresses. These features are elaborated in the following
sections.
1. The Arrow Notation in Mechanisms
Since
chemical reactions involve the breaking and making of bonds, a
consideration of the movement of bonding ( and non-bonding ) valence
shell electrons is essential to this
understanding. It is now common practice to show the movement of
electrons with curved arrows, and a sequence of equations depicting the
consequences of such electron shifts is termed a mechanism. In general, two kinds of curved arrows are used in drawing mechanisms:
A full head on the arrow indicates the movement or shift of an electron pair:
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A partial head (fishhook) on the arrow indicates the shift of a single electron:
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The use of these symbols in bond-breaking and bond-making reactions
is illustrated below. If a covalent single bond is broken so that one
electron of the shared pair remains with each fragment, as in the first
example, this bond-breaking is called homolysis. If the bond
breaks with both electrons of the shared pair remaining with one
fragment, as in the second and third examples, this is called heterolysis.
Bond-Breaking |
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Bond-Making |
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Other Arrow Symbols
Chemists also use arrow symbols for other purposes, and it is
essential to use them correctly.
The Reaction Arrow |
The Equilibrium Arrow |
The Resonance Arrow |
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The following equations illustrate the proper use of these symbols:
2. Reactive Intermediates
The products of bond breaking, shown above, are not stable in the
usual sense, and cannot be isolated for prolonged study. Such species
are referred to as reactive intermediates, and are believed to
be transient intermediates in many reactions. The general structures
and names of four such intermediates are given below.
A pair of widely used terms, related to the Lewis acid-base notation, should also be introduced here.
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Electrophile:
An electron deficient atom, ion or molecule that has an affinity for an
electron pair, and will bond to a base or nucleophile.
Nucleophile: An atom, ion or molecule that has an electron pair that may be donated in bonding to an electrophile (or Lewis acid). |
Using these definitions, it is clear that carbocations ( called
carbonium ions in the older literature ) are electrophiles and
carbanions are nucleophiles. Carbenes have only a valence shell sextet
of electrons and are therefore electron deficient. In this sense they
are electrophiles, but the non-bonding electron pair also gives
carbenes nucleophilic character. As a rule, the electrophilic character
dominates carbene reactivity. Carbon radicals have only seven valence
electrons, and may be considered electron deficient; however, they do
not in general bond to nucleophilic electron pairs, so their chemistry
exhibits unique differences from that of conventional electrophiles.
Radical intermediates are often called free radicals.
The importance of electrophile / nucleophile terminology comes from the
fact that many organic reactions involve at some stage the bonding of a
nucleophile to an electrophile, a process that generally leads to a
stable intermediate or product. Reactions of this kind are sometimes
called ionic reactions, since ionic reactants or products are often
involved.
The shapes ideally assumed by these intermediates becomes important
when considering the stereochemistry of reactions in which they play a
role. A simple tetravalent compound like methane, CH4, has a tetrahedral configuration.
Carbocations have only three bonds to the charge bearing carbon, so it
adopts a planar trigonal configuration. Carbanions are pyramidal in
shape ( tetrahedral if the electron pair is viewed as a substituent ),
but these species invert rapidly at room temperature, passing through a
higher energy planar form in which the electron pair occupies a
p-orbital. Radicals are intermediate in configuration, the energy
difference between pyramidal and planar forms being very small. Since
three points determine a plane, the shape of carbenes must be planar;
however, the valence electron distribution varies.
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