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organic reactions and addition based on theory of lewis base acid etc.
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• Overall reaction: reactants products
• Mechanism: Step-by-step pathway.
• To learn more about a reaction:• Thermodynamics
• Kinetics
The Study of Chemical Reactions
Chlorination of Methane
Understanding Organic Reactions
• Equations for organic reactions are usually drawn with a single reaction arrow ()
between the starting material and product.
• The reagent, the chemical substance with which an organic compound reacts, is
sometimes drawn on the left side of the equation with the other reactants. At other times,
the reagent is drawn above the arrow itself.
• Although the solvent is often omitted from the equation, keep in mind that most organic
reactions take place in liquid solvent.
• The solvent and temperature of the reaction may be added above or below the arrow.
• The symbols “h” and “” are used for reactions that require light and heat respectively.
Writing Equations for Organic Reactions
• When two sequential reactions are carried out without drawing any intermediate
compound, the steps are usually numbered above or below the reaction arrow.
This convention signifies that the first step occurs before the second step, and
the reagents are added in sequence, not at the same time.
Understanding Organic Reactions
Types of Organic Reactions
1) Substitution Reactions
2) Elimination Reactions
3) Addition Reactions
Substitution Reactions
• A substitution is a reaction in which an atom or a group of atoms is
replaced by another atom or group of atoms.
• In a general substitution, Y replaces Z on a carbon atom.
• Substitution reactions involve bonds: one bond breaks
and another forms at the same carbon atom.
• The most common examples of substitution occur when Z is
a hydrogen or a heteroatom that is more electronegative than
carbon.
Elimination Reactions
• Elimination is a reaction in which elements of the
starting material are “lost” and a bond is formed.
( X and Y are detached from two different carbon atoms that are vicinal to each other)
9
• In an elimination reaction, two groups X and Y are removed from a
starting material.
• Two bonds are broken, and a bond is formed between adjacent
atoms.
• The most common examples of elimination occur when X = H and Y
is a heteroatom more electronegative than carbon.
Addition Reactions
( X and Y add to two different atoms in a molecule that has one or more bonds)
• Addition reaction is the reaction in which elements are added to the
starting material.
• In an addition reaction, new groups X and Y are added to the
starting material. A bond is broken and two bonds are formed.
Bond Making and Bond Breaking
• A reaction mechanism is a detailed description of how bonds are
broken and formed as starting material is converted into product.
• A reaction can occur either in one step or a series of steps.
Changes in Bonding During a Chemical Reaction
• Regardless of how many steps there are in a reaction, 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.
Think about a simple example like H2.
• Homolysis and Heterolysis require energy.
• Homolysis generates uncharged reactive intermediates
with unpaired electrons.
• Heterolysis generates charged intermediates.
• To illustrate the movement of a single electron, use a half-headed
curved arrow, sometimes called a fishhook.
• A full headed curved arrow shows the movement of an electron pair.
Review of Using Curved Arrows in Organic Chemistry
Three major reactive intermediates resulting from
homolysis and heterolysis of a C – Z bond
Bond breaking forms particles called reactive intermediates.
Carbocation Structure
• Carbon has 6 electrons, positively charged.
• Carbon is sp2 hybridized with vacant p orbital.
Carbocation Stability (Continued)
• Stabilized by alkyl substituents in two ways:
1. Inductive effect: Donation of electron density along the sigma bonds.
2. Hyperconjugation: Overlap of sigma bonding orbitals with empty p orbital.
Free Radicals
• Also electron-deficient.
• Stabilized by alkyl substituents.
• Order of stability:3 > 2 > 1 > methyl
Carbanions
• Eight electrons on carbon: 6 bonding plus one lone pair.
• Carbon has a negative charge.
• Destabilized by alkyl substituents.
• Methyl >1 > 2 > 3
Carbenes
• Carbon is neutral.
• Vacant p orbital, so can be electrophilic.
• Lone pair of electrons, so can be nucleophilic.
Heterolytically cleave each of the carbon-hetratom bonds and label
the organic intermediate as a carbocation or carbanion
OH + OH
carbocation
b) H3CH2C Li H3C CH2+ Li
carbanion
a)
33
• Bond formation occurs in two different ways.
• Two radicals can each donate one electron to form a two-
electron bond.
• 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.
Understanding Organic Reactions
• Homolysis generates two uncharged species with unpaired electrons.
• A reactive intermediate with a single unpaired electron is called a
radical.
• Radicals are highly unstable because they contain an atom that does
not have an octet of electrons.
• Heterolysis generates a carbocation or a carbanion.
• Both carbocations and carbanions are unstable intermediates.
• A carbocation contains a carbon surrounded by only six electrons, and
• A carbanion has a negative charge on carbon, which is not a very
electronegative atom.
Bond Making and Bond Breaking
Use arrows to show the movement of electrons in the following reactions.
N N + N N
b) HO OH OH2
c)
+ Br Br
a)
37
Bond Dissociation Energy
• The energy absorbed or released in any reaction, symbolized by
H0, is called the enthalpy change or heat of reaction.
• Bond dissociation energy is the H0 for a specific kind of reaction—
the homolysis of a covalent bond to form two radicals.
• Because bond breaking requires energy, bond dissociation energies are
always positive numbers, and homolysis is always endothermic.
• Conversely, bond formation always releases energy, and thus is always
exothermic. For example, the H—H bond requires +104 kcal/mol to
cleave and releases –104 kcal/mol when formed.
• Comparing bond dissociation energies is equivalent to comparing
bond strength.
• The stronger the bond, the higher its bond dissociation energy.
• Bond dissociation energies decrease down a column of the periodic table.
• Generally, shorter bonds are stronger bonds.
Stronger bonds have a higher ΔHº
• Bond dissociation energies are used to calculate the enthalpy change (H0)
in a reaction in which several bonds are broken and formed.
Bond dissociation energies have some important limitations.
• Bond dissociation energies present overall energy changes only.
They reveal nothing about the reaction mechanism or how fast a
reaction proceeds.
• Bond dissociation energies are determined for reactions in the gas
phase, whereas most organic reactions occur in a liquid solvent
where solvation energy contributes to the overall enthalpy of a
reaction.
• Bond dissociation energies are imperfect indicators of energy
changes in a reaction. However, using bond dissociation energies
to calculate H° gives a useful approximation of the energy
changes that occur when bonds are broken and formed in a
reaction.
• For a reaction to be practical, the equilibrium must favor products and the
reaction rate must be fast enough to form them in a reasonable time. These two
conditions depend on thermodynamics and kinetics respectively.
• Thermodynamics describes how the energies of reactants and products compare,
and what the relative amounts of reactants and products are at equilibrium.
• Kinetics describes reaction rates.
• The equilibrium constant, Keq, is a mathematical expression that relates the
amount of starting material and product at equilibrium.
Thermodynamics and Bonding
• The size of Keq expresses whether the starting materials or products predominate once
equilibrium is reached.
• When Keq > 1, equilibrium favors the products (C and D) and the equilibrium lies to the
right as the equation is written.
• When Keq < 1, equilibrium favors the starting materials (A and B) and the equilibrium lies
to the left as the equation is written.
• For a reaction to be useful, the equilibrium must favor the products, and Keq > 1.
• The position of the equilibrium is determined by the relative energies of the reactants
and products.
• G° is the overall energy difference between reactants and products.
Summary of the relationship between ∆G° and Keq
•G° is the overall energy difference between reactants and products.
• G° is related to the equilibrium constant Keq by the following equation:
• When Keq > 1, log Keq is positive, making G° negative, and energy is released.
Thus, equilibrium favors the products when the energy of the products is lower than
the energy of the reactants.
• When Keq < 1, log Keq is negative, making G° positive, and energy is absorbed.
Thus, equilibrium favors the reactants when the energy of the products is higher
than the energy of the reactants.
• Compounds that are lower in energy have increased stability.
• The equilibrium favors the products when they are more stable
(lower in energy) than the starting materials of a reaction.
• Because G° depends on the logarithm of Keq, a small change in
energy corresponds to a large difference in the relative amount
of starting material and product at equilibrium.
• These equations can be used for any process with two states in
equilibrium. As an example, monosubstituted cyclohexanes exist
as two different chair conformations that rapidly interconvert at
room temperature, with the conformation having the substituent
in the roomier equatorial position favored.
• Knowing the energy difference between two conformations
permits the calculation of the amount of each at equilibrium.
Enthalpy and Entropy
• G° depends on H° and the entropy change, S°.
• Entropy change, S°, is a measure of the change in the randomness of a
system. The more disorder present, the higher the entropy. Gas molecules
move more freely than liquid molecules and are higher in entropy. Cyclic
molecules have more restricted bond rotation than similar acyclic molecules
and are lower in entropy.
• S° is (+) when the products are more disordered than the reactants. S° is (-)
when the products are less disordered than the reactants.
• Reactions resulting in increased entropy are favored.
• G° is related to H° and S° by the following equation:
• This equation indicates that the total energy change is due to two
factors: the change in bonding energy and the change in disorder.
• The change in bonding energy can be calculated from bond
dissociation energies.
• Entropy changes are important when
The number of molecules of starting material differs from the
number of molecules of product in the balanced chemical equation.
An acyclic molecule is cyclized to a cyclic one, or a cyclic molecule
is converted to an acyclic one.
• In most other reactions that are not carried out at high temperature, the entropy term (TS°) is
small compared to the enthalpy term (H0), and therefore it is usually neglected.
Free Energy Change
G = (energy of products) - (energy of reactants)
G is the amount of energy available to do work.
Negative values indicate spontaneity.
Go = -2.303 RT(log10Keq)
where R = 8.314 J/K-mol and T = temperature in kelvins.
Factors Determining G
Free energy change depends on:• Enthalpy
• H= (enthalpy of products) - (enthalpy of reactants)
• Entropy• S = (entropy of products) - (entropy of reactants)
G = H - TS
Enthalpy
• Ho = heat released or absorbed during a chemical reaction at standard conditions.
• Exothermic (-H) heat is released.
• Endothermic (+H) heat is absorbed.
• Reactions favor products with lowest enthalpy (strongest bonds).
Entropy
• So = change in randomness, disorder, or freedom of movement.
• Increasing heat, volume, or number of particles increases entropy.
• Spontaneous reactions maximize disorder and minimize enthalpy.
• In the equation Go = Ho - TSo the entropy value is often small.
Energy Diagrams• An energy diagram is a schematic representation of
the energy changes that take place as reactants are
converted to products.
• An energy diagram plots the energy on the y axis
versus the progress of reaction, often labeled as the
reaction coordinate, on the x axis.
• The energy difference between reactants and products
is H°. If the products are lower in energy than the
reactants, the reaction is exothermic and energy is
released. If the products are higher in energy than the
reactants, the reaction is endothermic and energy is
consumed.
• The unstable energy maximum as a chemical reaction
proceeds from reactants to products is called the
transition state. The transition state species can never
be isolated.
• The energy difference between the transition state and
the starting material is called the energy of activation,
Ea.
• For the general reaction:
• The energy diagram would be shown as:
• The energy of activation is the minimum amount of energy needed to break the
bonds in the reactants.
• The larger the Ea, the greater the amount of energy that is needed to break
bonds, and the slower the reaction rate.
• The structure of the transition state is somewhere between the structures of the
starting material and product. Any bond that is partially formed or broken is
drawn with a dashed line. Any atom that gains or loses a charge contains a
partial charge in the transition state.
• Transition states are drawn in brackets, with a superscript double dagger (‡).
• Consider the following two step reaction:
• An energy diagram must be drawn for each step.
• The two energy diagrams must then be combined to form an
energy diagram for the overall two-step reaction.
• Each step has its own energy barrier, with a transition state at
the energy maximum.
• Kinetics is the study of reaction rates.
• Rate of the reaction is a measure of how the concentration of the products increase while the concentration of the products decrease.
• A rate equation is also called the rate law and it gives the relationship between the concentration of the reactants and the reaction rate observed.
• Rate law is experimentally determined.
• Ea is the energy barrier that must be exceeded for reactants to be converted to products.
Kinetics
Rate Law
• For the reaction A + B C + D,
rate = kr[A]a[B]b
• a is the order with respect to A• b is the order with respect to B• a + b is the overall order
• Order is the number of molecules of that reactant which is present in the rate-determining step of the mechanism.
• The higher the concentration, the faster the rate.
• The higher the temperature, the faster the rate.
• G°, H°, and Keq do not determine the rate of a reaction. These quantities indicate
the direction of the equilibrium and the relative energy of reactants and products.
• A rate law or rate equation shows the relationship between the reaction rate and the
concentration of the reactants. It is experimentally determined.
Kinetics
• Fast reactions have large rate constants.
• Slow reactions have small rate constants.
• The rate constant k and the energy of activation Ea are inversely
related. A high Ea corresponds to a small k.
• A rate equation contains concentration terms for all reactants in
a one-step mechanism.
• A rate equation contains concentration terms for only the
reactants involved in the rate-determining step in a multi-step
reaction.
• The order of a rate equation equals the sum of the exponents of
the concentration terms in the rate equation.
• A two-step reaction has a slow rate-determining step, and a fast step.
• In a multi-step mechanism, the reaction can occur no faster than its rate-
determining step.
• Only the concentration of the reactants in the rate-determining step
appears in the rate equation.
Catalysts
• Some reactions do not proceed at a reasonable rate unless a catalyst is
added.
• Changes the speed of a reaction
• Changes the speed of a reaction: A catalyst is a substance that speeds up the
rate of a reaction.
• Does not appear in the product: It is recovered unchanged in a reaction, and it
does not appear in the product
• Many types of catalyst can easily be recovered and used again
Catalysts
• Some reactions do not proceed at a reasonable rate unless a catalyst is added.
• A catalyst is a substance that speeds up the rate of a reaction. It is recovered
unchanged in a reaction, and it does not appear in the product.
The effect of a catalyst on a reaction
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