6 RX Organic PDF

8/10/2019 6 RX Organic PDF

1/27

1

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.

Al though the solven t is of ten omit ted from the equat ion, 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

2



Figure 6.1Different ways of writing

organic reactions


2/27

3



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.

4


Kinds of Organic 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 substitut ion, Y replaces Z on a carbon atom.


3/27

5



Substitution reactions involve bonds: one bond breaksand another forms at the same carbon atom.

The most common examples of substitution occur when Z is

a hydrogen or a heteroatom that i s more electronegative than

carbon.

6



Elimination is a reaction in which elements of the

starting material are lost and a

bond is formed.


4/27

7



In an elimination reaction, two groups X and Y are removedfrom 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 .

8



Addition is a reaction in which elements are added to the

starting material.


5/27

9



In an addition reaction, new groups X and Y are added tothe starting material. A bond is broken and two

bonds are formed.

10



Addition and elimination reactions are exactly opposite.

A bond is formed in elimination reactions, whereas a

bond is broken in addition reactions.


6/27

11


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 o r a ser ies of steps.

12

Understanding Organic ReactionsBond Making and Bond Breaking

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.


7/27

13



Homolysis and heterolysis require energy.

Homolysis generates uncharged reactive intermediates with

unpaired electrons.

Heterolysis generates charged intermediates.

14



To illustrate the movement of a single electron, use a half-

headed curved arrow, sometimes called a fishhook.

A fu ll headed curved arrow shows the movement of an

electron pair.


8/27

15


Homolysis generates two uncharged species with

unpaired electrons.

A reactive intermediate wi th 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 anegative charge on carbon, which is not a very

electronegative atom.


16

Understanding Organic ReactionsBond Making and Bond Breaking

Figure 6.2Three reactive intermediates

resulting from homolysis andheterolysis of a C Z bond


9/27

17


Radicals and carbocations are electrophiles because theycontain an electron deficient carbon.

Carbanions are nucleophiles because they contain a carbon

with a lone pair.


18


Bond formation occurs in two different ways.

Two radicals can each donate one electron to form a two-

electron bond.

Al ternat ively , two ions with un like charges can come

together, with the negatively charged ion donating both

electrons to form the resulting two-electron bond.

Bond formation always releases energy.



10/27

19


A number of types of arrows are used in descr ib ing organic

reactions.


20


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

reactionthe homolysis of a covalent bond to form two

radicals.


11/27

21



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 HH bond requires +104

kcal/mol to cleave and releases 104 kcal/mol when formed.

22


12/27

23



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.

24



Bond dissociation energies are used to calculate the enthalpy

change (H0) in a reaction in which several bonds are broken

and formed.


13/27

25



26



Consider the oxidation of isooctane and glucose to yield CO2and H2O.

H is negative for both oxidations, so both reactions areexothermic.

Both isooctane and glucose release energy on oxidation

because the bonds in the products are stronger than the bonds

in the reactants.


14/27

27



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 dissociationenergies to calculate H gives a useful approximation of theenergy changes that occur when bonds are broken and

formed in a reaction.

28


Thermodynamics

For a reaction to be practical, the equilibr ium 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 atequilibrium.


15/27

29


Thermodynamics 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 equili brium l ies 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 andproducts.

30


Thermodynamics

Figure 6.3Summary of the relationship

between G and Keq


16/27

31


Thermodynamics

G is related to the equilibrium constant Keq by the followingequation:

When Keq > 1, log Keq is positive, making G negative, andenergy 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, andenergy is absorbed. Thus, equilibrium favors the reactants when

the energy of the products is higher than the energy of the

reactants.

32


Thermodynamics 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 inenergy corresponds to a large difference in the relative amount

of starting material and product at equilibrium.


17/27


18/27

35


Enthalpy and Entropy

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 acycl ic molecule is cycl ized to a cycl ic one, or a

cyclic molecule is converted to an acyclic one.

36


Enthalpy and Entropy

In most other reactions that are not carried out at high

temperature, the entropy term (TS) is small compared to theenthalpy term (H0), and therefore it is usually neglected.


19/27

37


Energy Diagrams

An energy diagram is a schematic representation of the energychanges 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 coord inate, on

the x axis.

The energy difference between reactants and products is H. Ifthe 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.

38


Energy Diagrams For the general reaction:

The energy diagram would be shown as:


20/27

39


Energy Diagrams

The energy of activation is the minimum amount of energy

needed to b reak the bonds in the reactants.

The larger the Ea, the greater the amount of energy that is

needed to b reak 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 ().

40


Energy Diagrams

Example 1Figure 6.4Some representative

energy diagrams


21/27

41


Energy Diagrams


energy diagrams

42


Energy Diagrams


energy diagrams


22/27

43


Energy Diagrams


energy diagrams

44

Understanding Organic ReactionsEnergy Diagrams

Figure 6.5Comparing H and Ea in two

energy diagrams


23/27

45


Energy Diagrams

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 anenergy diagram for the overall two-step reaction.

Each step has its own energy barrier, with a transition state at

the energy maximum.

46


Energy Diagrams


24/27

47


Energy Diagrams

48


Energy Diagrams

Figure 6.6Complete energy diagram for

the two-step conversion of


25/27

49


Kinetics

Kinetics is the study of reaction rates.

Recall that Ea is the energy barrier that must be exceeded

for reactants to be converted to products.

50


Kinetics

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 o f 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.


26/27

51

Understanding Organic ReactionsKinetics

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 equat ion contains concentrat ion terms for all reactants in

a one-step mechanism.

A rate equation contains concentrat ion terms for on ly 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.

52


Kinetics

A two-step reaction has a s low 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.


27/27

53


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 react ion. It

is recovered unchanged in a reaction, and it does not appear in

the product.

Figure 6.7The effect of a catalyst

on a reaction

54


Enzymes Enzymes are biochemical catalysts composed of amino acids

held together in a very specific three-dimensional shape.

An enzyme contains a region called i ts act ive si te which binds an

organic reactant, called a substrate. The resulting unit is called

the enzyme-substrate complex.

Once bound, the organic substrate undergoes a very specific

reaction at an enhanced rate. The products are then released.

Figure 6.8A schematic representation

of an enzyme at work

Date post:	02-Jun-2018
Category:	Documents
Upload:	khafsahs
View:	227 times
Download:	0 times

6 RX Organic PDF

Documents