Four Types of Organic Reaction - Yonsei Universitychem.yonsei.ac.kr/chem/upload/CHE2103-03-00/114307743360310.pdf · Ch.5 An Overview of Organic Reactions 5.1 Kinds of Organic Reactions

Ch.5 An Overview of Organic Reactions

5.1 Kinds of Organic Reactions

A + B C

These reactantsadd together ...

... to give thissingle product

Addition Reaction:

H

H H

H+ H-Br

H

H H

H

H

Br

example

Four Types of Organic Reaction


A + BC

This onereactant ...

... splits apart to givethese two products

Elimination Reaction:

H

H H

H+ H-Br

H

H H

H

H

Br

examplebase


A + D

These two reactantsexahange parts ...

... to give thesetwo new products

B C A + DC B

Substitution Reaction:

+ Cl-Clexample

CH

H HH

UV+ H-ClC

HH Cl

H


A B

This single reactant ... ... gives the isomeric product

Rearrangement Reaction:

example acid catalyst


5.2 How Organic Reactions Occur: Mechanisms

Reaction Mechanism: an overall description of how a reaction occurs

A B A + B

Homolytic bond breaking (radical):one electron stays with each fragment

Bond cleavage

radical reaction

polar reaction

A B A + B

Heterolytic bond breaking (polar):two electrons stay with one fragment


A BA B

Homolytic bond making (radical):one electron donated by each fragment

Bond making

radical reaction

polar reaction

A BA B

Heterolytic bond making (polar):two electrons donated by one fragment


5.3 Radical Reactions and How They Occur

• radical reactions are not as common as polar reactions, but important reaction

• radicals are highly reactive because one electron is deficient: try to achieve a valence-shell octet

A B A + BR R

unpaired electron unpaired electron

substitutionproduct

Substitution reaction: abstract an atom from another molecule


A B AR R

unpaired electron

unpaired electron

additionproductradical

B

Addition reaction: addition to multiple bonds

ABA RR

unpaired electron unpaired electron

B

Elimination reaction: eliminate another radical species


Radical Chain Reaction

H3C H + Cl Clhv

H3C Cl + H Cl

radical substitution reaction: three kinds of steps

Step 1. Initiation: production of a small number of reactive radicals

Cl Clhv (UV)

Cl2


Step 2. Propagation: repetition of chain reaction

H3C H + Cl CH3 +(a)

+ ClCH3 +Cl Cl(b) H3C Cl

H Cl

(c) Repeat steps (a) and (b) until termination

HCl

H3C HCl

CH3 Cl Cl

CH3Cl


Step 3. Termination: two radicals combine to form a stable product; such termination steps occur infrequently becacuse the concentration of radicals in the reaction at any given moment is very small.

+ Cl

CH3 H3C Cl

Cl Cl2

Cl+

CH3 + CH3 H3C CH3

possible termination steps:

All radical reactions involve odd number of electrons: bonds arebroken and formed by reaction of species that have unpaired electrons.


5.4 Polar Reactions and How They Occur

• polar reactions occur because of the attraction between positive and negative charges on different functional groups in molecules

• most organic molecules are electrically neutral; they have no net positive or negative charge

• Certain bonds, particularly the bonds in functional groups, are polar due to the electronegativity differences


H C N O F

PSi S Cl

Br

I

4.02.52.1 3.53.0

3.01.8 2.52.1

2.8

2.5

Li

Na

K

1.0

0.9

0.8

Be

Mg1.6

1.2Ca1.0

Cs0.7

δ-Y

Cδ+ δ-

M

C

δ+

Y = O, N, Cl, Br M = a metal


Polarity Patterns in Some Functional Groups

Alkane Alkene

C C

Alkyne

C C

Arene(aromatic ring)

C C

nonpolar groups


Halide

XC

(X= F, Cl, Br, I)Alcohol

OHC

Ether

OC C

Amine

NH2C

SulfideThiol

SHC SC C

Aldehyde

CC HO

Ketone

CC CO

Carboxylicacid

CC OHO

Ester

CC OO

C

Amide

CC NO

Carboxylicacid chloride

CC ClO

δ+ δ- δ+ δ- δ+ δ- δ+ δ-

δ+ δ- δ+ δ-δ+δ-

δ+δ-

δ+δ-

δ+δ-

δ+δ-

δ+δ-

δ+

δ+

δ- δ- δ- δ-

polargroups


CH3

OH

H-A

CH3

OH H

+ A-

wealky polar C-O bond protonated methanol(strongly polar C-O bond)

Polar bonds can also result from the interaction of functional groups with solvents and with Lewis acids or bases (activation).

For example, the polarity of C-O bond in methanol is greatly enhanced by protonation of the oxygen atom with an acid; much more reactive C-O bond


• As the electric field around a given atom changes because of changing interactions with solvent or with other polar molecules, the electron distribution around that atom also change. The measure of this response to an external influence is called the polarizability of the atom.

• Larger atoms with more loosely held electrons are more polarizable than smaller atoms with tightly held electrons. (polarizability; C-I > C-F)

Polarizability is another factor of bond reactivity

δ-I

Cδ+ Because of iodine's high polarizability, the C-I bond behaves as if it were polar.


Because unlike charges attract, the fundamental characteristic of all polar organic reactions is that electron-rich sites in one molecule react with electron-poor sites in another molecule.

Bonds are made when an electron-rich atom donates a pair of electrons to an electron-poor atom, and bonds are broken when one atom leaves with both electrons from the former bond.


A curved arrow shows where two electrons move when reactant bonds are broken and product bonds are formed.

Electron Movements

A B+ A B

Electrophile(electron-poor)

Nucleophile(electron-rich)

This curved arrow shows that electrons move from B- to A+

The electrons that moved from B- to A+ end up here in this new covalent bond

A generalized polar reaction


• nucleophile: a substance that is 'nucleus-loving' (Remember that a nucleus is positively charged); electron-rich atom; neutral or negatively charged

• electrophile: a substance that is 'electron-loving'; electron-poor; neutral or positively charged

some nucleophiles (electron-rich)

H3N H2O HO Br

some electrophiles (electron-poor)

H H3C Brδ+ δ-

CO

δ+δ-


Sometimes, a species could be both nucleophilic and electrophilic depending on the circumstances

HO

HH3C-MgBr

CH4AlCl4

- CH3+

H3C-OH

water as a nucleophile

water as an electrophile

Lewis bases are electron donors and behave as nucleophiles, whereas Lewis acids are electron acceptors and behave as electrophiles.

Therefore, much of organic chemistry is explainable in terms of acid-base reactions.


5.5 An Example of a Polar Reaction: Addition of HBrto Ethylene

• Electrophilic addition:

H

H H

H+ H-Br

H

H H

H

H

Br

Ethylene(nucleophile)

Hydrogen bromide(electrophile)

Bromoethane

• C=C double bond: σ-bond + π-bond;electron rich and more accessible electrons → nucleophilic


CC

C-C σ-bond: stronger; less accessible bonding electrons

• HBr is H+ donor: electrophile

C-C π-bond: weaker; more accessible bonding electrons


electrophilic addition:

The electrophile HBr is attacked by theπ-electrons of the double bond, and a new C-H σ-bond is formed. This leavesthe other carbon atom with a + charge and a vacant p-orbital

H

H H

H

H-Br

HH

HH

H

Br-

HH H

H

HBr

carbocationintermediate

Br- donates an electron pair to the positively charged carbon atom, forming a C-Br σ-bond and yielding the neutral addition product.


All polar organic reactions take place between electron-rich sites and electron-poor sites and involve the donation of an electron pair from a nucleophile to an electrophile


5.6 Using Curved Arrows in Polar Reaction Mechanisms

Electron movements in polar reaction mechanisms

O E N E

C E E

Electrons move from a nucleophile source (Nu:) to an electrophilicsink (E).

rule 1

The nucleophile source must have an electron pair available, usually either in a lone pair or a multiple bond.


The electrophilic sink must be able to accept an electron pair, usually because it has either a positively charged atom or a positively polarized atom in a functional group.

C

Nu:C Halogen

Nu:δ+ δ-

H O

Nu:

C O

Nu:δ+ δ-δ+ δ-


H3C O + H Br H3C O-H + Br

negativelycharged

neutral negativelycharged

The nucleophile can be either negatively charged or neutral.rule 2

If the nucleophile is negatively charged, the atom that gives away an electron pair becomes neutral.


If the nucleophile is neutral, the atom that gives away an electron pair acquires a positive charge.

H

H H

H

H-Br

HH

HH

HBr-

neutral positive negative


OC

HCN+

OH

CN

positive neutralnegative

The electrophile can be either positively charged or neutral.rule 3

If the electrophile is positively charged, the atom bearing that charge becomes neutral after accepting an electron pair.


OC

HCN+ O

CNH

unstable

OH

CNOH

CNneutral


If the electrophile is neutral, the atom that accept an electron pair acquires a negative charge. For this to happen, the negative charge must be stabilized by being on an electronegative atom such as aoxygen or a halogen.

H

H H

H H-BrHH

HH

HBr-

neutral positive stable, negativelycharged ion

++


The octet rule must be followed.rule 4

H

H H

H HH

HH

HBr-++ BrH

This hydrogen already has two electrons. When another electron pair moves to the hydrogen from the double bond, the electron pair in the H-Br bond must leave.


OC

H+

OH

CNC N

This carbon already has eight electrons. When another electron pair moves to the carbon from -CN, an electron pair in the C=Obond must leave.

H3C CH2

O

H3C CH2

O

+ H3C BrH3C C

H2

OCH3 + Br-

Practice Electron Movements


H3C CH2

OH3C Br

Practice Electron Movements


H

H H

H+ H2O

H

H H

H

H

Br

HO-

+ Br-

O

H3C OCH3

Cl H3C OCH3

O+ Cl-


5.7 Describing a Reaction: Equilibria, Rates, and Energy Changes

aA + bB cC + dD

Keq =[Products]

[Reactants]

[C]c[D]d=

[A]a[B]b

Chemical reactions can go in either forward or reverse direction. The position of the equilibrium is expressed by Keq (equilibrium constant)

Keq > 1: forward reaction Keq < 1: backward reaction


What determine the magnitude of the equilibrium constant?

• Keq > 1: forward reaction : ∆Go negativeKeq < 1: backward reaction: ∆Go positive

Gibbs free-energy change, ∆G: the energy change that occurs during a chemical reaction

standard Gibbs free-energy change, ∆Go: 1 atm, 298 K, 1 M reactant concentrations

∆Go = - RT ln Keq

∆Go = ∆Ho + Τ∆So∆H, enthalpy∆G, entropy


∆H (heat of reaction): measure of the change in total bonding energyduring a reaction

• The enthalpy term is frequently larger and more dominant than the entropy term.

∆Go = - 44.8 kJ/mol∆Ho = - 84.1 kJ/mol∆So = - 0.132 kJ/(K.mol)

H2C CH2 + HBr CH3CH2Br

T = 298 K


∆S (entropy change): measure of the change in the amount of molecular disorder, or freedom of motion, that accompanies a reaction.

∆So > 0A B + C

A + B C ∆So < 0

• ∆H < 0: exothermic: the bonds in the products are stronger (more stable) than the bonds in the reactants, heat is released• ∆H > 0: endothermic: the bonds in the products are weaker (less stable) than the bonds in the reactants, heat is absorbed


Explanation of Thermodynamic Quantities: ∆Go = ∆Ho - T∆So

Term Name

∆Go Gibbs free-energy change

The energy difference between reactants and products. When ∆Go is negative,the reaction is exergonic, has a favorable equilibrium constant, and can occur spontaneously. When ∆Go is positive, the reaction is endergonic, has an unfavorable equilibrium constant, and cannot occur spontaneously.


∆Ho Enthalpy change

The heat of reaction, or difference in strength between the bonds brokenin a reaction and the bonds formed. When ∆Ho is negative, the reaction releases heat and is exothermic. When ∆Ho is positive, the reaction absorbs heat and is endothermic.

∆So Entropy change

The change in molecular disorder during a reaction. When ∆So is negative, disorder decreases; when ∆So is positive, disorder increases.


• The equilibrium constant tells only the position of the equilibrium, or how much product is theoretically possible. It doesn't tell the rate of reaction, or how fast the equilibrium is established.

• Some reactions are extremely slow even though they have favorable equilibrium constants.

For example,Gasoline reacts with oxygen slowly at room temperature (stable)but, at higher temperature such as occur in contact with a lighted match, gasoline reacts rapidly with oxygen and undergoes complete conversion to the equilibrium products H2O and CO2.

Rate→ Is the reaction fast or slow?

Equilibrium→ In what direction does the reaction proceed?


5.8 Describing a Reaction: Bond Dissociation Energies

Bond dissociation energy (D): the amount of energy required to break a given bond to produce two radical fragments; in the gas phase at 25oC

A B A + B

∆H = bond dissociation energy (D)


Table 5.3 Bond dissociation energy (D)

H Cl 432 kJ/mol

Cl Cl 243 kJ/mol

H Br 366 kJ/mol

Br Br 193 kJ/mol

H I 298 kJ/mol

I I 151 kJ/mol

H3CO H 437 kJ/mol

H2C=CHCH2 H 361 kJ/mol

HO OH 213 kJ/mol

H3C Br 293 kJ/mol

PhCH2 H 368 kJ/mol

H3C I 234 kJ/mol


CH

H HH

+ Cl Cl CH

H ClH

+ H Cl

reactant bonds broken

C-HCl-Cl

D= 438 kJ/molD= 243 kJ/mol

total D= 681 kJ/mol

product bonds formed

C-ClH-Cl

D= 351 kJ/molD= 432 kJ/mol

total D= 783 kJ/mol

∆Ho= 681 - 783 = - 102 kJ/mol

• bond dissociation energies can be used for the rough calculation of ∆Ho

; exothermic by -102 kJ/mol


but, limitations with the calculations:- no information on ∆So and thus no information on ∆Go

- no information about the rate of reaction even if ∆Go is favorable- measured in the gas phase; but most organic reactions are carried in solution

Solvation effect:solvent molecules can surround and interact with dissolved reactants

- the entropy term ∆So also can be different in solution because the solvation of a polar reactant by a polar solvent causes a certain amount of orientation in the solvent and thereby reduces the amount of disorder

- can weaken bonds and cause large deviation from the gas-phase value of ∆Ho


5.9 Describing a Reaction: Energy Diagrams and Transition States

H

H H

H

H-BrHH

HH

H

Br-

HH H

H

HBr

carbocation

For a reaction to take place, reactant molecules must collide, and reorganization of atoms and bonds must occur.


reaction energy diagram:describes energy changes a reaction

CH3CH2 Br

H2C CH

Ener

gy

Reaction progress (reaction coordinate)

reactants

carbocation product

transition state (TS)

∆Go

∆G

activationenergy

+ HBr

∆Go controls the position of the equilibrium

∆G‡ controls the reaction rate


H HH H

H Br

transition state: the highest energy structure, unstable, can't be isolated

A hypothetical transition state structure for the first step.The C-C p bond is just beginning to break, and C-H bond is just beginning to form, and the H-Br bond is just beginning to break


Activation energy, ∆G‡ : the energy difference between reactants and transition state

- determine how rapidly the reaction occurs at a given temperature- collisions with energies greater than the activation energy can form products- the activation complex can proceed to the products or revert back to the reactants

• typical organic reactions: ∆G‡ ~10-35 kcal/mol (40-150 kJ/mol)

• Reactions with activation energy less than 20 kcal/mol (80 kJ/mol) take place at or below room temperature, whereas reactions with higher activation energies normally require a higher temperature.


- a large activation energy: slow reaction because few collisions occur with enough energy for the reacting molecules to reach the transition state.

∆Go

∆G

Ener

gy


a slow exergonic reaction


∆Go

∆G

Ener

gy


a slow endergonic reaction


- a small activation energy: fast reaction

Ener

gy


a fast exergonic reaction

∆Go

∆G


∆Go∆G

Ener

gy


a fast endergonic reaction


Ener

gy


What kind of a reaction can have a symmetric reaction energy diagram?


5.10 Describing a Reaction: Intermediates

H

H H

H

H-BrHH

HH

H

Br-

HH H

H

HBr

reaction intermediate

reaction intermediate: species exist momentarily during the course of the multi-step reaction


CH3CH2

Br

H2C CH2

Ener

gy


reactants ∆Go

∆G1

+ HBr

∆G2

TS1TS2

CH3CH2Br

can't be isolated, unstable but more stable than TS1 and TS2


Ener

gy


∆G1

∆G2

∆Go

hypothetical reaction energy diagrams for some two-step reactions: exergonic


Ener

gy


∆Go

∆G1∆G2

hypothetical reaction energy diagrams for some two-step reactions: endergonic

Biological Reactions

• Reactions in living organisms follow reaction diagrams too• They take place in very controlled conditions• They are promoted by catalysts that lower the activation barrier• The catalysts are usually proteins, called enzymes• Enzymes provide an alternative mechanism that is compatible with the condi

tions of life


Ener

gy

uncatalyzed

enzyme catalyzed

HO OHOH

+ 3 HNO3H2SO4 O2NO ONO2

ONO2

Nitroglycerin(highly unstable)

Glycerin

+ 3 H2O

1865, Alfred Nobel

commercial dynamite: a mixture of ammonium nitrate and nitroglycerin absorbed onto diatomaceous earth (stabilized)

Explosives

• spontaneous break down of molecules into fragments- usually stable gases such as N2, H2O, CO2• instantaneouse release of large quantaties of hot gases, which set up a devastingshock wave as they expand

• primary explosives: highly sensitive, ex) Pb(N3)2• secondary explosives: less sensitive to heat and shock, detonated by primary initiators

Chemistry @ Work

Explosives

• military explosives: stable

CH3O2N NO2

NO2

Trinitrotoluene(TNT)

O2NOH2CCH2ONO2

CH2ONO2

CH2ONO2

Pentaerythritol tetranitrate(PETN)

N

N

NO2N NO2

NO2

RDX(research department explosive)

• plastic explosive: PETN and RDX compounded with waxes or synthetic polymers

Chemistry @ Work

Four Types of Organic Reaction - Yonsei Universitychem.yonsei.ac.kr/chem/upload/CHE2103-03-00/114307743360310.pdf · Ch.5 An Overview of Organic Reactions 5.1 Kinds of Organic Reactions

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