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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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
Ch.5 An Overview of Organic Reactions
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
Ch.5 An Overview of Organic Reactions
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
Ch.5 An Overview of Organic Reactions
A B
This single reactant ... ... gives the isomeric product
Rearrangement Reaction:
example acid catalyst
Ch.5 An Overview of Organic Reactions
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
Ch.5 An Overview of Organic Reactions
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
Ch.5 An Overview of Organic Reactions
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
Ch.5 An Overview of Organic Reactions
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
Ch.5 An Overview of Organic Reactions
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
Ch.5 An Overview of Organic Reactions
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
Ch.5 An Overview of Organic Reactions
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.
Ch.5 An Overview of Organic Reactions
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
Ch.5 An Overview of Organic Reactions
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
Ch.5 An Overview of Organic Reactions
Polarity Patterns in Some Functional Groups
Alkane Alkene
C C
Alkyne
C C
Arene(aromatic ring)
C C
nonpolar groups
Ch.5 An Overview of Organic Reactions
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
Ch.5 An Overview of Organic Reactions
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
Ch.5 An Overview of Organic Reactions
• 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.
Ch.5 An Overview of Organic Reactions
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.
Ch.5 An Overview of Organic Reactions
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
Ch.5 An Overview of Organic Reactions
• 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-
Ch.5 An Overview of Organic Reactions
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.
Ch.5 An Overview of Organic 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
Ch.5 An Overview of Organic Reactions
CC
C-C σ-bond: stronger; less accessible bonding electrons
• HBr is H+ donor: electrophile
C-C π-bond: weaker; more accessible bonding electrons
Ch.5 An Overview of Organic Reactions
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.
Ch.5 An Overview of Organic Reactions
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
Ch.5 An Overview of Organic Reactions
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.
Ch.5 An Overview of Organic Reactions
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-
Ch.5 An Overview of Organic Reactions
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.
Ch.5 An Overview of Organic Reactions
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
Ch.5 An Overview of Organic Reactions
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.
Ch.5 An Overview of Organic Reactions
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
++
Ch.5 An Overview of Organic Reactions
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.
Ch.5 An Overview of Organic Reactions
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
Ch.5 An Overview of Organic Reactions
H3C CH2
OH3C Br
Practice Electron Movements
Ch.5 An Overview of Organic Reactions
H
H H
H+ H2O
H
H H
H
H
Br
HO-
+ Br-
O
H3C OCH3
Cl H3C OCH3
O+ Cl-
Ch.5 An Overview of Organic Reactions
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
Ch.5 An Overview of Organic Reactions
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
Ch.5 An Overview of Organic Reactions
∆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
Ch.5 An Overview of Organic Reactions
∆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
Ch.5 An Overview of Organic Reactions
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.
Ch.5 An Overview of Organic Reactions
∆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.
Ch.5 An Overview of Organic Reactions
• 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?
Ch.5 An Overview of Organic Reactions
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)
Ch.5 An Overview of Organic Reactions
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.5 An Overview of Organic Reactions
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
Ch.5 An Overview of Organic Reactions
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
Ch.5 An Overview of Organic Reactions
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.
Ch.5 An Overview of Organic Reactions
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
Ch.5 An Overview of Organic Reactions
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
Ch.5 An Overview of Organic Reactions
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.
Ch.5 An Overview of Organic Reactions
- 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
Reaction progress (reaction coordinate)
a slow exergonic reaction
Ch.5 An Overview of Organic Reactions
∆Go
∆G
Ener
gy
Reaction progress (reaction coordinate)
a slow endergonic reaction
Ch.5 An Overview of Organic Reactions
- a small activation energy: fast reaction
Ener
gy
Reaction progress (reaction coordinate)
a fast exergonic reaction
∆Go
∆G
Ch.5 An Overview of Organic Reactions
∆Go∆G
Ener
gy
Reaction progress (reaction coordinate)
a fast endergonic reaction
Ch.5 An Overview of Organic Reactions
Ener
gy
Reaction progress (reaction coordinate)
What kind of a reaction can have a symmetric reaction energy diagram?
Ch.5 An Overview of Organic Reactions
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
Ch.5 An Overview of Organic Reactions
CH3CH2
Br
H2C CH2
Ener
gy
Reaction progress (reaction coordinate)
reactants ∆Go
∆G1
+ HBr
∆G2
TS1TS2
CH3CH2Br
can't be isolated, unstable but more stable than TS1 and TS2
Ch.5 An Overview of Organic Reactions
Ener
gy
Reaction progress (reaction coordinate)
∆G1
∆G2
∆Go
hypothetical reaction energy diagrams for some two-step reactions: exergonic
Ch.5 An Overview of Organic Reactions
Ener
gy
Reaction progress (reaction coordinate)
∆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
Ch.5 An Overview of Organic Reactions
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
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