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..Organic Reactions and Their Mechanism
CHAPTER 1
Organic Reactions and their Mechanisms
A reaction mechanism is a detailed description of the bonding changes as a reaction proceeds. The reaction mechanism also includes the many important principles of organic chemistry. A plausible reaction mechanism must be consistent with the principles of organic chemistry.
Bond Making and Bond Breaking Processes: Heterolysis and Homolysis
.A covalent bond may break by either of two different processes: heterolysis or homolysis
A : B +ions
+ -A B .
Double-barbed arrowis used to show movementof an electron pair
Homolysis (Gr: homo-"the same" + lysis)
Heterolysis (Gr: hetero- "different" + lysis-"cleavage")
A : B . +
radicals
A B. ..
Single-barbed arrowis used to show movementof a single electron
Bond Heterolysis
.
Bond heterolysis typically is found with polar covalent bonds where the intrinsic electronegativities promote polarization of the bonding electrons
A : B ++ -A B
polar covalent bond
Assisted Bond Heterolysis
.Bond heterolysis may be assisted by a second molecule or chemical species that can either donate or accept an electron pair.
A : B C
+C A : B
+ B+ -C : A C donates an electron
pair in forming bond to Aas B departs as anion.
A : B D+
+A : B
D+ +A+ B : D D+ accepts an electron pairas it bonds to B releasing A+.
Electrophile (cations)
Nucleophile (anions)
Homolysis Cleavage
refer to breaking covalent bond between same/diff atoms with small electronegativity difference
form free radical,
Example…
endothermic - absorb energy to break down the covalent bond
provided by sunlight (UV light) / high temperature
Cl ●, H ●, ● CH3
Organic Reagents
Divide into 3 group:Electrophile
Nucleophile
Free radical
A reagent which can accept an electron pair in a rxn
Electrophile
means
A molecule/ion which is electron deficient (low e density) and attack the electron rich species (high e density)
or
Nucleophile
A reagent which donate an electron pair in a rxn
means
A molecule/ion which is electron rich and attack the electron deficient centre
or
are positive charged species
are negatively charged or a neutral molecule having an atom with a lone pair of electrons
Nucleophile Electrophile
Examples:
a) OH-, CN-, Cl-, Br-, I-
b) Double and triple bond of alkene and alkynes
c) Compound containing an atom with lone pair
c) H2SO4, HNO3, HNO2
e) Lewis base
a) H+ and other +ve ions
b) Halogen and halogen acids
d) R2O:, :NH3, R3N:, etc d) NO2, AlCl3, BF2, etc
e) Oxidising agent (Cl2, Br2)
Free Radical
Result of homolytic cleavage in a covalent bond
Molecules/atoms which contain unpaired electron which are neutral and reactive
Example: H•, CH3• and Cl•
Four General Categories of Organic Reactions
.Organic reactions tend to fall into four categories: substitutions, additions, eliminations, rearrangements
Substitutions
In a substitution reaction, one atom or group replaces another in a structure. This type of reaction is commonly observed in saturated hydrocarbons and aromatics.
an alkyl halide
+ Na+ -OH + Na+ X-R-X R-OHH2O
an aromatic+ Br2
+ HBrAr-HFe
Ar-Br
The mechanisms of the above two substitution reactions are completely different.
Nucleophilic substitution
Electrophilic substitution
Additions
.
Addition reactions are found in organic compounds with multiple bonds: alkenes, alkynes, carbonyl-containing compounds. In this reaction, the-component of the multiple bond is lost as new bonds are formed to the carbon (or other atomic) centers
+ Br2
Bromine adds to the alkene (ethene).
C C
H
H
H
H
C C
H
H
Br Br
H
H
A reaction in which atoms or groups are added to the reactant.
means
Eliminations
.
These reactions are the reverse of addition reactions. In an elimination reaction, a molecule loses atoms or groups from adjoining carbon (or other atomic) centers, forming a multiple bond
C C
H
H
H
H
+ KOH + K+ Br- + H2OC C
H
H
H Br
H
H
The above reaction is a dehydrohalogenation, loss of HBr, of an alkyl halide to form an alkene.
means A rxn that involves the removal of atoms or molecules from the reactant
Rearrangements
.In rearrangement reactions, there is a reorganization of the atoms or groups in a structure
H+
.In the presence of acid, the alkene on the left rearranges to the alkene on the right
A reaction in which involves the migration of an atoms or groups from site to another within the same molecule
means
Reaction Mechanisms and Chemical Intermediates
.
Reaction mechanisms are detailed descriptions of changes at the molecular level as reactants become products. Often the reactions involve a sequence of steps with one or more chemical species called intermediates that are formed and consumed
.Chemical intermediates typically are not stable structures that can be put in a bottle. Many exist for very short times (10-6 - 10-9 seconds)
Check this out!!!CH3 – CH2 – Cl + OH- CH3 – CH2 – OH + Cl-
CH3CH3 + Cl2 CH3CH2Cl + HCl
+ Cl2
ClAlCl3
CH3 – CH = CH2 + Br2 CH3CH(Br)CH2(Br)
uv
CCl4
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CH3 – C = O + HCN CH3 – C - OH
H
H
CN
CH3 – CH2OH CH2 = CH2 + H2O
(CH3)2C = CH2 + Br2 (CH3)2CBrCH2Br
CN- + CH3Br CH3CN + Br-
(CH3)2CHCl + NaOH (CH3)2CHOH + NaCl
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