..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

1)

2)

3)

4)

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

5)

6)

7)

8)

9)