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Organic ReactionsCourse: B.Tech.Subject: Engineering ChemistryUnit: IV(B)

Organic Reactions and MechanismsOrganic chemistry deals with organic compounds(which contains C,H,O)

Organic reactions are chemical reactions involving organic compounds.

A reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs.NucleophileA reagent which can donate an electron pair in a reaction is called a nucleophile.

The name nucleophile means nucleus loving and indicates that it attacks regions of low electron density (positive centres) in the substrate molecule.

Nucleophiles are electron rich.They may be negative ions including carbanions or neutral molecules with free electron pair.

A nucleophile can be represented by a by general symbol Nu:-

Examples Cl-, Br-, I-, CN -, OH-, RCH2-, NH3, RNH2, H2O, ROH

Electrophiles A reagent which can accept an electron pair in a reaction called an electrophile.

The name electrophile means electron-loving and indicates that it attacks regions of high electron density (negative centers) in the substrates molecule.

Electrophiles are electron deficient.They may be positive ions including carbonium ions or neutral molecules with electron deficient centres.

An electrophile can represented by E+.

ExamplesH+, Cl+, Br+, I+, NO2+, R3C+, +SO3H, AlCl3, BF3

Organic Reaction MechanismA reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs.

Although only the net chemical change is directly observable for most chemical reactions, experiments can often be designed that suggest the possible sequence of steps in a reaction mechanism. Electron displacement effectsInductive effect

Inductive effect is defined as permanent displacement of shared electron pair forming a covalent bond towards more electronegative atom or group.

3Types of Inductive effect :

1. Negative Inductive Effect : (I effect, Electron withdrawing effect) when an electronegative atom or group (more electro negative than hydrogen)is attached to the terminal of the carbon chain in a compound, the electrons are displaced in the direction of the attached atom or group.

-NO2 > -CN > -COOH > F > Cl > Br > I > OH > C6H5 > H2.Positive Inductive effect : (+I effect, Electronreleasing effect)When an electro positive atom or group (more electro positive than hydrogen)is attached to the terminal of the carbon chain in a compound, the electrons are displaced away from the attached atom or group.

(CH3)3C- > (CH3)2CH- > -C2H5 > - CH3Mesomeric/ Resonance Effect The flow of electrons from one part of a conjugated system to the other caused by phenomenon of resonance is called resonance effect or mesomeric effect.

The re-distribution of electrons which takes place in unsaturated and especially in conjugated systems via their -orbitals.

-M or -R effect : When the electron displacement is towards the group.

e.g :-NO2 , -CHO

+M or +R effect : When the electron displacement is away from the group.

e.g :-OH , -OR,-Cl

Electromeric EffectElectromeric effect is defined as the complete transfer of electrons of a multiple bond towards one of the bonded atoms at the demand of an attacking reagent.

Note : a) It is shown by those compounds containing multiple bond b) It is a purely temporary effect & is brought into play only at the requirement of attacking agent.Types of Electromeric Effect

+E effect : When displacement of electrons is away from the atom or group.

e.g : addition of H+ to alkene.

-E effect : When displacement of electrons is towards the atom or group.e.g : addition of cyanide ion(CN-) to the carbonyl group.

Inductomeric effect

Inductomeric effect is the temporary effect which enhances the inductive effect and it accounts only in the presence of an attacking reagent.Example

In methyl chloride the -I effect of set is further increased temporarily by the approach of hydroxyl ion.

4Organic intermediatesFree radicals A Free radical is a species which has one or more unpaired electrons.

It is paramagnetic .

It can be detected by electron spin resonance spectroscopy.CarbocationsCarbocations are the intermediates in which the positive charge is carried by the carbon atom with six electrons in the valence shell.

Carbocations are carbon atoms in an organic molecule bearing a positive formal charge. Therefore they are carbon cations. CarboanionsCarbanions arer thosev intermediates in which central carbon atom carries negative charge and they posses unshared pair of electrons.

Carbanions are units that contain a negative charge on a carbon atom.

CH3 >1 0>20>30CarbenesThe carbenes are neutral carbon intermediates in which the central carbon has six electrons,two of which are free.In chemistry, a carbene is a molecule containing a neutral carbon atom with a valence of two and two unshared valence electrons.

Ex,:CH2 ,MethyleneTypes of reactionsTypes of ReactionsReaction TypeSub-typeExamplesAddition reactionsElectrophilic Nucloephilicradicalhalognenation, hydrohalogenation and hydrationElimination reactionDehydrationSubstitution reactionsnucleophilic aliphaticSubstitutionnucleophilic aromatic substitutionnucleophilic acyl substitutionelectrophilic substitutionelectrophilic aromatic substitutionradical substitutionwith SN1, SN2 and SN reaction mechanisms Addition Reactions-Electrophilic additionAn electrophilic addition reaction is an addition reaction where, in a chemical compound, a bond is broken and two new bonds are formed. The substrate of an electrophilic addition reaction must have a double bond or triple bond.

The driving force for this reaction is the formation of an electrophile X+ that forms a covalent bond with an electron-rich unsaturated C=C bond. The positive charge on X is transferred to the carbon-carbon bond, forming a carbocation.Addition Reactions-Electrophilic addition

Addition Reactions-Electrophilic additionIn step 1, the positively charged intermediate combines with (Y) that is electron-rich and usually an anion to form the second covalent bond.

Step 2 is the same nucleophilic attack process found in an SN1 reaction. The exact nature of the electrophile and the nature of the positively charged intermediate are not always clear and depend on reactants and reaction conditions.Nucleophiic addition A nucleophilic addition reaction is an addition reaction where in a chemical compound a bond is removed by the creation of two new covalent bonds by the addition of a nucleophile.

Addition reactions are limited to chemical compounds that have multiple-bonded atoms

molecules with carbon - hetero multiple bonds like carbonyls, imines or nitriles

Nucleophiic additionAn example of a nucleophilic addition reaction that occurs at the carbonyl group of a ketone by substitution with hydroxide-based compounds, denoted shorthand. In this example, an unstable hemiketal is formed.

Substitution ReactionsThe reactions in which an atom or group of atoms in a molecule is replaced or substituted by different atoms or group of atoms are called substitution reaction. For example,

Nucleophilic SubstitutionNucleophilic substitution is a fundamental class of substitution reaction in which an "electron rich" nucleophile selectively bonds with or attacks the positive or partially positive charge of an atom attached to a group or atom called the leaving group; the positive or partially positive atom is referred to as an electrophile. Nucleophilic substitution reactions can be broadly classified as

Nucleophilic substitution at saturated carbon centresNucleophilic substitution at unsaturated carbon centres

Nucleophilic substitution at saturated carbon centresIn 1935, Edward D. Hughes and Sir Christopher Ingold studied nucleophilic substitution reactions of alkyl halides and related compounds.

They proposed that there were two main mechanisms at work, both of them competing with each other. The two main mechanisms are the SN1 reaction and the SN2 reaction. S stands for chemical substitution, N stands for nucleophilic, the number represents the kinetic order of the reaction.In the SN2 reaction, the addition of the nucleophile and the elimination of leaving group take place simultaneously.

SN2 occurs where the central carbon atom is easily accessible to the nucleophile. By contrast the SN1 reaction involves two steps.

SN1 reactions tend to be important when the central carbon atom of the substrate is surrounded by bulky groups, both because such groups interfere sterically with the SN2 reaction (discussed above) and because a highly substituted carbon forms a stable carbocation.Nucleophilic substitution at carbon atom SN1 Mechanism

Nucleophilic substitution at carbon atom SN2 Mechanism

Nucleophilic substitution at unsaturated carbon centers

Nucleophilic substitution via the SN1 or SN2 mechanism does not generally occur with vinyl or aryl halides or related compounds.

When the substitution occurs at the carbonyl group, the acyl group may undergo nucleophilic acyl substitution. This is the normal mode of substitution with carboxylic acid derivatives such as acyl chlorides, esters and amides.Nucleophilic Aromatic substitutionA nucleophilic aromatic substitution is a substitution reaction in organic chemistry in which the nucleophile displaces a good leaving group, such as a halide, on an aromatic ring.

5NitrationNitration is a general chemical process for the introduction of a nitro group into a chemical compound. Examples of nitrations are the conversion of glycerin to nitroglycerin and the conversion of toluene to trinitrotoluene. Both of these conversions use nitric acid and sulfuric acid.In aromatic nitration, aromatic organic compounds are nitrated via an electrophilic aromatic substitution mechanism involving the attack of the electron-rich benzene ring by the nitronium ion.

References1.Organic chemistry by Morrison and Boyd2. Engineering Chemistry by Jain and Jain,Dhanpat rai Publication3. http://chem-guide.blogspot.in/2010/04/inductive-effect.html4.http://www.expertsmind.com/learning/inductomeric-effect-assignment-help-7342872160.aspx5. http://nptel.ac.in/courses/Webcourse-contents/IIT-Delhi/chemistry/chap4/chp4.htm