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Alder Ene Reaction

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A chemical compound can involve in various chemical reactions to form new substances. A chemical reaction can be defined as the formation of new substances by bond breaking or formation from the starting substance. Here the starting substance is called as reactant and new substance that formed is called as product.

Hence a chemical reaction can be represented as;

REACTANT  $\rightarrow$ PRODUCT

It looks very easy to write a word form of chemical reaction but it is not so easy to occur as bond breaking requires a certain amount of energy whereas bond formation releases some energy to make a stable molecule.

The reagent is the substance which initiated the conversion of reactant to product. Each reaction is characterized by certain reactions conditions such as temperature, pressure and catalyst which speed up the reaction.
 
On the basis of energy change the chemical reactions can be classified as exothermic and endothermic reactions.
  • Exothermic reactions are chemical reactions which releases certain amount of energy in the form of heat / light such as combustion reactions.
  • Endothermic reactions are chemical reactions occur with the absorption of energy.
On the basis of bond breaking and formation; chemical reactions can be classified in following types.

Synthesis reactions: These reactions are also called as combination reaction as it leads to the formation of new substance with the combination of two or more substances. For example combination of hydrogen and oxygen gas leads to formation of water molecule. 

2$H_{2(g)} + O_{2(g)}  \rightarrow  2 H_{2}O_{(l)}$

Decomposition reactions: These reactions are just reverse reactions of combination reactions. They lead to decomposition of chemical compound to its components. For example; heating of mercuric oxide forms Hg and oxygen gas. 

2$HgO  \rightarrow 2 Hg + O_{2}$

Displacement reaction: Displacement reactions are mainly associated with salts. It involves the displacement of less reactive ion with more reactive ion. For example;  

$Fe+ CuSO_{4} \rightarrow FeSO_{4} + Cu$

Here Fe is more reactive than Cu therefore it displaces Cu from its salt; copper sulphate to form iron sulphate. 

Redox reaction: Redox reactions are characterized by simultaneous oxidation and reduction reactions. Oxidation reaction can be defined as removal of electrons whereas reduction leads to addition of electrons. For example;

$2 Mg + O_{2} \rightarrow 2 MgO$

Here Mg gets oxidize to Mg+2 ions whereas O gets reduced to O-2 ion. Hence Mg is reducing agent whereas O2 is oxidizing agent.

Alder Ene Reaction Definition

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Another type of classification is based on reaction mechanism. On the basis of mechanism, chemical reactions can be classified as;
  • Addition reactions
  • Elimination reactions
  • Substitution reactions
  • Pericyclic reactions. 
Addition reactions are mainly associated with unsaturated hydrocarbons (alkenes, alkynes etc.). In these chemical reactions, some reagent (electrophilie (E+ or nucleophile Nu-) added on unsaturated part of molecule.  On the basis of coming reagent, these reactions are classified as electrophilic addition and nucleophilic addition reactions. For example hydration of alkene is an example of electrophilic addition reactions whereas carbonyl compounds (aldehyde and ketones) exhibit the nucleophilic addition reactions. As name suggested, elimination reactions are reverse reactions of addition reactions as they involve the elimination of certain group from the molecule. It leads to the conversion of saturated compound to unsaturated compound. For example dehydrohalogenation of alkyl halide in the presence of base leads to the formation of alkenes. 

Dehydrohalogenation

Substitution reactions also associated with saturated compounds with functional groups such as halogen group. Here halogen acts as good leaving group and some Nu- (nucleophile) such as –OH-, -OCH3 etc. substituted at the same position. On the basis of mechanism, substitution reactions can be classified as SN1 and SN2 reactions.

SN1 reactions are uni-molecular nucleophilic substitution reactions which leads to the formation of carbocation as an intermediate. The coming nucleophile attack on carbocation from either sides to form substituted product. Since carbocation is a planer intermediate therefore optically active alkyl halide gives racemised product. Polar solvent are preferred for this mechanism and rate of reaction depends on the concentration of alkyl halides only as the formation of carbocation is rate determining step here. 
The stability order of carbocation determines the rate of reaction that is;

Tertiary alkyl halide > Secondary alkyl halide > Primary alkyl halide

SN2 reactions are bimolecular substitution reactions which leads to the formation of transition state as an intermediate which is highly unstable intermediate due to penta-valency of carbon atom.

This mechanism is mainly associated with primary alkyl halides due to less steric hindrance in the molecule. The rate of reaction depends on the concentration of both alkyl halide and nucleophile. Since transition state does not have complete charges therefore non-polar solvents are preferable for this mechanism. 

The rate of reaction with different alkyl halides follow given order:

Primary alkyl halide > Secondary alkyl halide > Tertiary alkyl halide

Another type of chemical reactions are called as Pericyclic reactions. These are concerted chemical reactions which occur with the formation cyclic transition state as an intermediate. These are generally photochemical or thermal reactions which requires light or heat and always stereospecific in nature. In other words a stereoisomer of the reactant forms a stereoisomer of the product. These reactions involves bonds formation and cleavage in a concerted cyclic transition state. Unlike other reaction mechanisms, these reactions are not effected by solvent, initiators or electrophilic or nucleophilic catalysts. There are mainly four types of pericyclic reactions;
  • Cycloaddition reactions
  • Electrocyclic reactions
  • Sigmatropic reactions
  • Ene Reactions.
All these reactions are reversible and proceeds by ring cleavage or conversion of two sigma-bonds to two pi-bonds. Diels-Alder reaction is well known cycloaddition reaction which forms cyclic alkene from dienes. Here diene reacts with dieophile to form cyclic alkene. It involves conversion of 2 Ï€-electron systems to a ring of atoms with 2 new σ bonds. 

2 À-Electron Systems
 
An electrocyclic reaction are also called as electrocyclic ring opening reactions. These reactions follow concerted cyclization of a conjugated π-electron system with the conversion of 1 pi bond to sigma bond. Another type of pericyclic reactions are Sigmatropic rearrangements which is a type of molecular rearrangement reaction.

It involves shifting of sigma and pi bond out of which [1, 5] and [1, 3]-shifting are more common. Cope and Claisen rearrangements is a type of Sigmatropic rearrangement reaction. 

Claisen Rearrangements

The ene reaction are also called as Alder-ene reaction. Alder-ene reaction can be defined as the pericyclic reaction which involve the bonding of alkene with an allylic hydrogen. Here alkene with allylic hydrogen is called as en and other compound is called as enophile which can have some other functional group with multiple bonds such as >C=O, -C=S, N=N etc. Here new sigma bond is formed with the migration of double bond of ene and [1,5]- H shift. 

1,5-Hydrogen Shift

Alder Ene Reaction Mechanism

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The mechanism of Alder Ene reaction involves the frontier-orbital interaction between HOMO (highest occupied molecular orbital) of ene with LUMO (Lowest unoccupied molecular orbital) of enophile. Here Alkene with –I group (negative inductive effect) and alkene with allylic hydrogen involve in reaction. Unlike other pericyclic reactions, no ring formation occurs here otherwise, reaction follows almost same wat as in Diels-Alder reaction. 

Diels-Alder Reaction