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Isomerization

The change which involves the change in chemical composition of the substance is called as chemical change. A chemical change can be shown in the form of chemical reactions. A chemical reaction represents the conversion of reactant molecules to product molecules. Here reactant molecules are chemical substances which involve in chemical change whereas products are the molecules which newly form during the chemical reactions. A chemical reaction can be represented in terms of chemical equation in which reactants and products are represented as their chemical symbols which are separated by a single or double headed arrow.

A chemical reaction can be many types such as combination reaction (synthesis reaction, decomposition, Redox, double decomposition, displacement, isomerization and Neutralization reactions. Synthesis or combination reactions lead to combination of more than one reactant molecules to form one molecule. For example reaction of hydrogen and iodine leads to the formation of hydrogen iodide (HI). On the contrary, decomposition reactions involve the decomposition or cleavage of one molecule to more than molecules such as decomposition of water leads to formation of hydrogen and oxygen gases. So we can say that decomposition reactions are just opposite to the synthesis or combination reactions. Displacement reactions involve displacement of one or more cation or anion between two molecules. Redox reactions are oxidation and reductions reactions and involve the oxidation and reduction of molecules. Neutralization reactions are the reactions between acids and bases to form salts and water. Since it neutralizes the effect of acids and bases with the formation of salt and water, these reactions are called as neutralization reactions. 

 

Define Isomerization

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Isomerization reactions can be defined as processes in which constitutional isomers inter-converted to other isomer. These reactions are very common in Enols, enolates, and enamines. Therefore in these reactions only the bond connectivity is altered and also stereochemical configuration is changed. Many enzymes interconvert constitutional isomers such as isomerases. Similarly some of the enzymes catalyze the interconversion of enantiomers and epimers such as racemases and epimerases.

Isomerization Reaction

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Isomerization reactions are the reaction in which chemical compounds convert to another by only change in the position of bonded atoms. In other words, isomerization reactions involve the conversion of one isomer to another. Let’s discuss carbonyl isomerization reaction. The glycolytic pathway involves successive keto-enol tautomerization steps.

They catalyzed by the enzymes phosphoglucose isomerase and triose phosphate isomerase enzymes. In both of these reactions, the carbonyl group of the sugar molecule is shifted back and forth by a single carbon and involve the conversion of ketones to aldehydes and back again - this is a conversion between two constitutional isomers. In the triosephosphate isomerase reaction ketone; dihydroxyacetone phosphate (DHAP) converts to enol tautomer in the presence of enzymatic acid/base pair. Next step is with glyceraldehyde phosphate (GAP) which is a tautomerization in reverse direction.

The conversion of glucose-6-phosphate (an aldehyde sugar) to fructose-6-phosphate (a ketone sugar) is also a isomerization reaction which is called as phosphoglucose isomerase reaction. This process is catalyzed by enzyme ribose-5-phosphate isomerase; an active enzyme in Calvin cycle as well as in pentose phosphate pathway

Isomerization Example

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Stereoisomerization reaction known as an epimerization are good example of isomerization reactions. In the epimerization reactions, the enolate form of a carbonyl group forms as an intermediate. Here epimer stands for the pair of diastereomers which shows difference at a single stereocenter.  For example; ribulose-5-phosphate and xylulose-5-phosphate are epimers. Both of these sugar phosphate have identical shape but different at stereochemistry of $C_3$. In Calvin cycle of plants, the C-atom of $CO_2$ passes to sugars and involves the interconversion of two sugar phosphate epimers; ribulose-5-phosphate and xylulose-5-phosphate at $C_3$. 

Isomerization Example

Isomerization Reaction Examples

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Deprotonation and reprotonation are other examples of isomerization reactions which occur at opposite sides of the planar, $sp_2$-hybridized intermediate. In the epimerization reaction of ribulose-5-phosphate, a coordinated pair of aspartate residues is placed at opposite sides of the enzyme's active site. At the start of the catalytic cycle Aspartate residues is ionized and abstracts the $C_3$ proton to converted $C_3$ from a chiral tetrahedral to an achiral planar group. During reprotonation, Asp$_2$ which is placed on the back side of the plane donates the proton and causes the inversion of stereochemistry at $C_3$.

Isomerization Process

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Isomerizations reactions involve the conversion of one isomer to another with the shifting of position of bonded atoms. No combination or decomposition takes place during such reactions. Let’s take some examples of isomerization reactions. The isomerization of alkene in the degradation of unsaturated fatty acids involves the formation of new isomers. 

It is an oxidation process in which unsaturated fatty acids (Fatty acids with long carbon chain and presence of multiple covalent bonds) shows shuffling of the position of $C = C$ bonds in the parent chain. This process is accomplished in the presence of enoyl CoA isomerase and with the formation of one of the intermediate; enolate intermediate. In this isomerization reaction, the cis $C_3 = C_4$ isomerizes to trans-bond between $C_2$ and $C_3$. Isomerization reactions also involve in the transformation of n-paraffins to iso-paraffins. Here n-paraffins are hydrocarbons with low octane number and iso-paraffins have very high octane number. Therefore the isomerization reactions are mainly involved in the production of high octane blending components.

Some other examples of isomerization processes are isomerisation of butane to iso-butane, conversion of pentanes and hexanes into higher- branched isomers.
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