The Arrow Notation in Mechanisms

Arrow pushing. Arrow pushing or electron pushing is a technique used to describe the progression of organicchemistry reaction mechanisms. ... In using arrow pushing, "curved arrows" or "curly arrows" are superimposed over the structural formulae of reactants in a chemical equation to show the reaction mechanism.

Since chemical reactions involve the breaking and making of bonds, a consideration of the movement of bonding ( and non-bonding ) valence shell electrons is essential to this understanding. It is now common practice to show the movement of electrons with curved arrows, and a sequence of equations depicting the consequences of such electron shifts is termed a mechanism. In general, two kinds of curved arrows are used in drawing mechanisms:

The use of these symbols in bond-breaking and bond-making reactions is illustrated below. If a covalent single bond is broken so that one electron of the shared pair remains with each fragment, as in the first example, this bond-breaking is called homolysis. If the bond breaks with both electrons of the shared pair remaining with one fragment, as in the second and third examples, this is called heterolysis.

{The term homolysis generally means breakdown (lysis) to equal pieces (homo = same). There are separate meanings for the word in chemistry and biology: ... Homolysis (chemistry), a chemical bond dissociation of a neutral molecule generating two free radicals.}

{In chemistry, heterolysis or heterolyticfission (from Greek ἕτερος, heteros, "different", and λύσις, lusis, "loosening") is the process of cleaving a covalent bond where one previously bonded species takes both original bonding electrons from the other species.}

 

Other Arrow Symbols

Chemists also use arrow symbols for other purposes, and it is essential to use them correctly.

The Reaction Arrow :

The equilibrium arrow. The equilibrium arrow is a combination of two, half headed arrows facing in the opposite direction to reflect the chemical reaction is a reversible reaction. It symbolizes equilibrium.

The Equilibrium Arrow : The equilibrium arrow is a combination of two, half headedarrows facing in the opposite direction to reflect the chemical reaction is a reversible reaction. It symbolizesequilibrium.

The following equations illustrate the proper use of these symbols:

 Reactive Intermediates

The products of bond breaking, shown above, are not stable in the usual sense, and cannot be isolated for prolonged study. Such species are referred to as reactive intermediates, and are believed to be transient intermediates in many reactions. The general structures and names of four such intermediates are given below.

{In chemistry a reactive intermediate is a short-lived, high-energy, highlyreactive molecule. When generated in a chemical reaction, it will quickly convert into a more stable molecule.}

A pair of widely used terms, related to the Lewis acid-base notation, should also be introduced here.

Electrophile:   In organic chemistry, an electrophile is a reagent attracted to electrons.Electrophiles are positively charged or neutral species having vacant orbitals that are attracted to an electron rich centre. It participates in a chemical reaction by accepting an electron pair in order to bond to a nucleophile.

Nucleophile:   A nucleophile is a chemical species that donates an electron pair to an electrophile to form a chemical bond in relation to a reaction. All molecules or ions with a free pair of electrons or at least one pi bond can act asnucleophiles. Because nucleophilesdonate electrons, they are by definitionLewis bases.

Using these definitions, it is clear that carbocations ( called carbonium ions in the older literature ) are electrophiles and carbanions are nucleophiles. Carbenes have only a valence shell sextet of electrons and are therefore electron deficient. In this sense they are electrophiles, but the non-bonding electron pair also gives carbenes nucleophilic character. As a rule, the electrophilic character dominates carbene reactivity. Carbon radicals have only seven valence electrons, and may be considered electron deficient; however, they do not in general bond to nucleophilic electron pairs, so their chemistry exhibits unique differences from that of conventional electrophiles. Radical intermediates are often called free radicals.

{A carbanion is an anion in which carbon is tervalent (forms three bonds) and bears a formal negative charge in at least one significant mesomeric contributor (resonance form).}

The importance of electrophile / nucleophile terminology comes from the fact that many organic reactions involve at some stage the bonding of a nucleophile to an electrophile, a process that generally leads to a stable intermediate or product. Reactions of this kind are sometimes called ionic reactions, since ionic reactants or products are often involved. Some common examples of ionic reactions and their mechanisms may be examined by Clicking Here

The shapes ideally assumed by these intermediates becomes important when considering the stereochemistry of reactions in which they play a role. A simple tetravalent compound like methane, CH4, has a tetrahedral configuration. Carbocations have only three bonds to the charge bearing carbon, so it adopts a planar trigonal configuration. Carbanions are pyramidal in shape ( tetrahedral if the electron pair is viewed as a substituent ), but these species invert rapidly at room temperature, passing through a higher energy planar form in which the electron pair occupies a p-orbital. Radicals are intermediate in configuration, the energy difference between pyramidal and planar forms being very small. Since three points determine a plane, the shape of carbenes must be planar; however, the valence electron distribution varies