HOW TO DRAW REACTION MECHANISM IN ORGANIC CHEMISTRY

Organic reactions follow a logical pathway involving the atoms and groups of atoms interacting with each other. The route followed by the reactants to produce products is known as the reaction mechanism. Organic chemists are usually asked to draw a suitable (plausible) mechanism for different chemical reactions. There are a few things that need to be kept in mind while drawing reaction mechanisms correctly, keeping in view the basic concepts of chemistry in general and organic chemistry in particular.

1.  Drawing the reactants and reagents

The first step for drawing a more probable reaction mechanism is to draw the reactants and reagents in such a way that the bonds between different atoms in a molecule are clearly seen and understandable. This helps a lot. For example, it gives you an idea about the functional groups present in the molecule and from that the reactivity of these groups towards different reagents or reaction conditions. Secondly, it helps you find the exact center (atom) that is involved in the reaction.

Writing ethyl acetate as C₄H₈O₂ will not tell you anything about the reaction centers, but drawing it like

  clearly shows the ester group, with the carbonyl carbon and the a-hydrogens, one of which might be the possible reaction center.

2.  Nature of Reaction (Polar/Non Polar)

Almost all reactions in organic chemistry (except those involving free radicals) involve a reaction between an electron rich center and an electron deficient center. An important step in drawing mechanism is to figure out the nature of the reaction. If the reaction is of polar nature, it will definitely involve electron rich and electron deficient centers. These sites can easily be figured out from the structural formula (given in step-1) and from the background knowledge of the subject. If the reaction is non-polar, it will involve free radicals, generated by homolytic cleavage of bonds. Which bonds be cleaved homolytically, comes from the knowledge of the subject.

3.  Reaction Conditions

The third step to know is the reaction condition. For example, acidic or basic conditions. The property of an acid is to give H⁺ in solution, in other words it provides hydrogen ion for protonation. Base is known for its electron rich nature and will abstract any acidic proton present in the molecule, such as the one attached to oxygens, nitrogens in the molecule or the a-hydrogens in carbonyl compounds.

If the reaction is carried out under acidic conditions, the very first thing that is bound to happen is the protonation of a heteroatom in the molecule, e.g., the carbonyl oxygen, oxygen of the alcohol, nitrogen in amines etc.

If the reaction conditions are basic, an acidic hydrogen is going to be abstracted first leading to the formation of intermediates after shifting of electrons.

4.  Electron Flow Arrows

Drawing of the electron flow arrows is an important, or probably the most important thing in drawing reaction mechanisms. The direction of these curved arrows show the direction of the flow of electrons. Students of organic chemistry sometimes draw them in a wrong direction.

The correct way to draw the arrow is to start from an electron rich center and end at an electron deficient center. This means that electrons are flowing from the richer center to the deficient center, which is more logical than the other way round.

TYPES OF ARROWS

These curved arrows are of different types. If the mechanism is polar there is usually flow of an electron pair. The arrow drawn in this case is a full headed arrow. In case of free radical reactions, there is homolytic cleavage involving the transfer of single electrons, a half headed arrow should be drawn.

5.  Which bond to break and make

To understand which bonds are to be broken and which formed, is very important. Normally the lone pairs on heteroatoms are more reactive and will react first to make sigma bonds. Pi bonds are weaker and more reactive than sigma bonds, so they will react first and are broken. The way they react depends upon the nature of the reagent and the conditions applied. Charged species are the most reactive ones, reacting rapidly to form bonds.

6.  Balanced Chemical Equation

Balancing the equation is necessary as it tells about the molar ratios of the reactants and the reagents.

An Example:

MECHANISM

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