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Structures & reactions of organic compounds

Compounds which incorporate carbon-carbon bonds or covalent bonds of carbon with other elements like hydrogen, nitrogen, oxygen, sulfur, phosphorous and halogens, are generally classified as organic compounds. The chemical properties of organic compounds are intricately related to their structures. Hence, the classification of organic compounds according to their structures, is an essential part of understanding the reactivity of these compounds.

The organic compounds can be classified into major groups as:

Aliphatic Compounds
Aromatic Compounds
Heterocyclic Compounds
Small Molecules

Apart from this classification, the reactivity of the organic compounds depends on the functional groups. The functional groups are defined as “specific groups (moieties) of atoms or bonds within molecules that are responsible for the characteristic chemical reactions of those molecules”. These functional groups, along with the class of organic compounds, shape the reaction outcomes.

Out of the many functional groups that can be encountered in various levels of chemistry, the organic chemistry study recognizes some of these as the most common functional groups. These are:


Hydrocarbons consist of only carbon and hydrogen as the reactive sites. These can be further classified as alkane, alkene, alkyne and benzene derivatives. The structures of each of these are given below in sequence.


Alkane                  alkene                  alkyne                       benzene derivatives

Halogen-containing groups

Two main classes of halogen-containing groups exist namely, haloalkanes and haloarenes (aryl halides). The general structure of these are respectively:


Haloalkanes       haloarenes (aryl halides)

Oxygen-containing groups

The impact of the second most electronegative element oxygen is clearly visible in different functional groups, depending on the location, bond strength and hybridisation of the covalent bond between C and O. The main classes of O containing groups are: Alcohol, aldehyde, ketone, acyl halide, carbonate, carboxylate, carboxylic acid, ester, methoxy, hydroperoxide, peroxide, ether, hemiacetal, hemiketal, acetal, ketal, orthoester, and heterocycles.

These are sequentially shown below:

Nitrogen-containing groups

The N containing groups are dependent not only on nitrogen, but occasionally on other atoms like O for their reactivity, but are still classified as such because of the crucial role of N. Some of the N-containing groups are: Amides, Amines (primary, secondary, tertiary and 4 degree ammonium ions), imines (ketimines and aldimines, each of which are classified as primary and secondary), imides, azides, azo (diimides), cyanates (and isocyanates), nitrates, nitriles (and isonitriles), nitrites, nitro, nitroso, oximes and pyridine derivatives. These are represented respectively as follows:

Sulfur, Phosphorous, and Boron containing groups

Organic chemistry also includes some of the S, P, and B containing groups. The frequently encountered ones are: thiol, sulfoxide, sulfide(thioether), disulfide, thiocyanate (and isothiocyanate), sulfonic acid, phosphines, phosphonic acid, phosphate, boronic acid and borinic ester.

These are represented sequentially, as follows:

All of these structures alter the course of a reaction in their own way, but still, the organic reactions may be classified into some major categories as follows:

Addition reactions

Elimination reactions

Substitution reactions

Rearrangement reactions

Radical reactions

Oxidation-Reduction reactions

These are characterized as follows:

Addition reactions

The reactions in which all the components of the reactants are added to the C atoms on adjacent sides, in lieu of multiple C-C bonds. No, atoms are released as byproducts.


Elimination reactions

These reactions are opposite to addition reactions, in which, the substituents are removed in one or two steps.



Substitution reactions

This is a type of chemical reaction where one functional group is replaced by another.


The substitutions reactions may further be divided into

Nucleophilic substitution - a reaction where an electron-rich nucleophile attacks a positively charged centre to replace another group (leaving group). These can be further classified as:

SN1 reactions: which are two-step processes where the first (slow) step is the leaving of the replaceable group and the second step is the fast attack by the nucleophile.

SN2 reactions: which are one-step processes where the attack by the nucleophile & leaving of the replaceable group happens simultaneously.


Electrophilic substitution - a reaction where an electrophile replaces another functional group.



Rearrangement reactions

The reaction of organic compounds where the product formed is a structural isomer of the reactant.

Example: Claisen rearrangement


Radical reactions

This class of reactions occurs through the free-radical mechanism by homolytic bond cleavage in presence of heat or light. The three steps of such reactions are initiation, propagation, and termination.

Example: Barton reaction


Oxidation-Reduction Reactions

In organic chemistry, the definition of redox reactions is slightly different from inorganic chemistry, since most of these do not involve actual electron transfer. Hence, the term oxidation means the addition of oxygen and/or loss of hydrogen. The term reduction means the addition of hydrogen or loss of oxygen.

Example: Birch reduction



  1. http://byjus.com/chemistry/organic-chemistry/
  2. http://www.chemistryexplained.com/Ny-Pi/Organic-Chemistry.html
  3. http://byjus.com/chemistry/types-of-organic-reactions/
  4. http://www.csus.edu/indiv/s/spencej/chem%2031%20summer%2014%20web/day%205%20lecture.pdf