ORGANIC COMPOUNDS: CHARACTERISTICS, CLASSIFICATION, NOMENCLATURE, EXAMPLES - CHEMISTRY - 2025

The organic compounds or organic molecules are those chemicals that contain carbon atoms. The branch of chemistry that is responsible for its study, then, is known as organic chemistry.

Virtually all the molecules that make cell life possible contain carbon, namely: proteins, enzymes, lipids, carbohydrates, vitamins and nucleic acids, etc. Therefore, all chemical reactions that occur in living systems are organic reactions.

Graphic representation of the structure of an alcohol, an organic compound (Source: SubDural12 / Public domain, via Wikimedia Commons)

Likewise, most of the compounds found in nature, on which humans depend for food, medicine, clothing, and energy, are also organic compounds.

There are natural and synthetic organic compounds, since chemists have managed to produce millions of organic compounds artificially, that is, within the walls of a laboratory and, therefore, these compounds cannot be found in nature.

Composition of organic compounds: carbon

Organic compounds are composed, among other things, of carbon atoms. Carbon is a very special element and this is due, in large part, to its position in the periodic table, since it is in the center of the second row of elements.

Carbon in the periodic table (Source: IUPAC / Public domain, via Wikimedia Commons)

Those elements to its left tend to give up electrons, while those to its right tend to receive them. The fact that carbon is in the middle of these elements implies that it neither completely gives up the electrons, nor accepts them completely, but rather shares them.

By sharing electrons and not taking them away or giving them to other elements, carbon can form bonds with hundreds of different atoms, forming millions of stable compounds with multiple chemical properties.

Characteristics of organic compounds

-All organic compounds are made up of carbon atoms in combination with atoms of hydrogen, oxygen, nitrogen, sulfur, phosphorus, fluorine, chlorine, bromine, among others.

However, not all compounds that have carbon atoms are organic compounds, such as sodium carbonate or calcium carbonate.

-They can be crystalline solids, oils, waxes, plastics, elastics, mobile or volatile liquids or gases. In addition, they can have a wide range of colors, smells and flavors (some of their characteristics will be described by groups)

-They can be natural or synthetic, that is, they can be normally found in nature or they can be artificially synthesized by man

-They have multiple functions, both from the cellular point of view and in the anthropocentric sense, since man exploits organic compounds in many aspects of his daily life

Classification

Organic compounds can be classified into a series of "functional groups." Of these functional groups, the most common and relevant are:

-Alkanes, alkenes and alkynes

-Sand or aromatic hydrocarbons

-Alcohols and phenols, ethers and epoxides

-Tiols, amines, aldehydes and ketones

-Halides

-Carboxylic acids

Alkanes

Alkanes are organic compounds made up only of carbon and hydrogen atoms joined by simple nonpolar covalent bonds, so they belong to a class of substances known as hydrocarbons.

The bonds that make up these compounds are generally the least reactive bonds that can be found in an organic molecule, and therefore the alkane sequences form the "inert framework" for most organic compounds.

Structure of some alkanes. Methane, ethane, propane and butane. (Source: 1840460mahesh / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0) via Wikimedia Commons)

These compounds can occur as hydrocarbon chains or as cyclic structures or rings. When a hydrocarbon chain is attached to more than one fundamental structural unit as a substituent, this is then known as an alkyl group.

The simplest alkanes are methane (CH4), which is one of the main compounds found in natural gas, ethane (C2H6), propane (C3H8) and butane (C4H10), used as liquid fuel in cigarette lighters. pocket.

Alkenes

Ethene By User: Bryan Derksen, from Wikimedia Commons

An organic compound is an alkene when it contains double bonds between the carbon atoms that make it up, so it is said that these are unsaturated, since they are not saturated with hydrogen atoms.

Alkenes are widely distributed in nature and some common examples include ethylene (used to make plastics), 2-methyl-1,3-butadiene isoprene (used to make rubbers or rubbers), and vitamin A.

Alkynes

Chemical structure of acetylene

Alkynes are hydrocarbons (compounds made up of carbon and hydrogen atoms) that have, among some of their carbon atoms, a triple bond, which has great strength and rigidity. They are not very abundant in nature.

Ethine, also known as acetylene, is one of the most representative examples of this group of molecules. It is used as fuel for the torches of oxyacetylene welders.

The boiling point of alkanes, alkenes and alkynes increases with increasing molecular weight, however, the melting point can be highly variable as it depends on the structure adopted by these molecules in the solid phase.

Aromatic hydrocarbons or arenes

Benzene

Also known as aromatic hydrocarbons, arenes are a set of organic molecules that contain a functional group made up of three pairs of atoms linked by double bonds, which are joined together forming a regular planar (flat) hexagon.

The hexagonal rings of these compounds are commonly represented in a sequence alternating single bonds with double bonds.

The smallest molecule that can form a functional group of this nature is benzene (C6H6) and arenes can have one or more benzene rings or others of similar structure. When they are named as substituents on other structural units they are known as aryl substituents.

The description of these as "aromatic hydrocarbons" has to do with the strong odor of benzene and other larger arenes.

A good example of these compounds is naphthalene, made up of two fused benzene rings, which corresponds to the active compound in mothballs, usually used as pesticides to drive away unwanted domestic insects.

Alcohols and phenols

Source: Generic structure of alcohols. Secalinum, from Wikimedia Commons

Alcohols are compounds formed by an alkane skeleton to which a hydroxyl group (-OH) is attached, while phenols are those in which the hydroxyl group, in turn, is attached to an aryl ring (aromatic hydrocarbon).

Both alcohols and phenols are extremely common in nature, but more abundant and important are alcohols.

Chemical structure of phenol. UAwiki / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

Due to the presence of a hydroxyl group, alcoholic and phenolic molecules have highly variable physical and chemical properties, since oxygen atoms are by far more electronegative than carbon or hydrogen atoms.

Consequently, the bonds between these three atoms are polar and it is these that are responsible for the main characteristics of alcohols and phenols.

The boiling point of alcohols is higher than the boiling point of alkanes or alkenes of comparable molecular weight, although the higher the alkyl group of these molecules, the more similar their properties are to those of alkanes.

Ethers and epoxides

General structure of organic esters

Ethers are organic molecules in which one oxygen atom is attached to two carbon atoms and they are extremely abundant in nature. Diethyl ether, for example, was previously used as an anesthetic and 2-ethoxynaphthalene is used in perfumery as an "orange flower-like flavoring".

There are straight chain ethers and cyclic ethers, and although the bonds between oxygen and carbon are polar, these compounds are less reactive than alcohols and phenols.

Epoxides, on the other hand, are cyclic ethers composed of a ring of three atoms, the simplest example being ethylene oxide, also known as oxirane, a flammable gas with a strong odor.

Example of epoxide, 2,3-epoxyhexane

Thiols

Sulfhydryl, thiol group

Thiols are very similar to alcohols, but instead of an oxygen atom they have a sulfur atom. Their main characteristic is that they have a very bad smell.

The simplest thiol is hydrogen sulfide (H₂S), a sulfur analog of water that smells like a rotten egg. Ethanethiol is another well-known thiol, as it is added to domestic gas to make leaks detectable.

When present as substituents on other structural units, thiols or SH groups are known as "mercapto" groups.

Amines

General formula for an amine. Source: MaChe, from Wikimedia Commons.

Amines are generally alkane (alkyl groups) or arene (aryl groups) compounds that contain at least one attached nitrogen atom.

If it is a framework made up of an alkyl group, then the compound is called an alkyl-amine. On the other hand, if the framework consists of an aryl group, the compound corresponds to an aryl-amine.

There are primary, secondary, and tertiary amines, depending on whether the nitrogen atom is attached to one, two, or three alkyl or aryl groups, correspondingly. These are very common in nature and many are physiologically active in living things.

Aldehydes and ketones

Representation of an aldehyde (Source: Wereldburger758, via Wikimedia Commons)

Both are compounds that have carbon atoms attached to carbonyl groups. A carbonyl group consists of an oxygen atom attached to a carbon atom through a double bond; in these groups the carbon atom is also attached to two other atoms in addition to oxygen.

Many other functional groups are produced by the presence of different atomic groupings on the carbon atom of the carbonyl group, but aldehydes and ketones are undoubtedly the most important.

Ketones are those compounds in which the carbonyl group carbon atom is attached to two other carbon atoms, while in aldehydes at least one of these atoms is hydrogen.

General formula of ketones

Many aldehydes and ketones are responsible for the taste and smell of many fruits and vegetables consumed by animals, so they are very abundant in natural environments.

Formalin, which consists of a mixture of formaldehyde in water, is a liquid commonly used for the preservation of biological specimens.

Benzaldehyde, for example, is the aromatic aldehyde responsible for the smell of almonds and cherries. Butanedione, on the other hand, is a ketone that has two carbonyl groups and is what imparts the characteristic odor of many cheeses.

Halides or halide

Lithium fluoride, a halide

They are compounds that contain carbon atoms linked to a halogen atom such as fluorine, iodine, bromine or chlorine, through polar bonds. They are very reactive compounds, since they have the carbon atoms that participate in the bond and have a slight positive charge.

Many of these compounds have been discovered in marine organisms, and others have a host of commercially important applications. Chloroethane or ethyl chloride, for example, is a volatile liquid used as a topical anesthetic.

Carboxylic acids

Structure of a carboxylic acid. R is a hydrogen or carbonate chain.

If a carbonyl group (C = O) joins with a hydroxyl group (-OH) it forms a functional group known as a carboxyl group (-COOH).

The hydrogen atom of a carboxyl group can be removed, forming a negative ion that has acidic properties, so the compounds that possess these groups are known as carboxylic acids.

These compounds are abundant in nature. They are in the vinegar that we use in the kitchen, in the citrus fruits that we consume, as well as in certain vegetables and even in many commonly used medicines.

The structural unit that contains an alkyl group attached to a carboxyl group is known as an acyl group and compounds derived from carboxylic acids are all those that contain an acyl group attached to different substituents.

These derivatives include esters, amides, acid halides, and anhydrides. Esters are formed by an alkoxy fragment (OR) attached to an acyl group, amides have amino groups (-NR2), acid halides have a chlorine or bromine atom, and anhydrides have a carboxyl group.

Some simple esters give a pleasant scent to fruits and flowers. Urea is a double amide of carbonic acid and is the main component of urine.

Acyl chlorides and anhydrides are the most reactive derivatives and are generally used as chemical reagents, but are not very important in nature.

In addition to the previously named groups, it is important to comment that there are also compounds called polyfunctional, since in their structure they harbor more than one functional group than those listed above.

Nomenclature

The most used nomenclature to name an organic compound is the IUPAC, which consists of giving the name to the longest chain of carbon atoms connected by double bonds of a molecule, regardless of whether it is a continuous chain or if it has a structure cyclical.

All "offsets", whether they are multiple bonds or atoms other than carbons and hydrogens, are indicated as prefixes or suffixes according to certain priorities.

Nomenclature of alkanes

Alkanes can be linear (acyclic) or cyclic (alicyclic) molecules. If you start with an alkane of five carbon atoms, the number of carbons in the chain is indicated by a Greek letter or the Latin prefix.

If they are cyclic alkanes (cycloalkanes) then the prefix "cyclo" is used. Depending on the number of carbon atoms, alkanes can be (linear or cyclic):

-Methane (CH4)

-Ethane (CH3CH3)

-Propane (CH3CH2CH3)

-Butane (CH3 (CH2) 2CH3)

-Pentane (CH3 (CH2) 3CH3)

-Hexan (CH3 (CH2) 4CH3)

-Heptane (CH3 (CH2) 5CH3)

-Octane (CH3 (CH2) 6CH3)

-Nonane (CH3 (CH2) 7CH3)

-Decano (CH3 (CH2) 8CH3)

-Undecane (CH3 (CH2) 9CH3) and so on

Nomenclature of compounds with functional groups

Functional groups are named according to their priority. The following list shows the different functional groups in order of decreasing priority (from most important to least important) and denotes both the prefix and the suffix that must be used to name the molecules with these characteristics:

For those that can be named using prefix or suffix:

- Carboxylic acid: R-COOH, prefix “carboxy- acid” and suffix “-oic”

- Aldehyde: R-HC = O, prefix "oxo-" or "formyl" and suffix "-al" or "carbaldehyde"

- Ketone: RC = OR, prefix “oxo-” and suffix “-one”

- Alcohol: ROH, prefix “hydroxy-” and suffix “-ol”

- Amine: RN-, prefix “amino-” and suffix “-amine”

For those that can be named using only suffixes:

- Alkene: C = C, suffix “-eno”

- Alkyne: C-triple bond-C, suffix "-ino"

For those that can only be named using prefixes:

- Alkyl (methyl, ethyl, propyl, butyl): R-, prefix "alkyl-"

- Alkoxy: RO-, prefix "alkoxy-"

- Halogens: F- (fluoro-), Cl- (chloro-), Br- (bromo-), I- (iodine-)

- Compounds with -NO2 groups: prefix "nitro-"

- Compounds with -CH = CH2 groups: prefix "vinyl-"

- Compounds with groups -CH2CH = CH2: prefix "allyl-"

- Compounds with phenolic groups: prefix "phenyl-"

According to the above, organic compounds that have substituents, for example, which can only be named with prefixes, must be named like this:

1. Locate the longest chain of carbon atoms and determine the “root” name for this parent chain, that is, the name of the single-chain alkane of the same number of carbon atoms.
2. Number the chain so that the first substituent occupies the first position, in other words, that the first substituent has the lowest number.
3. Determine the name and position of each substituent in the chain. If one of the substituents is a nitrogen, then an "N-" is used instead of a number.
4. Indicate the number of identical groups with the numerical prefixes "di", "tri", "tetra", etc.
5. Write the numbers of the positions and the names of the substituent groups in alphabetical order and before the “root name”. When sorting alphabetically, the prefixes "sec -", "tert -", "di", "tri", etc. are not taken into account, but the prefixes "cyclo-" and "iso" are taken into account.

Organic compounds that have substituents and that can only be named with suffixes must be named like this:

Alkenes are named the same as alkanes, except that:

1. The chain of carbon atoms that includes the double bond (C = C) is listed in such a way that these atoms have the "lowest position" possible, since they have higher priority than any substituent.
2. The suffix “-ano” is changed to “-eno”
3. The geometric isomer is designated by the prefixes "cis", "trans", "E" or "Z"
4. When C = C cannot be included, then the name of the substituent is used

Alkynes are also named after alkanes, with certain modifications:

1. The chain of carbon atoms containing the pair of carbons linked by three bonds is listed in such a way that the functional group has the "lowest" numerical position.
2. The suffix "-ano" is changed to "-ino" and the numerical position is assigned to the first carbon in the chain.

The nomenclature of the molecules that can be named with both prefixes and suffixes, those molecules with one or more functional groups are named with the suffix of the functional group with the highest priority and the others are indicated as a prefix, also in order of priority.

Examples of organic compounds

Throughout this text some classic examples of the different groups of organic compounds have been mentioned and it is important for the reader to remember that the macromolecules that make up our cells are also heterogeneous groups of these compounds.

Image of Tumisu at www.pixabay.com

Hence, examples of large and important organic compounds include:

-Nucleic acids such as deoxyribonucleic acid and ribonucleic acid

-All proteins and enzymes

-Simple and complex carbohydrates, that is, monosaccharides such as glucose or galactose and polysaccharides such as starch, cellulose or chitin

-Simple and complex lipids, which consist of a combination of carboxylic acids, alcohols and other functional groups, almost always polar

Image by Steve Buissinne at www.pixabay.com

2-Propanol is an organic compound, which we know commercially as isopropyl alcohol and which we usually use to clean wounds. So is the oil we use for cooking, whatever the vegetable origin.

The ethyl alcohol that we get in alcoholic beverages such as beer or wine is an organic compound, as is the sugar that we use to sweeten desserts and drinks.

References

1. Speight, JG (2016). Environmental organic chemistry for engineers. Butterworth-Heinemann.
2. Bruice, PY (2004). Organic Chemistry. International Edition.
3. Clayden, J., Greeves, N., Warren, S., & Wothers, P. (2001). Organic chemistry.
4. Leigh, GJ (Ed.). (2011). Principles of chemical nomenclature: a guide to IUPAC recommendations. Royal Society of Chemistry.
5. Usselman, M., Zumdahl, S., Norman, R., Noller, C. (2019). Encyclopaedia Britannica. Retrieved April 6, 2020, from britannica.com