Classification of Organic Compounds

Classification of Organic Compounds

• The topics covered include classification, acyclic/open chain compounds, cyclic/closed chain compounds, homocyclic/carbocyclic compounds, alicyclic compounds, aromatic compounds, heterocyclic compounds, and functional groups

Organic Compounds

• Organic compounds are defined as compounds whose molecules contain carbon and hydrogen, also referred to as hydrocarbons, or compounds that are derivatives of hydrocarbons

Classification Based on Structure

• Organic compounds are classified as either aliphatic (acyclic or open chain) or cyclic (closed chain).
• Aliphatic compounds can be saturated (alkanes) or unsaturated (alkenes and alkynes)
• Cyclic compounds can be homocyclic/carbocyclic or heterocyclic
• Homocyclic compounds can be alicyclic or aromatic
• Alicyclic compounds include cyclobutane and cyclohexane
• Aromatic compounds can be benzenoid or non-benzenoid

Classification Based on Functional Groups

• Organic compounds are classified based on functional groups like hydroxyl (-OH), aldehyde (-CHO), and carboxylic acid (-COOH)

Acyclic or Open Chain Compounds

• These compounds are also known as aliphatic compounds, and can have branched or straight chains
• Straight-chain compounds do not have any side chains
• Branched-chain compounds have a straight chain with one or more side chains attached

Saturated Compounds

• A hydrocarbon containing only carbon-carbon single bonds is called a saturated compound
• Four valency of carbon atom is saturated with different atoms or groups

Unsaturated Compounds

• A hydrocarbon having carbon-carbon double or triple bonds is called an unsaturated compound

Cyclic or Closed Chain Compounds

• Organic compounds with a closed chain of atoms are called closed chain or cyclic compounds
• Cyclic compounds are also known as "Ring compounds".
• Cyclic compounds where carbon atoms are connected to each other in a ring are called homocyclic
• When atoms other than carbon are also present in the ring, it is called heterocyclic

Homocyclic or Carbocyclic Compounds

• Cyclic compounds composed entirely of carbon atoms are called homocyclic or carbocyclic
• Homocyclic compounds are further divided into alicyclic and aromatic compounds

Alicyclic Compounds

• Some homocyclic compounds contain a ring structure but behave like aliphatic compounds, and are named alicyclic or cyclic aliphatic compounds
• Alicyclic compounds are cyclic and contain three or more carbon atoms in the ring
• The properties of alicyclic compounds resemble aliphatic or alicyclic compounds

Aromatic Compounds

• Aromatic compounds are specific types of compounds containing at least a closed chain of six carbon atoms
• Aromatic compounds initially had a specific smell
• The parent member of the aromatic family is benzene
• Aromatic compounds are divided into benzenoid and non-benzenoid compounds

Benzenoid Aromatic Compounds

• These compounds contain at least one benzene ring (six-membered carbon cycle) with alternate single and double bonds

Non-Benzenoid Aromatic Compounds

• The structural unit differs from the benzenoid type

Heterocyclic Compounds

• Cyclic compounds in which one or more carbon atoms are replaced by other atoms are called heterocyclic compounds
• The non-carbon atoms present in the ring are called hetero atoms
• Examples of hetero atoms are nitrogen, oxygen, and sulfur

Functional Group

• A functional group is an atom or group of atoms joined in a specific manner that is responsible for the characteristic chemical properties of organic compounds
• Examples of functional groups include hydroxyl group (-OH), aldehyde group (-CHO), and carboxylic acid group (-COOH)
• Prefixes and formulas for various functional groups, including single, double, and triple bonds, hydroxyl, ether, aldehyde, ketone, carboxylic acid, ester, acid anhydride, thio alcohol, thio ether, halides, amino, amide, nitro, cyanide, and isocyanide groups

Nomenclature of Organic Compounds

• Two systems are adopted for naming organic compounds: Common Names (or Trivial System) and the IUPAC system (or Geneva System)

Common Naming (Trivial System)

• Compounds were named based on their sources or the name of their discoverer
• These names are called common or trivial names
• The Trivial Nomenclature system involves a non-systematic approach to naming organic compounds
• There are no particular rules for writing the trivial naming of compounds

IUPAC System

• IUPAC (International Union of Pure and Applied Chemistry) nomenclature of organic compounds is a systematic approach for naming organic compounds
• The IUPAC nomenclature of organic compounds consists of three parts: stem name, prefix, and suffix
• The goal is to give each structure a unique and unambiguous name

IUPAC Nomenclature Components

• Secondary prefix, primary prefix, root word, primary suffix, and secondary suffix.
• Each component contributes specific information about the compound

Prefixes in IUPAC Nomenclature

• Prefixes indicate substituents or branches
• Cyclo for cyclic compounds
• Alkyl groups, and halo and nitro substituents

Chain Length and Root Word

• C1: Meth-
• C2: Eth-
• C3: Prop-
• C4: But-
• C5: Pent-
• C6: Hex-
• C7: Hept-
• C8: Oct-
• C9: Non-
• C10: Dec-

Primary Suffixes

• Indicate saturation
• C-C: -ane (Alkane)
• C=C: -ene (Alkene)
• C≡C: -yne (Alkyne)

Secondary Suffixes

• Identify the main functional group in the molecule
• Examples: -ol (alcohols), -amide (acid amides), -al (aldehydes), -oate (esters), -one (ketones), -nitrile (nitriles), -oic acid (carboxylic acids), -oyl chloride (acid chlorides), and -amine (amines)

IUPAC Name of Saturated Hydrocarbons

• Rule 1: Longest Chain Rule to select longest chain of carbon atoms
• Rule 2: Lowest Number Rule is used to assign principle carbon chain
• Priority order: Functional group > Double bond > Triple bond > Substituent.
• Rule 3: Lowest Sum Number is used to assign numbers when more than one substituent is present and maximum number of substituents, multiple bonds and functional groups
• Priority order: Functional group > Multiple bond > Substituent.
• Rule 4: Use prefixes such as di, tri, tetra, etc. to indicate multiple similar alkyl groups; positions of substituents are separated by commas but prefixes are not considered for alphabetizing
• Rule 5: If two different alkyl groups are located at equivalent positions. Numbering in the carbon chain is done so that the alkyl group which comes first in the alphabetical order, gets the lowest number.
• Rule 6: For compounds with two or more chains of the same length, the principle chain is selected to have greater number of substituents to be used as prefix.
• Rule 7: Some functional groups always work as a prefix. If the molecule contains more than one dissimilar functional group, then the chain is numbered such that the functional group of higher priority gets the lower number

IUPAC name of Unsaturated Hydrocarbons

• Longest chain of carbon atoms is selected that must include double or triple bond even if that is not the actual longest chain
• Numbering the Parent Chain is done so as to give the lowest number to that carbon atom linked by double or triple bond
• Alkyl groups or other substituents are numbered, named and placed as prefixes in alphabetical order.
• A primary suffix is added to the root word in order to indicate the presence of double or triple bond in the parent chain
• Double bond = Root word + -ene
• Triple bond = Root word + -yne

Nomenclature of Organic Compounds Containing A Monofunctional Group

• Numbering of carbon atom is done so that the functional group is attached to the carbon atom possessing the lowest possible number in the chain

Nomenclature of Organic Compounds Containing A Polyfunctional Group

• One of the functional groups is chosen as the principal functional group and the compound is then named on that basis
• The order of decreasing priority for some functional groups: -COOH, -SO3H, -COOR (R=alkyl group), COCl, -CONH2, -CN,-HC=O, >C=O, -OH, -NH2, >C=C<, -C≡C-
• Substituents that are always prefixes: -R, C6H5-, halogens (F, Cl, Br, I), -NO2, alkoxy (-OR) etc.

IUPAC Nomenclature of Alicyclic Compounds

• Rule-1: Name of monocyclic, saturated hydrocarbons are formed by attaching the prefix "cyclo" to the names of acyclic, saturated hydrocarbon with same number of carbons
• Rule-2: Name of unsaturated monocyclic hydrocarbons (with no side groups) is formed by substituting "-ene", "-yne", "-adiene". "-atriene", "-adiyne" etc. for "ane" in the name of corresponding cycloalkane e.g
• Rule-3: If a straight chain substituent attached to monocyclic hydrocarbon has greater number of carbon in the main chain then the cyclic ring becomes substituent

Isomerism

• Isomers are compounds that have the same molecular formula but differ in physical or chemical properties
• The phenomenon is called Isomerism
• Types of isomerism: Structural/Constitutional and Stereoisomerism
• Structural isomers have different structural formulas
• Stereoisomers have the same but atoms are arranged differently in space

Structural/Constitutional Isomerism

• Arises due to differences in the arrangement of atoms within the molecule itself, and is also known as constitutional isomerism
• Kinds such as chain, position, functional group, ring chain and metamerism
• Molecule has the same molecular formula with different orders and bonding's

Chain Isomerism

• Chain isomerism occurs when there is a difference in the atomic arrangement of the carbon to the carbon chain of a molecule
• Two or more compounds have the same molecular formula but different main chains

Positional Isomerism

• Positional isomerism arises when there is a difference in the positions occupied by the substituent atoms or a group of atoms or due to the unsaturation occurring in the chain
• When the position of the functional groups with respect to main chain atom changes

Functional Isomerism

• Occurs when presence of odd form of functional groups with the same chemical formula.
• Some compound has two different structures but the same chemical formula

Metamerism

• Isomerism in which compounds having the same molecular formula but different alkyl groups on either side of functional groups are called Metamerism

Tautomerism

• Tautomerism is a phenomenon where a single chemical compound tends to exist in two or more interconvertible structures that are different in terms of the relative position of one atomic nucleus which is generally the hydrogen
• The two structures are called tautomers
• Always is in dynamic equilibrium

Ring-Chain Isomerism

• Ring-chain isomerism describes isomers that have an open and ring structure
• Number of pi bonds is different in the ring vs open structure