Functional group classification:
A systematic learn of chemistry or for that matter any other branch of science, is not possible with no arranging the subject matter in a rational manner whenever enough data has accumulated. In case of inorganic chemistry, formulation of the periodic table stimulated not only the search for missing elements but also led to the understanding of the periodic behaviour. In organic chemistry, as the number of recognized organic compounds runs into millions, it is extremely difficult to study each and every compound independently. Therefore, via grouping alike compounds mutually in a class or a family, it is easier to understand their properties, reactions etc. One way of these classifications is based on the functional groups. A functional group can be described as an atom or a group of atoms in a molecule that exhibits trait chemical properties. These chemical properties exhibited via the functional group are more or less steady for several change of the functional group and do not affect the rest of the molecule. The benefit of such a classification based on functional groups is that in addition to logically systematizing the organic compounds, the properties of the compounds can be expected just via looking at their structures, for example, through knowing the kind of functional group present. Table lists a number of significant functional groups.
Table: Functional Groups
The compounds that are listed in the 1st category in Table are the compounds that contain only carbon and hydrogen. Such compounds are as well termed hydrocarbons. The hydrocarbons can be classified as aromatic, alicylic or aliphatic. In the aliphatic hydrocarbons, the carbon atoms are relation to each other to shape chains (straight of branched). The aliphatic hydrocarbons can be additional classified as saturated or unsaturated. The saturated hydrocarbons have the carbon and hydrogen atoms linked to each other by single bonds and are termed alkanes. The unsaturated hydrocarbons are of 2 kinds: the one containing double bond as the functional group is named as alkenes; the other containing a triple bond as the functional group are recognized as alkynes.
In the alicylic hydrocarbons, the carbon atoms are arranged in rings to give up cyclic structures. Such compounds are as well recognized as cycloalkanes.
The aromatic hydrocarbons consist of benzene and those compounds that resemble benzene in their properties.
Actually, the hydrocarbons provide a backbone to those diverse functional groups may be attached to yield an enormous assortment of organic compounds.
The alkyl groups, usually symbolized via R, are derived from alkanes via removing one hydrogen. The simplest alkyl group is methyl group (CH3-C) which is derived alkane, methane (CH4). Common alkyl groups are listed in Table, likewise, aryl groups denoted through Ar, are attained from benzene and its derivatives via removing on hydrogen.
The simplest aryl group is phenyl group (C6H5- and is abbreviated as Ph. In general, aryl halide (Ar-X) can refer to any of the following:
Let us now learn about the structural features of several classes of aliphatic compounds. The compounds in that the carbon and oxygen atoms are connected via a single bond can be classified as alcohols or ethers, based upon the number of alkyl groups joined to oxygen. In alcohols, oxygen is connected to only 1 alkyl group and 1 hydrogen; but in ethers, oxygen has 2 alkyl groups attached to it. The compounds containing carbon and oxygen linked through a double bond (such as C = O, that is termed carbonyl group, can be classified as aldehydes or ketones, depending on whether the number of alkyl groups attached to carbonyl carbon is one or two, correspondingly. If instead of an alkyl group, one hydroxyl (-OH) group is attached to the carbonyl group, a class of compounds known as carboxylic acids
is gained. A number of carboxylic acid derivatives are obtained by replacing the hydroxyl group by halogens groups.
Accordingly, these compounds are termed acid halides, amides, anhydrides and esters.
They are as well termed functional derivatives of carboxylic acids, as they are attained via the transforms in the functional group. In a alike manner, compounds having carbon-nitrogen single bond are termed amines. The amines can be of 3 kinds: primary, secondary and tertiary amines depending on whether the number of alkyl groups attached to nitrogen is one, two or three. The carbon-nitrogen double bond is trait of the class of compounds recognized as imines whereas compounds having carbon-nitrogen triple bond are termed nitriles. Then we contain alkyl halides which contain their unique significant in the change of functional groups. The sulphur analogs of alcohols and carboxylic acids are recognized as thiols and suphonic acids, correspondingly.
Parallel to the classes we familiar above for aliphatic compounds, we contain aromatic compounds in that benzene forms the backbone to which several functional groups revealed above can be attached to yield comparable classes of aromatic compounds, like aryl halides, arylamines, phenols, aromatic carbonyl compounds, aromatic acids and their derivatives, and so on. As we have seen in Table, R is usually utilized to symbolize an alkyl group; the analogous aromatic compounds are attained via replacing R through Ar that denotes an aryl group; this is shown in Table in case of alcohol and phenol.
Nomenclature of Organic Compounds:
The earliest attempts to given name organic compounds were depend either on their origin or on their properties. For instance citric acid was named so since of its occurrence in citrus fruits. The aromatic compounds were termed so as of their trait odour (Greek: 'aroma, fragrant smell'). Instances are oil of wintergreen and vanillin (a constituent of vanilla as well utilized as a flavoring agent) which were termed aromatic due to their trait fragrance. With the improvement and growth in the knowledge of chemistry, the number of recognized organic compounds has amplified speedily. As well, with the enhance in the number of carbon atoms, the number of possible isomers for hydrocarbons (without any functional group) becomes extremely large.
Isomers are the compounds that contain identical molecular formulas but differ in the ways in that the atoms are bonded to each other. For instance, 4 carbons in a hydrocarbon having molecular formula C4H10 can be arranged in the 2 different ways:
Straight chain H3C - CH2 - CH2 - CH3 ordinary name; normal butane or n-butane Branched chain
H3C - CH - CH3 with a common name 150 - butane.
Therefore, n-butane and isobutene are isomers.
Table: Possible Number of Isomers for Hydrocarbons
Having studied about the variety of functional groups, we can imagine that the nature and position of functional groups present can increase such numbers many fold. Under such a circumstances, it is next to impossible to study the names randomly assigned to the compounds, especially when there is no correlation of the name to the structure of the compound. This necessitated the require to contain a systematic nomenclature for that the International Committee of Chemists met at Geneva in the year 1892. The work was carried on via the International Union of Chemists (I.U.C.) that gave its report in the year 1931, recognized as the I.U.C. system of nomenclature. As the nomenclature is forever undergoing modifications and revisions, the latest rules which are broadly accepted were recommended by the Commission on Nomenclature of Organic Chemistry of the International Union of Pure and Applied Chemistry (I.U.P.A.C.). We will now study this system in detail.
Because the nomenclature of other classes of compounds is depending on the nomenclature of alkanes, permit us start the learn of nomenclature by the alkanes. Alkanes are symbolized via the general formula CnH2n+2 here n can be 1, 2, 3, 4... etc. The first four alkanes preserve their original or nonsystematic names. The names of alkanes higher than such start by a prefix (Greek or Latin words) that indicates the number of carbon atoms in the chain and end with suffix-ane. The IUPAC names for several alkanes having different chain lengths are given in Table. The unbranched alkanes have their ordinary names as normal alkanes or n-alkanes. Compounds that differ from each other in their molecular formulas via the chapter - CH2- are termed members of a homologous series. Therefore, the compounds listed in Table belong to a homologous series.
Table: IUPAC Names of straight chain alkanes having general formula CnH 2n+2
The branched chain alkanes are named via using the subsequent steps:
1. The longest constant chain of carbon atoms is in use as the parent hydrocarbon. For instance, in the compound given beneath, the parent hydrocarbon is heptanes and not the hexane.
2. Classify the substituent alkyl groups attached to the parent chain. Several ordinary alkyl groups are listed in Table. We can locate that both the substituent in the instance cited above are methyl groups.
Table: Common Alkyl groups
The IUPAC system of nomenclature has retained several of the older names for branched alkyl groups these as isopropyl, isobutyl, sec-butyl, and neopentyl.
* The numbering of carbon atoms is from the point of addition of the group to the parent chain.
3. The parent carbon chain is then numbered in such a mode that the substituent gets the lowest possible numbers. The carbon atoms in the above compound can be numbered as;
Can we guess which of the 2 numbering schemes is accurate: The 1st possibility locates the methyl groups at carbons 4 and 5 and the second, at carbons 3 and 4. Certainly, the second way of numbering the carbon chain is correct.
4. Prefixes di, tri, tetra, penta etc, are utilized when the substituent take place more than once. Given that in the above compound the methyl substituent is occurring two times, the name is prefixed by di for the above compound.
5. The name of the compound is written via writing the position and name of the substituent followed through the name of the parent alkane. Therefore, the above compound can be symbolized as 3, 4-dimethylheptane.
As a reminder that a comma is utilized to divide the 2 numbers and the numbers are separated from names of groups via a hyphen. As well memo that there is no vacant space between the name of the end substituent and the parent alkane.
6. Whenever more than one sort of alkyl groups is present, then they are cited in the name in the alphabetical array, regardless of their location in the principal chain.
The numerical prefixes di, tri, tetra, etc. and hyphenated prefixes these as sec-tert- aren't considered in determining the alphabetical order but prefixes iso, neo, cyclo are considered for alphabetizing. To realize it, let us consider the instances following beneath
Note that here ethyl is cited before methyl, in spite of its higher location number. Similarly, the compound shown below, can be named as 4-isopropyl-5,5-dimethylnonane or 4-(1-methylethy1)-5, 5-dimethylnonane.
7. The branched chain substituent, these as 1-methylethyl given in step 6, is numbered beginning from the carbon connected unswervingly to the parent chain. Table shows the numbering for the branched substituent registered there. The longest carbon chain is preferred and the substituents are symbolized according to the rules listed above for compounds having unbranched substituent. As a reminder that the name and numbering of branched substituent is written in brackets in sort to separate it from the numbering of the main chain.
8. The alkyl substituent can be further classified as primary, secondary or tertiary. An alkyl group is termed a primary alkyl group if the carbon atom at the point of attachment is bonded to only one another carbon. For instance, R -CH2- is a primary alkyl group. Likewise, a secondary alkyl group has 2 alkyl groups bonded to the carbon atom in use as the point of attachment to the main chain. Therefore, a secondary alkyl group can be written as given below:
Likewise, a tertiary alkyl group has 3 carbon atoms bonded to the carbon atom in use as point of attachment. Therefore, a tertiary alkyl group can be symbolized as shown below:
9. Whenever more than one carbon chains of equivalent length are accessible, the numbering is done considering the given points:-
(a) The principal chain should contain the greatest number of side chains. For instance, in the compound given below;
The chain having numbering in red color has 4 side chains whereas the chain marked by numbers in black color has 3 side chains. So the principal chain is the one that is marked in the red color. Therefore, the name is 3-ethyl-2, 5, 6-trimethyloctane
(b) The chain having the lowest number for substituent is chosen as the principal chain. In the compound shown below;
If the numbering is finished as shown in black color, the name would contain substituent at positions 3, 4 and 5. But, if the carbon chain numbered in red color is in use as the principal chain, then the substituent obtains the numbers 2, 3 and 4 that is clearly the correct choice. Till now we were studying the nomenclature of alkanes.
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