Organic- Nomenclature-II(Modified)
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Concepts in Chemistry CHAPTER-II IUPAC NOMENCLATURE OF ALIPHATIC COMPOUNDS CONTAINING ONE FUNCTIONAL GROUP ALKANES There are two types of alkanes: (i) Straight chain alkanes(unbranched alkanes): If all the carbon atoms are connected in series one after the other and nowhere there is any branching, we call it a straight chain alkane. Note that you may write such an alkane in a twisted way, but it remains as a straight chain. See these examples. C CC CC CC CCCCCC CC CC C In all the above cases the carbon atoms are connected in straight chain although in the second and third cases the carbon skeletons have been written in a twisted manner. There is no branching in any of them. If you attach the required number of hydrogen atoms, each structure is same and it is hexane. So the alkanes- methane, ethane, propane, butane, pentane, hexane, heptane etc. that we learnt in Chapter-I belong to the category of straight chain alkanes and these are their IUPAC names. But never think that their structures are really linear. Each carbon atom is tetrahedral in geometry i.e the the bond angles are 1090 28'. So the shape of alkanes is 3-dimensional. (ii) Branched alkane : If larger number of carbon atoms connected in series make the main or principal carbon chain and fewer number of carbon atoms are attached to the principal chain at different positions forming branches, we call it a branched alkane. See these examples. 5 1 C 1 2 3 4 56 4 3 2 1 C 2 3 4 CCCCCC CCCC C CC C C C C In the first example, there is a branch of one carbon atom attached to the 2nd carbon atom(from LHS) of the main chain. In the second example, there is a branch of one carbon atom attached to the 2nd carbon atom counted from the RHS of the main chain. In the third case, there are two branches, one at the 2nd carbon atom from LHS and the other at 2nd carbon atom from RHS(4th carbon atom from LHS) of the main chain containing 5 carbon atoms. NAMING OF BRANCHED ALKANES: (i) For branched alkane, first the longest continuous carbon chain is selected. For doing so, one has to start from one terminal(end) carbon(CH3) and go upto the other terminal carbon(CH3) and see in which direction, the number of carbon atoms is maximum. (ii) Number the carbon atoms with numerals 1,2,3..... This is the principal carbon chain of alkane. For example, if the principal chain contains 5 carbon atoms, it is pentane, and if it contains 4 carbon atoms it is butane and so on. Nomenclature of Organic Compounds (iii) The other carbon atoms which do not belong to the main chain form the branches or substituents. Mark these substituents as shown below. 1 2 3 4 5 6 CH3 CH CH2 CH2 CH2 CH3 2-methylhexane(correct) CH3 Here the longest chain contains six carbon atoms. Hence the parent alkane is hexane. But there is a branch at carbon number 2. The name of the substituent is methyl. You already know the names of different alkyl groups like methyl(CH3-), ethyl(CH3CH2-), propyl(CH3CH2CH2-) and so on. (iv) The position(number) of carbon atom to which the branch(sustituent) is attached is called the locant of the substituent. Here the locant of methyl group is 2 as it is attached to carbon number 2 of the main chain. (v) First the name(s) of the branch is(are) written. Branch name is preceded by its locant followed by '-"(hyphen) mark e.g 2-methyl in this case. Then the name of the alkane is written. There should be no space between the branch name and the name of parent alkane. Hence the name of the above alkane is : 2-methylhexane ( not 2-methyl hexane) PROCEDURE FOR GENERAL NOMENCLATURE branches + Word root +suffix In alkane, suffix is -ane. So, for alkanes the name becomes branches + alkane BRANCHES : Locants are prefixed to each branch(substituent) using hyphens (-) to separate the locants from the names of branches, e.g 2-methyl, 4-ethyl etc. If there are two or more of the same branch(say methyl or ethyl or any other) in a given molecule, then the multiplying prefixes di, tri, tetra, penta, hexa, hepta etc., are used together with a set of locants, one for each of the branches with the locants separated by commas. In such case the hyphen(-) is given after the last locant i.e before the name of the branch.For example, if there are three methyl groups out of which two are attached to C-2 and the third is attached to C-3, then the branches are to be written as 2,2,3-trimethyl(neither 2,3-trimethyl nor 2,2-dimethyl-3-methyl). Note that each and every branch must have a locant, even if it requires repeating the identifier as in 2,2-dimethyl. (vi) Alphabetical ordering: The substituents(branches) are arranged according to alphabetical order of the first letters of the branches. The terms di-, tri- tetra- etc., used as multiplying prefixes of the substituents are not considered for alphabetical ordering. Wrong order : 2,3, 3-trimethyl-3,4-diethyl-4-butyl-5-propyl Correct order : 4-butyl-3,4-diethyl-2,3,3-trimethyl-5-propyl Note that the prefixes di, tri are not considered for alphabetical ordering of branches. Full name: 4-butyl-3,4-diethyl-2,3,3-trimethyl-5-propyldodecane (correct) 4-butyl-3,4-diethyl-2,3,3-trimethyl-5-proply dodecane (incorrect) Concepts in Chemistry There should not be any space between the name of the last branch and the name of the parent alkane. In the second case, there is a space between the last branch propyl and the parent alkane dodecane. To be specific, there should not be any vacant space anywhere in the name. (vii) LOWEST LOCANT RULE OR LOWEST SET OF LOCANTS RULE: Lowest Locant Rule: When there is only one branch this rule is followed. According to this rule, that direction of numbering the principal carbon chain is correct which gives the lowest locant to the substituent(branch) and the corresponding name is the correct name. This is achieved by writing names two times, first with one direction of numbering and next with reverse direction of numbering. The numbering of carbon atoms is reversed with respect to first numbering in the same chain. The last carbon becomes the first carbon while the first carbon becomes the last carbon in the new numbering system. Look at this example. 1 2 345 6 5-methylhexane(wrong) CH3 CH CH2 CH2 CH2 CH3 6 5 432 1 2-methylhexane(correct) CH3 According to numbering from left to right the methyl(CH3) group is attached to carbon No. 2. So its locant is 2. According to the numbering from right to left methyl(CH3) group is attached to carbon No. 5. So, its locant is 5. According to this rule, the name having lowest locant of the branch is the correct name. Hence the correct name of the alkane is 2-methylhexane, (not 5-methyhexane). Note that the direction of numbering the carbon chain can be done by mere inspection without reversing the numbering system. IMPORTANT : The direction in which a substituent is ecountered first is the correct direction of numbering. Lowest set of locants rule : When there are more than one branches, lowest set of locants rule is followed. The numbering of the main chain is done in the direction which gives the lowest locant to the first branching point. In other words, the direction in which a branch is met earlier is the correct direction of numbering. That gives the lowest set of locants. See this example. Example CH3 2 4 CH 1 3 CH2 CH2 5 CH H3C CH CH 3 6 8 5-ethyl-2,3-dimethyloctane(correct) CH3 CH2 7 CH3 CH2 The longest chain contains eight carbon atoms. So the parent alkane is octane. Let us now look to the branches. There are three brances, two methyl(CH3) attached to C-2, C-3, and one ethyl(-CH2-CH3) attached to C-5. So, the name is 5-ethyl-2,3-dimethyloctane (correct) Nomenclature of Organic Compounds Reverse the number system: CH3 7 5 CH 8 6 CH2 CH2 4 CH H3C CH CH 3 3 1 4-ethyl-6,7-dimethyloctane(wrong) CH3 CH2 2 CH3 CH2 According to this numbering, the name is 4-ethyl-6,7-dimethyloctane. Lowest Set of Locants Rule: Write the locants of all the substituents in each name in increasing numerical value and make a set. In the first name, the set is : 2, 3, 5 (lowest set of locants) In the second name, the set is : 4, 6, 7 When comparison is made between the two sets from the beginning, locant by locant, the set giving the lower locant at the first point of difference is called the lowest set of locants irrespective of the locants appearing at later positions. This lowest set gives the correct name. So, the correct name is 5-ethyl-2,3-dimethyloctane. Example : CH3 2 4 6 8 10 1 CH2 3 CH 5 CH2 7 CH2 9 CH3 9 7 5 3 1 H3C CH CH2 CH2 CH 10 8 6 4 2 CH3 CH3 The longest chain contains ten carbon atoms, hence the parent alkane is decane. Two names that can be proposed with respect to both forward and backward numbering systems are 3,4,9-trimethyldecane (wrong) 2,7,8-trimethyldecane (correct) When we compare the two sets i.e (3,4,9) and (2,7,8) we find the difference at the first point.