Concepts in Chemistry CHAPTER-IV TRIVIAL OR COMMON SYSTEM OF NOMENCLATURE Naming an organic compound on the basis of the rules recommended by IUPAC is absolutely essential for big and complex molecules. However, small molecules are more popularly addressed by their common names or the trivial or the primitive names. We already know before that IUPAC has accepted a few common names as official names e.g acetic acid, formic acid etc. for some compounds and isopropyl, sec-butyl, tert-butyl etc. as some groups. A detailed logically genesis of the common system is discussed below. TYPES OF CARBON ATOMS: There are four types of carbon atoms. (i) Primary or 10 carbon atom: carbon atom attached with 3 or 4 H atoms. (ii) Secondary(sec-) or 20 carbon atom: carbon atom attached with 2 H atoms. (iii) Tertiary(tert-) or 30 carbon atom: carbon atom attached with 1 H atom (iv) Quaternary or 40 carbon atom: carbon atom attached with no H atom. 0 0 0 1 1 1 CH3 0 0 CH3 0 0 CH3 1 3 0 4 2 3 Example: H3CCH C CH2 CH 0 0 1 1 CH3 CH3 In the above example, all the carbon atoms have been assigned with their types. SAQ IV.1. (i)Name the molecule in which a primary carbon atoms is linked with 4 H atom. (ii)Draw the structure and give the IUPAC name of the compound which contains one 40 and four 10 carbon atoms. TYPES OF ALKYL GROUPS: In common system alkyl groups are prefixed with the symbols n-, iso, sec-, tert- and neo. While naming the alkyl group all the carbon atoms present in the alkyl group are counted. Numbering of carbon atoms is not done in this case like IUPAC system. (i) normal or n-alkyl group: If all the carbon atoms of the alkyl group are connected in straight chain and there is no branching, it is called n-alkyl group. H3CCH2 CH2 H3CCH2 CH2 CH2 H3CCH2 CH2 CH2 CH2 n-propyl n-butyl n-pentyl or n-amyl (N.B: Note that methyl and ethyl groups are named as such; not as n-methyl and n-ethyl, because they do not have any other types like iso, sec- , tert- or neo) Nomenclature of Organic Compounds H3C (ii) isoalkyl group: The group beginning with CH unit and other carbon atoms if H3C any are connected in straight chain with no further branching is called isoalkyl group. H3C H3C H3C CH CH2 CH2 CH (isopropyl), CH CH2 (isobutyl) H3C H3C H3C isopentyl or isoamyl (iii) secondary or sec-alkyl group: When one H atom is removed from a secondary carbon atom, a sec- alkyl group results. Butane gives sec-butyl and propane gives sec-propyl groups. 0 2 H3CCH2 CH2 CH3 H3CCH2 CH CH3 H3CCH2 CH butane sec-butyl CH3 In sec-alkyl group the unfulfilled valency of the alkyl group(shown by dash mark) is connected to that carbon atom which is linking with one H atom. 0 2 H3C CH H3CCH2 CH3 H3CCHCH3 H3C propane sec-propyl Note that sec-propyl group is same as isopropyl as it satisfies the requirements of both types. Between the two names isopropyl is more popular and widely used. Note also that sec-alkyl group is restricted to sec-butyl only because larger sec-alkyl groups do not have unique structure. For example, sec-pentyl group can have two different structures which can be obtained from pentane by removing one H atom from C-2 and C-3. But butane gives only one sec-butyl group as C-2 position is unique. (iv) tertiary or tert-alkyl group: When one H atom is removed from a tertiary carbon atom, a tertiarty or tert- alkyl group results. CH3 CH3 CH3 CH3 0 0 3 3 H3CCH H3CC H3C CH2 CH H3C CH2 C CH3 CH3 CH3 tert-pentyl CH3 tert-butyl In tert-butyl group the unfulfilled valency of the alkyl group(shown by dash mark) is connected to that carbon atom which does not link with any H atom. The use of tert-alkyl group is limited to tert-butyl and tert-pentyl groups only as for higher tert-alkyl groups unique structures are not possible. (v) neopentyl group: The alkyl group beginning with the following part structure(I) and other carbon atoms if any are connected in straight chain is called neoalkyl group. Concepts in Chemistry CH3 CH3 CH3 H C C CH (neopentyl) H C C CH CH (neohexyl) H3CCCH2 (I) 3 2 3 2 2 and CH3 CH3 CH3 so on Note that while iso and neo prefixes are integrated with the names of the alkyl groups, n-, sec- and tert- prefixes remain separated from alkyl groups by a hyphen. SAQ IV.2: Draw the structures of the following alkyl groups (i)n-hexyl (ii)isohexyl (iii)neoheptyl NAMING OF COMPOUNDS USING COMMON OR TRIVIAL SYSTEMS: 1. ALKANES: (i) normal or n-alkane: When the carbon atoms are connected in straight chain and there is no branching, it is called a n-alkane. In other words one one H atom is attached with a n- alkyl group it becomes a n-alkane. H3CCH2 CH2 CH3 (n-butane), H3CCH2 CH2 CH2 CH3 (n-pentane) (ii) isoalkane: When one H atom is attached with an isoalkyl group, an isoalkane results. H3C H3C CH CH3 (isobutane) CH CH2 CH3 (isopentane) H3C H3C (iii) neoalkane: When one H atom is attached with a neoalkyl group, a neoalkane results. CH3 CH 3 H CCCH H CCCH CH 3 3 (neopantane) 3 2 3 (neohexane) CH3 CH 3 IMPORTANT: The names isobutane, isopentane and neopantane have been accepted as sytstematic IUPAC names for those alkanes. OLEFINES (ALKENES): The first member of alkene is called ethylene and all other alkenes are expressed as substituted ethylenes. Monosubstituted ethylene: If there is one alkyl substituent attached to any carbon atom of the C=C in place of one H atom, then it is called alkylethylene. It is a monosubstitued alkene. : H2CCH2 ethylene(ethene); H2CCHCH3 propylene or methyl ethylene(propene) Disubstituted ethylene Disubstituted alkenes are classifed as (i)symmetrical or sym-alkene and (ii)unsymmetrical or unsym-alkene. Nomenclature of Organic Compounds Symmetrical alkene: If one H atom from each of the two doubly bonded carbon atoms is substituted by same alkyl groups, it is a symmetrical alkene. Unsymmetrical alkene: If two H atoms of any one doubly bonded carbon atom are substituted by two same alkyl groups, it is an unsymmetrical alkene. The IUPAC names have been given inside parentheses for comparison. CH3 CH CH CH3 CH3 CCH2 CH3 sym-dimethylethylene unsym-dimethylethylen (but-2-ene) (2-methylpropene) Unsym-dimethylethylene(2-methylpropene) is also called isobutene or isobutylene. ALKYNE: The first member of alkyne is called acetylene and all other alkenes are expressed as substituted acetylenes- (i)monosubstituted- if one H atom is replaced by an alkyl group or (ii)disubstituted- if both the H atoms are replaced by alkyl groups H3C CH CCH HC CH H3CC CH H3CCCCH3 acetylene methylacetylene dimethylacetylene H3C isopropylacetylene SAQ IV.3: Draw the structures of the following compounds. Also give their IUPAC names. (i)isohexane (ii)neoheptane (iii)sec-butylethylene (iv)sym-diisopropylethylene (v)tert-butylacetylene (vi)ethylisobutylacetylene (vii)n-heptane ALKYL HALIDE(HALOALKANE): The haloalkanes are called alkyl halides in the common system. First the name of alkyl group according to common system is written which is followed by the term halide(fluoride, chloride, bromide, iodide) with a space in between. CH3 H3C CH Cl H3C C Br H3CCH2 CH2 I H3C CH3 isopropyl chloride tert-butylbromide n-propyl iodide (2-chloroprapane) (2-bromo-2-methylpropane) (1-iodopropane) SAQ IV.4: Give the common names of the following alkyl halides, also write their IUPAC names. H3C CH CH2 Br (i)CH3Br (ii)CH3-CH2-Cl (iii) (CH3)3C-I (iv) H3C SAQ IV.5: Draw the structures of the following and also give their IUPAC names. (i)sec-butyl chloride (ii)neopentyl iodide (iii)isohexylbromide (iv)tert-pentyl chloride (v) n-propyl iodide ALCOHOL: In common system, an alcohol is named as alkyl alcohol. First the name of the alkyl group is written according to common system, followed by the term alcohol with a space in between. Concepts in Chemistry CH3 H3C CH OH H3CCH2 CH OH H3CC CH2 OH CH3 H3C CH3 isopropyl alcohol sec-butyl alcohol neopentyl alcohol (propan-2-ol) (butan-2-ol) (2,2-dimethylpropan-1-ol) THIOALCOHOL: In common system thioalcohol is named as alkyl mercaptan. First the name of the alkyl group is written according to common system, followed by the term mercaptan with a space in between. H3C 3 SH 2 SH H3C CH H3C 3 1 sec-butyl mercaptan(butane-2-thiol) isopropyl mercaptan (propane-2-thiol); SALT OF ALCOHOL: RONa or ROK are the salts of alcohol which are named as metal alkoxides. The trivial name of the alkyl group is used to name the alkoxide. H3C O Na sodium isopropoxide(sodium propan-2-olate); H3C CH3CH2OK potassium ethoxide (potassium ethanolate) SAQ IV.6: Give the names of the following according to common system.Also give their IUPAC names. (i) CH3-CH2-OH (ii)CH3-OH (iii) CH3-CH2-CH2-OH (iv)CH3-CH(OH)-CH2- CH3 (v)(CH3)3COH (vi)(CH3)2CHCH2CH2OH SAQ IV.7: Draw the structures of the following alcohols. Also give their IUPAC names, (i) tert-pentyl alcohol (ii) n-butyl alcohol (iii) isopropyl alcohol (iv)isobutyl mercaptan (v) sodium tert-butoxide (vi)potassium methoxide KETONES: Ketone is named as alkyl alkyl ketone or dialkyl ketone having a space between any two words.The two alkyl groups attached to the carbonyl group are written first in alphabetical order followed by the term ketone.