Classification & Nomenclature Notes

Page # 1 CLASSIFICATION & NOMENCLATURE CLASSIFICATION 1. KEKULE’S PRINCIPLE (a)Carbon has four valencies. (b)Carbon has a property of catenation. It can make a large chain with addition of other carbons. (c)A carbon atom can share 2, 4 or 6 electrons with other carbons & can form single, double or triple bond. (d)For a carbon atom, it is not possible to make more than 3 bonds with adjacent carbon atom because a carbon atom complete its octet from overlapping which consists directional property. 2. CLASSIFICATION OF ORGANIC COMPOUNDS Organic Compounds & NOMENCLATURE 2.1.2 Unsaturated Hydrocarbons : – (a)There will be a double bond or a triple bond between any two carbon atoms, CH2 = CH – CH3 (Propene), CH  C – CH3 (Propyne) (b)Gen. formula is CnH2n or CnH2n – 2 . (c)Alkenes are also called as olefins because they reacts with halogens to form oily substances olefins (Oleum + fines i.e. Oil forming). (d)Due to presence of  bonds these are more reactive. 2.2 Closed chain compounds : – In these compounds first & last carbons are attached with each other. example. cyclopropane . These are of two types – Open chain or Acyclic or Aliphatic compound Sa turated Heterocyclic Closed chain or Cyclic or Ring compound Unsaturated Homocycl i c 2.1 Aliphatic or Open chain compounds :– Are those compounds in which first & last carbon atoms are not connected with each other. Branched or unbranched chains are possible in these compounds. For example : , C – C – C – C (unbranched) C | , C–C–C C–C–C–C | | C C (branched) There are two varieties in these compounds 2.1.1 Saturated Hydrocarbons :– – (a)In such type, adjacent carbons are attached with single bonds. Example - CH3 – CH2 – CH3 (b)General formula of these compounds is CnH2n + 2 . (c)These are also called as paraffins (Parum + Affinis i.e. little reactivity) because these are less reactive due to absence of  – bonds. 2.2.1 Homocyclic compounds : – These are the compounds in which the complete ring is formed by carbon atoms only. These are also of two types (a) Alicyclic compounds : – These are the compounds having the properties like aliphatic compounds. These may be saturated or unsaturated like aliphatic compounds. Cyclopropane, Cyclobutene Cyclopropene, (b)Aromatic compounds : – These compounds consist of at least one benzene ring i.e. a six-membered carbcyclic ring having alternate single and double bonds. These compounds have some fragrant odour and hence, named as aromatic (greek word aroma means sweet smell) eg. Benzene Toluene Styrene 2.2.2 Heterocyclic Compounds : – These are cyclic compounds having ring or rings built up of more than one kind of atoms. Furan Thiophene Page # 2 CLASSIFICATION & NOMENCLATURE 3. GROUPS Atom or a group of atoms which possess any ‘charge’ on it or any ‘free valency’ are called as Groups. 3.1 Alkyl group : – When a hydrogen is removed from saturated hydrocarbon then alkyl group is formed. It is represented by R & its general formula is CnH2n +1. A bond is vacant on alkyl group on which any functional group may come.   CH3 – Methyl H  CH3–CH2 –ethyl CH3– CH3  H CH4 (a) C3H7 has following two isomers – (i) Normal propyl CH3 – CH2 – CH2 – (ii) Isopropyl (1-methyl ethyl) CH3  CH  | CH3 (b) C4H9 has following four isomers – (i) n - butyl H3C – CH2 – CH2 – CH2 – (ii) Iso butyl (2-methyl propyl) CH3  CH  CH2  | CH3 CH3  CH2  CH  | CH3 (1,1-dimethyl ethyl) CH3 | CH3  C  | CH3 (c) C5H11 has following eight isomers – (i) H3C – CH2 – CH2 – CH2 – CH2 – n-pentyl (ii) H3 C  CH  CH2  CH2  | H3 C Isopentyl (iii) (3-methyl butyl) H3 C  CH2  CH  CH2  | H3 C Active amyl (2-methyl butyl) (iv) H3 C | H3 C  CH2  C  | H3 C H3 C | (v) H3 C  C  CH2  | H3 C Neo pentyl (2,2-dimethyl propyl) (vi) H3 C  CH2  CH2  CH  | H3 C Active secondary amyl (1-methyl butyl) (vii) H3 C  CH2  CH  | H2 C | H3 C Secondary amyl (1-ethyl propyl) (viii) H3 C  CH  CH  | | H3 C H3 C Active Isosecondary amyl (1,2-dimethyl propyl) (iii) Secondary butyl (1-methyl propyl) (iv) Tertiary butyl Tertiary pentyl (1,1-dimethyl propyl) 3.2 Alkenyl group : – CH2 = CH – Vinyl CH2= CH – CH2– Allyl CH3 – CH = CH – Propenyl(1-propenyl) CH3  C  CH2 | Isoprop enyl (1-methyl -1- ethenyl) 3.3 Alkynyl group – CH  C – Ethynyl CH  C – CH2– Propargyl (2-propynyl) CH3 – C  C – Propynyl (1-propynyl) 3.4 Normal group : – (a)It is represented by ‘n’. (b)Straight chain of carbon atom is known as normal group. (c)Free bond will come either on Ist carbon atom or on last carbon atom. n – butyl C – C – C – C – n – propyl C – C – C – 3.5 Iso group : – (a)It is represented by following structure ; H3 C  CH  | CH3 (b)When two methyl groups are attached to the same carbon atom, group is named as iso Page # 3 CLASSIFICATION & NOMENCLATURE 3.6 Secondary group : – (a)It is represented by following structure. C  C  C | C (b)When ethyl & methyl groups attached to the terminal carbon atom, group is named as secondary3.7 Tertiary group : – (a)It is represented by following structure - C | C  C  | C (b)When three (alkyl groups) (similar or dissimilar) are attached to the same carbon atom, group is named as tertiary. 3.8 Neo group : – (a)When a carbon atom is attached to other four carbon atom group is named as neo group. (b)It is represeted by following structure - C | CCC | C C | for eg. C  C  C – Neo pentyl | C 3.9 Amyl group : – It is of four types (a)Active amyl 4. NOMENCLATURE Mainly three systems are adopted for naming an organic compound – Common Names or Trivial System Derived System IUPAC system or Jeneva System 5. COMMON OR TRIVIAL SYSTEM Initially organic compounds are named on the basis of source from which they were obtained. eg. S. Organic Trivial Name Source N. Compound 1. CH3OH Wood spirit Obtain ed by or Methyl destructive spirit distillation of wood. 2. NH2CONH2 Urea Obtained from urine 3. CH4 Marsh gas It was produced (fire damp) in marsh places. 4. CH3COOH Vinegar Obtained from Acetum - i.e. Vinegar COOH 5. | COOH Oxalic acid Obtained from oxalis plant. 6. HCOOH Formic acid Obtained from formicus [Red ant] 7. CH3 – CH – COOH Lactic acid | OH 8. CH2 – COOH Malic acid | CH(OH)COOH H | CH3  CH2  C  CH2  | CH3 9 f (b)Secondary amyl CH3  CH2  CH – | CH2 | CH3 (c)Active secondary amyl CH3  CH2  CH2  CH  | CH3 (d)Active iso secondary amyl CH3  CH  CH  | | CH3 CH3 CH3CH2CH2COOH Butyric acid Obtained r o m butter. 10. CH3(CH2)4COOH Caproic acid O b t a i n e d from goats. Some typical compounds in which common & trivial names are also differ. S. Compound Trivial Common N. Name Name 1. CH4 Marsh gas Methane 2. CH3OH Woodspirit Methyl alcohol 3. CH3COOH Vinegar Acetic acid 4. 5. 1 AcetoneDimethyl ketone Acrolein Acryl Aldehyde Page # 4 CLASSIFICATION & NOMENCLATURE 6. Pivaldehyde Tertiary valer aldehyde (Common – Names S.N. Compound 1. R – X 2. R – OH 3. R – SH 4. R – NH2 5. R–O–R 6. R–S–R 7. 8. Dialkyl ketone R–NH–R 9. 10. Dialkyl amine Trialkyl amine R–O–R’ 11. 12. 13. R is termed as alkyl ) Name Alkyl halide Alkyl alcohol Alkyl thio alcohol Alkyl amine Dialkyl ether Dialkyl thioether Alkyl alkyl’ ether Alkyl alkyl’ ketone R–S–R’ R–NH–R’ 14. Alkyl alkyl’ thio ether Alkyl alkyl’ amine Alkyl alkyl’ alkyl” amine 5.1 Position of double bond : – In an unsaturated hydrocarbon if the position of double bond is on Ist or last carbon then it’s prefix will be  (alpha) if it is on 2nd carbon it is termed as  (Beta) & the  (gamma) &  (delta) and so on. eg. H2C = CH – CH2 – CH3  - butylene H3C – CH = CH – CH3  - butylene H3C – CH2 – CH = CH2  - butylene H2C = CH – CH3 or H3C – CH = CH2 (Both are same positions, propylene) H3 C  C  CH2 | CH3 Isobutylene CH3–CH2–CH=CH–CH2–CH3  - hexylene CH3– CH2–CH2–CH=CH–CH2 –CH2 – CH3  - octylene 5.2 Common – Naming of dihalides : – (a)When two same halogen atoms are attached to the same carbon such compounds are called Gemdihalides. (b)Common names of such compounds are alkylidene halides (c)When two same halogen atoms are attached to adjacent carbon, these are called as vicinal dihalides. Common names of such compounds are alkylene halide. eg CH3  CH  CH2 Propylene Iodide | |   Cl | H3 C  C  CH2  Cl | CH3 Isobutylene chloride (d)When two same halogen atoms are attached at the two ends of a carbon chain its common naming will be polymethylene halide. ‘poly’ word indicates the number of –CH 2 – groups. –CH2– 2 3 4 5 6 Poly di tri tetra penta Hexa eg. CH2  CH2  CH2 | |   Trimethylene Iodide CH2  CH2  CH2  CH2  CH2 | | Br Br Pentamethylene Bromide Exception : – CH2 – X dimethylene halide (wrong) | CH2 – X ethylene halide (right) 5.3 Common - Naming of di-hydroxy compounds: (a)When two –OH groups are attached to adjacent carbon’s they are termed as alkylene glycol. CH3  CH2  CH  CH2 | | OH OH Butylene glycol OH | CH3  CH2  C  CH2  OH | CH3 Active amylene glycol Page # 5 CLASSIFICATION & NOMENCLATURE (b)When two –OH group are attached at the two ends of a carbon chain, these compounds are named as polymethylene glycol. Poly  Number of CH2 groups. 5.4 Common - Naming of the functional group having carbon : – Chart - 1 Functional group Suffix group Functional Suffix O ||  C  OH -ic Acid eg. : CH2  CH2  CH2  CH2 | | OH OH O || C  H -aldehyde Tetra methylene glycol O || C  X -yl halide – C  N -o-nitrile O || C  NH2  N  C O || C  O  R -ate -ic anhydride CH2  CH2  CH2  CH2  CH2  CH2 | | OH OH Hexamethylene glycol Dimethylene glycol (wrong) Prefix : – 1 Carbon  Form3 Carbon  Propion- Ethylene glycol 4 Carbon  5 Carbon  Exception : CH2 – OH | CH2 – OH eg. (right) Make the structure of following organic -amide -o-isonitrile 2 Carbon  Acet-  Normal Butyr |  Iso compounds 1. Isopropylidene Bromide Valer 2. Active amylene Iodide 3. Isobutylene glycol 3 C + (=) double bond = Acryl 4 C + double bond = Croton- 4. Isobutylene 5. Trimethylene glycol eg. Sol.  | 2. CH3  C  CH2   | CH2  CH3 OH | 3. CH3  C  CH2  OH | CH3 O || H C H O || CH3  C  O  H Formaldehyde Acetic Acid O || CH3  CH2  C  Cl O || CH3  CH  C  NH2 | CH3 Propionyl chloride O || CH3  C  H Acetaldehyde Isobutyr amide 5.5 Nomenclature of Ester : – 4. H3 C  C  CH2 | CH3 O || C  O  R The group which is attached to the oxygen is written as alkyl & the remaining structure is named same as defined in chart-1. 5. CH2  CH2  CH2 | | OH OH eg. 1 O || CH3  C  O  CH3 Methyl acetate Page # 6 CLASSIFICATION & NOMENCLATURE O || CH3  CH2  C  O  CH2  CH3  C  COOH 7.  C  COX | | Ethyl propionate O || CH3  C  O  CH2  CH3 Ethyl acetate O || H  C  O  CH3 Methyl formate O || CH3  O  C  H Methyl formate O || CH3  C  O  H Acetic acid O || CH2  CH  C  O  CH2  CH3 Ethyl acrylate O || CH 3  CH  CH  C  O  CH 3 Methyl crotonate Alkanoyl halide 8. Alkanamide Acetyl halide Acetamide | –C– CONH2 | 5.6 Nomenclature of Anhydride : – Rule : – Add the total number of carbon atoms & divide it by 2, the substract will give you the number of C - atom. Now name it according to Chart - 1. 4 Total = Substract = Number of C atom = 2 2 2 Acetic anhydride 6. DERIVED SYSTEM According to this system name to any compound is given according to the parrent name of the homologous series. This system is reserved for the following nine homologous series. Chart - 2 Series Name of Homologous series | 6. Alkanoic acid Acetic acid 9. Alkanone Acetone |  C C C | || O | 6.1 Types of Ethylene:– (Symmetrical & Unsymmetrical) (a) Symmetrical : – In the given two alkyl groups one group is attached to the one carbon of ethylene & next on the next carbon. (b) Unsymmetrical : – When both the given groups are attached on the same carbon. Note : – Symmetrical & Unsymmetrical : – terms are used only when two alkyl groups are given. eg. Symmetrical dimethyl Unsymmetrical ethylene dimethyl ethylene Symmetrical ethyl methyl ethylene Tri methyl ethylene Tetra methyl ethylene Derived Structure Name of group | C 1. Alkane Methane 2. Alkene 3. Alkyne Ethylene Acetylene C  C  4. Alkanol Carbinol  C  OH 5. Alkanal Acetaldehyde  C  CHO | >C=C< | | | 1 | CH3 – C  C – CH3 Dimethyl acetylene 7. I.U.P.A.C SYSTEM OR GENEVA SYSTEM (International Union of Pure & Applied Chemistry) This system is based on the total number of carbons in the selected carbon chain in any organic compound. Following word root are used in this system. 1C 2C3C 4C 5C 6C 7C Meth Eth Prop But Pent Hex Hept 8C 9C10C 11C 12C 13C 14C Oct Non Dec. Undec Dodec Tridec Tetradec 19C 20C 21C Page # 7 CLASSIFICATION & NOMENCLATURE Nonedec Eicosane Uneicosane 22C 23C 30C Twoeicosane Trieicosane Tricontane 7.1 Rules for IUPAC Nomenclature : – (Using only preffix) Rule-1. Logest Chain Rule Select the longest continuous chain of carbon atoms as the parent chain. If some carboncarbon multiple bond is present, the parent chain must contain the carbon atoms involved in it. CH3–CH2–CH2–CH2–CH–CH3 CH2–CH3 Preffix : Word root p.suffix : methyl hept ane : CH3 Rule-4. Alphabetical Arrangement of Prefixes If there are different alkyl subsitituents present in the compound, their names are written in the alphabetical order. However, the numerical prefixes such as di, tri etc, are not considered for the alphabetical order. For example , C2H5 1 Naming different alkyl substituents at the equivalent positions. If two different alkyl groups are located at the equivalent positions, then numbering of the chain is done in such a way that the alkyl group which comes first in alphabetic order gets the lower position. C2H5 CH3 1 2 1 ethyl ; methyl but ene Rule-5. Rule for large number of substituents If a compound has two or more chain of the same length, then parent chain selected in such a way that greater number of substituent works as preffix. 1 2 3 3-iso propyl pentane one substituent - wrong name 2,2,4-Trimethylpentane (Correct) 5 4 3 CH3–CH2–CH––CH2–CH3 2 CH3 1 2 CH3–C–CH2–CH–CH3 3-ethyl-2-methyl pentane two substituent - correct name CH3 2,4,4-Trimethylpentane (Wrong) Rule-3. Use of Prefixes Di, Tri, etc. If the compound contain more than one similar alkyl groups, their positions are indicated separately and an appropriate numerical prefix, di, tri etc., is attached to the name of the substituent. The positions of the substituents are separated by commas, for example, 2 3 Rule-6. Naming the Complex Alkyl Substituents If the alkyl substituent is further branched, it is named as substituted alkyl group. For this purpose, the carbon atoms of the alkyl group are separately numbered in such a way, that the carbon atom directly attached to the parent chain is given number 1. The prefix/name of such a substituent is enclosed in brackets. CH3 9 CH3 CH3 1 CH–CH3 1 CH3 3 5 CH3 CH3 CH3 4 CH3–CH2–CH––CH2–CH3 5 4 6 CH–CH3 4 2 CH3–C– CH2–CH–CH 3 5 5 4 3-Ethyl-4-methylhexane CH3 3 3 CH3–CH2–CH––CH–CH2–CH3 Rule-2. Lowest Number Rule The numbering is done in such a way that the substituted carbon atoms have the lowest possible numbers. CH3 5 4 3-Ethyl-2,3-dimethylpentane CH2 : 3 CH3 CH3 CH3–CH–C–CH2–CH3 Preffixes : Word root p.suffix : 2 CH3–CH–C–CH2–CH3 8 7 6 5 4 3 2 1 CH3 –CH2–CH–CH2–CH2–CH–CH2–C–CH3 4 5 CH3–CH–CH–CH2 –CH3 2,3-Dimethylpantane 1 CH3 1 CHCH3 CH3 2CH 3CH 3 2 Page # 8 CLASSIFICATION & NOMENCLATURE Here, substituted propyl group is present at carbon number 4 of the parent chain. The propyl group has methyl substituent at carbon number 1 of the alkyl group. Thus, the name of the above compound is : 2,2,7-Trimethyl-4-(1-methyl propyl) nonane 8. NOMENCLATURE OF COMPOUNDS CONTAINING FUNCTIONAL GROUPS : : In case some functional group (other than C=C and C  C) is present in molecule, it is indicated by adding secondary suffix after the primary suffix. The terminal ‘e’ of the primary suffix is generally removed before adding the secondary suffix. The terminal ‘e’ of the primary suffix is removed if it is followed by a suffix begining with ‘a’, ‘i’, ‘o’, ‘u’ or ‘y’ . Some of functional group always works as preffix Functional group Preffix name –X halo – OR alkoxy – C–C– epoxy O – NO 2 nitro – NO nitroso –N=N– azo Commonly used group, which works as suffix Functional group Suffix name – COOH oic acid – CH = O al –C– one 2. In order to decide the primary suffix, the presence of carbon-carbon multiple bond in chain is observed. 3. The functional group present is identified. This enables the selection of appropriate secondary suffix or prefix. 4. The carbon atoms of the parent chain are numbered in such a way so that the carbon atom of the functional group gets the lowest possible number. In case the functional group does not have the carbon atom, then the carbon atom of the parent chain attached to the functional group should get the lowest possible number. 5. The name of the compound is then arrived at by arranging prefixes and suffixes along with their positions as follows. 9. NOMENCLATURE OF COMPOUNDS WITH MORE THAN ONE SIMILAR FUNCTIONAL GROUPS : : If the organic molecule contains more than one similar functional groups, then in addition to various rules the numerical prefixes di (for 2), tri (for 3), etc., are added before the secondary suffix which indicates the functional group. While adding such words the vowel e of the primary suffix is retained. For example. | | | OH OH OH Here, word root is but, primary suffix is ane and secondary suffix is ol. The —OH groups are positioned at carbon number 1,2 and 3. Thus, the name is : Butane-1, 2, 3-triol 3 O – OH ol – CN nitrile Name of an organic compound containing functional group is derived through the following steps. 1. Select the longest continuous chain of the carbon atoms as parent chain. The selected chain must include the carbon atoms involved in the functional groups like –COOH, –CHO, –CN etc., or those which carry the functional groups like –OH, –NH2, –Cl, –NO2, etc. The number of carbon atoms in parent chain decides the word root. 1 | 4 CH2– CN Butane-1, 4-dinitrile 1 2 3 4 HOO C – C H  C H – C OOH But-2-en-1, 4-dioic acid Note: If an unbranched chain is linked directly to more than two carboxyl groups, these groups are named by substitutive use of suffix “tricarboxylic acid” , atc. The principal chain selected should be linked directly to maximum possible number of carboxyl groups. For example. COOH | HOOC— CH2— CH—3CH2—4CH2—COOH 1 2 Butane-1, 2, 4-tricarboxylic acid Page # 9 CLASSIFICATION & NOMENCLATURE COOH COOH | | 6 5 4 3 2 1 CH3— CH— CH2— CH— CH— CH2—COOH | CH2COOH 3-(Carboxymethyl) hexane-1, 2, 5tricarboxylic acid Similarly, the substitutive prefixed for aldehydes. cya nides, acylc hl orides and ami des are carbaldehyde, carbonitrile, chlorocarbonyl and carboxamide respectively. 10.NAMING THE COMPOUNDS WITH TWO OR MORE DIFFERENT FUNCTIONAL GROUPS : : If the molecule contains more than one dissimilar functional groups, the parent chain must contain maximum possible number of functional groups. The numbering of the parent chain is done in such a way so that the functional group of higher priority gets the lower number. The order of priority of various groups for the sake of numbering is given in following table. 11.Nomenclature of hydrocarbon having more than one multiple bond 1. When more than one double or triple bond is present then word root  Alka eg. CH2 = CH–CH = CH2 : buta -1,3-diene 2. When double and triple bonds are present, the hydrocarbon is named as alkenyne 3. The numbering of the parent chain should always be done from that end which gives lowest sum for the multiple bonds. For example, 2+4=6 3 5 1+3=4 4 3-Penten-1-yne not 2-Penten-4-yne 4. If, however, there is a choice in numbering, the dobule bond is always given preference over the triple bond. 1 3 2 4 5 2 1 CH3 – CH = CH – C  CH 5 4 3 2 1 1 2 3 4 5 CH  C – CH 2 – CH = CH 2 1-Penten-4-yne not 4-Penten-1-yne 4 3 2 1 1-Hexene-4-yne not 4-Hexene-1-yne 12 Nomenclature of alicyclic compounds : : Names of alicyclic compounds are derived by putting another preffix ‘cyclo’ before the word root which depends upon the number of carbon atoms in the ring. The suffixes ane, ene or yne are written depending upon saturation or unsaturation in the ring. 1 If some substituent or functional group is present, it is indicated by some appropriate prefix or suffix and its position is indicated by numbering the carbon atoms of the ring. The numbering is done in such a way so as to assign least possible number to the functional group or substituent in accordance with the rules already discussed. Some examples are. NO2 3 2 2 3 1 CH3 CH3 3-Methylcyclo- 1-Methyl-3hexanol Nitrocyclohexene 1OH CH3 3 2 CH3 CH3 C2H5 1 1-Ethyl-22-3-DimenthylcycloMethylcyclobutene pentene O 1 2 1 4 3 CH3 – CH – CH = CH2 2 3 OH 3-Hydroxycyclo hexanone 3-Cyclopentyl-1butene 13. Naming Compounds With Bond Line Formula In this representation of organic molecules, carbon and hydrogen atoms are not shown and the lines representing carbon – carbon bonds are drawn in zig-zag manner. A single line (–) represents a single bond, two parallel lines (=) represents a double bond and three parallel lines () represent a triple bond. The only atoms specitically written are those that are neither carbon nor hydrogen bound to carbon. The intersection of lines represent carbon atoms carrying appropriate number of hydrogen atoms. Some bond line structures along with their IUPAC names are given below. Page # 10 CLASSIFICATION & NOMENCLATURE 2 1 15.NAMES OF BRIDGED BICYCLIC HYDROCARBONS Saturated bicyclic systems having two or more atoms in common are named by prefixing ‘bicyclo’ to the name of the cyclic parent hydrocarbon system containing the same total number of carbon atoms in the skeleton. The number of carbon atoms in each of the three bridges, connecting the two tertiary carbon atoms is indicated in parentheses, in descending order and arabic numerals are used to indicate the number of carbon atoms and the numbers are separated by full stops. The bicyclic system is numbered starting with one of the tertiary bridging cabon and proceeding through longest bridge to the second bridging carbon continuing back to the first bridging carbon through the second longest chain. Numbering is completed by numbering the shortest bridge beginning with the atom next to the first bridging carbon. For example : 1 1 2 6 2 7 8 3 9 3 6 5 4 5 4 Bicyclo [2.2.1]heptane Bicyclo [3.2.2] Nonane 4 3 5 Hexa-1, 3, 5-triene 2 1 3 OH 4 5 Pent-2-en-3-ol Br 4 2 3 1 5 O 2-Bromopentan-3-one 14. Naming of aromatic compounds : : (a) Mono substituted benzene 3 4 5 2 spiro [2.4]heptane (b) Disubstituted Benzene X ortho (o –) ortho (o –) meta (m –) meta (m –) para (p –) OH CHO o-hydroxy benzaldehyde NH2 16. NAMES OF SPIRO BICYCLIC HYDROCARBONS Spiro bicyclic hydrocarbons contain two rings consisting of carbon atoms only and the two rings are linked by a common carbon. These compounds are named by placing prefix ‘spiro’ before the name of the acyclic parent hydrocarbon with same number of skeletal carbon atoms. The numbers of skeletal atoms linked to the spiro atom are indicated by arabic numbers, separately by a fullstop. The numbers are written in ascending order and enclosed in square brackets. Ex. 1 7 6 NH2 Numbering of a spiro bicyclic hydrocarbon starts with a ring carbon next to the spiro atom and proceeds first through the smaller ring and then through the spiro atom and around the second ring. For example : Ex. COOH m- amino benzoic acid 1 SO3H p-amino benzene sulphonic acid 8 9 10 1 5 2 CH3 3 4 2-Methylspiro [4, 5] deca-1, 6-diene 7 6