Basics Of Organic Chemistry PDF

l C = C < CH2 = CH2, CH3 - CH = CH 2 , C4 H 8 , C 5 H 10 etc. ( double bond) ,.,. - C =C -.). Alkynes...

l C = C < CH2 = CH2, CH3 - CH = CH 2 , C4 H 8 , C 5 H 10 etc. ( double bond) ,.,. - C =C -.). Alkynes BC = CH, CH3 - C = CH, C 4H 6, C 5H 8 etc. (triple bond) 4. Haloalkanes - X(F, Cl, Br, I.) CH3Cl, C2H 5Br, C2 H 5I, (CH3) 2 CHBr, (CH3)3C- Br etc. 5. Alcohols or Alkanols - OH (Hydroxy) CH3OH, C2H5OH, C3H7OH, (CH3)2CHOH,. (CH3)3C - OH etc.. 6. Ethers OR (alkoxy) CH3O.CH3, CH3O- C2Hs, C2H5OC2Hs, CI-l3OCH(CH 3)i etc. 1-2 '  -6 7. -Aldehydes or Alkanals - CHO or - CO-· --Ctr3 I (Cl 10, --- - Cl IO, Cl I3Cl r2CHO' Cl I3CH2CI-l2CHO 8. etc. C ll Kctones or Alkanoncs > C= O CJ-l3COCl 13, CI l3COC2I Is, C2I f5CO 2 s etc. 9. Carboxylic acids - COOi-I HCOOH, Cl-I3COOf-l, Cl l3CI l2COOI I, (Alkanoic acids) (Carboxyl) C3l £7Cl-l2COOI-I etc. l 0. Esters - COOR Ester or I-lCOOCH3, Cl-l3COOCH3, Cl l3COOC2I Is etc. alkoxy Carbonyl 11. Acid halides -COCl CI-T3COCJ, CH3Cl-I2COCI etc. (Chloroformyl) 12. Acid amides - CONH 2 (amide) HCONI-12, CI-13CONH2 etc. 13. 14. Nitrites Nitroalkanes - C = N (Cyano) CH 3C =N, CI-13CH2C = N etc. - N02 (nitro) CH 3N02, CH3CH2N02, etc. 15. Amines NI-12 CH3NH2, CH 3CI-I2NI-I2 etc. NOMENCLATURE OF ORGANIC COMPOUNDS In the early days of the development of organic chemist1y, any new co1npound that was discovered, was given a separate name. These ·names often depend on one or the other property or source of the co1npound. Thus, CH4 was named as marsh gas because it was produced in 111arshy places. Formic acid (HCOOH) was named so because it was first prepared by the distillation of red ants (Latin fonnica). Such names are called Trivial or Common names. Comn1on or trivial names suffer from a big defect that Remember (IUPAC nomenclature) they are individual and unsystematic. The same compound CH3CH 2CH 2CH 3 is Butane and not n-Butane CH 3CH(CH 3)CH 2CH 3 is was given different names by different persons in different 2-Methyl butane and not isobutane. countries (CH4 = methane-fire damp, marsh gas) which gives Chloropropane, methoxyethane and I \ rise to confusion. To avoid confusion in literature, it became nitroethane are written as one word. necessary to have a systematic nomenclature for the organic But in alcohols,.ethers, ketones, name compounds. The first attempt for systematic naming of the of each part is separately written CH3CH 2OH is ethyl alcohol and not organic compounds was made by the International Chemical ethylalcohol. Congress at Geneva (1892). This was revised and extendedby CH3COC2H 5 is.methyl ethyl and not 11 the International Union of Chemistry (fUC) at Liege (1930). methylethyl l The IUC syste1n has been further revised from time to CH30C2Hs is methyl ethyl ether and not methyl ethylether. ti1ne by the International Union of Pure and Applied Chen1istry (IUPAC). The latest version was released in 1997. IUPAC system. The s~stem of nomenclature accepted and used all over the world today is that approved by IUPAC. It 1s commonly referred to as the llJPAC System of Nomenclature. The IUPAC rules are also under constant review in the light of practical difficulties. Table 1.2 The Chain length and the word root In the IUPAC system, the name of an Chain length Word root organic compound consists of three parts : Chain length Word root C1 (i) Prefix (ii) Word root and (iii) Suffix. C2 Meth c6 Hex Eth C1 Of these, the word root is the basic unit of Hept C3 Prop the name. It depe nds upon the number of carbon Cs Oct C4 But atoms in th e lon gest carbon chain selected, called C9 Non Cs Pe nt (' I 0 Dec  BASICS OF ORGANIC CHEMISTRY 7 the parent chain. Depending upon th e number or cnrbo ns lll Lhc longest chain , tl,c compound is assigned a '"'ord root as described in tabk l.2. Tablo 1.3 Tho primary suffix after the word root The word root is foil owed by appropriate Nature of bond P. Suffix Ceneral name suffix or suffixes to represent the nature of C-C bonds and the functional group present. Saturated hydrocarbon - anc Alkanc A prirnary (P) suffix is added after the word chain AJkcne root to indicate the nature of the carbon-carbon C = C double bond - enc bonds as given in the table 1.3 C = C triple bond - yne Alkyne Group (R-) - yl A lkyl A secondary suffix is added after the primary suffix to indicate the nature of the functional group as illustrated below in table 1.4. Table 1.4 A secondary suffix. Functional group Sec. Suffix Functional group Sec. Suffix Alcohols (- OH) - ol Carboxylic acid (- COOI-I) - oic acid Aldehyde (CHO) - al Acid chloride (- COCI) - oyl chloride Ketone (> C = 0) - one Acid amide (- CONI-12) - amide Nitrile (- C = N) - Nitrile Ester (- COO -) - oate The prefixes are added before the word root to indicate the nature of the side chains or substituents attached to the parent chains as described below in table 1.5. Table 1.5 The substituents and their prefixes. Substituent Prefix Substituent Prefix F Fluoro - N02 Nitro Cl Chloro - CN Cyano Br Bromo ➔ Isocyano -N =C l lodo - OH Hydroxy CH 3 Methyl >C=O Keto 0 - c-1/ Ethyl Carboxy ' C2Hs ' OH C3H7 Propyl - NH2 Amino CH30- Methoxy - NHR N-Alkyl-amino C 2 Hs0- Ethoxy - NR2 N, N-Dialkyl-an1ino NOMENCLATURE ON THE BASIS OF FUNCTIONAL GROUPS 1. Longest continuous carbon chain Rule. (i) Select the longest continuous carbon chain, The carbon atom/atoms which are not included in the chain are substituent and denoted as prefixes. (ii) If the two chains carry the same number of carbon atoms, then the one with more numh~r of substituents is selected. Substitutents may be named in alphabetical order.  MODERN COLLEGE CHEMISTRY (C.B.C.S. SEM. 11) 8 Exan1ples. CI-1 3 _CH_ Cl-I - CH 3 1CH3 - 2cH - 3cH - 4CI-I2- 5CH3 I I I I CI-1 2 CH 2 CH3 CI-1 2 - CH3 I I CH 3 CI-1 3.. ti 11 3, 4- D une 1y 1cxane. 3-Ethyl-2-methylpentane (iii) If the substituent is branched, then it is named as substituted alkyl group CH 3 1 6 cH 7 1 CH - 2 CH - 3 CH - 4 CH - SCH 2 - 2 - Cf:J -i 3 3 2 I H 3 C-CH- CH 3 2-Methyl-3(1-mcthylethyl) heptane. 2. For unsaturated hydrocarbons. (i.e. for C = C, C = C etc. compounds) (i) Select the longest continuous carbon chain which contains the maximum nmnber of multiple bonds. (ii) If the carbon chain contains one double or triple bond, then number the chain. so as to give the lowest numeral to the carbon containing the 1nultiple bond. (iii) If numbering of the chain from either end give the saine set of locants to the 1nultiple bonds, then select the set which gives lower locant to the double bond i.e. ene comes before yne and 'e' of ene is deleted if it is followed by suffix starting with vow~l (a, i, o, u or y). Examples. 1 CH - 2 CH= 3 CH- 4 CH- 5 CH - 6 CH 3 2 3 I CH 4-Methyhex-2-ene. 3 ICH2 II 4 CH 3 - CH2 - 2C- 3CCH2 - C- CH2 - CH 3 II 5CH2 2, 4-riethyl penta-1, 4-diene (For dienes, diynes, trienes etc. 5 'a' is added to the word root) CH 2 - CH 3 I 1 CH 2 3 4 2 = CH - CI = C -. CH 3 7 6 CH3= c=sc_4CH-3CH-2CH=1CH 2 CH 3 I I 3, 4-dimethylhexa-l , 3-diene _ CH 3 CH 3 3, 4-dunethyl hepta-1-en-S- yne 3. For Orga1uc Compounds conta1n1ng one PrinciJlal F t·.... unc 1onal Group ( 1') S 1 longest carbon cham which mcludes the functional group and 11. · e ect t 11e 1ax1mun1 nun1ber of l ·. I b d (ii) Numbering of carbon chain is done so as to give the 1 nlu tip e on s. owe st 1ocant to the t"i t. I (iii) The characteristic functional group like _ CHO, _ COOH _ unc iona group. should always get number 1. ' COOR, - COCI, C = N etc. Example. CH3 - CH2 - 2CH - 3CH2- 4CH I 3 1 CHO 2-Ethyl butanal  ·--- - - - - - -- - - " ' ". _.........,tliila!U(Ull:fflUUlliUUl..-.&:rJIUl.!f.lillJCUt,1,JfM!l.l.i.Nh,,~jJ;l]UUU!;~~l.;l:J'Jn't.'GL"l:JJUL!UtnJ'-=L."l♦GUUU~:.wu.... uuw.u..n_ _ _ __ _ _ _ _ _ BASICS OF o ·R GANIC CHEMISTRY 9 6 o CH= 7 CJ-I 2 I 2 II 4 j 3 5 CH3 - CH 2 - C - CH 2 - CH - CI--I 2 - CH 3 5-Ethyl hept-6-en-J -one CH 3 s 4 3 I 1 CH3 - CH= CH - f\CH - COOH CH 3 - CH = CH - C = N 2-Methyl pent-3-en-l~ic acid But-2-enenitrile or 2-Methyl pent-3-en6 ic acid \ ' \ Cl 0 4 3 2 I I II CH3 - Cl-I - C = C - COOCH 2 CH 3 CH 3 - CH - CH - C - Cl I. I CH 3 \ OC 2 H 5 Ethyl, 4-methylpent-2-ynoate 2-Chloro-3-ethoxybutanoyl chloride Non1enclature of Organic Co1npounds having Multiple Bonds, Functional Grou ps, Substituents/Side Chain For naining organic co1npounds having functional groups, multiple bonds, substituents or side chain, following order of preference is adopted. Functional group > Double bond > Triple bond > Substituent side chain Following examples· are given for clarity. 6 5... 4 3 2 1 6 5 4 3 2 I CH 3 - CH 2 - CH= CH- CH 2 CHO CH 3 - CH 2 - C = C - CH 2 - COOH Hex-3-enal Hex-3-yn-oic acid 4 3 2 I 7 6 5 4 3 2 1 CI--1 3 - CH= CH - CH 2 0H CH= C - CH 2 - CH= CH - CH 2 - CHO But-2-en-1-ol Hept-3-en-6-ynal NOMENCLATURE OF POLYFUNCTIONAL ORGANIC COMPOUNDS (a) If the functional groups are of same kind. The organic compounds which contain n,vo or more functional groups in the same molecule are called polyfunctional co1npounds. For assigning IUPAC names to the polyfunctional compounds containing two or more functional groups of t he same kind, the parent chain or continuous chain is chosen in such a way that the selected chain contains maxi1num number of functional groups. According to 1993 IUPAC recommendations for TUP AC nomenclature, if an unbranched chain is directly linked to more than two functional groups of the same kind, the IUPAC name is obtained by adding the suitable suffix to the name of the parent alkane (without including the carbons of the functional groups). e.g., I CH OH CH2CN I 2 I 2 CHOH CHCN C001-I I I I CH 2 0H 3 CI-1 2 CN COOH Propane-I, 2, 3-triol Propane-I, 2, 3-tricarbonitrile Etham:dio ic acid  EGE CHEMISTRY (C.B.C.S. SEtv1. 11) 10 MODERN COLL - CH 3 I COOH C=O 1 CH I I 2 CHO CH CH2 I 2 II I CI-ICHO CH C=O I I I 3 cH 2 CHO COOH CH3 Propane-I , 2, 3 tricarbaldehyde But-2-en-l, 4-dioic acid Pentane-2, 4-dione · (b) If the functional · · d h l · nun1ber along the hcarbonbelchain (Jroups are of unhke Ion , t e owet · ts · gtven b to one of the groups chosen as principal functional group m tiie Order of preference s own ow ·· 1. Chain terminating groups will have precedence in the following order : (i) Acid groups : - S03H, - C02H 0 0 0 II II II I (ii) Acid derivatives : - C - O - C -, - C - 0, - COX, CONH2 (iii) Fonnyl, - CHO (iv) Cyano, - C = N. 2. The order of preference for groups that are not chain terminating shall be (i) Oxo, > C = 0 (ii) Hydroxy, - OH (iii) Mercapto, - SH I (iv) Amino, - NH2, N-alkyl amino - NHR ; N, N-dialkyl atnino - NR2 Thus the order of preference for the selection of the principal functional groups is : rSulphonic - - - -acids --- --------------------------7 (v) Alkoxy, - OR. > carboxylic acids > acid anhydride > esters > acid chlorides > amides > I aldehydes > nitrites > ketones > alcohols > thioalcohols > Amino > N-alkyi amino, N, N-dialkyl I I amino > ethers > alkene > alkyne. I L---------------------------------_J The other groups are regarded as the substituents or secondary functional groups. If an organic compound contains - COOH and - NH2 groups, then - COOH group will be the principal functional group and - NH2 group a substituent. The principal functional group is designated by a suffix while the substituent is designated by a prefix. For exan1 ple, :NH 2 ·1.....: 5 CH 3 - 4 CH - 3 CH 2 - 2 CH 2 :ic.00·1-1: 4-Aminopentanoic acid. ·· ··· · ··· (c) When two ~rincipal functi~n_al groups are si~uated sym1netrically in the parent compound, the numbering is estabhshe? by the pos1t1on of unsatt~ratton. Groups other than principal functional group are considered as subst1tuents and denoted by suitable prefixes in the naine of the parent compound. H HH H HHH 1-lH I I 3 I 4 s I I I I I I H - C1 - 2C = C - CH 2 - C - 1-I H- cs - C 4= C 3 - C 2- Cl - H I I I I I OH OH OH H OH Pent-2ene-l , 5-diol or2-Pentcne-l, 5-diol Pent -3-ene- l, 5-diol or 3- pcntcne-1 , 5-diol (Correct) (Wrong) BASICS OF ORGANIC CHEMISTRY 11 (d) Numerical prefixes for complex entities such as substituted substitucnts are obtained by adding the ending -'kis' to the numerical term. However, the e nding '-kis' is not used with 'mono'. For example, in case of a compound having four identical groups, the term used is tetrakis and for five si1nilar groups, it is pentakis. As exceptions, 'bis' is used for the number 2 and 'tris' for 3. For example, CH2CH2CI I CH 3 - CH 2 - CH 2 - CH 2 - C - CH 2 - CH 2 - CH 3 I CH2CI-I2Cl 4, 4-Bis (2-chloro ethyl) octane (correct) 4, 4-Di (2-Chloroethyl) octane (incorrect) ( e) When a su bstituent is itself substituted, all the subsidiary substituents are represented by suitable prefixes. The substituent bearing the subsidiary substituents is regarded as a parent substituent. The name of the whole substituent is obtained by using the rules applicable to c01npounds. However, while na111ing substituents, no characteristic group is expressed as a suffix and the point of attachment of the c0111plex substih1ent has the lovvest permissible locant. Names of s0111e complex substituents are as follows : - CI-I 2 OH - CH 2 CHO -CH 2 COOH - CH 2 - C =N Hydroxy methyl Formyl methyl Carboxy methyl Cyano methyl - - Table 1.6 Suffixes and Prefixes for some important characteristic groups Class Formula Suffix Prefix Carboxylic acids - COOH - oic acid Carboxy Esters - COOR R... oate Alkoxycarbony l or carboalkoxy Acid chloride - COX (X =F, Cl, Br, I) Carbonyl ha lide Halofonnyl Amides - CONH2 amide Carbamoyl Aldehydes - CHO - al Fo1myl Nitriles - C = N - nitrile Cyano Ketones >C= O - one Oxo or keto Alcohols, Phenol - OH - ol Hydroxy Amines - NH2 - amme An1ino Ethers - OR - Alkoxy Thioalcohol - SH Thiol 1\11 ercapto Imine = NH. - 1m111e Imino  12 MODERN COLL EGE CHEMISTRY (C.B.C.S. SEV. II) HYBRI DISATION OF BOND-ORBITALS AND SHAPES O f ORGANIC MOLECULES Ilybridisatio.. ,· h ·brid is the product of l'xo different species ha,. a- n in ~a~bon atoms. Literally speakm~.. :. ns the intermixing of two or n1ore ~~.b-_charactenst1cs.. of both ~e parents. "Hybnd1sation n;~s ,Yhich result in their r itals of an a torn haY1ncr nearlv the same ener O b ·b · d b "t l ~ earrangcn1ent to forn1 altooether diff:rent b~t ide ntical orbitals, called ~. n or I a s.. h !he hybrid orbitals fom1:d are exactlv similar in shape and energy" hich acnially c en.11cal bonding. The hybrid orbitals. are different in shape and energy from the parent or Ita s. in take/a7 T_he phenon1enon of hybridisation inYolYes the following 51eps :.. /1) ";romotion or excitation. The electronic configuration of carbon aton1 in the ground state ts 1~ 2s- '2p_J 2P_J '2pJ. Since a carbon atom combines ;.ith four monoYalent aton: 5 four half filled orbitals are needed. It is explained bY savin2: that one of the electrons from ? 5 orb1tal ~ets promoted to 2p-:. orbital. Thus: the electronic ;onfigu;;tion of carbon atom in the excited slate is as folio,, s : ls2 2s 1 2p_J 2p1~ 2p]. (ii) Reorientation. In this step: the orbitals ~ontaining four unpaired electro~s rearrange :-hems.elYes into an altogether ne\Y order and give rise to the hYbrid orbitals \Yhich are identical among lhemselve~. Conditions of hybridisati;n. Follo\Yin2: a;e the conditions for hybridisation. (i) The orbitals of one and the same at~11 participate in hybridisation. Only the orbitals and not the electrons get hybridised. (ii) The ener2:v --- difference beh\·een the hvbridisino...=: orbitals should be small. Characteristics of hYbridisation.., Followino are the characteristics of hYbridisation ~... (i) The number of hybrid orobitals formed is equal to the number of hybridising orbitals. (ii) The hybrid orbitals are all equiYalent in shape and energy. (iii) A hybrid orbital \vhich is to take pa.rt in bond formation must contai.n one electroo in it. (ii-) Due to the electronic repulsions beh,·een the hybrid orbitals: they tend to rem3in at the ma-xi1num distance apart. Types of hybridisation. Hybridisation and bond formation of the carbon aton1s t 3.k.e pb.(e in three \Yavs.,. : (i) sp 3 hybridisation. (ii) sp2 hybridisation and (iii) sp hybridisation. (i) sp3 hybridisation. \Vhen 2s, '2px, 'lpy and V z 2pz orbitals of the carbon atom get mixed up. four ne,v orbitals called sp3 hybrid orbitals result. These orbitals make an angle of 109°28' with one another. The orbitals arrange themselves in space in the form o + ~~ x of regular te trahedron. Each sp 3 hybrid orbital contai~s 25% s-character and 75% p-character. sp3 hybridisation involved in th~ formation ~f methane 2s (CH4 ) molecule is explained m the follo\vmg steps : sp 3 HY6R D :Z.\TIOi\ (a) Hybridisation. In the excited carb_on atom~ the orbitals are for a 1nomenl as shown (Fig. 1.1). 0 109-28 The four half filled orbitals of carbon, before takinu part in bond formation 1nix up and distribute O t h en1se 1\ es. eq uallv; to oo-ive four new. and. absolutdv~3.d. t _.t ls Each of the four.orbitals is. called 1 ent1ca 01 6 1 a.~ sp. d h onienon of their fo1111nt1on b called orbitals an t e P11en Fig. 1.1 Four sp3 hybrid orbitals. sp3 hybridisation. l  BASICS OF ORGANIC CHEMISTRY 13 (b) ~dop~ing new orientation. The sp3 hybrid orbitals arrange themselves about the carbon nucleus, dir~ctmg t~wards the four corners of the regular tetrahedron. In this position, the hybrid orbitals are at a maxunun1 distance apart and the angle between the1n is 109° 28' (see Fig. 1. 1). B~nd formation ,vith hydrogen. Now the four hydrogen atoms with ls orbital each, approach the excited carbon aton1s. The overlapping takes place between the hybrid sp3 orbitals of carbon and ls orbital of H-aton1. The four 1nolecular orbitals (sp 3 - s) or bonds are formed. The orbital structure of 111ethane is shown in Fig 1.2. t In ethane (C2H6), two carbon ato1ns are bonded to ea~h other by a sigtna bond fanned by the overlapping of two sp? hybrid orbitals of carbon, each directed towards the nucleus of the other carbon atom. The ren1ain ing sp3 orbitals on the two carbon aton1s· are used in fonning sign1a bonds with hydrogen atoms. "'- · 1s In ethane, C-C bond length is 1.54 A and C-I-I bond length in 1.09 A. (ii) sp 2 hybridisation. In this type, the excited atom involves hybridisation of the 2s orbitals with two 2p orbitals while the third 2p 1s orbital re1nains unchanged. The new sp2 hybrid orbitals result. sp2 hybrid Fig. 1.2 Orbital structure of Methane. orbitals 1nake an angle of 120° with one another. These orbitals are directed in space towards the corners_of a regular y z 2 triangle. Each sp orbital has 33% s-character and 66% p-character. sp2 hybridisation involved in the formation of ethene (C2H 4 ) 1nolecule is explained in I the following steps: (a) Hybridisation. In the excited carbon atom, the orbitals are for a m01nent as shown : 2px 2py j 2p 2 The 2s, 2px and 2py orbitals mix up and form i sp HYBRIDIZATION 2 LEFT UNHYBRID IZED 2pz three exactly identical orbitals. The fourth orbital 2pz, remains as such (Fig. 1.3 ).. ( b) Orientation. The hybrid orbitals direct !he1n- THREE sp HYBRID 2 selves towards the three corners of a regular triangle ORBITALS AND THE 2pz UNHYBRIDIZED d the P- orbital perpendicular to the planar triangle. ORBITAL ;111s, the-hybrid orbitals stretch to m_1 angle of 1200 in I pane. The fowih p.,,- orbital is perpendicular to the plane. Fig. 1.3 Orientations of sp2 hybrid orbitals. Bond formation in Ethene. On~ sp2 orbital to 1s 1s each carbon atom overlaps to fonn a s1g1na bond. The p- BOND · · g two s:n2 rema1nm :r orbitals of two carbon atoms overlap with the s- orbitals of hydrogen ato1ns. The pure p-orbitals left on the carbot~ ato1ns ( one on each C-atom) overlap laterally formmg n-bond. The sum of the sp2 _ sp2 (a-bond) and,1/J - p) (n bond~ betw~en two carbon atoms is called the double bond. 1 he orbital s- BOND picture of ethene molecul~ is shown in Fig 1.4 ln 1s 1s ethene, C = C bond length 1s 1.34 A and C - I I bond Fig. 1.4 Formation of Ethe_ne involves a f length is 1.09 A. sigma-bond and a p1- bond. 14 MODER N COLLEGE CHEM ISTRY (C.B.C.S. SEM. II) (iii) sp h) bridisation. In this type, the excited 1 z carbon aton-1 involves hybridisation of 2s orbital with one 2p orbita l, while the other two 2p orbitals remain unchanged. The new sp orbitals result. sp orbitals make an angle of 180° \ivith each other. Each sp orbital has 50% s-character and 50% p-charactcr. An sp hybridised carbon atom is also called a diagonal carbon atoin. The shape of a diaoonal carbon atom is 2Px J L 2 Pv 2P1 J I.mear. sp hybridisation involvedo in the formation of acetylene molecule is explained in the following steps. i LL~2:_:s:__-,-___ _ __ sp HYBRIDIZATION ---------- LEFT UNHYBRIDIZED 2Py (a) flybridisation. The 2s and one 2p orbitals n1ix up to form t\1/o new exactly identical hybrid orbitals. The other two 2pv and 2p= orbitals remain as they are: · TWO sp HYBR ID ORBITALS (b) Orientation. The hybrid sp orbitals direct AND THE TWO UNHYBRIDI ZE D p-ORBITALS thcn1selves in opposite directions whereas the Py and P= orbitals remain in their original positions (Fig. l.5). (c) Bond formation. One of the sp orbitals of Fig. 1.5 Orientation of sp hybrid orbitals. one carbon ato1n overlaps with sp n BOND orbital of the other carbon to form 2 a sign1a (a) bond. The remaining a 8 ~=1==;::= -:::. 2pz sp- > sp. I fence, we say that carbon-carbon 1c bond kngth 111 the compounds decrease in the order. 3 3 ? 2 sp C - esp > stJ -e - esp > sp e - C:,p 15-J pm I ,JS pm 138 pm In e = C and C = C bonds, the bond length decreases due to addition sidewi se overlap of two p-orbi tals. Due to sidr\\'ise overlapping, the tvvo carbon atoms come closer. For exampl e : - C-e-> -C=C -> -e=C- 15-l pm 134 pm 120 pm The bond length of C - H bond also decreases when the state of hybridisation of carbon atom changes from sp 3 ➔ sp2 ➔ sp. sp3C - H > sp2C - H > sp C - I-I 110 pm 107 pm 106 pm 2. Bond angle. The bond angles in a compound depends upon (i) the type of hybridisation and (ii) the number of shared and lone pairs of electrons around the central ato1n in a molecule. The magnitude of repulsive interactions has the order : Ip - Ip > Ip - bp > bp - bp. Clearly, as the number of lone pairs increases, distortion from the regular geometry occurs and the bond angles decrease from their expected values. For example, consider n1ethane (CH 4 ), ammonia (NH 3 ) and ,vater (H 2 O: ) 1nolecules.. Jn all these compoun~~ the central ato1ns (C, N, 0) are sp 3 hybridised. But CH-i has no lone pair, NH 3 has one and H 2 0 : has two lone pairs. Thus, Ip - bp and Ip - Ip interactions increase from Nl-13 to H20. Thus, bond angle decreases fron1 el-14 ( 109°28 ') to NH 3(107°) to water (104.5°) Also deviations are found fro111 ideal values of 120° in compounds containino 0 sp 2 hybridised carbon. For example, in ethene,. : (i) I-I - C - C bond angle is 121. 7° whereas that (ii) in H - e- H bonding is 1I 6.6°. 3. Bond Energy. It may be noted that as the percentage o f s-character increases from sp sp3 ➔ 2 ➔ sp, the size of the hybrid orbital decreases. Due to this, bond length decreases and bond energy mcreases. For example, consider carbon-carbon bonds : Bond energy : 3 3 2. 2 sp C - esp < sp C - esp < sp C - c ·''P 347 kl mol- l 383 kJ mol - l 433 kJ mol- t Also consider carbon-hydrogen bonds. 3 Bond energy. Jp C _ J[ < sp 2e _ Il < sp c _lI I 1- I 4 16 kJ mo )- 4t13 kl mol - 1 507 Id mo 1 6 MODERN COLLEGE CHEMISTRY (C.B.C.S. SEM. 11) 1 ELECTRON DISPLACEMENT IN A MOLECULE_(: JMP). erall observed in organic molecules. There are four types of electron displacement mechantSms gen Y (i) Inductive effect (ii) Electromeric effect, ' (iii) Mesomeric or Resonance effect, and (iv) Hyperconjugation. -~___::__----;----- A. Inductive Effect. In case of C - H covalent o- bond, the two atoms involve equal sharing of the pair of bonding electrons and the shared pair of electrons is assumed to be symmetrical between carbon and hydrogen atoms. It is due to their very small electronegativity difference. The unequal sharing of the bonding electrons of a covalent bond gives rise to a Fig 1.7 Inductive effect. dipole in t~e molecule. If hydrogen atom is replaced by.. \ atom or group X (more electronegative than carbon), then the shared patr of e~ectrons will ?e l l displaced more towards X. The more electronegative group X acquires some negative charge while j the carbon atom acquires the same amount of positive charge. On the other hand, if an atom or group Y has lower electronegativity than carbon (then the shared pair of electrons of the bond (C - Y) will be displaced more towards the carbon atom (Fig. 1.7). The carbon atom directly attached to the group Y acquires some partial negative charge and group Y acquires partial positive charge. This polarization ~ a permanent state of the molecule and is called the inductive effect. The partial ionic character of a covalent bond due to inductive effect is shown by the symbol ➔ , the arrow head pointing towards the more electronegative atom. The partial positive and negative charges developed due to inductive effect are shown by o+ and o- respectively as represented below. H I I -C :H; -C-H ( symmetrical between C and H). I I H I a+\ 0- -C:X; -c ➔ x (X is more electronegative, - I effect) I I I o-\ a+ -C:Y; -C~Y (Y is electropositive, +l effect) I I In a carbon chain, slightly positively charged C 1 attracts tl.. le e 1ectrons be. h c 1 and c2. As a result, a sma11 pos1t1ve charge is developed on C H 2 tng s ared between charge will be less than that on C1.. · owever, the magnitude of this Similarly, the pair of electrons shared between c 2 and C 3.11. more electronegative group X. wi get displaced in the direction of III II I o+ o+ o+ I II IU 0- c:5 - == o,.. + o_ ,..,,. + ~ U+ - C3 ➔ Ci ·-+ Ci ➔ X L BASICS OF ORGANIC CHEMISTRY 17 Similarly, the magnitude of the negative charge goes ~n decreasing as we move away from the carbon directly attached to the group Y. I II III c5+ = c5- + c5- + c5 - C3 ~ C2 ~ C1 ~ Y The influence of a permanent polarization is generally considered almost negligible beyond two carbon atoms from the carbon atom directly attached to X or Y. The permanent displacement of the shared electron pairs in a carbon chain towards the more electronegative group is known as the inductive effect. Inducti v~ effect or simply I effect "Th of two types. (i) + I effect and (ii) - I effect. - I effect. If a group is more electronegative than hydrogen, then the group withdraws electrons from the C-chain towards itself (as compared with hydrogen). It is said to have - I effect. The various groups with - I effect compared to that of hydrogen are given in decreasing order: + + NR3, - NH3, - N02, - S0 2 R, - CN, - COOH, - F, - Cl, - Br, - I, - OC6Hs, - COOR, - OR, - OH, - C = CH, - C6Hs, - CH= CH2 +I effect. If a group attached to the carbon is less electronegative than hydrogen, it donates electrons to the C-chain. It is then said to have +I effect. The various groups with +I effect compared to that of hydrogen are given below in decreasing order: - 0, COO, C (CH3)J, - CH(CH3)i, - CH2CH3, - CH3 Charactet:,istics.-. of inductive effect. Inductive effect. causes bond polarization and exhibits the following prominent characteristics : 1. There is virtuaIJy no displacement of electrons in a C - H bond. It is because the electronegativities of both the atoms are very nearly the same (C = 2.5, H = 2.1 ), Hence, the C - H bond is nearly covalent. 2. Inductive effect always takes place between a carbon atom and an electronegative atom or group sharing only a pair of electrons. In other words, inductive effect involves only a single bond. 3. Inductive effect is permanent and irreversible. 4. The electron displacements progressively decreases along a C- chain as the distance from the carbon atom directly attached to the substituent increases. o'+ > o"+ > c5"'+ and so on. It is almost negligible after two carbon atoms from the carbo~ atom directly attached to the more electronegative element. 5. The displaced electrons do not leave their orbital. Only the orbital is defonned a bit and this causes polarization. 6. Different groups cause polarity to different extents.  18 MODERN COLLEGE CHEMISTRY (C.B.C.S. SEM. II) Applications 0 f I nd uctive effect. Following are a s:: 1ew examp Jes in which inductive effect exp lams the val f d· ue o 1pole moment in compounds:. 1. It explains the polarity of covalent bonds in organic compouo d s· (i) The unequal :::n_g of t~e bonding electrons of a covalent bond gives to the molec~le a charge sep_ar_ation reflected a1 mduct1ve_ effect and hence a permanent dipole. The difference m elcctronegat1v1t1es of ca~bo_n 1d h~drogen IS very small and C - C and C - H bonds may be considered as purely covalent. fh1s explains why the hydrocarbons such as Methane, Ethane, Propane, etc., have no d ipole moment.. (ii) The s ubstitution of a hydrogen atom in methane by chlorine to form methyl chloride produces a dipole moment in the molecule and explains its polar character. H H Table 1.7 Bond moments I I Bond Dipolemoment H- C - H I-I - C ➔ Cl (µ in D) I I H H H-F 1.90 (µ = 0) (µ = 1.87D) H-Cl 1.03 NOTE Dipolemoment is measured in Debye units designated as D. H-Br 0.78 H-I 0.38 (iii) Though there are four C - Cl bonds in carbon tetrachloride C-F 1.51 it has no dipolemoment as the vectorial sum of the individual C - Cl C-CI 1.56 bond moments is zero. C-Br 1.48 Cl C-I 1.29 i C-N * 0.40 Cl~ C ~ Cl (µ = 0) C-O 0.86 J, Cl (iv) Highly electronegative atom or group (- I- effect) increases the polarity of the molecule and hence , the value of dipole moment increases. For example, a few bond dipole moments are given in table 1.7 : c( : , ,, Q c1 :, I This explains why HF(µ = 1.90 D) is more polar Cl c1 1 than HCI (µ = I.03 D) and water (µ = 1.84 D) is MORE POLAR LESS POLAR LEAST POLAR 2 30 more polar than ammonia (µ = 1.40 D). The - I effect (u ~ · D) (u=1.48 D) (u=ZERO) 1 due to oxygen atom is more compared to nitrogen atom. Fig. ·8 Dipole moment in different dichloro- benzenes. (v) Similarly, CH3 OH is more polar compared to C2H5QH The rea h ethyl group is more compared to methyl group. · son ts t at +I effect due to (vi) Let us see the values of dipole moment in ortho meta and d" ' para- tchloro be A · is a vector quantity, the value of dipole moment of para isomer is zero Th nzenes. s it 1 in one direction cancels due to - I effect of second CI atom in the · el - effe~t due to Cl atom exact y opposite direction. Dipolemoment (µ) = ✓ µf + µf + 2µf cos 0 where µ 1 == c - Cl bond moment. 0 = angle between two c _ CI bond s · · compound. the value of net dipolemoment o f t he 1somenc · More th e ang Ie, lesser is 2 Inductive Effect explains the relative strength of acid A.d · _ An acid is said to be strong' if it ndency to give up a proton. th e te h:s· great ct tenden may be defined.;.;.: s_n_·en_gt;;___h~-d;--____ as..----- - - - - - - - - - cy to onate a proton. , "'' t t t l 1111F BASICS OF ORGANIC CHEMISTRY 19 j The inductive effect due to the groups close to the carboxyl group has profound effect on the acid strength. For comparing the relative strength of some substituted acids their acidity constants have been detem1ined. It has been found that : ' (i) the presence of electron withdrawing groups attached to - COOi-I group make the acid stronger. Greater the electronegativity of the atom or group attached (- I effect); stronger is the acid, r 1' I (ii) the presence of electron donating groups attached to - COOH group make the acid weaker. I Greater the electropositive character (+ I effect), weaker the acid. Examples. (a) Chloroacetic acid is stronger than acetic acid. In chloroacetic acid, the chlorine atom causes - I effect and makes the release of proton easier. 0 0 II II (a) CH 3 - C - OH Cl ~ CH 2 ~ C - OH Acetic acid Chloroacetic acid (stronger) Similarly, dichloroacetic acid and trichloroacetic acid are increasingly stronger acids than monochloroacetic acid since the electron attracting power of two or three such halogen atoms is greater than that of one halogen. 0 0 II II (b) Cl - CI-1 2 - C - OH F.CH2 - C -OH Chloroacetic acid (weaker) Fluoroacetic acid (stronger) It is due to the fact that Fluorine atom is more electronegative than chlorine atom : 0 0 II II (c) Cl - CH 2 - CH 2 - C - OH CICI-12 - C - OH Chloropropanoic acid (weaker) Cbloroacetic acid (st~onger) In chloropropionic acid, the - I effect due to chlorine atom decreases with increase in distance from - COOH group. Many other groups besides halogens exhibit an acid enhancing (electron withdrawing) or -I effect. Among these are nitro (-NO2), methoxy (CH3O), carbonyl [> C = O] and nitrite (C = N) (d) A comparison of acidities of formic and acetic acids furnishes an illustration of the operation of inductive effect. 0 0 II II H- C-OH CH 3 ➔ C ➔ O-H Formic acid (stronger) Acetic acid (Weaker) Alkyl groups_ methyl, ethyl, isopropyl, etc., _are the sub~titu~nt~ that are acid w~a~ening relative to hyd rogen. C ompare tile K a values of formic acid and acetic acid.. m Table 1.7. This ts because. the alkyl group release electrons to the carboxyl group and thus exh1b1t ~ +I e_ffect. The magnitude of the chemical effects of alkyl groups does not app~ar _to chan~e gr~atl~ tn gomg from methyl to ethyl to propyl (compare the Ka values of acetic/prop1onic, butyric acids m Table 1.8). !, NOTE Higher the Ka values, (Dissociation constant of acid) stronger is the acid. I: r 20 MODERN COLLEGE CHEMISTRY (C.8.C.S. SEM. 11) Table 1.8 Acidity constants (Ka of aliphatic carboxylic acids) Acid K*a (Value x 1o-5) Acid Ka (Value x 103) HCOOH 24 CICH2COOH 155 CH3COOH 1.85 Cl2CHCOOH 5140 CH3CH2COOH 1.52 Cl3C.COOH 24000 CH3CH2CH2C02H I.SO CH 3 CH(C l)CH2C02H 8.9 3. Inductive effect also explains the relative strengths of bases. Base strength may be defined as the tendency to accept a proton. A base is said to be strong if it has a greater tendency to accept a proton. It should be noted that a base in Lewis theory is the same as in the Lowry-Bronsted theory ; both are substances with an available pair of electrons. Since base strength of the usual organic compounds depends upon the availability of the unshared pair of electrons on the nitrogen atom, the presence of electron donating groups (causing +I effect) make the electron pair more available on the nitrogen atom and hence increase the basic strength. (a) CH3 - NH2 CH CH - NH 3 2 2 Methyl amine (Weaker) Ethyl amine (stronger) +I effect due to C2 H5-group is more compared to - CH3 group. CH 3 (b) ) NH CH 3 - CH 2 - NH 2 CH 3 Dimethyl amine (stronger) Ethyl amine (weaker) The +I effect due two methyl groups is more compared to an ethyl group. (c) @ Aniline (weaker) ~ CH3 p- toludine (stronger) The +I effect of methyl group causes the increase in basic strength. In the same way, we say that the presence of electron withdrawing group causing _ I effect, makes the base weaker. (d) Aniline (stronger) m-N1troaniline (weaker) m-Nitroani l.ine is weaker due to the presence of - I effect on th ·. e aromatic rmg. Influence on D1polemoment Value : The inductive effect cause th I · · h' I · h th e po Ianty, I · s e po ansat1on of bond. More 1g 1er 1st e va ue of d1polemoment. In case of diatomic I I.. f. mo ecu es, more the difference m the electronegat1v1t1es o t\vo atoms, more 1s the polarity. For example ·d d. · _ _ , cons1 er I1y rogen halide molecule. Greater the value of Ka, stronger 1s the acid. In case of aromatic acids th makes the acid more stronger. The presence of -I group at ortho or met · e presence of -I group at ortho position of strength as ortho > para > meta. For example, ortho nitrobenzoic ~o.r para to - COOH group shows the order acid which in turn is slightly more acidic than meta nitrobenzoic aci;ci IS much stronger than para nitrobenzoic  BASICS OF ORGANIC CHEMISTRY Molecule Dipolemoment 21 1 HF 1.90 D (D stands for De bye) HCI 1.03 D HBr 0.78 D As dipolemoment is a vector quantity, the dipolemoment of polyatamic molecule depends upon the geometry of the molecule. An unsymmetrical molecule is polar while a molecule with symmetrical molecule is non-polar. B. Electromeric Effect. In case of compounds having a double or triple bond, the pair of n-electrons gets completely transferred to the more electronegative element under the influence of I an attacking reagent. For example, when CN- attacks the carbon of the carbonyl group, electron -pair of the.n-bond gets completely transferred to the oxygen atom as shown below : I, I' 0 0~-tdn-d "' /0 CN + /C=O ----. /C "'- CN fomplete transfer of a shared pair of electrons of a multiple bond to one of the bonded atoms under the influence of the attacking reagent is known as the electromeric effect. The electromeric effect is indicated with the help of a curved arrow in the direction of the atom which acquires the pair of electrons. The electron displacement can take place in any direction as shown (in A or B) below, provided the molecule is non-polar. If the molecule already contains a polar bond, the displacement of pi-electrons takes place in one direction only. Electromeric effect is of two types : (i) + E-effect, and (ii) - E-effect. (i) + E-effect. When the transfer of n-electrons takes place towards the attacking reagent (clectrophile), then th~ effect is called + E effect An Example is : ~ + Step(i) 0 = H2C CH2 + H - - - -. CH2- CH3 Consider the addition of HBr to prcpene. Br r---w 0 Br- cH3CH=CH--- CH3-CH-CH3 ~ C H3-CH- CH3 I Br (ii) _ E-effect. When the transfer of n-electrons take~ place away from the attacking reagent (nucleophile), the effect is called - E effect. An example 1s : '--c=o +/cN----. '--c-6 /~ - /1 CN Consider the addition of HCN to acetone CH3 '-. ~ _ step (i) CH3 '-... _ step (ii) CH3'-. C=O+CN - - ~ /C-Q: H+ C-OH CH3 / tr / CH3 I CH3/I ~ CN CN It may be noted that if inductive effect (I effect) and electromeric effect (E effect) occur together in a molecule, they may be reinforcing or opposing each other. In case there are opposing, then E-effect dominates. ~I 3  MODERN COLLEGE CHEMISTRY (C.B.C.S. SEM,...!!) 22. lay in the presence of an attacking a t and comes mto p Electromeric effect is a temporary ei iec. withdrawn. th reagent and ceases as soon as e a ac tt king reagent ,s. k' d the attack of the attac mg reagent. f h lectron pair an b I Important. In this effect, the shift o t e n-e.. fi d In case of car ony compounds. I ec1es ts onne · · · I occur simultaneouly and at no stage d ,po ar sp. The n-electron shift 1s comp eted only (> C = 0) we donot get c+ - C- at any stage of th e reactionh. arbon atom of the carbonyl group. h ·1 ) approaches t e c w h en the attacking reagent (say nuc Ieop 1 e t ated compounds, i.e., alkenes...... tions : The unsa ur.. Apphcat10ns. (i) Electroph1hc addition reac _. bond in presence of electroph1les like and alkynes involves the polarisation of carbon-carbon mu 1tip 1e H+ involving + E effect... b yl compounds involve. dd.. reactions m car on s (ii) Nucleopbilic addition reactions. The a ,t,on h'l through -E effect.. the polarisation of carbon-oxygen bond in presence of nuc Ieop I e of Inductive and Electromeric effect "J~L ~ Inductive effect Electromeric effect -~-__________________-F~T~a~b~le~-~~1-~9~~C~o=m=p=a=n=s=on======i==========J~~~~~~~~~~~~~~!~~~~~;~~~j It involves the pennanent displacement of an It involves only a temporary displacement of a pair electron pair of a molecule. of n-electrons. Presence of an attacking reagent is not essential. Presence of an attacking reagent is essential. Presence of a multiple bond is not essential but Presence of a multiple bond is essential. polarity of bond is essential. The displaced electron pair does not leave its The electron pair which gets transfe,,-ed completely molecular orbital. There is only a dist01tion in the leaves the molecular orbital and takes up a new shape of the molecular orbital. position. There is a partial charge separation. There is a complete charge separation. No ions are formed. Ions are formed. C. Resonance. In case of certain compounds, more than one structures can be written. However, none of them is able to explain all the known properties of the compound. For example, consider two electronic structures of benzene. Each structure contains three double bonds. lt should give addition reactions 00 I II easily. However, it gives substitution reactions more easily. Moreover, the bond length of three (C - C) single bonds should be 1.54A. The bond length of three (C = C) double bonds should be 1.34 A each. Actually, the bondlength of all the six carbon-carbon bonds in case of benzene is the same (1.39A). This value of bond lengths lies inbetween the value for (C - C) single bonds and (C = C) double bonds. Thus, none of the structures, I and TI, is consistent with the known properties of benzene. In such cases _where more than one structure can be written for a compound but none of them is able to explain all the knol1!!!_properties of the compound completely, the aifferent structures written are known as the resonating or contributing or canonical structures. *This phenomenon is known as resonance or mesomerism. The actual strncture known as the resonance hybrid lies somewhere inbetween these structures. However, we have got no physical means to represent that structure on the paper. !he resonance h~brid may be represented by 0-0 putting double headed arrows mbetween the vanous contributing structures as illustrated below for benzene. j BASICS OF ORGANIC CHEMISTRY 23. The resonance hybrid neither oscillates between the contributing structures nor consists of a mixture of the contributing forms. It is an entirely new and individual structure. Conditions for Resonance: Following are ·the Conditions for writing Resonating structures: 1. Same atomic skeleton. The different contributing structures should have the same position of the constituent atoms, they may have different electronic arrangements. 2. The structure having maximum number of bonds is most stable. 3. Same number of unpaired electrons. The number of unpaired electrons niust be the same in each contributing structure. We should not draw structures in a manner that one structure may not contain even a single unpaired electron and another contains two unpaired electrons. 4. Same energy. The conh·ibuting structures should have nearly the same energy. 5. Close~ bond lengths. The bond lengths and bond angles should be close to the real structure. 6. Those structures in which negative charge resides on the most electronegative atom and positive charge on the most electropositive atom will be closer to the true structure. 7. The contributing structures should be so written that unlike charges reside on neighbouring atoms. The contributing structures involving charge separation have little contribution to resonance hybrid. 8. The greater the number of covalent bonds, -greater is the contribution of that resonating stmcture. 9. Planarity unchanged. The compounds exhibiting resonance must be planar in nature. Characteristics of Resonance hybrid. Some of the characteristics of resonance hybrid are: 1. Stability. A resonance hybrid is always more stable than any of the contributing sh·uctures. Greater the number of contributing structures for a molecule, greater will be the stability of the resonance hybrid. All the resonating stmctures may not contribute equally towards the hybrid. In case they are equivalent, the resonance hybrid is more stable, (e.g., Benzene) than when they are non-equivalent. 2. Resonance energy. Benzene is found to be 35 kcal/mole more stable than any of its contributing structures. It is supported experimentally on the basis of heats of combustion and heats of hydrogenation values. This e~tra stabilisation due to resonance is known as resonance energy. The resonance energy gives a measure of the stability of the resonance hybrid. Greater its value, greater is the stability of the hybrid. 3. Bond lengths. In a resonance hybrid, the bond lengths are always different from those of any of the contributing structures. For example, the (C - C) bond length in case of benzene is 1.39 A, which is different from the bond length of 1.54 A for (C - C) single bonds and 1.34 A for (C = C) double bonds. Some other examples of molecules which exhibit resonance are.... e.. e e.. e :o=c = o: ~ :o.. - c =o : ~ :o=c-o:...... e 0 0.. e :N ==N: ~ :N = N ~ N= N: e (±) (!).. e :c=o:.--. :c!::'::::::o:.-► :c-o:  - - - - - n.,.zz- iai.~. - 1:111- -- - 24 MODERN COLLEGE CHEMISTRY (C.B.C.S. SE~ · EXAMPLE 1. The correct stability order of the fiO II owmg resonance structure is : + - + + + H 2 C-N=N H2C =N =N H 2 C = N == N I-l2C - N = N JV I IT HI (a) (I) > (II) > (IV) > III (b) (J) > (111) > (IJ) > (IV) (c) (11) > (I) > (llI) > (IV) (d) (UI) > (I) > (IV) > (ll) Ans. (b) (1) is most stable due to maximum number of bonds in it. Also negati_ve charge resides on electronegative nitrogen atom. (III) has more number of covalent boll d S th an 111 (II) and (N). Also (II) is more stable than (IV) as negative charge is present on nitrogen atom. Resonance effect or Mesomeric effect. In case of compounds having conjugated n-systems (having alternate sigma and pi-bonds), electrnns can flow from one part of th e syS t em to the other due to resonance. This flow of electrons from one part of conjugated n-system to the other, caused by phenomenon o resonance is cal ed resonance effect or mesomeric effect. This is of two types : (i) +R or +M effect, and (ii) - R or - M effect. +M effect. If a group donates electrons to the conjugated n-system through resonance, then it is said to have +R or +M effect. For example, Cl atom, OH group or any other group havino lone pair of electrons have got +R or +M effect because they donate electrons to the conjt~gated n - systems as shown : Some other examples of the atoms or groups which cause +R or +M effect are : F, Br, I, NH , 2 SH and OR.. 6-◄- ~~ (> e.()"e _◄_....., Q - M Effect. Groups which withdraw electrons towards themselves tl,i·ot1g I1 resonance are sa1'd to have - R or - M effect, such as - N(0 group. 0 ~ e e e ~o /o /o 0 ;J::,o ~o ):::o < v-®tu-llJjJ-6@ 0 El) Some other examples of the groups having - R or _ M ~ e11ect are : EB - NR3, - COOH, CN, CHO, COR and S02 0H. l L BASICS OF ORGANIC CHEMISTRY 25 Applications. 1. Resonance explains the relative acidic strength. 1. Phenols. The acidic nature of phenols, cresols etc. can be explained on the basis of the resonance concept. '. For example, phenol is a resonance hybrid of the following structures. A. Reactions involving phenolic (- OH) group. 1. Acidic nature. Acidic nature of phenols, I ,, (vrz., phenols, cresols, catechol, resorcinol , etc.) can be explained on the basis of resonance concept. For example, phenol is a resonance hybrid of the following structures... © © ©.. b◄ Qe ► o ◄ 0:6 OH OH OH !OH ►.& ◄ ► :::::-.... I◄ ►6 0 II 111 IV V The above structures are the contributing structures to the resona.n ce hybrid. Due to resonance, - OH group acquires a partial positive charge. Due to the pat1ial positive charge on oxygen atom, it attracts the bond pair of O - H bond more towards itse lf making the release of a proton easier. This explains the acidic character of phenols. Also the phenoxide ion formed by the loss of a proton from phenol is resonance-stabilised. Phenoxide ion is a resonance hybrid of the following structures. 5-& --1)-~,6 -6 I II 0 0 III IV V The phenoxide is much more stable than phenol. It is due to the fact that the last three resonating structures in case of phenol involves charge separation, i.e., they carry both positive and negative charges. However, there are no corresponding structures in case of phenoxide ion. Since energy is needed to separate opposite charges, the last three structures in case of phenol contain more energy. Thus, the phenoxide ion is more stable than phenol and therefore, can be easily formed from phenol by the loss of proton. This explains the acidic character of phenol. On the other hand , alcohols are neutral and their acidity constants are nearly 10- 16 (even less than water). The reason is that the formation of alkoxide ion is not favoured as it is not resonance stabilised. ROH ~ R - 0 :8 + H+ Alcohol (Alkoxide ion) (Not reasonance stablised) It can also be explained by saying that as alkoxide is OH :o:e not resonance stabilised, its formation requires more energy. Phenoxide ion, being more stable is easily formed. Hence phenol is much more acidic than alcohol. Similarly, benzyl alcohol (C 6 H 5CH 2 OH) is also neutral as benzylate ion © PHENOL - ~ © PH E NOXI0E ION + H+ (C 6 H 5CH 2o-) is also not resonance stabilised. ,.- - 26 MODERN COLLEGE CHEMI STRY C.8.C.S. SEM. II ~----------------~~~~~,~~t en round t 1nl :l~~~11~r~c:sc:,n:c:c:o~f~c~lc:'c~·td~n:m~rc~' 1C. l~c;a·is:~ng Effect of nuclcnr substitucnts. It l,us bc. rrl1. ·111 crcnscs the electron cnsity on the groups in the benzene nucleus pushes electrons t O ti ' e nng· ,. ,s.. , the phcnoxidc ·,on. T t1c release of 1 thus destnl'>l 1::;cs. "leasing groups decrcasc t I1e acid oxygen atom intensifies the ncgutivc chnrgc nn d. , - ' , " of electron t l; proton will be less probable. Hence, tho presence strength of phenols. G1 itlidraws el0ctro11 s from tl, o ring , G releases electrons towards the ring,_ t:bilises penoxid e ion. It becomes destabilises phenoxlde ion and hence it s 'die th an th e parent pll enol. ls less acidic compared to parent pl·1enol. more aci X COOl-l ·ld · · ·oup (such as - NO-,, - , - " etc.) On the other hand the presence of electron wit 1 raw mg gt. ,. _. :- t d M. on the benzene nucleu; withdrm,v electrons and thus, the release of proton 18 ac t1t1a c · orcovet, l1le phenoxide ion gets stabilised. The effect of substituents is shown in the fo llowing Table l. l 0. The strength of phenol is in terms of K 0 values. Higher the value of K0 , stronger is the acid. 10 Table 1.10 Dissociation constants of Phenols, K 8 for Phenol = 1.3 X 10-. Phenolic Compound K8 value Phenolic Compounds K 8 value o-Cresol 0.3 X 10- 11 o-N itrophenol 6.8 X 10- 2 m-Cresol 9.8 X 10- l l m-Nitrophenol 5.3 X 10- 2 p-Cresol 6.7 X 10- 11 2,4-Dinitrophenol 1.1 X \0-4 o-ChlorophenoI 7.7 X 10- 11 2,4,6-Trinitrophenol 4.2 X 10- 1 2. Carboxylic acids (Aliphatic). Carboxy lic acids are stronger acids than phenols. The reason is that carboxylate ion is more stable than phonate ion. Carboxylic acid ionises to give carboxylate ion. 0 R-C ~ -- '-- o-H Carboxylic acid Carboxylic acid Both carboxylic acid and carboxylate ion can be written in the follow·111 o- 1·e sonat·mg st rue tures: 0 Co ~.. /9. :.e R- C ◄ ► R C ~-- - ~0 (:_;Q-H ~0-H I (Carboxylic acid) 11 Structure II.111 carboxy 1·1c ac1'd carries. charge separation. Henc ·t l b.. d. energy and is less stable. e, 1 s 1Y 11 structure carrtes more 0 - R-c1e Carboxylic ion n "''0 (Hybrid)  BASICS OF ORGANIC CHEMISTRY 27. In carboxylate ion, negative charge is dispersed on two oxygen atom and both strnctures are eqmvalent. D~e to greater delocalisation, carboxylate ion is more stable. Thus, carboxylic acid releases a proton to give more stable carboxylate ion.. - Ph_eno! is less acidic since phonate ion is less stable than cru·boxylate ion. In phonate ion, negative charge 1s d tspersed only on one oxygen atom. Aro~at_ic acids. Benzoic acid is a stronger acid than acetic acid (aliphatic). The reason is that ben~~ate ton is more stable than acetate ion. The reason is that due to benzene ring, benzoate ion has add1t1onal resonating structures. Benzoate ion Benzoate ion u= Benzoate ion c~·· '-6: ··G ofd Effect of substituents. The presence of electron releasing groups in benzoate ion destabilises etc. the anion and acidic strength falls. Also the presence of electron withdrawing group stabilises the anion and the acidic strength increases. 2. Resonance explains the relative basic strength of aromatic amines. Aniline is a weak base as compared to aliphatic amines and ammonia. In case of aniline, - NH2 group exerts + M effect. Aniline can be written in the following resonating structures... 0 0 ~ (NH2 u- e-A + NH2