introduction of organic chemistry by eyes of ajnish kumar gupta (akg)

Organic Chemistry:Organic Chemistry:Chemistry of carbon & its compounds

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Reason for formation of large number of compoundsReason for formation of large number of compounds

� Atoms combine to form molecules to complete their octet and attain lowerenergy state so as to become more stable.

� Elements on the left hand side of carbon have more tendency to lose electrons.� Elements on the right hand side of carbon have more tendency to gain

electrons.� As carbon is in the middle of the second period so it has equal tendency to

loose or gain 4 electrons. But generally it is seen that carbon neither gains norlooses electrons, instead it shares electrons with other atoms like H, F, Cl, Br,looses electrons, instead it shares electrons with other atoms like H, F, Cl, Br,I, N, O, S, P, etc and forms covalent compounds.

� Due to sharing of electrons with other atoms many different functional groupsare formed resulting in large number of organic compounds.

� Carbon not only shares its electrons with other atoms but it can also share withother carbon atom thereby forming long chain carbon compounds having single,double or triple bond. This property is called catenation.

� Hence position of carbon in the periodic table which includes its size,electronegativity, electron affinity, property of catenation etc is the reasonbehind the formation of large number of organic compounds.


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functional groups� A functional group is an atom or group of atoms in a

molecule that gives the molecule its characteristicchemical properties i.e. they are the action group or reactivesite in a chemical reaction and the remaining hydrocarbonpart remains inert.

� Each functional group shows its characteristic chemical� Each functional group shows its characteristic chemicalreactions.

� Functional group helps in nomenclature of organiccompounds.

� Functional group serves to classify organic compounds intodifferent classes or families i.e. compounds with samefunctional group belong to same class.



� Functional group with ó bond only:C-C/H Alkane, C-X Alkyl halide, C-OH Alcohol, C-O-C Ether, C- NH2Amine, C-SH Thiol, C-S-C Thioether etc.

� Functional group with one ð bond:� Along C=O bond:

CHO Aldehyde, COC Ketone, COX Acid halide, COOH Carboxylic acid,COOC Ester, COOCOC Acid anhydride, CONH2 Amide

Different functional groups

COOC Ester, COOCOC Acid anhydride, CONH2 Amide

� Along C=N bond derieved from aldehyde or ketonesC=N-H Imine, C=N-OH Hydroxyl amine, C=N-NH2 Hydrazene,

C=N-NHPh Phenyl hydrazene etc.� Along C=C bond.

C=C Alkene� Functional group with two ð bond.

C≡C Alkyne, C ≡N Nitrile, N≡C Isonitrile



Reasons for chemical reactions in organic chemistry

� By looking at different functional groups we find that carbon iseither bonded with less electronegative atom or moreelectronegative atom to complete its octet. Due to theseelectronegativity differences some electronic displacement takesplace in the molecule. The effect which creates electronicdisplacement in organic molecules is called as electronic effect.

� These effects may be permanent or temporary.� These effects may be permanent or temporary.� They are of following types:

� Inductive effect.

� Resonance or Mesomeric effect.

� Hyperconjugation.

� Electromeric effect.www.OrganicChemistry.co.in


Something for chemical reactionSomething for chemical reaction� Reactivity of a position in a molecule is determined by the charge density

at that position, i.e. an electron deficient species (electrophilic reagent) willattack preferentially at the site of highest electron density and electron richspecies (nucleophilic reagent) will attack at the site of lowest electron density.

� This charge distribution is ascertained by the examination ofconsequence of electronic factors such as Resonance,Hyperconjugation & Inductive effect.

� Some reactions proceed via formation of intermediate such as cabocation,� Some reactions proceed via formation of intermediate such as cabocation,carbanion, free radical, carbene, nitrene & benzyne while others do not formintermediate but proceeds through transition state (TS).

� But one point is common to both i.e. reactivity of different positions in themolecule are determined only by their relative activation energies. Sinceactivation energy depends on the structures of transition state so generallymore stable intermediate have lower energy of the transition state &consequently the lower is the activation energy leading to that intermediateformation.



Something for chemical reactionSomething for chemical reaction� Organic chemistry is very-very simple subject if you have a good

command over these tools of organic chemistry which are only fewin numbers. So never try to learn the question but try to find theposition of electron rich & electron deficient centre in substrateand reagents using the concept of electronic factors and follow myrule i.e. Ajnish rule.

� Ajnish Rule: During any organic reaction if an� Ajnish Rule: During any organic reaction if anintermediate is formed in the rate determining step thenstability of intermediate will decide the formation ofmajor product. Greater is the stability of intermediate,major is the corresponding product and the stability ofintermediate is generally decided by Resonance,Hyperconjugation & Inductive effect.



Inductive effectInductive effect� When ó bond is formed between two atoms of different electronegativity thenó bond pair electron density is shifted towards more electronegative atom anddue to shifting of electron cloud dipole is created between the two atoms. Dueto this dipole in ó bond, electron density in the chain is also shifted andprogressively moves into the chain. This permanent effect of polarization inthe chain due to a dipole is called as inductive effect.

� Inductive effect is represented as I effect.

� I effect is a permanent effect i.e. it will always be present when carbon is � I effect is a permanent effect i.e. it will always be present when carbon is attached to a more electronegative or less electronegative atom.

� This effect is distance dependent and its strength is generally negligible after 3 carbon atom in the chain.

� I effect is additive in nature i.e. when two groups of similar nature are present in molecule then their effects are added and if two groups of dissimilar nature are present in molecule then their effects are subtracted.

� One point should always be kept in mind that I effect always operate along ó bond. Never think of inductive effect in ð bonds because in ð bond some other effects are operative.



Inductive effectInductive effect� The C � H sigma bond is taken as a reference for I effect. Shifting of

electron density or polarity in C � H sigma bond is considered to be negligibleand I effect of H is taken to be zero.

� I effect is weak effect since the ó bond electrons are strongly held.

� A crossed arrow ( ) is often used to indicate the direction of movement ofelectron. By convention the electrons are displaced in the direction of thearrow. The tail of the arrow (which looks like a plus sign) is electron poor ä+

& the head of arrow is electron rich (ä-).& the head of arrow is electron rich (ä ).

� It is also important to remember that I effect creates only polarity in thesigma bond and not the ionization in the bond.

� Inductive effect is also called as transmission effect.

� In case of organometallic compounds for example lithium alkyl (R-Li) &Grignard reagent (R-MgX) carbon is bonded to the less electronegative metalatom. Due to this electronegativity difference between metal & Carbon,metals are electron donating in nature leading to creation of partial negativecharge over carbon



Inductive effectInductive effect� It is also important to remember that inductive effect is atom�s ability to

polarize the bond i.e. shifting of electrons in a sigma bond in response to theelectronegativity of nearby atoms.

� Types of I effect.� On the basic of direction of electron flow along sigma bond which either

make carbon electron deficient or electron rich in nature. So on the basis of it,inductive effect is of two types.

� � I effect:� � I effect:� The atom or group which withdraws electron density (EWG group) from the

carbon making carbon electron deficient are called as � I groups and effect iscalled as � I effect.

� � I groups create partial positive charge over the carbon & partial negativecharge over more electronegative atom.

� Various groups with decreasing order of � I strength follows:

-NR3+ > -SR2+ > -NH3+ > -NO2 > -SO3H > -CN > -CHO > -COOH > -F > -Cl >-Br > -I > -OR > -OH > C≡CH > -NH2 > -C6H5 > -CH=CH2 > -H



Inductive effectInductive effect� + I effect:� The atom or group which release, repel or donate electron density (EDG

group) to the carbon making carbon electron rich are called as + I groups andeffect is called as + I effect.

� + I groups create partial negative charge over the carbon & partial positivecharge over less electronegative atom.

� Various groups with decreasing order of + I strength follows:� -O- > - COO- > C(CH ) > - CH(CH ) > - CH CH > -CH > �D > - H� -O- > - COO- > C(CH3)3 > - CH(CH3)2 > - CH2CH3 > -CH3 > �D > - H

� Capsule:� If there is any electronegativity difference between two atoms along a sigma

bond inductive effect is always operative.

� In organic chemistry always look for the carbon whether substituent attachto it donate electron to it or withdraws electron from it. Electron donatinggroup of any form is + group for carbon & electron withdrawing group ofany form is � group for carbon.



ResonanceResonance� There are so many organic molecules (neutral molecules, ions, free radicals)

where all the observed properties i.e. physical & chemical properties can�t besatisfactorily explained with the help of a single structural formula i.e. Lewisstructure or valence bond structure.

� In order to satisfactorily explain all the properties of such molecules werepresent the molecule by two or more structures just by shifting of p orbitalelectrons or delocalization of electrons without changing relative positionsof the atomic nuclei. These structures are known as resonating structure,canonical structure, contributing structure or resonance unperturbedstructures and phenomenon is called as resonance.

� The resonating structures are only the hypothetical structures and the realstructure lies in between them. The resonating structures contribute to the realstructure which is called as Resonance hybrid. The resonance hybrid is morestable than any of the resonating structures.

� Compound showing resonance is not a mixture of some molecules having thestructures shown by one canonical form and some having structures shown byanother but the compound showing resonance have the same structure allthe time www.OrganicChemistry.co.in

General cases of resonance1. A=B-C=D 2. A=B − C+3. A=B-C¯ 4. A=B − C˚5. A=B − C˚˚ 6. A=B − C≡D7. A ¯ − B 8. A ˚˚ − B9. A ≡ B − C ≡ D 10. A ≡ B − C +11. A ≡ B − C ¯ 12. A ≡ B − C ˚13. A ≡ B − C ˚˚

Capsules:1. A=B −C+/ ¯/ ˚/ ˚˚/=/ ≡2. A ¯/ ˚˚ − B3. A ≡ B − C+/ ¯/ ˚/ ˚˚/ ≡



Points to draw resonating structures1. Resonating structures differs only in placement of their pi

electrons or non bonding electrons i.e. sigma bond skeletonremains same in all resonating structures.

2. The position of each atom in all resonating structures must besame.

3. All the resonating structures must be a proper Lewis structure i.e.elements of 2nd period (C, N, O, F) cannot expand their octet in anyelements of 2 period (C, N, O, F) cannot expand their octet in anyof the resonating structures.

4. All the resonating structures must have same numbers of pairedelectrons if they have pi bonds, positive, negative or lone pair ofelectrons and same numbers of unpaired electron if they are freeradical.

5. If any conjugate position has more than one lone pairs ofelectrons then only one lone pair of electron will take part inconjugation. www.OrganicChemistry.co.in

Points to draw resonating structures6. If any conjugate position has more than one pi bonds then only

one pi bond will take part in conjugation.

7. If any conjugate position has pi bonds along with positive charge, negative charge, odd electron or lone pair of electrons then only pi bond will take part in conjugation.

8. Electron of negative charge or lone pair of electrons behaves as two pi electrons if it is in conjugation with pi bonds.two pi electrons if it is in conjugation with pi bonds.



Points to check stability of resonating structures

1. Neutral molecules are generally more stable than chargeseparated structures.

2. Greater the number of pi bonds in resonating structure,more stable the molecule will be.

3. Resonating structures with complete octet for each atom ismore stable no matter if electronegative atom has positivecharge or electropositive atom has negative charge.charge or electropositive atom has negative charge.

4. Resonating structures with electronegative atom havingnegative charge and electropositive atom having positivecharge are more stable than those in which electronegativeatom having positive charge and electropositive atom havingnegative charge when number of pi bond is same.




5. Resonating structures with similar charge on adjacent atoms areinsignificant due to electronic repulsion and result into instability inmolecule. Structures with two fully positive charge or partial positivecharge on adjacent atom; two fully negative charge or partial negativecharge on adjacent atom; or negative charge and lone pair on adjacentatom are insignificant.

6. Resonance stabilization is more when at least two equivalentresonating structures are possible for a molecule.resonating structures are possible for a molecule.

7. As conjugation increases in the molecules, stability increases.

8. If conjugation is same in aliphatic and aromatic compounds thenaromatic compound is more stable.

9. Structures with linear conjugation is more stable than those structureswhich have cross conjugation (if two groups are in conjugation witha particular group but not in conjugation with each other then thesystem have cross conjugation)




10.Dissimilar canonical structures vary widely in their energycontents (stability) i.e. those canonical structures having higherenergy (less stability) contribute less to the resonance hybrid andcanonical forms having lower energy (more stability) contributemore to the resonance hybrid. Equivalent resonating structurescontribute equally to the resonance hybrid.

11. Resonance is distance independent and delocalization of electrons11. Resonance is distance independent and delocalization of electronsoccurs from one place to another till the conjugation is present.

12. Resonance energy � Potential energy difference between the moststable resonating structure and the real structure (resonancehybrid) is called as resonance energy. It has a fix value for amolecule.

13. Resonance stabilizes the molecule i.e. it lowers the energy contentwhich can be revealed by lower heat of combustion & lower heatof hydrogenation values. www.OrganicChemistry.co.in

Mesomeric effect� It is simply the resonance occuring in a molecule when any atom

or group of atoms is in conjugation with organic molecule. Ingeneral there is no difference between resonance and mesomericeffect. Heinsenberg (1926) called it as resonance and Ingold(1933) called it as mesomeric effect.

� A very minute difference is there between resonance andmesomeric effect i.e. when a molecule which do not have anymesomeric effect i.e. when a molecule which do not have anypolarity, such as CH2=CH-CH=CH2 ,is considered then pielectrons may move in either direction (i.e. from left to right orfrom right to left), in such case resonance is operating in themolecule but when the same compound is attached with CHOgroup i.e. CH2=CH-CH=CH-CH=O then electrons will alwaysmoves from left to right, in such case due to CHO group carbonaquires positive charge and oxygen aquires negative charge.Therefore in such case resonance is called as mesomeric effect.



Types of Mesomeric effect� Similar to I effect, mesomeric effect is also of two types:

+ M effect:

� Those atoms or groups which donate electron density towards theconjugated system are called as + M groups and the effect is called as +Meffect.

+ M groups in decreasing order of power:

-O- >-NH2 >-NHR >-NR2 >-OH > -OR > NHCOR > OCOR > Ph > -2 2CH=CH2 > -F > -Cl > -Br > -I

�M effect:

� Those atoms or groups which withdraw electron density from theconjugated system are called as - M groups and the effect is called as -Meffect.

- M groups in decreasing order of power:

� -NO2 > -SO3H > -CN > -CHO > -COR > -COX > -COOCOR > -COOR > -COOH > -CONH2 > -CONHR > -CONR2



Capsule of Mesomeric effect� Atom directly attach to the conjugated

system have at least one lone pair ofelectron or a negative charge withcomplete octet are said to be + M groups.

� Atom directly attach to the conjugated� Atom directly attach to the conjugatedsystem have multiple bond (double ortriple) with more electronegative atomwithout lone pair of electrons are said tobe �M groups.



Hyperconjugation� Inductive effect of alkyl group follows following order (CH3)3C- >

(CH3)2CH- > CH3CH2- > CH3- when they are attached to saturatedcarbon. But when they are attach to unsaturated carbon i.e. C=C oraromatic system then order is reversed. This abnormal behavior ofelectron release can be explained by a new concept other thanresonance and inductive effect which is called as Hyperconjugation.

� Here electron releasing effect can be explained by assuming that óorbital of á C-H bond overlap with the adjacent ð orbital. Thisorbital of á C-H bond overlap with the adjacent ð orbital. Thisdisplacement of electron pairs of á C-H bond cause a partial positivecharge on the H atom without the actual proton release.

� ability of sigma bonded electrons of an alpha C-H bond to undergoconjugation with the adjacent pi electron is called ashyperconjugation.

� Concept of hyperconjugation is generally applied to carbocation ,free radical & C=C double bonded system such as alkene andaromatic compounds. www.OrganicChemistry.co.in

Hyperconjugation� In case of carbocations & free radicals, the carbon atom having

positive charge or unpaired electron respectively is á carbon.Therefore in that case only the electrons in the C�H ó bond that areâ to the positively charged carbon or free radical showhyperconjugation & can stabilize them.

� In case of alkenes the carbon adjacent to double bonded carbon is ácarbon. Here the electrons in the C�H ó bond that are á to thecarbon. Here the electrons in the C�H ó bond that are á to thedouble bonded carbon atom stabilize it by hyperconjugation.

� Hyperconjugation is also permanent effect similar to Resonance &Inductive effect.

� It is distance independent similar to resonance.

� Hyperconjugation also increases the stability of molecule. Greateris the number of hyperconjugation greater is the stability because itis also a stabilizinh phenomenon similar to resonance.



Hyperconjugation� As no bond structures are obtained similar to resonance so

hyperconjugation is also called as no bond resonance.

� One point should always kept in mind that the stability of carcanionis not decided by hyperconjugation in general case. For that we usethe concept of M effect or I effect.

� To calculate the number of hyperconjugation or no bond resonancealways count the hydrogen present over the carbon adjacent to thealways count the hydrogen present over the carbon adjacent to thecarbon having + charge, free radical or double C=C bond.

� If proton is released from the system completely then such effect iscalled resonance & if proton is present in the vicinity with no bondingwith any atom then same effect is called as hyperconjugation.

� One most important point must always kept in mind that concept ofhyperconjugation is only applied to carbocation, free radical andalkenes. So never think the concept of hyperconjugation in C=O, C=N,

C=S, C≡N etc. www.OrganicChemistry.co.in

introduction of organic chemistry by eyes of ajnish kumar gupta (akg)

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