Introduction of Organic Chemistry by Eyes of Ajnish Kumar Gupta (AKG)

Organic Chemistry: Chemistry of carbon & its compounds

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Reason for formation of large number of compounds • Atoms combine to form molecules to complete their octet and attain lower energy 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 nor 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 groups are formed resulting in large number of organic compounds. • Carbon not only shares its electrons with other atoms but it can also share with other 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 reason behind 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 characteristic chemical properties i.e. they are the action group or reactive site in a chemical reaction and the remaining hydrocarbon part remains inert. • Each functional group shows its characteristic chemical reactions. • Functional group helps in nomenclature of organic compounds. • Functional group serves to classify organic compounds into different classes or families i.e. compounds with same functional group belong to same class. www.OrganicChemistry.co.in

Different functional groups

• Functional group with ó bond only: C-C/H Alkane, C-X Alkyl halide, C-OH Alcohol, C-O-C Ether, C- NH2 Amine, 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

• Along C=N bond derieved from aldehyde or ketones C=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

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Reasons for chemical reactions in organic chemistry • By looking at different functional groups we find that carbon is either bonded with less electronegative atom or more electronegative atom to complete its octet. Due to these electronegativity differences some electronic displacement takes place in the molecule. The effect which creates electronic displacement in organic molecules is called as electronic effect. • 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 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) will attack preferentially at the site of highest electron density and electron rich species (nucleophilic reagent) will attack at the site of lowest electron density. • This charge distribution is ascertained by the examination of consequence of electronic factors such as Resonance, Hyperconjugation & Inductive effect. • Some reactions proceed via formation of intermediate such as cabocation, carbanion, free radical, carbene, nitrene & benzyne while others do not form intermediate but proceeds through transition state (TS). • But one point is common to both i.e. reactivity of different positions in the molecule are determined only by their relative activation energies. Since activation energy depends on the structures of transition state so generally more stable intermediate have lower energy of the transition state & consequently the lower is the activation energy leading to that intermediate formation.

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Something 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 few in numbers. So never try to learn the question but try to find the position of electron rich & electron deficient centre in substrate and reagents using the concept of electronic factors and follow my rule i.e. Ajnish rule.

• Ajnish Rule: During any organic reaction if an intermediate is formed in the rate determining step then stability of intermediate will decide the formation of major product. Greater is the stability of intermediate, major is the corresponding product and the stability of intermediate is generally decided by Resonance, Hyperconjugation & Inductive effect. www.OrganicChemistry.co.in

Inductive effect • When ó bond is formed between two atoms of different electronegativity then ó bond pair electron density is shifted towards more electronegative atom and due to shifting of electron cloud dipole is created between the two atoms. Due to this dipole in ó bond, electron density in the chain is also shifted and progressively moves into the chain. This permanent effect of polarization in the 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 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. www.OrganicChemistry.co.in

Inductive 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 negligible and 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 of electron. By convention the electrons are displaced in the direction of the arrow. The tail of the arrow (which looks like a plus sign) is electron poor ä+ & the head of arrow is electron rich (ä-). • It is also important to remember that I effect creates only polarity in the sigma 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 metal atom. Due to this electronegativity difference between metal & Carbon, metals are electron donating in nature leading to creation of partial negative charge over carbon www.OrganicChemistry.co.in

Inductive 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 the electronegativity 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: • The atom or group which withdraws electron density (EWG group) from the carbon making carbon electron deficient are called as – I groups and effect is called as – I effect. • – I groups create partial positive charge over the carbon & partial negative charge 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 www.OrganicChemistry.co.in

Inductive 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 and effect is called as + I effect. • + I groups create partial negative charge over the carbon & partial positive charge over less electronegative atom.

• Various groups with decreasing order of + I strength follows: • -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 attach to it donate electron to it or withdraws electron from it. Electron donating group of any form is + group for carbon & electron withdrawing group of any form is – group for carbon. www.OrganicChemistry.co.in

Resonance • There are so many organic molecules (neutral molecules, ions, free radicals) where all the observed properties i.e. physical & chemical properties can’t be satisfactorily explained with the help of a single structural formula i.e. Lewis structure or valence bond structure. • In order to satisfactorily explain all the properties of such molecules we represent the molecule by two or more structures just by shifting of p orbital electrons or delocalization of electrons without changing relative positions of the atomic nuclei. These structures are known as resonating structure, canonical structure, contributing structure or resonance unperturbed structures and phenomenon is called as resonance. • The resonating structures are only the hypothetical structures and the real structure lies in between them. The resonating structures contribute to the real structure which is called as Resonance hybrid. The resonance hybrid is more stable than any of the resonating structures. • Compound showing resonance is not a mixture of some molecules having the structures shown by one canonical form and some having structures shown by another but the compound showing resonance have the same structure all the time www.OrganicChemistry.co.in

General cases of resonance 1. A=B-C=D 3. A=B-C¯ 5. A=B − C˚˚ 7. A ¯ − B 9. A ≡ B − C ≡ D 11. A ≡ B − C ¯ 13. A ≡ B − C ˚˚

2. 4. 6. 8. 10. 12.

A=B − C+ A=B − C˚ A=B − C≡D A ˚˚ − B A≡B−C+ A≡B−C˚

Capsules: 1. A=B −C+/ ¯/ ˚/ ˚˚/=/ ≡ 2. A ¯/ ˚˚ − B 3. A ≡ B − C+/ ¯/ ˚/ ˚˚/ ≡ www.OrganicChemistry.co.in

Points to draw resonating structures 1. Resonating structures differs only in placement of their pi electrons or non bonding electrons i.e. sigma bond skeleton remains same in all resonating structures. 2. The position of each atom in all resonating structures must be same. 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 any of the resonating structures. 4. All the resonating structures must have same numbers of paired electrons if they have pi bonds, positive, negative or lone pair of electrons and same numbers of unpaired electron if they are free radical. 5. If any conjugate position has more than one lone pairs of electrons then only one lone pair of electron will take part in conjugation. www.OrganicChemistry.co.in

Points to draw resonating structures 6. 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. www.OrganicChemistry.co.in

Points to check stability of resonating structures 1. Neutral molecules are generally more stable than charge separated 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 is more stable no matter if electronegative atom has positive charge or electropositive atom has negative charge. 4. Resonating structures with electronegative atom having negative charge and electropositive atom having positive charge are more stable than those in which electronegative atom having positive charge and electropositive atom having negative charge when number of pi bond is same. www.OrganicChemistry.co.in

Points to check stability of resonating structures 5. Resonating structures with similar charge on adjacent atoms are insignificant due to electronic repulsion and result into instability in molecule. Structures with two fully positive charge or partial positive charge on adjacent atom; two fully negative charge or partial negative charge on adjacent atom; or negative charge and lone pair on adjacent atom are insignificant. 6. Resonance stabilization is more when at least two equivalent 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 then aromatic compound is more stable. 9. Structures with linear conjugation is more stable than those structures which have cross conjugation (if two groups are in conjugation with a particular group but not in conjugation with each other then the system have cross conjugation) www.OrganicChemistry.co.in

Points to check stability of resonating structures 10.Dissimilar canonical structures vary widely in their energy contents (stability) i.e. those canonical structures having higher energy (less stability) contribute less to the resonance hybrid and canonical forms having lower energy (more stability) contribute more to the resonance hybrid. Equivalent resonating structures contribute equally to the resonance hybrid. 11. Resonance is distance independent and delocalization of electrons occurs from one place to another till the conjugation is present. 12. Resonance energy – Potential energy difference between the most stable resonating structure and the real structure (resonance hybrid) is called as resonance energy. It has a fix value for a molecule. 13. Resonance stabilizes the molecule i.e. it lowers the energy content which can be revealed by lower heat of combustion & lower heat of 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. In general there is no difference between resonance and mesomeric effect. Heinsenberg (1926) called it as resonance and Ingold (1933) called it as mesomeric effect. • A very minute difference is there between resonance and mesomeric effect i.e. when a molecule which do not have any polarity, such as CH2=CH-CH=CH2 ,is considered then pi electrons may move in either direction (i.e. from left to right or from right to left), in such case resonance is operating in the molecule but when the same compound is attached with CHO group i.e. CH2=CH-CH=CH-CH=O then electrons will always moves from left to right, in such case due to CHO group carbon aquires positive charge and oxygen aquires negative charge. Therefore in such case resonance is called as mesomeric effect. www.OrganicChemistry.co.in

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 the conjugated system are called as + M groups and the effect is called as +M effect. + M groups in decreasing order of power: -O- >-NH2 >-NHR >-NR2 >-OH > -OR > NHCOR > OCOR > Ph > CH=CH2 > -F > -Cl > -Br > -I – M effect: • Those atoms or groups which withdraw electron density from the conjugated system are called as - M groups and the effect is called as -M effect. - M groups in decreasing order of power: • -NO2 > -SO3H > -CN > -CHO > -COR > -COX > -COOCOR > COOR > -COOH > -CONH2 > -CONHR > -CONR2 www.OrganicChemistry.co.in

Capsule of Mesomeric effect • Atom directly attach to the conjugated system have at least one lone pair of electron or a negative charge with complete octet are said to be + M groups. • Atom directly attach to the conjugated system have multiple bond (double or triple) with more electronegative atom without lone pair of electrons are said to be – M groups. www.OrganicChemistry.co.in

Hyperconjugation • Inductive effect of alkyl group follows following order (CH3)3C- > (CH3)2CH- > CH3CH2- > CH3- when they are attached to saturated carbon. But when they are attach to unsaturated carbon i.e. C=C or aromatic system then order is reversed. This abnormal behavior of electron release can be explained by a new concept other than resonance and inductive effect which is called as Hyperconjugation. • Here electron releasing effect can be explained by assuming that ó orbital of á C-H bond overlap with the adjacent ð orbital. This displacement of electron pairs of á C-H bond cause a partial positive charge on the H atom without the actual proton release. • ability of sigma bonded electrons of an alpha C-H bond to undergo conjugation with the adjacent pi electron is called as hyperconjugation. • Concept of hyperconjugation is generally applied to carbocation , free radical & C=C double bonded system such as alkene and aromatic 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 are â to the positively charged carbon or free radical show hyperconjugation & can stabilize them. • In case of alkenes the carbon adjacent to double bonded carbon is á carbon. Here the electrons in the C–H ó bond that are á to the double 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. Greater is the number of hyperconjugation greater is the stability because it is also a stabilizinh phenomenon similar to resonance. www.OrganicChemistry.co.in

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 carcanion is not decided by hyperconjugation in general case. For that we use the concept of M effect or I effect. • To calculate the number of hyperconjugation or no bond resonance always count the hydrogen present over the carbon adjacent to the carbon having + charge, free radical or double C=C bond. • If proton is released from the system completely then such effect is called resonance & if proton is present in the vicinity with no bonding with any atom then same effect is called as hyperconjugation. • One most important point must always kept in mind that concept of hyperconjugation is only applied to carbocation, free radical and alkenes. So never think the concept of hyperconjugation in C=O, C=N, C=S, C≡N etc. www.OrganicChemistry.co.in