1 polar effects - 30-01-2016.docx

B. Tech. Chemical Engineering Organic Chemistry (IV Sem./2015-16)

UNIT I: SYLLABUS

Polar effects – Inductive effect, electromeric effect, resonance or mesomeric effect, Hyper conjugation, examples.Types of intermediates: Carbocation – Carbanion - Free radical (factors influencing the reactions)Types of reagents: Nucleophiles – electrophiles - Free radicals; some reagents of synthetic importance – N-bromosuccinimide (NBS), Grignard reagent, Lithium Aluminium Hydride

Self learning topics: Boron trifluoride, Aluminium isopropoxide

NOTES:

Polar effects: It is well known fact that a covalent bond is formed by the sharing of electrons between the two atoms. If the covalent bond is formed between two similar atoms, the bonding electrons are located at the centre of the two bonding atoms and the bond is called as non polar bond. For example,

H : H or Cl : Cl

On the other hand, if the covalent bond is formed between two different atoms, the bonding electrons are closer to more electronegative atom and such bonds are called as polar bonds. Since the attaching/attacking reagent normally bears charge (either positive or negative), for a reaction to take place on the covalent bond, the substrate molecule should possess opposite charge. The substrate molecule although as a whole is electrically neutral, must develop polarity on the atoms of the covalent bond which participate in the chemical reaction, by the partial or complete displacement of the bonding electrons. The electronic displacement of bonding electrons thus shows some effects which are called as polar effects. Some of these are permanent and are known as polarization effects e.g. inductive effect and mesomeric effect while others are temporary and are known as polarisability effects e.g. electromeric and inductomeric effects. Let us discuss one by one.

Inductive Effect:

If the covalent bond is formed between two dissimilar atoms, the bonding electrons are never shared equally between the two atoms and are attracted a little more towards more electronegative atom of the two, and displace permanently towards more electronegative atom. The more electronegative atom possesses partial negative charge and the other atom possesses partial positive charge. Such covalent bond is called as polar bond. For example, in the compound C—X where X is more electronegative than C, the C—X bonding electrons displace towards the atom X with the result X attains partial negative charge (denoted by – δ) and carbon atom attains partial positive charge (denoted by + δ). Such displacement of electrons is called as inductive effect.

C X

1

+ -

C4 C3 C2 C1 X

C4 C3 C2 C1 Y


Thus inductive effect may now be defined as the permanent displacement of σ bonding electrons towards more electronegative element or group of the bond.

Inductive effect causes a certain degree of polarity in the bond and facilitates to attack the other charged groups. The inductive effect is denoted by the symbol ‘ ’, the arrow pointing towards the more electronegative atom or group and represented by ‘I’. Since electro negativity values of carbon and hydrogen are almost same, the inductive effect of hydrogen is considered as zero and the inductive effect of other groups are compared with the hydrogen.

The inductive effect of the group is always transmitted along the carbon chain in the molecule. For example, consider a carbon chain in which one terminal carbon atom is joined to a group X which is more electronegative than H.

Now since X is more electronegative than carbon, the electron pair between C1 and X will be displaced towards X group, with the result X will acquire partial negative charge and C1 carbon will acquire partial positive charge. Now since C1 carbon has positive charge it attracts the bonding electrons from C2 carbon with the result C2 carbon also will acquire positive charge. But the positive charge developed on the C2 carbon is smaller than that of C1 carbon because the effect of X is transmitted to C2 through C1. Similarly C3—C2 bonding electrons migrate towards C2 carbon and the C3 carbon acquires positive charge but than on C2. This effect can be proceeding further, but the inductive effect will be considered in all practical purpose up to C3

carbon. The decrease in the effect is denoted by greater number of δ signs. Any group which attracts the bonding electrons more strongly than hydrogen is known as – I group and its effect is called as – I effect. Some important – I effect groups and their order is given below.

- I effect groups

−N+( Me )3 > −N

+H3 > −NO2 > − CN > − COOH > − X > − OAr >

− COOR > −OR > −OH > −C6 H5 > −H

Similarly, consider a carbon chain in which one terminal carbon atom is joined to a group Y which is less electronegative than H.

Now since Y is less electronegative than carbon, the electron pair between C1 and Y will be displaced towards C1 carbon, with the result Y will acquire partial positive charge and C1

carbon will acquire partial negative charge. Now since C1 carbon has negative charge it pushes the bonding electrons towards C2 carbon with the result C2 carbon also will acquire negative charge. However the negative charge developed on the C2 carbon is smaller than that of C1

2

++ + + -

- - - - +


carbon. Similarly C3—C2 bonding electrons migrate towards C3 carbon, C4—C3 bonding electrons migrate towards C4 carbon and so on. The group which is less electronegative than hydrogen is known as + I group and effect is known as + I effect. Some important + I effect groups and their order is given below.

+ I effect groups

−O− > −COO− > R3 C− > R2 HC− > RH 2 C− > −CH 3 > −H

Applications of Inductive Effect: The phenomenon of inductive effect is very important in organic chemistry as it explains several effects, some of the important of which are given below.

1. Effect on bond lengths: The average distance between the nuclei of the bonding atoms is known as bond length. Usually ionic compounds have shorter bond lengths than the covalent molecules. Inductive effect leads to the polarity of the bond and thus the ionic character in the bond. So, increase in inductive effect increase the ionic character and decrease the bond length. For example, the C—X (halogen atom) bond length in alkyl halides decreases from alkyl iodide to alkyl fluoride as – I effect increases from iodine to fluorine.

2. Dipole moment: Dipole moment of the bond is the product of bond length and charge of one of the bonding atoms. It is a measure of the ionic character of the molecule. Since the inductive effect leads to a polarity polarity in the molecule, it develops some dipole moment in the molecule. Since polarity increases with increasing the inductive effect, dipole moment also increases with inductive effect. For example, C—Cl bond moment in the methyl chloride is more than the C—Br moment in the methyl bromide because the – I effect of chlorine is greater than the bromine. The dipole moments of alkyl halides are given below

Dipole Moment 1.64D 1.79D 1.83D

3. Reactivity of alkyl halides: Alkyl halides, in general, are more reactive than the corresponding alkanes. For example, ethyl chloride on treating with aqueous KOH gives ethyl alcohol but ethane does not give ethyl alcohol with KOH. It can be explained by inductive effect.

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- +


Since halogen atom (X) is more electronegative than carbon atom, it attracts the C—X bonding electrons from carbon atom with the result a partial positive charge on carbon atom and negative charge on the halogen atom will develop. The positive charge on the carbon atom facilitates the attack of nucleophile on it. Further, primary alkyl halide (ethyl chloride) is more reactive than methyl halide because the + I effect of the alkyl group enhances the – I effect of the halogen group. The reactivity order of the alkyl halides is as follows:

4. Strength of carboxylic acids: An acid is a species that has the tendency to loose proton. Further, the substance which looses proton easily is known as strong acid while the substance which does not loose proton easily is called as weak acid. Since the oxygen is more electronegative than the hydrogen, it attracts the electron pair of the O—H bond towards itself and facilitates to lose the hydrogen as proton. Hence carboxylic acids behave as acids.

a) Acidity of carboxylic acids: Since alkyl group is more electron repelling than hydrogen (+ I group), it increases the electron density on the oxygen of —O—H group and decreases the migration of O—H bonding electrons towards oxygen atom and thus decreases the acidity of carboxylic acid. Thus formic acid is much stronger acid than other acids. Further, as number of carbon atoms in the alkyl group increases the + I effect of the group increases and therefore the acidity of carboxylic acid decreases. The dissociation constant values of fatty acids show this phenomenon.

b) Acidity halogenated fatty acids: Since halogens are electron withdrawing groups, they attract the electrons from neighboring carbon, decreases the electron density on oxygen

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atom of –O—H group and facilitate to migrate the O—H bonding electrons towards oxygen and thus increase the acidity. So halogenated acids are much stronger acids than the parent acids. For example chloro acetic acid (Ka = 155 x 10-5) is much stronger than the acetic acid (Ka = 1.84 x 10-5).

However, since the inductive effect decreases rapidly as the group responsible for the effect (i.e. halogen atom) moves from the –O—H group of carboxylic acid, the strength of the acid will also proportionately decreases. For example, α-chlorobutyric acid is stronger than the β-chlorobutyric acid. Thus the chlorobutyric acids follow the following acidity order which is evident from their dissociation constants.

Further, as number of halogen atoms increases, the inductive effect also increases markedly and with the result these acids are progressively more stronger than the corresponding mono-halogenated acids. For example, tri-chloroacetic acid is stronger one than the di-chloro acetic acid and than that of mono-chloro acid.

Acidity

Similarly, since the electronegativity decreases from fluorine to iodine, the – I effect and thus the acidity of halogenated acids increases from fluorine to iodine. For example, fluoro

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acetic acid is stronger acid than the chloro acetic acid. The acidity order of these acids as follows.

c) Acidity of dicarboxylic acids: In case of dicarboxylic acids since the inductive effect of one carboxyl group ( - I effect of – COOH) enhances the acidity of the other, the acid strength of dicarboxylic acids is higher than that of the monocarboxylic acids. For example, acetic acid is stronger (Ka = 1.8 x 10-5) than the oxalic acid (K1 = 3500 x 10-5). Further, the + I effect of carboxyate anonic portion, hinders the further ionization of the carboxylate monoanion of the dicarboxylic acid to dianion and proton, with the result the monoanions of dicarboxylic acids (except oxalic acid) is weaker acids than the monocarboxylic acids. It is evident from the first dissociation constants of dicarboxylic acids are higher than that of the second dissociation constants. Furthermore, it is expected that the dissociation constant values fall off as the distance between the two carbonyl groups increases. Because the – I effect of the carboxylic acid group decreases as the distance between two carboxylic groups.

Dicarboxylic acid Formula K1 x 10-5 K2 x 10-5

Oxalic acid HOOC—COOH 3500 5.3Malonic acid HOOC—CH2—COOH 171 0.22Succinic acid HOOC—(CH2)2—COOH 6.6 0.25Glutaric acid HOOC—(CH2)3—COOH 4.7 0.29Adipic acid HOOC—(CH2)4—COOH 3.7 0.24

5. Basic character of amines: According to Lewis concept, the substance which donates pair of electrons and forms a dative bond with proton is called as a base. The basic character of amines is due to the presence of loan pair of electrons on nitrogen atom, which accepts proton. The readiness of the loan pair of electrons is available to accept proton determines the relative basic strength of amines.Due to + I effect of alkyl groups, the nitrogen atom in amines becomes rich in electrons with the result the loan pair of electrons on nitrogen atom is more easily available than in ammonia and thus amines are stronger bases than ammonia.

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Since tertiary, secondary and primary amines have 3, 2 and 1 alkyl groups respectively, the relative basic character of these amine may be in the following order.

Tertiary > Secondary > Primary > AmmoniaHowever, dissociation constants of these bases showed that the basicity of amines is in the following order

Secondary > Primary > Tertiary > AmmoniaThe reason for this is believed to be sterric effect of alkyl groups

Further, if electron withdrawing group if present, it decreases the electron density on the nitrogen atom and thus decreases the basic character of the amine. For example chloramine and aromatic amines are less basic than ammonia

6. Stability of carbonium ions: Since alkyl groups are + I groups, they decrease the positive charge of the carbonium ion by repelling the electrons and thus increase the stability of carbonium ion. Therefore primary carbonium ion is more stable than that of methyl carbonium. Further secondary and tertiary carbonium ions have 2 and 3 alkyl groups respectively with the result the stability order of carbonium ions is as follows:

Tertiary > Secondary > Primary > methyl carbonium ion

Inductometric (inductomeric) effect: We know that inductive effect is permanent and is always present in a molecule. Sometimes it may be increased temporarily prior to some

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reaction or change, by approach of a charged ion (attaching reagent). For example, in nitromethane the – I effect of —NO2 group is further increased temporarily by the approach of hydroxyl group to the hydrogen atom which is ultimately removed as water. Such a temporary effect is known as inductomeric effect. Thus inductomeric effect may be defined as a temporary effect which enhances the inductive effect and is brought into play only in the presence of charged attacking reagent.

In general, the – I effect will be enhanced by the approach of a negatively charged ion and the + I effect will be increased by the approach of positively charged ion. Since inductomeric effect is temporary effect, it will exist as long as the attacking ion present in the system.

Resonance: It has been observed that a single structural formula of certain molecules cannot satisfactorily explain all the properties of the molecule. Such a compound can be represented by two or more hypothetical structures, each of which separately accounts for most, but not all, of its properties.

Kekule proposed the structures I and II for benzene. Each of the two structures has three C—C single bonds and three C=C double bonds, i.e. they have two types of carbon-carbon bonds. But it has been experimentally determined that all carbon-carbon bonds in benzene are identical and have the bond length 1.39Ao. Furthermore, it is the intermediate between the normal carbon-carbon single bond (1.54Ao) and carbon-carbon double bond (1.20Ao). Thus neither of the above two structures is a correct representation of benzene. It was, therefore assumed that the true structure of the compound is an intermediate between the two structures.

Thus two are more structures are proposed to explain some of the properties of a compound, which are known as resonating or canonical structures. But no structure is the actual structure; the actual structure of molecule is the intermediate of all the structures known as resonance hybrid that can not be depicted exactly on paper. In such a case the molecule is said to be in a state of resonance. Thus resonance may be defined as the phenomenon in which two or more structures involving identical positions of atoms can be written for a particular compound. Thus following the concept of resonance theory, the benzene molecule may be expressed as a resonance hybrid of the two contributing Kekule structures. Such contributing structures are commonly known as canonical structures. The resonance hybrid may be represented as structure III.

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Thus we may now defined resonance hybrid as the actual structure of all the different possible structures that can be written for the molecule without changing the relative positions of its atoms and without violating the rules of covalence maxima for the atoms.

To indicate the forms contribute to the resonance hybrid, a double headed arrow of type⃗ is placed between the two forms. Each individual canonical structure contributes some stability to the resonance hybrid. Since in the resonance hybrid structure electrons are more delocalized than in any of the individual canonical structures, it will be more stable than any of the individual resonating structures. The difference in energy between the resonance hybrid and more stable resonating structure is referred to as the resonance energy of the molecule and the molecule is said to be stabilized by the resonance energy.

It is observed that larger the number of the possible canonical structures greater will be the resonance energy and hence stability of the compound. Furthermore, all structures do not contribute same stability more stable resonating structure contributes more stability. Resonance energy is quite large when the contributing forms have the same structural feature.

Mesomeric Effect: The mesomeric effect takes place in unsaturated conjugated system via their π orbitals. For example, the properties of carbonyl group may be explained by representing the classical structure, I and extreme polar structure, II. None of these are the actual structure, but the actual structure of this group is some what intermediate of these structures, which is known as resonance hybrid.

Now if the carbonyl group is in conjugation with C == C type of system, the above displacement of electrons is transmitted further via the π electrons as shown in the structures (i) and (ii) below. The resonance hybrid may be represented as structure (iii). It can also be represented by molecular orbital concept.

The same type of displacement of electrons may also be observed when an atom having at least one lone pair of electrons is in conjugation with π electrons, or the π electrons are in conjugation with other π electrons. Such displacement of electrons is known mesomeric effect. For example,

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Thus, mesomeric effect may be defined as the permanent displacement of π electrons from a multiple bond to an adjacent atom or π electrons from multiple bond to a single covalent bond or lone pair of electrons from an atom to a adjacent single bond. In other words, according to molecular orbital approach, mesomeric effect may be defined as the delocalization π electrons of multiple bond or lone pair of electrons of an atom with the other π electrons.

Mesomeric effect is represented by ‘M’. Like inductive effect, if this displacement of electrons takes place away from the group, it is known as + M effect, and if the displacement of electrons takes place towards the group, then it is known as – M effect.

The groups like − O

. .

. .H , −X

. ., − O

. .

. .R , −N

. .H2 , −N

. .HR , −N

. .R2 containing at least one lone

pair of electrons which are capable of delocalization with an attached unsaturated system, show

+ M effect. Similarly, groups like −C=O , −NO2 , −COOH , −CN , −SO3 H , −N+

R3 which are strong electron withdrawing groups show – M effect.

Since mesomeric effect is a permanent effect and always operates in the molecule, like inductive effect it effects physical properties of the molecule. Since mesomeric effect is due to displacement of loosely bonded π electrons, it is more dominant than inductive effect.Influence of mesomeric effect: The phenomenon of mesomeric effect causes several effects on the properties of the molecules.

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+M effect


a) Bond length: Concept of mesomeric effect explains the abnormal bond lengths in certain compounds. For example, the C – Cl bond length in vinyl chloride (1.72A0) is shorter than that in ethyl chloride (1.78A0). This is due to the mesomeric effect, due to this effect, the C – Cl bond in the vinyl chloride is neither single nor double i.e it has partial double bond character.

b) Strength of acids: Mesomeric effect explains the strength of carboxylic acids. When carboxylic acid gives carboxylate ion and proton when it dissociates. Once carboxylate ion is formed it stabilizes due to resonance. The following resonating structures are possible for the ion. Since carboxylate is stable, it forms very easily on dissociation, and therefore, carboxylic acid which is the conjugate acid of carboxylate ion exhibits acidic nature.

c) Acidity of phenols: Both phenols and alcohols contain – OH group, but phenols are acidic in nature while alcohols are neutral. It can be explained on the basis of resonance or mesomeric effect. On dissociation phenols give phenoxide ions and alcohols give alcoxides. Both phenol and phenoxide are the resonance hybrids of several structures.

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However, since oxygen is more electronegative atom and cannot accommodate (+)ve charge, the resonance structures of phenol which possess (+)ve charge at oxygen atom are less stable. So, the resonance hybrid of phenol is relatively less stable than that of phenoxide ion. With the result, phenol which is the conjugate acid of phenoxde ion, is acidic in nature. Such resonance will not exist in the alkoxide ion, further, alkoxide ion is less stable due to (+) I effect of alkyl group.

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Further, electron withdrawing groups if present in the increase the stability of phenoxide ion by decreasing (-)ve charge on oxygen atom and therefore, increase the acidity of phenols. For example, nitro-phenol is more acidic in nature than phenol. Similarly, electron releasing groups, since, they increase (-) charge on the oxygen atom, will decrease the stability of phenoxides. Therefore, aminophenol is less acidic than that of phenol.

d) Basicity of aromatic amines: We can explain why aromatic amines are weaker bases as compared to the aliphatic amines. In case of aromatic amines, the lone pair of electrons on the nitrogen atom are involved in the resonance and therefore lesser available for protonation. Further, such resonance is not possible in the aliphatic amines. Moreover, the alkyl groups, since they are electron releasing groups, increase electron density on nitrogen atom and easily available for protonation.

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Electron withdrawing groups if attached to the aromatic ring, they increase the displacement of lone pair of electrons towards the benzene ring, and thus, the basicity of amine decreases further. Similarly, if electron releasing groups present, the resonance effect will decrease and therefore the basicity of amine increases.

e) Formation of 1,4- addition product in conjugated dienes: When one mole reagent capable of adding alkenes, reacts with conjugate dienes two products are formed, a normal 1,2-addition product and 1,4-addition product. It can be explained on the basis of resonance.

Since alkenes normally undergo electrophiclic addition reactions, carbonium ion is formed as intermediate. The following mechanism explains the formation of 1,4-addition product

Hyper Conjugation or No-bond resonance or Baker-Nathan effect:

Baker & Nathan in 1935 proposed a special type of resonance (called Hyper conjugation) in which delocalization of electrons takes place through overlap between sigma bond orbital and pi-bond orbital or p- orbitals. For example in propene molecule, Hyper conjugation arise due to partial overlap of sp3-s sigma bond orbital and the empty p-orbital or pi-bond orbital of an adjacent carbon atom. Here one of the carbon-hydrogen bonds of methyl group can lie in the plane of pi-bond orbital, hence partial overlap with pi-bond orbital. This results the delocalization of pi-electrons and increase the stability of molecule.

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In resonating structures of propene, there is no bond between carbon and hydrogen ion, therefore hyper conjugation is also called as no bond resonance. As the number of methyl group bonded in double bonded carbon atom increases, the possibility of Hyper conjugation increases which results more stability. That is the reason, more substituted alkene are more stable than less substituted alkene. The increasing order of stability of some alkenes is as follow:

(CH3)2C=C(CH3)2 > (CH3)2C=CHCH3 > CH3CH=CHCH3 > CH3CH=CH2

There are many molecules and reaction intermediates which can show hypercojugation. Some of the common examples are following.

1. Ethyl carbocation

In carbocation such as ethyl carbocation, the sigma electrons of Csp3-hydrogen bond are delocalized with empty p-orbital of positively charged carbon atom and show four contributing structures

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2. Iso-propyl carbocation

Like ethyl carbocation, iso-propyl carbocation also shows hyper conjugation. In this intermediate, there are six carbon- hydrogen sigma bonds which can interact with pi-electron therefore it shows seven contributing structures and more stable than ethyl carbocation.

3. Free radicalsSimilarly free radicals get stabilized through hyper conjugation. Like carbocation, the sigma electrons of carbon-hydrogen bonds of methyl group next to carbon atom contain odd electron interact with p-orbital having odd electron. As the number of alpha carbon-hydrogen bond increases, contributing structures increases results more stability.

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4. Nitromethane

The nitrogen-oxygen pi-bond interacts with alpha carbon-hydrogen bond and show hyper conjugation.

5. Acetonitrile and propyne

The presence of triple bond between carbon-carbon or carbon-nitrogen also show hyper conjugation with alpha carbon-hydrogen bond like in acetonitrile and propyne molecule.

6. Toluene

The carbon - hydrogen sigma bond interacts with pi-bond of aromatic ring to form four contributing structures of toluene.

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7. 2-Butene

The interaction of pi-bond with alpha carbon-hydrogen bond forms six resonating structure of 2-butene. While 1-butene shows only two contributing structures and less stable than 2-butene.

Applications of Hyper conjugation:1. Stability of unsaturated hydrocarbons

The stability of unsaturated hydrocarbons like nitriles, alkenes effects with hyper conjugation. The more possible contributing structures in hyper conjugation increase the stability of molecule.

Since hyper conjugation mainly involves alpha carbon-hydrogen sigma bond and pi-electrons, therefore as the number of alpha sigma bonds increases, hyper conjugation increases.

For example, 2-butene consists of six alpha carbon-hydrogen sigma bonds while there are only 2 carbon-hydrogen bonds next to double bonded carbon atom in 1-butene.

Hence 2-butene shows six contributing structures while 1-butene shows only two which make 2-butene more stable compare to 1-butene.

This rule is applicable on other alkenes also. Hence as the number of alkyl group on double bonded carbon atoms increases, hyper conjugation increases and stabilizes the molecule or more substituted alkenes are more stable than less substituted alkenes.

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2. Stability of reaction intermediates

Intermediates like carbocations and free radicals also show hyper conjugation due to the presence of empty p-orbitals or p-orbital with odd electrons. The stability of carbocation and free radical get affected with the number of contributing structures. For example; in primary carbocation like ethyl carbocation there are three alpha carbon-hydrogen bonds which delocalized in to the empty p-orbital of C+. While in secondary carbocation like iso-propyl carbocation, there are six alpha carbon-hydrogen bonds and in tertiary carbocation like tert-butyl carbocation, there are nine alpha carbon-hydrogen bonds. Hence the increasing order of stability of carbocations can be given as:

Primary < Secondary < Tertiary carbocation

Similarly the increasing order of stability of free radical is primary < secondary < tertiary free radical.

3. Dipole moment & bond length

Hyper conjugation induces polarity in molecule which affects the dipole moment and bond length of molecule. As the polarity increases, dipole moment increase and bond length decreases.

4. Orientation effect of electrophilic substitution reactions on benzene ring

The methyl group of toluene shows positive inductive effect and hyper conjugation which releases electrons towards aromatic ring. These two effects increases the electron density on aromatic ring atortho and para-positions, therefore coming electrophile effectively attack on these two positions to give ortho and para-substituent products.

The delocalization of electrons creates inductive effect, mesomeric effect and hypercojugation which further effect stability and other properties of molecule. It provides stability to many reaction intermediates like carbocation, free radical and anions. For example,

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cyclopentadiene carbocation gets stabilised due to delocalization of the positive charge and the pi bonds occurs over the entire ring.

Delocalization of electrons can be possible.

1. In the presence of conjugated system:2. Presence of pi-bond next to positive charge on an adjacent atom.3. The presence of an atom bearing lone pairs of electrons next to a positive charge.4. The presence of a pi bond with an adjacent atom bearing lone pairs of electrons.

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1 polar effects - 30-01-2016.docx

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