NEW CHAPTER BASIC CONCEPTS BINDING AND RESONANCE

Bonding Patterns Localized Electrons: When electrons are restricted on single atom (as lone pair) or between two bonded atoms only. Delocalized Electrons: When electrons spread over an area, such as in benzene,  electrons spread over whole ring. Resonance structures Mechanisms

A valid organic mechanism uses: (a) curved arrows (b) Lewis structures A mechanism is step by step description of how reactants become products (Or Pathway to product) A valid organic mechanism uses: (a) curved arrows (b) Lewis structures (c) and reactions This is a mechanism

Curved Arrows Notice the tail of the arrow must start at an electron pair.

Resonance Structures: Resonance Structures are two Lewis structures having the Same placement of atoms but a different arrangement of electron.

Drawing Resonance Structures (continued) Rule [3]: Resonance structures must be valid Lewis structures. Hydrogen must have two electrons and no second-row element can have more than eight.

Rules for Drawing Resonance Structures Rule [1]: The placement of atoms and single bonds always stays the same. Rule [2]: Two resonance structures must have the same number of unpaired electrons. C is not a resonance str of A and B

Resonance None of these structure represents benzene. The real structure of benzene is a resonance hybrid of these contributing structures. The contributing structures have no real structure existence but these only exist in our imagination. All the molecules of benzene have one structure which can't be drawn, but which resembles the contributing structure to varying extent. Example: Mule

What is resonance? Resonance is defined as the representation of a real structure as a weighted average of two or more contributing structure is called resonance. The contributing structure is called canonical form and the real structure is called resonance hybrid. The resonance hybrid is similar to each of the contributing structures but identical to non of them. For example the various structure of benzene

Resonance The energy of the actual molecule is less than the contributing structures . The contributing structure has different energies. Some have high and some have low energies. The contributing structure which has the lowest energy is more resemble to the actual molecule and is called major contributor. The contributing structure which has the highest energy is less contributing to the hybrid one and is called the minor contributor.

Resonance Energy The difference of energy between the actual molecule and the contributing structure of the lowest energy is called the resonance energy. The double headed arrow represents resonance which means that molecule is better represented by several Lewis structure than by one. The contribution of structure (1) and (2) to the actual structure has been calculated to be 39 % each where as (3), (4) and (5) is 7.3% each

Resonance Energy Complete Hydrogenation of benzene to Cyclohexane delivers 208.4 kJ/mol. While hydrogenation of 1 double bond required 119.7 kj/mol. Difference in energy is 359.7 ─208.4 = 150.7 kJ/mol.

Resonance Effect The decrease in electronic density at one position and the corresponding increase else where is called the resonance ( or mesomeric effect). For example in ammonia resonance is absent, the unshared pair of electrons is located on the nitrogen atom. However, if one of the hydrogen atoms of ammonia is replaced by a benzene ring. The pair of nitrogen is delocalized over the ring, resulting in the decrease of electron density o the nitrogen atom and the corresponding increase electron density on the ring as shown below. NH2 group in aniline donates to the ring by the resonance effect.

Electron Withdrawing Group (-M): Similarly, nitrobenzene the –NO2 group withdraws electron from the ring by a similar effect Some of the common electron donating group (+M ) and electron withdrawing (-M) groups are listed below Electron Withdrawing Group (-M): Electron Donating Group (+M ) :

Inductive Effect The permanent diapole induced by a bond on another bond is called as inductive effect. This effect also the effect the reactivity of a compound. If a carbon atom is joined to an atom X of higher ectronegativity than carbon The bonding electrons of the C  X bond will be displaced away from the carbon atom. The bonding electrons of the C  X bond will be displaced towards atom X. Carbon atom will exhibit a partial positive charge Atom X will exhibit a partial negative charge.

Inductive Effect If a carbon atom is joined to an atom Y of lower electronegativity than carbon The bonding electrons of the C  Y bond will be displaced away from atom Y The bonding electrons of the C  Y bond will be displaced towards the carbon atom Carbon atom will exhibit a partial negative charge Atom Y will exhibit a partial positive charge

Inductive Effect When an electron-withdrawing group (X) is linked to carbon  the group develops a partial negative charge  exert a negative inductive effect (-I)

Inductive Effect Represented by an arrow head in the middle of the covalent bond pointing in the direction of the displacement of electrons

Inductive Effect When an electron-releasing group (Y) is linked to carbon  the group develops a partial positive charge  exert a positive inductive effect (+I)

Alkyl groups like CH3, C2H5, C3H7 Inductive Effect 1. Groups which exhibit negative inductive effect (i.e. electron-withdrawing groups) NO2 > F > COOH > Cl > Br > I 2. Groups which exhibit positive inductive effect (i.e. electron-releasing groups) Alkyl groups like CH3, C2H5, C3H7

Inductive Effect The stabilities of the carbocations in decreasing order:

Why is tert-Butyl Carbocation the Most Stable among the Four Carbocations? Three electron-releasing methyl groups surrounding the central carbon atom Help reduce the positive charge on the central carbon atom by exerting positive inductive effects

Inductive Effect The greater the number of alkyl groups attached to the central carbon atom  the more dispersion the charge  the more stable the carbocation

Formic acid is 10 times stronger acid than acetic acid. It is due to the inductive effect of (CH3) methyl group attached to carbon in acetic acid. As CH3 group is an electron donating group. This it donates electron though inductive effect to the carbon and as result the carbon shift electrons to (OH) group and becomes electron rich and thus cant be replaced from acid and hence the strength of the acid is decreased. Because we know that an acid will be strong if donate (-H+) more easily.

On the other hand hydrogen attached to carbon in formaic acid not an electron donating group and thus the Oxygen of OH group attracts electron from the O-H bond and hydrogen becomes electron deficient and so easily b replaced This is the reason that formic acid is stronger acid that acetic acid and this is due to the permanent dipole Other example. Chloro acetic acid is 100 times acidic than acetic acid.

Electron withdrawing inductive effect (-I) or electron donating inductive effect (+I) The functional groups responsible for producing inductive effect can be classified with an electron withdrawing inductive effect (-I) or with an electron donating inductive effect (+I) relative to Hydrogen depending on whether they are more electronegative or less electronegative than H. some of the common –I and +I groups are listed below approximately in the order of decreasing inductive effect. Electron with drawing groups (-I) -NR3, -NH3, -NO2, -CN, -CO2H, -F, -Cl, -Br, -I, -OAr -CO2R, -OR, -COR, -OH, etc Electron donating groups (+I) -O-, -CO-, -CR3, -CHR2, -CH2R, -CH3, -D

Hyperconjugation Sigma bond in conjugation with p orbital The system in which the sigma electrons are in conjugation Sigma bond in conjugation with p orbital is also called hyperconjugation Explanation The molecule of propylene in which C-H is in conjugation with an unsaturated system. The sigma electron of the C-H bond are delocalized and a canonical form such as (b).

In this canonical form there is no bond between the carbon and hydrogen atom. This type of resonance is called no bond resonance and this system of conjugation where the sigma electron of a C-H bond are in conjugation with Pi electron of unsaturated system or with a p orbital is known as hyperconjugation. This no bond resonance between C-H is partially delocalized through conjugation.

Hydrogen Bonding Hydrogen bonding is an attractive force which occurs in any compound whose molecules contain O-H or N-H bonds (as in water, alcohols, acids, amines and amides. The O-H bond, for example, is a highly polar bond. Oxygen is more electronegative than H and pulls the bonding electrons closer to it. As a result of this displacement, the oxygen atom acquires a small negative charge and H small d- d+

Hydrogen bonds in Ethane Alcohols have abnormally high boiling points relative to their molecular weights due to their ability to hydrogen bond Hydrogen bonds in Ethane Hydrogen bonds

R-O-H has a structure similar to that of water Hydroxyl group is very polar Hydrogen bonds can form readily

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