Factors Influencing Electron Availability Dr. A. G Factors Influencing Electron Availability Dr. A.G. Nikalje Pharmaceutical Chemistry Dept Inductive effect

Factors Influencing Electron Availability Inductive Effect Electromeric Effect Resonance Mesomeric Effect Hyperconjugative Effect Steric Effect Tautomerism

The dipole moment is a property of the molecule that results from charge separations. However, it is not possible to measure the dipole moment of an individual bond within a molecule; we can measure only the total moment of the molecule, which is the vectorial sum of the individual bond moments.

+I and -I

Aplications: Dipole moment- increases with increase in inductive effect. Chemical reactivity- dominant role, eg.nitration of m-dinitrobenzene, toluene. In comparing basicity of bases. In comparing acidity of organic acids. Problem solving-1,4-dimethyl benzene, 4-chloro toluene, 1,4- dichloro toluene.

Elecromeric Effect (E) The temporary or time-variable effect involving the complete transfer of a shared pair of Pi electrons to one of the atoms joined by a multiple bond, double or triple , at the requirement of an attacking reagent is known as electromeric effect. The effect may be called polarisability of multiple bond; because as soon as the attacking reagent is removed, the transferred pi electron pair again form the bond & the molecule reverts to its ground state electronic condition. The electromeric effect has no specific direction: however the transfer of electrons occurs on more electronegative atom. Inductive effect also plays an imp. Role in determining the direction of of the E effect. Application: The E effect, ie, the close approach of the reagent to a multiple bond enhances the reactivity of the reactant molecule and explains its addition reaction product.

Hyperconjugation This is a conjugation involving sigma electrons. The excessive or extended conjugation involving sigma orbitals , generally of C-H, C-X or C-C bond, is called hyperconjugation. Sigma-pi type of orbital overlap gives rise to conjugation.

Resonance The compounds contain one or more bonding orbitals that are not restricted to two atoms, but that are spread out over three or more. Such bonding is said to be delocalized. In the VB method, several possible Lewis structures (called canonical forms) are drawn and the molecule is taken to be a weighted average of them. This representation of a real structure as a weighted average of two or more canonical forms is called resonance. For benzene, the canonical forms are 1, and 2. Double-headed arrows (<->) are used to indicate resonance. Resonance structures of Benzene

The Rules of Resonance In the resonance picture, these structures are considered to contribute: 1. All the canonical forms must be bona fide Lewis structures 2. The positions of the nuclei must be the same in all the structures. This means that when we draw the various canonical forms, all we are doing is putting in the electrons in different ways. 3. All atoms taking part in the resonance, that is, covered by delocalized electrons, must lie in a plane or nearly so 4.All canonical forms must have the same number of unpaired electrons. 5. The energy of the actual molecule is lower than that of any form, obviously. Therefore, delocalization is a stabilizing phenomenon.

6. All canonical forms do not contribute equally to the true molecule 6. All canonical forms do not contribute equally to the true molecule. Each form contributes in proportion to its stability, the most stable form contributing most. a. Structures with more covalent bonds are ordinarily more stable than those with fewer . b. Stability is decreased by an increase in charge separation. Structures with formal charges are less stable than uncharged structures. c. Structures that carry a negative charge on a more electronegative atom are more stable than those in which the charge is on a less electronegative atom. d. Structures with distorted bond angles or lengths are unstable

The Resonance Effect/ Mesomeric Effect Resonance always results in a different distribution of electron density than would be the case if there were no resonance. For example, if 18 were the actual structure of aniline, the two unshared electrons of the nitrogen would reside entirely on that atom. Since the real structure is not 18 but a hybrid that includes contributions from the other canonical forms shown, the electron density of the unshared pair does not reside entirely on the nitrogen, but is spread over the ring. This decrease in electron density at one position (and corresponding increase elsewhere) is called the resonance or mesomeric effect.

Tautomerism:

Nitroso-oxime Tautomerism:

Ring-Chain Tautomerism When 1,5-dienes are heated, they isomerize, in a [3,3] sigmatropic rearrangement known as the Cope rearrangement

Any 1,5-diene gives the rearrangement; for example, 3-methyl-l,5-hexadiene heated to 300°C gives 1,5-heptadiene However, the reaction takes place more easily (lower temperature required) when there is a group on the 3 or 4 carbon with which the new double bond can conjugate. The reaction is obviously reversible and produces an equilibrium mixture of the two 1,5-dienes,which is richer in the thermodynamically more stable isomer. Cope

The interesting thing is that at 180°C the NMR spectrum shows that what exists is an equilibrium mixture of the two structures. That is, at this temperature the molecule rapidly (faster than 103 times per second) changes back and forth between the two structures. This is called valence tautomerism and is quite distinct from resonance, even though only electrons shift.519 The positions of the nuclei are not the same in the two structures. Molecules like 105 that exhibit valence tautomerism (in this case, at 180°C) are said to have fluxional structures