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Basics of organic chemistry reactions mechanism

What is chemical reaction mechanism?

It is the detailed processes by which chemical substances are transformed into other substances.

As in Inorganic compounds the chemical reactions occur as:

  • The opposite charged reagents attract each other, an example: A solution contains Ag+ and other contains Cl so these ions attract each other and combine fromg the AgCl ppt.
  • Orbitals overlap: The uncharged molecules or have no dipole molecules can react with each other by there orbitals overlapping as the compounds which have one lone pair or more can form a bond with the vacant orbital in the another molecule. such as reaction of alkene with Br2.
fig.1

Here the Br vacant antibonding σ* accept the electrons of π bond of alkene.

As mentioned above How chemical reaction occurs In organic chemistry. The flow of electrons is a chemical rection also in organic chemistry:

fig.2

Orbital overlap and successful reaction:

fig.3

As we see in fig.3 not all Nu orbitals make successful overlap with any orbital of electrophie, but insteadly The energies of the filled Nu orbital and the empty E orbital should be almost the same.

fig.4

The suitable orbitals between Nu and E are HOMO and LUMO for them respectivelty.To make these orbitals as close as possible in energy, we want the nucleophile to have a high-energy HOMO and the electrophile to have a low-energy LUMO.

ِExamples of Nucleophiles:

1- Lone pair species:
The most common type of nucleophile has a non-bonding lone pair of electrons. Non-bonding electrons are typically high in energy because they don’t benefit from the stabilization bonding electrons get from being shared between two nuclei.

fig.4

fig.5

2- Anions:

Anions which have lone pairs are often good nucleophiles too, partly because they can be attracted electrostatically by positively charged electrophiles. The anionic centre is usu- ally O, S, or halogen, each of which can have several identical lone pairs. For example, hydroxide has three lone pairs—the negative charge cannot be assigned to one of them in particular. It’s convenient just to draw the negative charge, and not the lone pairs as well. Negative charges like this actually represent a pair of electrons.

fig.6

3- π bond:

π bond of alkene (C=C double bonds) is a weak nucleophile for strong electrophiles such as Br2

fig.7

4- σ bond:

σ bond of a nucleophile to donate electrons, provided it is a σ bond associated with electropositive atoms such as B, Si, or the metals, along with C or H.

fig.8

Summary of Nucleophiles:

fig.9

ِExamples of Electrophiles:

1- Empty or vacant orbital:

Electrophiles are neutral or positively charged species with an empty atomic orbital (such as the empty p orbital in borane) or a low-energy antibonding orbital that can easily accept electrons. exmples: hydronium ion (H+) fig.10 and Aluminium and Boron (fig.11 , fig.12 and fig.13)

fig.10

BF3 , AlCl3

Boron triflouride and Aluminium trichloride are an example of empty orbital electophiles.

Representation with orbital structure of ( 5B ):

fig.11

The LUMO of B is empty 2p orbitalwhile the LUMO of Al is 3p vacant orbital

fig.12
fig.13

2- Single (sigma) bond:

  • In most rganic electophile molecules the LUMO is low energy antibonding orbitals π* orbitals or σ* associated with electronegative atoms by π or σ bonds respectively.
  • The electronegative atoms are such as: O, N, Cl or Br , ….
fig.14

Lone pair enters σ* of iodo metane as in fig.14

  • In sigma electrophile examples also are Halogens: F2 , Cl2 , Br2 , I2
  • In all of these halogens σ* orbital is vacnat Which represents the LUMO fig.15
fig.15

fig.16

3- Double (Bi) bond:

The most common organic electrophile withdouble bond is carbonyl group [>C=O] ,but we are talking about carbonyl group of Aldehyde and Ketons only (not acid or ester).

carbonyl group has vacant σ* and π* orbitals but π* is higher in energy than σ* in energy so it represents the LUMO.

fig.17

Due to O is more electronegative than C then the partial positive charge on carbon attracts the nucleophile toward it then the reaction occur as in fig.17

Notice that:

When a nucleophile attack the carbonyl and enters the electrons in π* to form a new σ between carbon and nucleophile. That leads to break π bond leavign (C–O)σ bond. And move electrons on the electronegative atom oxygen.

fig.18

Summary of Electrophiles:

fig.19

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