Organic Chemistry

s bond andp bond | Types of reactions in organic chemistry | Isomers | Other pages

s bond and p bond

s (sigma) bond is formed when two S orbitals overlap. The shape of sigma bond is also the region occupied by electrons in S orbitals. In other worlds electrons in S orbitals can move around freely within this region.
p (pi) bond is formed when P orbitals overlap in sideways. This is also the region occupied by electrons in P orbitals. C2H4 (ethene)is a good example to explain these to kind of bondings. sigma and pi bond of ethene

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Types of reactions in organic chemistry

The reactions listed so far I believe are quite familiar for people who did their chemistry in junior high, high school or GCSE. Reactions after this point are the new bit for you (I hope).

Nucleophilic substitution / addition
Electrophilic substitution / addition
Free radical substitution / addition

Before we get into details of these reactions, first of all what are nucleophile, electrophile and free radicals??

  1. Nucleophile have a pair of electrons ( a lone pair of electron or a p bond if you like ) that attack an electron deficient site ( gap ) site on another molecule.
    Nucleophile can be negative or neutral

    OH-, and (:CºN (can be written as CN-) are examples of nucleophile with negative charge

    CH3CH2(:NH2, H2O(water), NH3, and CH2=CH2 are examples of neutral nucleophile.

  2. Electrophile have an electron deficient site (gap) and attack on electron concentrated site.
    Electrophile can be positive or neutral

    CH3+, H+, and NO2+ are examples of positive electrophile.

    Br2 and HBr are examples of neutral electrophile.

  3. Free radical is very reactive partical with an unpaired electron. It is formed when a covalent bond is split up by a very strong energy source such as UV (Ultra Violet) ray.

    When Cl2 is split up by UV, homolitic fission occurs and free radicals are formed. Cl-Cl® Cl* *Cl

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Isomers

Isomers can be grouped as follow types of isomers
Note: Positional Isomer can also be grouped within structural isomer

Structural isomers

Structural isomers have the same molecular formulas but differ in structure.
For example: 2-methylbutane and 2,2-dimethylpropane. They are both C5H12 isomers. structural isomers

Positional isomers

Positional isomers have the same molecular formula and the same functional group but differ in the position of the functional group.
For example: propane-1-ol and propane-2-ol
positional isomers

Stereo isomers

Stereo isomers have the same molecular formula and the same structure but have different arrangement in space.
Stereoisomers can be further subdivided into Geometrical and Optical.

Geometrical isomers (cis and trans isomerism)

Geometrical isomers have the same structure and molecular formula but differ in the geometric arrangement.
For example: 1,2-dibromoethene geometrical isomers

Optical isomers

Optical isomers have the same molecular formula and structure but different arrangement in space, as the isomers are mirror image to each other and cannot be superimposed.
For example: butane-2-ol optical isomers
Optical isomers must at least have one chiral center. Chiral is a carbon with four atoms or groups attached to it which no matter how molecule is rotated, twisted or turned, it cannot superimpose to its mirror images. chiral center

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