Aromatic compound

 The closed cycle compound with delocalized electron is called aromatic compound.

Aromatic compound are these closed cycle organic compound that have at least 4n + 2 number of the delocalized  pi electron. 

According to Huckel rule aromatic compound should have 4n + 2 number of pi electron. 

Properties of aromatic compound is called aromaticity. All of the closed compound are not aromatic. 

Though, in general thought compound with one or more benzene ring that have delocalized electron is aromatic. But in really compound except benzene ring may be aromatic compound.

Example, Pyrole is a aromatic compound. 

Unsaturated organic compound

 Organic compound with at least on double bond or triple bond in carbon carbon chain is called unsaturated organic compound or unsaturated hydrocarbon.

Alkene is unsaturated hydrocarbon because it have carbon carbon double bond. A one sigma bond and one pi bond.

Alkyne is also an unsaturated hydrocarbon because it have carbon carbon triple bond. A one sigma bond and two pi bond.

Position isomerism

 Structural isomerism due to different position of pi bond or functional group but same molecular formula is called position isomerism.

Functional group isomerism

 The isomerism due to different functional group but same molecular formula is functional group isomerism.

Metamorism

 Metamorism is a special types of structural isomerism due to differently division of carbon chain in both side by same functional group but having same molecular formula.

Tautomerism

 Structural isomerism due to spontaneous movement of an atom from one carbon to another carbon in carbon carbon chain.

Optically active molecule

 The molecule that responses to plane polarized light while passings through it in a Mannar that it rotates pane polarized light is called optically active molecule. 

D lactic acid and l lactic acid is optically active molecule. 

d lactic acid rotates plane polarized light to right and l lactic acid rotates plane polarized light to left.

The condition of being optically active is as bellow

Must have to be chiral carbon

Two isomer will be coinside on each other

Isomer will be mirror image of each other

Two isomer of optically active molecule may be enantiomers of each other or may not be enantiomers. If two isomer is not enantiomers these will be diasteriomer of each other.

Any optically active molecule or the mixture of molecules may seems optically inactive. For example, though a meso compound optically active but it does not rotates plane polarized light.  As same as though in racemic mixture there have two optically active isomer but shows no rotation of plane polarized light. In fact in racemic mixture rotation by one isomer eliminate by rotation by other isomer.