Hi Venkatesha,

Six member heterocyclic non aromatic compounds are more stable than five member ring. the six member can be in  chair form in which there is no angle or eclipse strain .   

I agree with Salim.

In theory, if the cycles had a unique 2D conformation, the angles of the 5-membered rings are more closely to the angles of carbon atoms with sp3 hybridation (108º versus 109º, please consider that heteroatoms can modify this angles). A 6-membered planar ring would have 120º, further from the C sp3 tetrahedral angle.

But the 6-membered rings can have 3D conformations, such as the chair conformation (the more stable) and the boat conformation. These conformations relaxe the angles, getting them closer to the tetrahedral angle. Thus, cycle gains stability.

Other advantage of this spacial ordenation is that the cycle can orientate the substituents in the axial and equatorial positions: normally the voluminous substituents goes to the equatorial as the axial positions may have steric hindrance.

please provide me a references...??

six membered

Six membered heterocyclic aromatic compounds are more stable than five membered counterparts. In the case of alicylic heterocyclic compounds six membered rings are more stable than five and four membered rings. Six membered ring compounds stability can be attributed to the puckered nature of the rings, that eliminates the ring strain imparting more stability to the compounds. Five membered rings do have such stabilising factors, but not equal to that of six membered counter parts. Four and three membered rings do not have such stability because they are planar and the atoms experience severe strain.. 

I agree with Salim six membered heterocyclic compounds are more stable than three and four.

Hi Venkatesha NJ 

I agree with Salim Y Hanna, Syed Shafi and with Sankaranarayanan Sethuraman 

Good luck 

Prof. Dr. Talaat El-Emary

six membered ring no angle strain and all ubove answers are enough

Six member heterocyclic  aromatic compounds are more stable than five member ring systems. Read Huckel theory of aromaticity.

Dear

Venkatesha NJ

Six membered ring of the heterocyclic compounds are more stable (less reactive) than the five membered ring due to the bigger bond angle of six membered than the five membered ones according to Huekel theory.

Good luck

Between five membered or six membered heterocyclic compounds  six member ring systems are more stable due to less ring strain  ?

@ Viktor Barat. The bond angle is essential to determine reactivity of the heterocyclic rings, e.g 3-membered ring is more reactive than 4-membred and  4-membred is more reactive than 5-membered and 5-membered is more reactive than 6-membred. 

Please, read well.

six membered ring is more stable

Yes, Prof. Madhukar and Dr. Mohamed Elsabaei, six membered ring is more stable than five-membered.

five membered heterocyclic ring is more stable it has more aromatic property than six membered heterocyclic ring because of the heteroatom.