Dear Kifayat Bukhari thank you for posting this interesting technical question. I think you did exactly the right thing when you carried out a reaction of n-butanol with 4-hydroxy benzoic in the presence of sulfuric acid as acidic catalyst. This is exactly the way in which butylparaben is manufactured in industry. I assume that you perhaps did not heat the reaction mixture long enough. The following detailed description of the synthesis calls for 8 hours of heating to reflux:

"Butanol and p-hydroxybenzoic acid are heated together for being dissolved, slowly added dropwise of sulfuric acid, continue the refluxing for 8 h. After cooling, add 4% sodium carbonate solution, separate the water layer, steam out the butanol, let it cool, filter to obtain the crude product, and then carry out ethanol recrystallization (solubility in ethanol: 200g/100ml)."

The synthetic procedure has been mentioned in the following link:

https://www.chemicalbook.com/ChemicalProductProperty_EN_cb5466788.htm

Good luck with your work and best wishes, Frank Edelmann

You can reflux the reaction mixture until the starting material get consumed.

Use strongly acidic macroporous cation-exchanger (based on sulfonated PS-DVB copolymer) in amount 5-10 mol% as catalyst. https://www.merckmillipore.com/CZ/cs/products/analytics-sample-prep/analytical-sample-preparation/ion-exchangers/strongly-acid-cation-exchanger/4Yyb.qB..5sAAAE_QwV3.Lxj,nav

Butyl paraben is a white solid that is soluble in organic solvents used as antimicrobial preservative in cosmetics. It is prepared by the esterification of 4-hydroxybenzoic acid with 1-butanol in the presence of an acid such as sulfuric acid.

1.Cosmetic Microbiology. CRC Press. pp. 163–180. doi:10.3109/9781420003321-12. ISBN 978-0-8493-1453-7.

2.Applied and Environmental Microbiology. 72(8): 5556–61. doi:10.1128/AEM.00494-06

3.Parabens as Preservatives" (PDF). UENO FINE CHEMICALS INDUSTRY LTD. Archived from the original(PDF) on 2012-04-25..