carbocations pentanol reagent zncl alcohols fuson butanol carbocation oily rcl chromatographic halides shriner nucleophile turbidity turbid reactivity hcl protonated colourless anhydrous alkyl solubility appreciably reagents roh spectroscopic hydrochloric chloride hydroxyl
why does the lucas test fail when the alcohol is not soluble in the reagent
Lucas ' reagent - wikipedia Lucas ' reagent Jump to : navigation , search Lucas test : negative ( left ) with ethanol and positive with t - butanol Lucas ' reagent is a solution of anhydrous zinc chloride in concentrated hydrochloric acid . This solution is used to classify alcohols of low molecular weight . The reaction is a substitution in which the chloride replaces a hydroxyl group . A positive test is indicated by a change from clear and colourless to turbid , signalling formation of a chloroalkane . Also , the best results for this test are observed in tertiary alcohols , as they form the respective alkyl halides fastest due to higher stability of the intermediate tertiary carbocation . The test was reported in 1930 and became a standard method in qualitative organic chemistry . The test has since become somewhat obsolete with the availability of various spectroscopic and chromatographic methods of analysis . It was named after Howard Lucas ( 1885 -- 1963 ) , an American chemist . Lucas test ( edit ) Lucas test in alcohols is a test to differentiate between primary , secondary , and tertiary alcohols . It is based on the difference in reactivity of the three classes of alcohols with hydrogen halides via an S 1 reaction : ROH + HCl → RCl + H O The differing reactivity reflects the differing ease of formation of the corresponding carbocations . Tertiary carbocations are far more stable than secondary carbocations , and primary carbocations are the least stable . An equimolar mixture of ZnCl and HCl is the reagent . The alcohol is protonated by this mixture , and H O group attached to carbon is replaced by the nucleophile Cl , which is present in excess . Tertiary alcohols react immediately with Lucas reagent as evidenced by turbidity owing to the low solubility of the organic chloride in the aqueous mixture . Secondary alcohols react within five or so minutes ( depending on their solubility ) . Primary alcohols do not react appreciably with Lucas reagent at room temperature . Hence , the time taken for turbidity to appear is a measure of the reactivity of the class of alcohol , and this time difference is used to differentiate among the three classes of alcohols : no visible reaction at room temperature and forming an oily layer only on heating : primary , such as 1 - pentanol solution forms oily layer in 3 -- 5 minutes : secondary , such as 2 - pentanol solution forms oily layer immediately : tertiary , such as 2 - methyl - 2 - butanol References ( edit ) Jump up ^ Shriner , R.L. ; Fuson , R.C. ( 1956 ) . The Systematic Identification of Organic Compounds ( 5th ed . ) . New York : John Wiley . pp. 117 -- 119 . OCLC 732878490 . Jump up ^ Lucas , H.J. ( 1930 ) . `` A New Test for Distinguishing the Primary , Secondary and Tertiary Saturated Alcohols '' . Journal of the American Chemical Society . 52 ( 2 ) : 802 -- 804 . doi : 10.1021 / ja01365a053 . ^ Jump up to : Kjonaas , R.A. ; Riedford , B.A. ( 1991 ) . `` A Study of the Lucas Test '' . Journal of Chemical Education . 68 ( 8 ) : 704 . doi : 10.1021 / ed068p704 . Retrieved from `` https://en.wikipedia.org/w/index.php?title=Lucas%27_reagent&oldid=787604989 '' Categories : Chemical tests Analytical reagents Talk About Wikipedia Afrikaans Deutsch Español Français Italiano Bahasa Melayu Nederlands Polski தமிழ் Українська 中文 Edit links This page was last edited on 26 June 2017 , at 11 : 23 . Text is available under the Creative Commons Attribution - ShareAlike License ; additional terms may apply . By using this site , you agree to the Terms of Use and Privacy Policy . Wikipedia ® is a registered trademark of the Wikimedia Foundation , Inc. , a non-profit organization . About Wikipedia