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
1. Lucas reagent
2. Classification of alcohols
3. Substitution reaction
4. Tertiary carbocations
5. Reactivity of alcohols