A Study of the Lucas Test Richard A. Kjonaas Indiana State University, Terre Haute, IN 47809 Bernie A. Riedford Mt. Vernon Senior High School, Mt. Vernon, IN 47620 Because of modern instrumentation, most of the classical wet tests used for organicqualitative analysis no longer have any value in chemical research and quality control. Some of them do, however, continue t o serve as valuable tools for teaching organic functional group reactivity in sophomore organic and allied health chemistry. One very well known example, the Lucas test,' distinguishes between primary, secondary, and tertiary ( l o ,ZO, and 3') alcohols, and demonstrates the relative ease of lo,ZO,and 3' carhocation formation. Even thouah the Lucas test is venr easv to undersand and perfurm,.st;dentsoften fail t o r e p o ~ a p & i t i v etest with Z0 alcoholn. This observation holds true when using either a macro- or microscale procedure. Furthermore, lab manuals are unclear in defininr! the range of alcohols for which the test is applicable. Theoriginal paper by Lucas is not of much help because it is based upon so few examples. In an effort to optimize its reliability for the teaching laboratory, we have undertaken a study of the Lucas test and our observations are reported herein. T o review the Lucas test briefly, an unknown alcohol is treated with several volumes of Lucas reagent (ZnCls and concentrated hydrochloric acid in a 1:l molar ratio). As shown in the equation below, this reagent transforms the alcohol into an alkyl chloride. ROH + HCl-
RCL
+ H,O
Since the alkyl chloride is not soluble in the Lucas reagent, the test mixture becomes cloudv and eventuallv a senarate layer appears. The appearance of cloudiness is usually what the student is instructed to watch for. Hecause of the relative ease of 3 O carbocation formation, 3 O alcohols react instantly, whereas loalcohols require much longer than 1 h. For Z0 alcohols, test times such as "3-4 min", "about 5 min", and "3-10 min" are usually cited by the authors of lab manuals. Alcohols that are not sufficiently soluble in the reagent might lead to a false conclusion since their insolubility can bemistaken for alkyl chloride formation. As a follow-up teat to confirm 3 O alcohols, the test can be repeated using concentrated HCI rather than the Lucas reagent; 3' aliohole should react within 5min, whereas loand 2' alcohols should not react within any reasonable length of time. For this study we performed the Lucas test using cyclouentanol. cvclohexanol. and all 33 (ienorine stereoisomerism) of the saturated aciclic monofun~ional&ohols having six or fewer carbonatoms (all C,H~,,~Oalcoholswithn 3 6). Also, in keeping with the trend toward microscale procedures, drops rather than milliliters were used as the volume unit. I t should be noted, however, that a certain alcohol/ reagent volume ratio measured in drops might not be precisely the same milliliter ratio since the size of the drops varies among different compounds. Presented In palt In the Division of Chemlcal Educatlon Poster Session at the 24th Mldwest Reglonal Meetlng of the American Chemical Soclety, November 1-3, 1989. Lucas. H. J. J. Am. Cham. Soc. 1930,52.802-804.
'
704
Journal of Chemical Education
Experimental Pasteur pipets C'disposahle pipets") were used for dispensing drops of the Lucas reagent and all of the alcohols except neopentyl alcohol, which is a solid at room temperature. The Lucas reagent wasprepared by dissolving 1.00 mol(136 g, Fisher certified ACS) of zinc chloride in 1.M) ma1 (100 g or 84.0 mL, Fisher reagent ACS) of concentrated hydrochloric acid with cooling. In performing the Lucas test, the alcohol (1,2,3, or 4 drops) and reagent (10 drops) were placed into a 13 mm X 100 mm test tube followed immediately by vigorous swirling of the test tube for about 3 to 5 s to mix the contents and then setting it aside. The amount of time between the initial addition and the first appearance of a definite eloudineas was taken as the test time. The temperatureof the room and test materiala was closely held at either 20 'C or 25 "C. When necessary to distinguish between insolubility and a very fast reaction, the cloudy test mixture was immediately centrifuged,and the organic layer was withdrawn with a Pasteur pipet and analyzed hy IR and NMR spectroscopy and hy gas chromatography.
Observatlm and Dlscuoslon Table 1is a list of the 1"and 3 O alc'ohols used in this studv along with their cost and boiling points. As expected, the 3; alcohols reacted instantlv, and the loalcohols failedto eivea positive test within 1 h . I n fact, with these alcohols-it is almost impossible to do the test in such a way as to get a n incorrect result! Table 1. Cost and Bolllng Polnt of all 22. Nonsacondary C#)I,*,O ( n 5 6 ) Alcohols Alwhol
Boillng Polnt. 'C
Cost, @/go
memano1 PRIh4ARYALCOhwLS
ethanol 1-propanol 2-methyl-lpropanol 2,2dime~yl-l-propanol l-butanol 2-methyl-1-butanol 3-melhyl-l-butanol 22dimelhyl-l-butan01 1-pentanal 3.3dlmeMyC1-butanol 2.3dlmelhyI-1-butand 2-melhyl-l-pamano1 24h+l-bulanol 4-melhyl-l-pentan01 3-melhyl-l-pantand l-hexanol
78 97 10s 113(mp = 52 'C) 117 129 132
1 1 2 45 1 1 2
ERTIARY ALCOHOLS
2-melhyl-2-pmpanol 2-melhyl-2-butanol 2.3dlmelhyl-2-butand 2-melhyl-2-pentanol 3-methyl-3-pentanol Ignoring sterecl~somar~sm.
,-"
'eased on me beat 1990 plce in qusmitles of 2 kg a leu d at marial.
,,
least 97%
pure
Table 2. Lucaa Test TImm ol Cyclownlanol, Cyclohexanol, and All 11. ol the Secondary C , J i 2 d r 0 ( n 5 6) Alcohols wlth 10 Drops ol
Alcohol
Boiling
Order of
Point, "C
Additionb A to R R to A
l(20 'C)
3.0
A to R R to A A to R R to A A to R R to A AIoR R to A AIoR
0.5
0.5
1.3
Cost.Q/g
1.0 1.0 0.25 0.25 0.25 0.3 0.5 0.75
0.25 0.2
0.1 0.1 0.1 0.1
1.3
0.5 0.7
0.2
0.2
RIoA RtoA AtoR R to A A to R R to A AtoR R to A
q25 ' C )
15 20
-L
AtoR R to A A to R RtoA
A to R
Test Time.'Minutes Drops of Alcohol (TempraNre.OC) l(25 ' C ) 2(25 OC) 3(25 'C)
0.2 0.25 1 0.25
0.2
0.3 0.5 0.5 0.15 0.15
-
'Ign~ring~ t e r B O i m r i S m .
'"A tow means mat the alcohol was added to ma reagent: "R M A" means that the reagent was added to the alcohol.
