用户:Zhantongz/沙盒4

卢卡斯试剂是无水氯化锌的浓盐酸溶液。在有机分析中，卢卡斯试剂用作六碳和六碳以下的伯、仲、叔醇的鉴别试剂。卢卡斯试剂与醇发生亲核取代反应，氯原子将羟基取代，通过反应速度来鉴别。^[1] The test was reported in 1930 and became a standard method in qualitative organic chemistry.^[2] The test has since become somewhat obsolete with the availability of various spectroscopic and chromatographic methods of analysis.

Lucas test

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:^[3]

ROH + HCl → RCl + H₂O

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 ZnCl₂ and HCl is the reagent. The alcohol is protonated by this mixture, and H₂O 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.^[3] 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 between the three classes of alcohols:

no visible reaction: primary, such as normal amyl alcohol (1-Pentanol)
solution turns cloudy in 3-5 minutes: secondary, such as sec-amyl alcohol (2-Pentanol)
solution turns cloudy immediately, and/or phases separate: tertiary, such as tert-amyl alcohol (2-Methyl-2-butanol)

References

^ Shriner, R. L.; Fuson, R. C. The Systematic Identification of Organic Compounds 5th. New York: John Wiley. 1956: 117–119. OCLC 732878490.
^ Lucas, H. J. A New Test for Distinguishing the Primary, Secondary and Tertiary Saturated Alcohols. Journal of the American Chemical Society. 1930, 52 (2): 802–804. doi:10.1021/ja01365a053.
^ ^3.0 ^3.1 Kjonaas, R. A.; Riedford, B. A. A Study of the Lucas Test. Journal of Chemical Education. 1991, 68 (8): 704. doi:10.1021/ed068p704.

[1] Shriner, R. L.; Fuson, R. C. The Systematic Identification of Organic Compounds 5th. New York: John Wiley. 1956: 117–119. OCLC 732878490.

[2] Lucas, H. J. A New Test for Distinguishing the Primary, Secondary and Tertiary Saturated Alcohols. Journal of the American Chemical Society. 1930, 52 (2): 802–804. doi:10.1021/ja01365a053.

[Kjonaas-3] 3.0 ^3.1 Kjonaas, R. A.; Riedford, B. A. A Study of the Lucas Test. Journal of Chemical Education. 1991, 68 (8): 704. doi:10.1021/ed068p704.

[1]

[2]

[3]