Lucas' reagent
Encyclopedia
Lucas' reagent is a solution of zinc chloride
in concentrated hydrochloric acid
. This solution is used to classify alcohol
s 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 colourless to turbid, signalling formation of a chloroalkane. 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.
s:
The differing reactivity reflects the differing ease of formation of the corresponding carbocation
s. 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. 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:
Zinc chloride
Zinc chloride is the name of chemical compound with the formula ZnCl2 and its hydrates. Zinc chlorides, of which nine crystalline forms are known, are colorless or white, and are highly soluble in water. ZnCl2 itself is hygroscopic and even deliquescent. Samples should therefore be protected from...
in concentrated hydrochloric acid
Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....
. This solution is used to classify alcohol
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....
s of low molecular weight. The reaction is a substitution in which the chloride replaces a hydroxyl
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...
group. A positive test is indicated by a change from colourless to turbid, signalling formation of a chloroalkane. 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.
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 halideHalide
A halide is a binary compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound. Many salts are halides...
s:
- ROH + HCl → RCl + H2O
The differing reactivity reflects the differing ease of formation of the corresponding carbocation
Carbocation
A carbocation is an ion with a positively-charged carbon atom. The charged carbon atom in a carbocation is a "sextet", i.e. it has only six electrons in its outer valence shell instead of the eight valence electrons that ensures maximum stability . Therefore carbocations are often reactive,...
s. Tertiary carbocations are far more stable than secondary carbocations, and primary carbocations are the least stable.
An equimolar mixture of ZnCl2 and HCl
Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....
is the reagent. The alcohol
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....
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
Turbidity
Turbidity is the cloudiness or haziness of a fluid caused by individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality....
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 between the three classes of alcohols:
- no visible reaction: primary, such as normal amyl alcoholAmyl alcoholAn amyl alcohol is any of 8 alcohols with the formula C5H11OH. A mixture of amyl alcohols can be obtained from fusel alcohol. Amyl alcohol is used as a solvent and in esterfication for example in the production of amyl acetate...
(1-Pentanol1-Pentanol1-Pentanol, , is an alcohol with five carbon atoms and the molecular formula C5H12O. 1-Pentanol is a colorless liquid with an unpleasant aroma. There are 8 alcohols with this molecular formula . The ester formed from butanoic acid and 1-pentanol, pentyl butyrate, smells like apricot...
) - solution turns cloudy in 3-5 minutes: secondary, such as sec-amyl alcohol (2-Pentanol2-Pentanol2-Pentanol is an organic chemical compound. It is used as a solvent and an intermediate in the manufacture of other chemicals. 2-Pentanol is a component of many mixtures of amyl alcohols sold industrially....
) - solution turns cloudy immediately, and/or phases separate: tertiary, such as tert-amyl alcohol (2-Methyl-2-butanol2-Methyl-2-butanol2-Methyl-2-butanol, also known as tert-amyl alcohol or amylene hydrate, is one of the isomers of amyl alcohol. It is a clear, colorless liquid with a strong odor of peppermint or camphor. In humans it possesses sedative, hypnotic, and anticonvulsant effects similar to ethanol through ingestion or...
)