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Organic Chemistry

Hot goIITian

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3 Apr 2009 14:43:15 IST

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wat is lucas test????

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wat us lucas test????

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A$PIRing HiGh - dEv aDitYa

Blazing goIITian

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3 Apr 2009 14:56:47 IST

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lucas test(ZnCl2 and HCl) distinguishes betn 1,2and 3 degree alcohols. 3 degree alcohol gives turbidity immediately. 2 degree alcohol gives it after 5 mins. and 1degree alcohol doesn't produce turbidity at room temp.

hari

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3 Apr 2009 14:58:19 IST

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In this experiment you will test the relative ease of

conversions of several alcohols into alkyl chlorides

with Lucas reagent (HCI + ZnCl₂ ).

tertiary alcohols react immediately with Lucas reagent to

produce turbidity while secondary alcohols do so in five

minutes

Primary alcohols do not react appreciably with Lucas

reagent at room temperature

The reaction may occur by one of two mechanisms, S_N1 or

S_N2, depending on the structure of the R

group.

In either mechanism, the first step is a rapid protonation of

the alcohol to form an oxonium ion. What

happens next depends on the

nature of R. If R is a group that readily forms a carbocation,

then the slow, rate-determining step will

be the loss of a water molecule

from the oxonium ion. The carbocation then reacts rapidly

with a halide ion to form the alkyl halide. This mechanism,

called S_N1

the mechanismfollows in the following manner

Bipin Dubey

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Joined: 23 Jan 2007

Posts: 7942

3 Apr 2009 15:29:12 IST

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The Lucas test is a test in alcohols that is conducted to test and differentiate between the types of primary, secondary or tertiary alcohol. It uses the differences in reactivity of hydrogen halides and the three classes or types of alcohol. In the reaction the hydrxy group is replaced by chlorine. The time it takes in the appearance of turbidity is used as a measure for determining the class of alcohol. A primary alcohol generally implies that no reaction will occur, secondary alcohols result in the solution turning cloudy after a period of three to five minutes and if the solution immediately turns cloudy it signifies that the alcohol present is tertiary, benzyl alcohol or allyl alcohol.

Reagent used : zinc chloride (ZnCl₂) in concentrated aqueous hydrochloric acid (HCl)

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