Alcohols

[Mr.Chemist]

Good day! I am an undergraduate student and one of my subjects is organic chemistry. It was really a challenging subject especially when we do the laboratory experiments. One experiment is testing for alcohols. We used ethanol, n-butanol,sec-butanol,tert-butanol, benzene alcohol, and methyl chloride. I tried to read discussions about these lucas test, chromic acid test, iodoform test, and methyl alcohol test but i can't fully grasp the material. Can you please help me about the solubility of the alcohols to water? How about the reactivity of the primary, secondary, and tertiary alcohols? Did my teacher get it correctly that when we dip a hot copper wire, only the oxygen is being transferred into the methyl chloride not the CuO itself? Thank you.

[Suxamethonium]

Um, so there is a lot to address in this post. Firstly, the alcohols you listed.

 

Ethanol (EtOH) is a primary alcohol, as is n-butanol (nBuOH). Sec-butanol (sBuOH) is a secondary alcohol and tert- butanol (tBuOH) is a tertiary alcohol. benzene alcohol? Possibly benzyl alcohol or phenol- either way representing aromatic alcohols. Methyl chloride is obviously not an alcohol.

 

The lucas test rarely works for primary alcohols (very very slow), is slow (15 mins) for secondary alcohols and is relatively fast for tertiary alcohols. The chromic acid test will react with primary and secondary alcohols but not tertiary. The iodoform test is for methyl ketones (RCOMe). I do not know of the methanol test.

 

Solubility: Generally as the chain length increases the alcohol becomes less soluble in water.

 

As for testing for chlorides, we used to clean the copper wire, flame it and then dip it in the sample and reflame. If the flame was greenish a halogen was present.

[Mr.Chemist]

Um, so there is a lot to address in this post. Firstly, the alcohols you listed.

 

Ethanol (EtOH) is a primary alcohol, as is n-butanol (nBuOH). Sec-butanol (sBuOH) is a secondary alcohol and tert- butanol (tBuOH) is a tertiary alcohol. benzene alcohol? Possibly benzyl alcohol or phenol- either way representing aromatic alcohols. Methyl chloride is obviously not an alcohol.

 

The lucas test rarely works for primary alcohols (very very slow), is slow (15 mins) for secondary alcohols and is relatively fast for tertiary alcohols. The chromic acid test will react with primary and secondary alcohols but not tertiary. The iodoform test is for methyl ketones (RCOMe). I do not know of the methanol test.

 

Solubility: Generally as the chain length increases the alcohol becomes less soluble in water.

 

As for testing for chlorides, we used to clean the copper wire, flame it and then dip it in the sample and reflame. If the flame was greenish a halogen was present.

 

 

 

yah, it's benzyl alcohol... thank you for clarifying things to me. I really have to learn a lot of things about organic chemicals.

Edited February 7, 2012 by Mr.Chemist

[Suxamethonium]

Oh, also I missed a bit about reactivity in my above post (I was a little hurried). Primary alcohols undergo a reaction known as S_N2 where a nucleophile (say cyanide CN^-) attacks the alcoholic carbon from the opposite side of the OH group. As the CN^- gets closer the OH gets further away and eventually leaves as OH^-. Secondary alcohols can also undergo this mechanism (but at a reduced rate due to steric hinderance). Tertiary alcohols are too hindered for this mechanism to work, and instead the OH^- leaves first (because the carbo-cation is stable enough to remain independantly). When the CN^- encounters the carbocation a the new compound forms (however the product is racemic). Again secondary alcohols can also react in this way.

 

As such, tertiary alcohols are more reactive as the rate limiting step is the first step dependant on carbocation stability. Secondary alcohols usually follow (depending on the stability of the carbocation formed) and primary alcohols, having no stable carbocations, are soley rate dependant on the concentration of CN^- (as well as to drive the reaction forward) which is available to 'attack'.