Alcohols are very similar to halogenoalkanes, in the sense that these also have a polar group attached to the Carbon and they perform more or less the same reactions. The difference that alcohols have from halogenoalkanes is that apart from having a polar element, which is the Oxygen attached to the Carbon chain, they also have a Hydrogen atom attached to the Oxygen, resulting in separation of charge, with the Oxygen taking electrons from the Hydrogen. The resultant charge on the Hydrogen cannot be stabilised and thus this would be open for attack, giving alcohols their reactivity.
Apart from reactivity, this charge separation introduces Hydrogen bonding which is attractive forces between adjacent molecules in between the Oxygen and Hydrogen atoms. This Hydrogen bonding increases the boiling point of alcohols, and all alcohols can be found as liquids:
The naming of alcohols is similar to that of halogenoalkanes but the suffix would be an –ol, to show that a hydroxyl group is attached to the molecule.
Reactivity of alcohols is similar to that of halogenoalkanes, in the sense that tertiary alcohols would react in an SN1 mechanism while primary alcohols would react in an SN2 mechanism.
Preparation of Alcohols
Alcohols can be prepared using several methods, with the most common being:
From Carbonyl compounds
Alcohols can react from two different positions, either on the Oxygen or on the α Carbon. The difference between the two reactions is that when a reaction is taking place on the Oxygen the alcohol is acting as a nucleophile while when it reacts on the Carbon it would be a nucleophilic substitution.
In order to act as a nucleophile the alcohol would have to be reacted with Sodium, and this would produce an alkoxide or an alcohol which would have lost a Hydrogen from the hydroxide group.
This test can also be used as a test for alcohols, due to the fact that Hydrogen is produced.
This is the household reaction for the production of an ester, although it must be noted that it is not the most efficient, especially considering that the equation is in equilibrium.
The sulfuric acid would act both as a catalyst and as a dehydrating agent.
Production of halogenoalkanes
The hydroxyl group can take a proton from the solution to produce a hydroxonium ion, which is a very good leaving group.
In order to produce a halogenoalkane then a halide would have to be introduced into the solution, which would act as a nucleophile and produce the halogenoalkane.
Reaction with Sulfuric acid
As seen in the chapter of alkenes, alkenes can be produced from alcohols using sulfuric acid as a dehydrating agent. The sulfuric acid would protonate the alcohol, producing a nucleophile, which would be the sulfonate (HSO4-). This nucleophile would then react with the protonated alcohol, which would produce the following intermediate:
This reaction can then continue in either of two ways:
production of an alkene
production of an ether
If excess sulfuric acid is added and the temperature is more then 170oC then the alkene would be formed.
If excess alcohol is added and the temperature is just 140oC then the ether would be formed.
Alcohols can be oxidised to ketones, aldehydes or carboxylic acids, according to the starting products. Primary alcohols can be oxidised to both the carboxylic acids or aldehydes. If sodium dichromate is used then the aldehyde is formed. If potassium permanganate is used then the acid would be obtained. It must be noted that if sodium dichromate is heated with the alcohol then the carboxylic acid would still be formed.
Secondary alcohols would form a ketone when they are heated with sodium dichromate. Tertiary alcohols cannot be oxidised.
Test for alcohols
A test for alcohols is the Lucas’ Test, in which one can analyse whether an alcohol is a primary alcohols, secondary alcohols or tertiary alcohol.
The Lucas test is a test analysing the type of alcohol, by producing a substitution reaction with HCl in the presence of dry ZnCl2. The reaction is as follows:
Due to the fact that the reaction favours the SN1 mechanism, tertiary alcohols would react faster than secondary alcohols while these would have reacted faster than primary alcohols.
- no visible reaction: primary
- the solution turns cloudy in 3-5 minutes: secondary
- the solution turns cloudy immediately, and/or phases separate: tertiary alcohols
Apart from the Lucas’ test, which can be performed on all alcohols, the Iodoform test can be produced on any alcohol which has got –CH(OH)CH3 group, or better still the C=OCH3 group.
The iodoform test is as follows:
It must be noted that for ketones and aldehydes the same reaction can take place, but the first step, ie. the oxidation of the alcohol group to a carbonyl group would not take place.