Alcohols

Introduction

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:

alcohol 1

Naming

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

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 alkenes

CH₃CH₂OH $\xrightarrow[170^oC]{H_2SO_4}$ CH₂=CH₂

From Halogenoalkanes

CH₃CH₂I + NaOH_(aq) → CH₃CH₂OH + NaI _(reflux)

From Esters (Future Chapters)

From Carbonyl compounds (Future Chapters)

Reactions

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 alcohol which would have lost a Hydrogen from the hydroxide group.

$2CH_3CH_2OH + 2Na\rightarrow 2CH_3CH_2O^-Na^+ + H_2$

This test can also be used as a test for alcohols, due to the fact that Hydrogen is produced.

Esterification

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.

Difference-Between-Protonation-and-Deprotonation-1

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 hydrogensulfate (HSO₄^–). This nucleophile would then react with the protonated alcohol, which would produce the following intermediate:

$C_2H_5OH + H\cdots OSO_3H \xrightarrow[]{140^oC} C_2H_5\cdots OSO_3H + H_2O$

This reaction can then continue in either of two ways:

production of an alkene

$C_2H_5OSO_3H \xrightarrow[]{170^oC}CH_2=CH_2 + H_2SO_4$

production of an ether

$C_2H_5OSO_3H + HOC_2H_5 \xrightarrow[]{140^oC}C_2H_5OC_2H_5 + H_2SO_4$

If excess sulfuric acid is added and the temperature is more then 170^oC then the alkene would be formed.

If excess alcohol is added and the temperature is just 140^oC then the ether would be formed.

Oxidation

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.

$CH_3CH_2OH\xrightarrow[reflux]{KMnO_4}CH_3COOH$

$CH_3CH_2OH\xrightarrow[distil]{K_2Cr_2O_7}CH_3CHO$

Secondary alcohols would form a ketone when they are heated with sodium dichromate. Tertiary alcohols cannot be oxidised.

$CH_3CH(OH)CH_3\xrightarrow[reflux]{Na_2Cr_2O_7}CH_3C(O)CH_3$

Reaction with PCl5

Solid phosphorus(V) chloride (phosphorus pentachloride) reacts violently with alcohols at room temperature, producing clouds of hydrogen chloride gas. It isn’t a good choice as a way of making chloroalkanes, although it is used as a test for -OH groups in organic chemistry.

$CH_3CH_2OH + PCl_5 \rightarrow CH_3CH_2Cl$

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