Alcohol: classification and reactions

In a nutshell

Alcohols are molecules with the functional group $$-OH$$ and have the prefix 'hydroxy-' or the suffix '-ol.' The hydroxy group enables alcohols to form hydrogen bonds with other molecules such as water. Alcohols undergo a number of reactions, such as dehydration to form alkenes. 

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Equations 

These are general chemical equations. You will need to be able to use these to write equations with specific substances.

​$$\begin{aligned}ROH \space + PCl_5 &\rightarrow RCl \space+ \space HCl \space+ \space POCl_3 \\\\ROH \space + \ HCl &\rightarrow RCl \ + \ H_2O \\\\ROH &\xrightarrow {Br^-, \ 50\% \ c.H_2SO_4} RBr \ + \ H_2O \\\\3 \,ROH \space + \ PI_3 &\rightarrow 3 \,RI \ + \ H_3PO_3 \\\\ alcohol &\xrightarrow {H_2SO_4} alkene \ + \ water\end{aligned}$$​​

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Classifying alcohols

​Alcohols are molecules with the functional group $$-OH$$ and can be classed as primary, secondary or tertiary alcohols. They have the general formula $$C_nH_{2n+1}OH$$. 

Classification of alcohols

Reactions of alcohols

Alcohols can act as a starting molecule for various reactions to form a product with the desired functional group.

Chloroalkanes

Alcohols can react with phosphorus pentachloride or hydrochloric acid to form chloroalkanes.

Reaction with phosphorus pentachloride

​$$ROH \space + PCl_5 \rightarrow RCl \space+ \space HCl \space+ \space POCl_3$$​​

Example

​$$ethanol \space + phosphorous \space pentachloride \rightarrow chloroethane \space+ \space hydrogen \space chloride \space+ \space phosphoryl \space chloride\\ \ \\C_2H_5OH \space + PCl_5 \rightarrow C_2H_5Cl \space+ \space HCl \space+ \space POCl_3$$​​

Reaction with hydrochloric acid

​$$ROH \space + \ HCl \rightarrow RCl \ + \ H_2O$$​​

Example

$$propanol \space + \ hydrochloric \ acid \rightarrow chloropropane \ + \ water\\ \ \\C_3H_7OH \space + \ HCl \rightarrow C_3H_7Cl \ + \ H_2O$$​​

Bromoalkanes

Alcohols can undergo a two-step substitution reaction with bromide ions, in the presence of an acid catalyst, such as  $$50 \%$$ concentrated sulfuric acid, to form bromoalkanes. The bromide ions react with the sulfuric acid to form hydrogen bromide, which goes on to react with the alcohol to form a bromoalkane.

​$$ROH \xrightarrow {Br^-, \ 50\% \ c.H_2SO_4} RBr \ + \ H_2O$$​​

Example

Below is an example of a reaction mechanism using ethanol.

Firstly, potassium bromide, which is a source for bromide ions, reacts with sulfuric acid.

​$$potassium \ bromide \ + sulfuric \ acid \rightarrow hydrogen \ bromide \ + potassium \ sulfate \\ \ \\2KBr \ + H_2SO_4 \rightarrow 2HBr \ + \ K_2SO_4$$​​

The hydrogen bromide formed goes on to react with ethanol.

​$$ethanol \ + \ hydrogen \ bromide \rightarrow bromoethane \ + \ water \\ \ \\C_2H_5OH \ + \ HBr \rightarrow C_2H_5Br \ + \ H_2O$$​​

Iodoalkanes 

Alcohols react with phosphorus triiodide to form iodoalkane. Phosphorus triiodide is made in situ when refluxing the reaction mixture of red phosphorus, iodine and the alcohol.

​$$3 \,ROH \space + \ PI_3 \rightarrow 3 \,RI \ + \ H_3PO_3 \\$$​​

Example

​$$ethanol \ + phosphorous \ triiodide \rightarrow iodoethane \ + \ phosphorous \ acid\\ \ \\3 \, C_2H_5OH \ + \ PI_3 \rightarrow 3 \,C_2H_5I \ + \ H_3PO_3 \\$$​​

Dehydration

Alkenes can be formed from alcohols by dehydrating them, in the presence of an acid catalyst, such as concentrated sulfuric acid. Polymers can be made by using alcohol as a starting material as it is a renewable resource. 

​$$alcohol \xrightarrow {c.H_2SO_4} alkene \ + \ water$$​​

Example

​$$ethanol \xrightarrow{c.H_2SO_4} ethene \ + \ water \\ \ \\C_2H_5OH \xrightarrow{c.H_2SO_4} C_2H_4 \ + \ H_2O$$​​

You need to know the mechanism for the dehydration of an alcohol.

Example

Mechanism for the dehydration of ethanol.

Unsymmetrical alcohols can give rise to a mixture of products, including E/Z isomers.

Example

Butan-$$2$$-ol dehydrated to give a mixture of alkenes including E/Z isomers.

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Hydrogen bonding

The polar nature of the hydroxyl group allows smaller alcohol molecules to be soluble in water. Hydrogen bonds form between the hydroxyl group of an alcohol and the water molecules. Alcohols may also form hydrogen bonding with each other via the hydroxyl groups. The presence of hydrogen bonds explains their low volatility, in comparison to similar sized organic compounds, such as alkanes. 

Solubility in water decreases as chain length of an alcohol increases, as larger alcohols primarily consist of a non-polar carbon chain.