# 18.5 Reactions of Alcohols Alcohols are versatile organic compounds that undergo various chemical transformations. Their reactivity is primarily due to the presence of the polar hydroxyl (-OH) group. Reactions generally involve either the cleavage of the C-OH bond or the O-H bond. ### (a) Combustion Alcohols are highly flammable compounds that undergo complete combustion in the presence of excess oxygen to produce carbon dioxide, water, and heat. Due to their high octane rating, they are used as fuels, especially in high-performance engines like those in racing cars. * **Methanol Combustion:** $$ 2\mathrm{CH_3OH} + 3\mathrm{O_2} \rightarrow 2\mathrm{CO_2} + 4\mathrm{H_2O} + \text{heat} $$ $$ \mathrm{CH_3OH} + \frac{3}{2}\mathrm{O_2} \rightarrow \mathrm{CO_2} + 2\mathrm{H_2O} + \text{heat} $$ * **Ethanol Combustion:** $$ \mathrm{CH_3CH_2OH} + 3\mathrm{O_2} \rightarrow 2\mathrm{CO_2} + 3\mathrm{H_2O} + \text{heat} $$ ### (b) Formation of Alkyl Halides Alcohols can be converted to alkyl halides through reactions with various halogenating agents. The hydroxyl (-OH) group is substituted by a halogen atom. These reactions involve C-OH bond cleavage. #### Reaction with Thionyl Chloride ($\mathrm{SOCl_2}$) When alcohols react with thionyl chloride ($\mathrm{SOCl_2}$), typically in the presence of a base like pyridine, they form alkyl chlorides. This method is highly efficient as the by-products, sulfur dioxide ($\mathrm{SO_2}$) and hydrogen chloride ($\mathrm{HCl}$), are gases and escape, driving the reaction to completion. $$ R\text{-}\mathrm{OH} + \mathrm{SOCl_2} \xrightarrow{\text{Pyridine}} R\text{-}\mathrm{Cl} + \mathrm{SO_{2(g)}} + \mathrm{HCl_{(g)}} $$ #### Reaction with Phosphorus Halides ($\mathrm{PX_3}$ and $\mathrm{PX_5}$) Alcohols react with phosphorus trihalides ($\mathrm{PCl_3}$, $\mathrm{PBr_3}$) and phosphorus pentahalides ($\mathrm{PCl_5}$) to yield the corresponding alkyl halides. * **With Phosphorus Trichloride ($\mathrm{PCl_3}$):** $$ 3R\text{-}\mathrm{OH} + \mathrm{PCl_3} \rightarrow 3R\text{-}\mathrm{Cl} + \mathrm{H_3PO_3} $$ * **With Phosphorus Pentachloride ($\mathrm{PCl_5}$):** $$ R\text{-}\mathrm{OH} + \mathrm{PCl_5} \rightarrow R\text{-}\mathrm{Cl} + \mathrm{POCl_3} + \mathrm{HCl} $$ #### Reaction with Hydrogen Halides ($\mathrm{HX}$) The -OH group of an alcohol can be replaced by a halogen atom by reacting it with a hydrogen halide ($\mathrm{HCl}$, $\mathrm{HBr}$, $\mathrm{HI}$). For more on halogens, see . $$ R\text{-}\mathrm{OH} + \mathrm{HX} \rightarrow R\text{-}\mathrm{X} + \mathrm{H_2O} $$ #### The Lucas Test This is a classic test used to distinguish between primary ($1^\circ$), secondary ($2^\circ$), and tertiary ($3^\circ$) alcohols based on their reaction rate with the **Lucas reagent** (a solution of anhydrous zinc chloride, $\mathrm{ZnCl_2}$, in concentrated hydrochloric acid, $\mathrm{HCl}$). The reaction produces an alkyl chloride, which is insoluble in the reagent and appears as a cloudy suspension or an oily layer. $$ R\text{-}\mathrm{OH} + \mathrm{HCl} \xrightarrow{\mathrm{ZnCl_2}} R\text{-}\mathrm{Cl} + \mathrm{H_2O} $$ * **Tertiary ($3^\circ$) Alcohols:** React almost *immediately* to form a cloudy solution or oily layer. $$ (\mathrm{CH_3})_3\mathrm{COH} + \mathrm{HCl} \xrightarrow{\mathrm{ZnCl_2}} (\mathrm{CH_3})_3\mathrm{CCl} + \mathrm{H_2O} \quad (\text{Immediate turbidity}) $$ * **Secondary ($2^\circ$) Alcohols:** React more slowly, forming a cloudy solution in *5 to 10 minutes*. $$ (\mathrm{CH_3})_2\mathrm{CHOH} + \mathrm{HCl} \xrightarrow{\mathrm{ZnCl_2}} (\mathrm{CH_3})_2\mathrm{CHCl} + \mathrm{H_2O} \quad (\text{Turbidity in 5-10 min}) $$ * **Primary ($1^\circ$) Alcohols:** Do not react at room temperature. A reaction occurs only upon *heating*. $$ \mathrm{CH_3CH_2OH} + \mathrm{HCl} \xrightarrow{\mathrm{ZnCl_2}, \Delta} \mathrm{CH_3CH_2Cl} + \mathrm{H_2O} \quad (\text{No reaction at RT}) $$ #### Reaction with Potassium Bromide ($\mathrm{KBr}$) Bromoalkanes can be prepared by heating an alcohol with potassium bromide and concentrated sulfuric acid. $$ R\text{-}\mathrm{OH} + \mathrm{KBr} + \mathrm{H_2SO_4} \xrightarrow{\Delta} R\text{-}\mathrm{Br} + \mathrm{KHSO_4} + \mathrm{H_2O} $$ ### (e) Dehydration of Alcohols Dehydration is the elimination of a water molecule from an alcohol. The product depends on the reaction conditions (temperature and concentration of the acid catalyst). Common dehydrating agents include concentrated sulfuric acid ($\mathrm{H_2SO_4}$) or aluminum oxide ($\mathrm{Al_2O_3}$). * **Formation of an Alkene (Intramolecular Dehydration):** Occurs at higher temperatures. $$ \underset{\text{Ethanol}}{\mathrm{CH_3CH_2OH}} \xrightarrow[\text{170-180}^\circ C]{\text{conc. } \mathrm{H_2SO_4}} \underset{\text{Ethene}}{\mathrm{CH_2=CH_2}} + \mathrm{H_2O} $$ * **Formation of an Ether (Intermolecular Dehydration):** Occurs at lower temperatures with excess alcohol. $$ 2\mathrm{CH_3CH_2OH} \xrightarrow[\text{140}^\circ C]{\text{conc. } \mathrm{H_2SO_4}} \underset{\text{Diethyl ether}}{\mathrm{CH_3CH_2OCH_2CH_3}} + \mathrm{H_2O} $$ ### (f) Formation of Esters (Esterification) Alcohols react with carboxylic acids in the presence of a strong acid catalyst (like conc. $\mathrm{H_2SO_4}$) to form esters. This reversible reaction is known as *esterification*. Esters often have pleasant, fruity smells. This reaction involves O-H bond cleavage of the alcohol. $$ \underset{\text{Carboxylic acid}}{R\text{-}\mathrm{COOH}} + \underset{\text{Alcohol}}{R'\text{-}\mathrm{OH}} \rightleftharpoons \underset{\text{Ester}}{R\text{-}\mathrm{COOR'}} + \mathrm{H_2O} $$ * **Example:** Formation of ethyl acetate from acetic acid and ethanol. $$ \underset{\text{Acetic acid}}{\mathrm{CH_3COOH}} + \underset{\text{Ethanol}}{\mathrm{C_2H_5OH}} \xrightarrow{\mathrm{H_2SO_4}} \underset{\text{Ethyl acetate}}{\mathrm{CH_3COOC_2H_5}} + \mathrm{H_2O} $$ ### (g) Oxidation of Alcohols Alcohols can be oxidized using oxidizing agents like potassium dichromate ($\mathrm{K_2Cr_2O_7}$) or potassium permanganate ($\mathrm{KMnO_4}$) in an acidic solution. The product depends on the type of alcohol. The oxidizing agent generates nascent oxygen, [O], which oxidizes the alcohol. $$ \mathrm{K_2Cr_2O_7} + 4\mathrm{H_2SO_4} \rightarrow \mathrm{K_2SO_4} + \mathrm{Cr_2(SO_4)_3} + 4\mathrm{H_2O} + 3[\mathrm{O}] $$ During the reaction, the orange dichromate ion ($\mathrm{Cr_2O_7^{2-}}$) is reduced to the green chromium(III) ion ($\mathrm{Cr^{3+}}$), causing a distinct color change. * **Primary Alcohols:** Are first oxidized to aldehydes, which can be further oxidized to carboxylic acids upon continued heating. $$ R\text{-}\mathrm{CH_2OH} \xrightarrow{[\mathrm{O}]} \underset{\text{Aldehyde}}{R\text{-}\mathrm{CHO}} \xrightarrow{[\mathrm{O}]} \underset{\text{Carboxylic Acid}}{R\text{-}\mathrm{COOH}} $$ *Example:* $$ \mathrm{CH_3CH_2OH} + [\mathrm{O}] \xrightarrow{\mathrm{K_2Cr_2O_7}/\mathrm{H_2SO_4}} \mathrm{CH_3CHO} + \mathrm{H_2O} $$ $$ \mathrm{CH_3CHO} + [\mathrm{O}] \xrightarrow{\mathrm{K_2Cr_2O_7}/\mathrm{H_2SO_4}} \mathrm{CH_3COOH} $$ * **Secondary Alcohols:** Are oxidized to ketones. Ketones are resistant to further oxidation under mild conditions. $$ R\text{-}\mathrm{CH(OH)}\text{-}R' \xrightarrow{[\mathrm{O}]} \underset{\text{Ketone}}{R\text{-}\mathrm{CO}\text{-}R'} + \mathrm{H_2O} $$ *Example:* $$ \underset{\text{2-Propanol}}{\mathrm{CH_3CH(OH)CH_3}} + [\mathrm{O}] \xrightarrow{\mathrm{K_2Cr_2O_7}/\mathrm{H_2SO_4}} \underset{\text{Propanone}}{\mathrm{CH_3COCH_3}} + \mathrm{H_2O} $$ * **Tertiary Alcohols:** Are resistant to oxidation by mild oxidizing agents because they have no hydrogen atom attached to the carbinol carbon. Under harsh conditions (strong heating), they are dehydrated to form alkenes, which can then be oxidized. ### (h) Iodoform Reaction The iodoform test is used to identify the presence of a methyl group attached to a hydroxyl-bearing carbon ($\mathrm{CH_3\text{-}\mathrm{CH(OH)}\text{-}}$) or a methyl ketone ($\mathrm{CH_3\text{-}\mathrm{CO}\text{-}}$). Ethanol is the only primary alcohol that gives a positive test. When treated with iodine and an alkali (like $\mathrm{NaOH}$), these compounds form a pale yellow precipitate of tri-iodomethane (iodoform, $\mathrm{CHI_3}$). * **Alcohols that give a positive test:** * Ethanol ($\mathrm{CH_3CH_2OH}$) * Secondary alcohols with the structure $\mathrm{CH_3CH(OH)R}$ (e.g., 2-propanol, 2-butanol). * **Reaction with Ethanol:** $$ \mathrm{CH_3CH_2OH} + 4\mathrm{I_2} + 6\mathrm{NaOH} \rightarrow \underset{\text{(Iodoform - yellow ppt.)}}{\mathrm{CHI_3}} + \underset{\text{Sodium formate}}{\mathrm{HCOONa}} + 5\mathrm{NaI} + 5\mathrm{H_2O} $$ * Tertiary alcohols do not give this test. ## Possible Questions/Answers * **Q:** How can you distinguish between primary, secondary, and tertiary alcohols? **A:** The **Lucas Test** is used. Tertiary alcohols react immediately with Lucas reagent ($\mathrm{ZnCl_2}/\mathrm{HCl}$) to form a cloudy solution. Secondary alcohols react in 5-10 minutes. Primary alcohols only react upon heating. * **Q:** What are the products of the oxidation of 1-propanol and 2-propanol? **A:** 1-propanol (a primary alcohol) is oxidized first to propanal (aldehyde) and then to propanoic acid (carboxylic acid). 2-propanol (a secondary alcohol) is oxidized to propanone (a ketone). * **Q:** Which of the following alcohols would give a positive iodoform test: methanol, ethanol, 2-propanol? **A:** **Ethanol** and **2-propanol** would give a positive test, forming a yellow precipitate of iodoform. Methanol would not. * **Q:** How can you convert ethanol into bromoethane? **A:** By heating ethanol with potassium bromide ($\mathrm{KBr}$) and concentrated sulfuric acid ($\mathrm{H_2SO_4}$). $$ \mathrm{C_2H_5OH} + \mathrm{KBr} + \mathrm{H_2SO_4} \rightarrow \mathrm{C_2H_5Br} + \mathrm{KHSO_4} + \mathrm{H_2O} $$