Reactions of Alcohols (Lucas, HX, Dehydration)

CHEAlcohols: Preparation & Reactions

This subtopic covers how alcohols react at the C–OH bond — the Lucas test that ranks $1^\circ/2^\circ/3^\circ$ , conversion to alkyl halides with HX, $SOCl_2$ or $PCl_5$ , and acid-catalysed dehydration to alkenes. The recurring exam theme is carbocation stability controlling both reactivity and which alkene forms.

Acid-catalysed dehydration

C_2H_5OH \xrightarrow[443\ K]{\text{conc. } H_2SO_4} CH_2{=}CH_2 + H_2O

Alcohol to alkyl chloride with thionyl chloride

R\text{-}OH + SOCl_2 \rightarrow R\text{-}Cl + SO_2\uparrow + HCl\uparrow

Reactivity orders

\text{alcohol with HX: } 3^\circ > 2^\circ > 1^\circ \qquad \text{HX strength: } HI > HBr > HCl

Lucas test reaction

R\text{-}OH + HCl \xrightarrow{\text{anhyd. } ZnCl_2} R\text{-}Cl\ (\text{turbidity}) + H_2O

Reaction with sodium metal

2\,R\text{-}OH + 2Na \rightarrow 2\,R\text{-}O^-Na^+ + H_2\uparrow

reactivity

1^\circ > 2^\circ > 3^\circ

(more acidic

\text{-}OH

reacts faster)

Fischer esterification

R\text{-}OH + R'COOH \underset{\Delta}{\overset{H^+}{\rightleftharpoons}} R'COOR + H_2O

Intermolecular dehydration to an ether

2\,C_2H_5OH \xrightarrow[413\ K]{\text{conc. } H_2SO_4} C_2H_5\text{-}O\text{-}C_2H_5 + H_2O

Lucas reagent = conc. HCl + anhydrous $ZnCl_2$ . $3^\circ$ alcohol: turbidity immediately; $2^\circ$ : turbidity in $\sim5\text{-}10$ min; $1^\circ$ : no turbidity at room temperature.
Alcohols react with active metals (Na, K) liberating $H_2$ and forming the alkoxide; reactivity is $1^\circ > 2^\circ > 3^\circ$ since the $\text{-}OH$ becomes progressively less acidic as more alkyl groups are attached.
Fischer esterification ( $R\text{-}OH + R'COOH \xrightarrow{H^+,\Delta} R'COOR + H_2O$ ) is a reversible, acid-catalysed equilibrium; with an acyl chloride or acid anhydride instead, esterification goes essentially to completion in one direction (e.g. $ROH + CH_3COCl \rightarrow CH_3COOR + HCl$ ).
Reactivity toward Lucas reagent / HCl: $3^\circ > 2^\circ > 1^\circ$ , because the reaction is $S_N1$ and the $3^\circ$ carbocation is the most stable.
Dehydration is acid-catalysed (conc. $H_2SO_4$ , $\sim443\,\text{K}$ ): protonation of $\text{-}OH$ , loss of water to a carbocation, then loss of $\beta\text{-}H$ to give the alkene.
Ease of dehydration of alcohols: $3^\circ > 2^\circ > 1^\circ$ (more stable intermediate carbocation = faster $E1$ ).
Saytzeff's rule: when more than one alkene can form, the more substituted (more stable) alkene is the major product — e.g. butan-2-ol gives but-2-ene over but-1-ene.
$SOCl_2$ is preferred over $PCl_5$ for $R\text{-}OH \rightarrow R\text{-}Cl$ because the by-products $SO_2$ and HCl are gases that escape, giving a pure alkyl chloride.
Reactivity of HX with a given alcohol: $HI > HBr > HCl$ ; with $1^\circ$ alcohols, anhydrous $ZnCl_2$ is needed to make even HCl react.
The turbidity in the Lucas test is the insoluble alkyl chloride separating out; the test works only for alcohols soluble in the reagent, i.e. those with up to about six carbons.
$PCl_5$ also converts alcohols to chlorides ( $R\text{-}OH + PCl_5 \rightarrow R\text{-}Cl + POCl_3 + HCl$ ); $PCl_3$ gives $3R\text{-}OH + PCl_3 \rightarrow 3R\text{-}Cl + H_3PO_3$ .
$1^\circ$ alcohols react with HX largely by $S_N2$ (no free carbocation), whereas $2^\circ$ and $3^\circ$ go by $S_N1$ through a carbocation — explaining both the rate order and possible rearrangements.
Dehydration is intramolecular elimination at high temperature ( $443\,\text{K}$ ); at lower temperature ( $413\,\text{K}$ ) the same conc. $H_2SO_4$ instead gives intermolecular dehydration to an ether — temperature decides the product.
With Lucas reagent, allyl and benzyl alcohols also give immediate turbidity because they form resonance-stabilised carbocations, so they can mimic a $3^\circ$ result.

Where the marks go

Reversing the Lucas timing — $3^\circ$ is immediate and $1^\circ$ shows no turbidity at room temperature; stating $1^\circ$ reacts fastest is a classic error.
Applying Markovnikov's rule to dehydration — elimination follows Saytzeff (most substituted alkene), not Markovnikov.
Forgetting the catalyst: $1^\circ$ alcohols need anhydrous $ZnCl_2$ to react with HCl, and dehydration needs an acid catalyst — pure heating alone is wrong.
Choosing $PCl_5$ over $SOCl_2$ when purity matters — $PCl_5$ leaves $POCl_3$ liquid behind, while $SOCl_2$ gives only gaseous by-products (the Darzens method).
Confusing the temperatures: $413\,\text{K}$ with conc. $H_2SO_4$ gives diethyl ether, $443\,\text{K}$ gives ethene — swapping them loses marks.

How the board asks it

Distinguishlucas test timing and turbidity
How will you distinguish between propan-1-ol, propan-2-ol and $2$ -methylpropan-2-ol using a single chemical test? Give the observation for each.
Give reasonscarbocation stability controlling ease of dehydration
Account for the fact that the ease of dehydration of alcohols follows the order $3^\circ > 2^\circ > 1^\circ$ .
Predict the productsaytzeff's rule in acid-catalysed dehydration
When butan-2-ol is heated with conc. $H_2SO_4$ at $443\,\text{K}$ , predict the major alkene formed and give the balanced equation, stating the rule you have applied.
Conversion $R\text{-}OH \rightarrow R\text{-}Cl$ with $SOCl_2$ and acid-catalysed dehydration
How will you convert ethanol into ethyl chloride and ethanol into ethene? Give the reagents and conditions for each step.
Mechanismacid-catalysed dehydration of ethanol with conc. $H_2SO_4$
Give the mechanism for the acid-catalysed dehydration of ethanol with conc. $H_2SO_4$ to ethene, showing all the steps and the intermediate formed.
Assertion–Reasontemperature control of product with conc. $H_2SO_4$
Assertion: Ethanol with conc. $H_2SO_4$ gives diethyl ether at $413\,\text{K}$ but ethene at $443\,\text{K}$ . Reason: The product of the reaction of an alcohol with conc. $H_2SO_4$ depends on the reaction temperature. State whether both are true and whether the Reason correctly explains the Assertion.

Practise this topic

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Written for Sublevo. Question text quoted anywhere in these notes is the Council’s and carries its year and paper; the board’s own diagrams are not reproduced.