Conversions & Roadmaps

CHEExam Practice & Reasoning

Roadmap questions ask you to climb or descend the oxidation ladder (alcohol $\rightarrow$ aldehyde $\rightarrow$ acid) and interconvert ketones, alcohols and alkanes with the right reagent at each arrow. The skill is choosing a reagent that is selective enough to stop at the required stage.

Oxidation ladder up

RCH_2OH \xrightarrow{PCC} RCHO \xrightarrow{KMnO_4/H^+} RCOOH

PCC stops at the aldehyde; acidic

KMnO_4

goes all the way to the acid

Chain extension via cyanohydrin

RCHO \xrightarrow{HCN} RCH(OH)CN \xrightarrow{H_3O^+} RCH(OH)COOH

adds one carbon as

\text{-COOH}

; gives an

\alpha

-hydroxy acid

Carbonyl to alkane (full deoxygenation)

{>}C{=}O \xrightarrow{Zn\text{-}Hg/HCl \ \text{or}\ NH_2NH_2,\ KOH,\ \Delta} {>}CH_2

Clemmensen (acid) or Wolff-Kishner (base) removes O entirely

Oxidation ladder up: $1^\circ$ alcohol $\xrightarrow{\text{PCC}}$ aldehyde $\xrightarrow{KMnO_4/H^+}$ carboxylic acid; e.g. methanol $\rightarrow$ methanal $\rightarrow$ methanoic acid. Use PCC (not $KMnO_4$ ) when you must stop at the aldehyde.
Toluene $\rightarrow$ benzaldehyde uses Etard ( $CrO_2Cl_2$ ) or controlled oxidation; pushing further with $KMnO_4/H^+$ gives benzoic acid.
Down the ladder (reduce carbonyl): $\text{C=O} \xrightarrow{NaBH_4\,\text{or}\,LiAlH_4}$ alcohol (propanone $\rightarrow$ propan-2-ol); full removal of O to $\text{CH}_2$ uses Clemmensen ( $Zn\text{-}Hg/HCl$ ) or Wolff-Kishner ( $NH_2NH_2$ , $KOH$ ).
Lengthen the chain: carbonyl + $RMgX$ then $H_3O^+$ adds a carbon and an $\text{-OH}$ ; carbonate a Grignard ( $CO_2$ then $H_3O^+$ ) to reach the next carboxylic acid.
Ascend ethanol to lactic acid: $CH_3CH_2OH \rightarrow$ ethanal $\rightarrow$ via HCN to the cyanohydrin $\rightarrow$ hydrolysis gives 2-hydroxypropanoic acid (the cyanohydrin route adds one carbon as $\text{-COOH}$ ).
From benzene: Friedel-Crafts acylation with $CH_3COCl/AlCl_3$ gives acetophenone, which $KMnO_4/H^+$ oxidises to benzoic acid; alternatively methylate benzene to toluene and oxidise that.
Make an oxime/derivative as a roadmap endpoint: propanone + $NH_2OH$ gives propanone oxime - a quick way to cap a sequence at a recognisable carbonyl derivative.
Step-down (one carbon shorter): a carboxylic acid $\rightarrow$ its sodium salt $\xrightarrow{\text{soda lime}}$ alkane with one fewer carbon (decarboxylation), the standard "descend" move.
Reagent selectivity is the whole game: PCC/Etard/Rosenmund/DIBAL stop at the aldehyde; $KMnO_4$ / $K_2Cr_2O_7$ go to the acid; $NaBH_4$ / $LiAlH_4$ stop at the alcohol; Clemmensen/Wolff-Kishner go to the alkane.
Chain-LENGTHENING reagents (Grignard $+$ carbonyl, Grignard $+ CO_2$ , $KCN$ then hydrolyse, cyanohydrin) add one carbon; chain-SHORTENING moves (decarboxylation, haloform) lose one - pick by how the carbon count must change.
Functional-group protection appears in roadmaps: convert an aldehyde to its acetal with excess alcohol/dry HCl, carry out a reaction elsewhere, then hydrolyse the acetal back to regenerate the $\text{-}CHO$ .
Plan a roadmap backwards from the target: fix the oxidation level and carbon count of the product first, then choose each arrow's reagent to reach it without over- or under-oxidising.

Where the marks go

Using $KMnO_4$ where the target is the aldehyde - it over-oxidises to the acid; PCC (or Etard/Rosenmund/DIBAL) is required to stop at $\text{-}CHO$ .
Forgetting that $NaBH_4$ / $LiAlH_4$ stop at the alcohol, while only Clemmensen/Wolff-Kishner reach the $\text{CH}_2$ (alkane) stage.
Losing track of the carbon count - Grignard/ $CO_2$ /cyanohydrin ADD a carbon; decarboxylation and haloform REMOVE one.
Omitting the acidic $H_3O^+$ work-up after Grignard or cyanohydrin steps, which leaves the wrong (salt/alkoxide) intermediate.
Trying to oxidise toluene to benzaldehyde with hot $KMnO_4$ - that gives benzoic acid; use Etard ( $CrO_2Cl_2$ ) or controlled oxidation to stop at the aldehyde.

How the board asks it

Conversionoxidation ladder and reagent selectivity
How will you convert: (i) ethanol into ethanoic acid, (ii) propan-2-ol into propanone, and (iii) benzaldehyde into benzoic acid? Give the reagents and conditions for each step.
Predict the productmulti-step interconversion via the cyanohydrin route
Identify $A$ , $B$ and $C$ in the sequence $CH_3CH_2OH \xrightarrow{\text{PCC}} A \xrightarrow{HCN} B \xrightarrow{H_3O^+} C$ . Name the final product $C$ and give the reagents used at each step.
Conversiongrignard carbonation and cyanohydrin routes
How will you obtain propanoic acid from ethanol, increasing the carbon chain by one carbon? Show each intermediate with the reagent used.
Give reasonsreagent selectivity and over-oxidation
Account for the following: (i) $PCC$ is preferred over acidified $KMnO_4$ when a primary alcohol is to be converted into an aldehyde; (ii) hot $KMnO_4$ fails to give benzaldehyde from toluene.
Predict the productreduction to alcohol versus full deoxygenation
Predict the product and write the equation when propanone is treated with (i) $NaBH_4$ , and (ii) $Zn\text{-}Hg/HCl$ . Explain why the two products differ.
Distinguishchain-shortening versus chain-lengthening moves
State the product formed when (i) sodium ethanoate is heated with soda lime, and (ii) ethanal is treated with $HCN$ followed by hydrolysis. Compare the two reactions with reference to the change in carbon count.

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.