Mixed, Long Answer & Previous-Year Style

CHEExam Practice & Reasoning

These are the high-mark composite questions that stitch the whole chapter together — "give reasons" one-liners, multi-step $A \rightarrow B \rightarrow C$ identifications, distinguishing-test pairs, and short mechanisms. Mastery here means recalling the right reagent, product, and crisp reasoning instantly across alcohols, phenols, and ethers.

Iodoform reaction

CH_3CH_2OH \xrightarrow{I_2,\ NaOH} CHI_3\downarrow + HCOO^-Na^+

yellow

CHI_3

ppt from

CH_3CO\text{-}

CH_3CH(OH)\text{-}

groups

Phenol with bromine water

C_6H_5OH + 3Br_2 \rightarrow C_6H_2Br_3OH\downarrow + 3HBr

white 2,4,6-tribromophenol, no catalyst needed

Excess HI cleavage of ether

C_2H_5OC_2H_5 \xrightarrow{\text{excess HI}} 2\,C_2H_5I + H_2O

Cumene process

C_6H_5CH(CH_3)_2 \xrightarrow{O_2} \text{cumene hydroperoxide} \xrightarrow{H^+} C_6H_5OH + (CH_3)_2CO

Iodoform test: $CHI_3$ (yellow ppt) forms with $I_2$ /NaOH only from ethanol, any $CH_3CH(OH)\text{-}$ alcohol (e.g. propan-2-ol), or a $CH_3CO\text{-}$ group (acetone). Methanol and propan-1-ol give no iodoform.
Phenol gives 2,4,6-tribromophenol (white ppt) with $Br_2$ water without a catalyst because $\text{-}OH$ strongly activates the ring; benzene needs $Br_2$ with a Lewis-acid catalyst.
Phenol is weakly acidic: it dissolves in NaOH to give sodium phenoxide and reacts with Na liberating $H_2$ , but it is too weak to react with $NaHCO_3$ (no $CO_2$ ).
Distinguish quickly: phenol vs ethanol by neutral $FeCl_3$ (phenol gives violet colour); phenol vs benzoic acid by $NaHCO_3$ (only the acid gives $CO_2$ effervescence).
Distinguish propan-1-ol vs propan-2-ol by the iodoform test (only propan-2-ol gives $CHI_3$ ); ether vs ethanol by Na (only ethanol liberates $H_2$ ).
Excess HI cleavage of diethyl ether gives ethyl iodide twice: $C_2H_5OC_2H_5 \xrightarrow{\text{excess HI}} 2\,C_2H_5I + H_2O$ (via protonation then $S_N2$ attack of $I^-$ on the alkyl carbon).
Hydrogen bonding explains physical trends: glycerol (3 $\text{-}OH$ ) is viscous, ethanol H-bonds (high b.p.), and phenol is only sparingly water-soluble but fully dissolves in NaOH as the ionic phenoxide.
Cumene sequence: cumene $\xrightarrow{O_2}$ cumene hydroperoxide $\xrightarrow{H_2SO_4}$ phenol + acetone; the phenol then gives 2,4,6-tribromophenol with $Br_2$ water.
Markovnikov hydration of propene gives propan-2-ol ( $2^\circ$ , oxidises to acetone, then no further); hydroboration–oxidation gives propan-1-ol ( $1^\circ$ , oxidises to propanal then propanoic acid) — a common $A,B,C,D,E$ identification.
tert-Butanol gives a negative iodoform test because it has no $CH_3CH(OH)\text{-}$ group; it cannot be oxidised to a methyl ketone — a frequent "give reason".
Ethers are used as solvents because they are largely inert (stable to bases, mild oxidising/reducing agents and dilute acids) yet dissolve a wide range of organic compounds, including Grignard reagents.
Glycerol is viscous (three $\text{-}OH$ groups give extensive intermolecular H-bonding) whereas propan-1-ol, with one $\text{-}OH$ , is a mobile liquid; diethyl ether dissolves in cold conc. $H_2SO_4$ by protonation of its lone-pair oxygen to give an oxonium salt.

Where the marks go

Claiming propan-1-ol or methanol gives iodoform — neither has the $CH_3CH(OH)\text{-}$ or $CH_3CO\text{-}$ unit, so both are negative.
Writing a brown/red precipitate for phenol with $Br_2$ water — the 2,4,6-tribromophenol precipitate is WHITE, and it needs no catalyst.
Using $NaHCO_3$ to confirm phenol as acidic — phenol does NOT react with $NaHCO_3$ ; that test is positive only for carboxylic acids ( $CO_2$ effervescence).
Forgetting to state the colour/observation in distinguishing tests — examiners award the mark for the observation (violet colour, white ppt, gas evolution), not merely the reagent.
Mixing up which alcohol forms in the two propene routes — Markovnikov hydration gives the $2^\circ$ alcohol, anti-Markovnikov hydroboration gives the $1^\circ$ alcohol.

How the board asks it

Give reasonsiodoform negatives, phenol activation, acidity ladder
Give reasons: (i) $tert$ -butyl alcohol does not give a positive iodoform test. (ii) Phenol reacts with $Br_2$ water without any catalyst whereas benzene needs a Lewis-acid catalyst.
Distinguishneutral $FeCl_3$ , iodoform and $Na$ tests
How will you distinguish between the following pairs using a simple chemical test, stating the reagent and the observation? (i) Phenol and ethanol. (ii) Propan-1-ol and propan-2-ol.
Identify / classifymarkovnikov vs hydroboration of propene, oxidation
An alkene $A$ ( $C_3H_6$ ) on Markovnikov hydration gives $B$ , which on oxidation gives $C$ that answers the iodoform test. The same $A$ on hydroboration-oxidation gives $D$ , which on oxidation gives the acid $E$ . Identify $A$ to $E$ with equations.
Conversioncumene process and excess $HI$ ether cleavage
How will you obtain: (i) phenol from cumene, and (ii) ethyl iodide from diethyl ether? Write the equations involved.
Mechanismprotonation then $S_N2$ cleavage of ethers by $HI$
Write the mechanism for the cleavage of diethyl ether $C_2H_5OC_2H_5$ by excess $HI$ , clearly showing the protonation step and the $S_N2$ attack of $I^-$ on the alkyl carbon.

PreviousAcidity Orders, "Why" Questions & Reasoning

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.