Alcohols, Phenols and Ethers — JEE Mains Chemistry Formulas and Traps

Classification, Preparation and Physical Properties

Classes, common names and boiling points

Boiling point at similar molar mass

\text{alkane} < \text{ether} < \text{aldehyde, ketone} < \text{alcohol} < \text{carboxylic acid}

Routes that make alcohols, and routes that do not

Route	Reagents	Product	Watch for
Acid hydration of an alkene	Dilute $\mathrm{H_2SO_4}$ (water, $\mathrm{H^+}$ )	Markovnikov alcohol: $\mathrm{CH_3CH{=}CH_2 \to CH_3CH(OH)CH_3}$	Goes through a carbocation, so methyl and hydride shifts can occur
Hydroboration–oxidation	$\mathrm{B_2H_6}$ , then $\mathrm{H_2O_2/OH^-}$	Anti-Markovnikov alcohol: $\mathrm{CH_3CH{=}CH_2 \to CH_3CH_2CH_2OH}$	No carbocation, so no rearrangement
Reduction of aldehydes and ketones	$\mathrm{H_2/Pd}$ , $\mathrm{NaBH_4}$ or $\mathrm{LiAlH_4}$	Aldehyde → 1° alcohol; ketone → 2° alcohol	Hydrogen adds across the C=O
Reduction of acids and esters	$\mathrm{LiAlH_4}$ or $\mathrm{B_2H_6}$ for acids; esters also by $\mathrm{H_2}$ over a catalyst	Primary alcohol $\mathrm{RCH_2OH}$	$\mathrm{NaBH_4}$ leaves a COOH group alone
Grignard reagent + carbonyl	RMgX in dry ether, then $\mathrm{H_3O^+}$	HCHO → 1°; RCHO → 2°; $\mathrm{R_2CO}$ → 3°	The new C–C bond forms at the old carbonyl carbon
Hydrolysis of an alkyl halide	Aqueous NaOH or KOH	Alcohol with the OH where the halogen was	Aryl halides do not react under these conditions
Controlled oxidation of alkanes	$\mathrm{2CH_4 + O_2}$ , Cu, 523 K, 100 atm; $\mathrm{(CH_3)_3CH + KMnO_4}$	$\mathrm{CH_3OH}$ ; $\mathrm{(CH_3)_3COH}$	With $\mathrm{Mo_2O_3}$ methane gives HCHO; with $\mathrm{(CH_3COO)_2Mn}$ alkanes give acids
Methanol from water gas	$\mathrm{CO + 2H_2}$ , $\mathrm{ZnO{-}Cr_2O_3}$ , 573–673 K, 200–300 atm	$\mathrm{CH_3OH}$	The industrial route to methanol
Fermentation	Sugar with yeast (invertase, then zymase)	Ethanol and $\mathrm{CO_2}$	Air must be kept out, or ethanol is oxidised to ethanoic acid
Ozonolysis of an alkene	$\mathrm{O_3}$ , then Zn and water	Aldehydes and ketones	Never an alcohol: the C=C is cut in two
Hydration of an alkyne	Water with $\mathrm{HgSO_4/H_2SO_4}$	A ketone; ethyne alone gives the aldehyde ethanal	The enol formed first tautomerises; no alcohol survives

Match the product class to the carbonyl: HCHO, other aldehydes and ketones give 1°, 2° and 3° alcohols with a Grignard reagent.

Common traps

Acid hydration can move a methyl group

3,3-Dimethylbut-1-ene with dilute acid gives 2,3-dimethylbutan-2-ol, not 3,3-dimethylbutan-2-ol: the secondary cation takes a 1,2-methyl shift first. Hydroboration of the same alkene gives 3,3-dimethylbutan-1-ol, with no shift.

NaBH₄ does not reduce a carboxylic acid

An acid or ester needs

\mathrm{LiAlH_4}

(or

\mathrm{B_2H_6}

for an acid).

\mathrm{NaBH_4}

reduces only aldehydes and ketones.

Isomers can differ by 80 K

Butan-1-ol (391 K) and ethoxyethane (308 K) have the same formula,

\mathrm{C_4H_{10}O}

. The gap comes only from hydrogen bonding between alcohol molecules.

Ring carbons count as carbon neighbours

A ring OH on a CH is secondary; a ring OH on a carbon that also carries a methyl group is tertiary. Count every carbon bonded to the carbinol carbon, inside the ring or outside it.

The chelated isomer melts lower

o-Nitrophenol (about 45 °C) melts far below p-nitrophenol (about 114 °C). Its hydrogen bond is inside one molecule, so it does not hold molecules together.

Acidity of Alcohols and Phenols

Learn this subtopic in the notes

Why phenol is a stronger acid than an alcohol

Counting active hydrogens with a Grignard reagent

\mathrm{ROH + CH_3MgI \to ROMgI + CH_4\uparrow} \qquad M = \dfrac{m}{n(\mathrm{CH_4})} \ \ (\text{one O–H per molecule})

Ranking substituted phenols and alcohols by pKa

The pKa ladder (NCERT values)

p\text{-NO}_2\,(7.1) < o\text{-NO}_2\,(7.2) < m\text{-NO}_2\,(8.3) < \text{phenol}\,(10.0) < p\text{-CH}_3\,(10.2) < \text{ethanol}\,(15.9)

Solubility and colour screens with NaOH, NaHCO₃ and FeCl₃

Screen	Passes	Fails	Why
Sodium metal ( $\mathrm{H_2}$ evolved)	Alcohols, phenols, carboxylic acids	Ethers and alkanes	Sodium replaces the hydrogen of an O–H group
Aqueous NaOH (dissolves as a salt)	Phenols, cresols, nitrophenols, carboxylic acids	Alcohols such as cyclohexanol and benzyl alcohol; ethers	Hydroxide removes a proton only from acids stronger than water
Aqueous $\mathrm{NaHCO_3}$ ( $\mathrm{CO_2}$ evolved)	Carboxylic acids, picric acid, 2,4-dinitrophenol	Phenol, cresols, m- and p-nitrophenol, alcohols	Needs an acid stronger than carbonic acid, pKa below about 6.4
Neutral $\mathrm{FeCl_3}$ (colour)	Phenol (violet), salicylic acid, other phenols and enols	Alcohols, benzyl alcohol, anisole	Iron(III) forms a coloured complex with an OH on the ring

Read the screens in order: NaOH tells a phenol from an alcohol, and NaHCO₃ tells a carboxylic acid (or a strongly nitrated phenol) from a phenol.

Common traps

Lower pKa means the stronger acid

Phenol (10.0) is the stronger acid and ethanol (15.9) the weaker. A statement that ethanol is the stronger acid reverses this, even when it quotes the right numbers.

A phenolic O–H uses up a Grignard reagent

With one equivalent of Grignard reagent, a hydroxy aldehyde or ketone is only deprotonated. After work-up you get the starting compound back, not an alcohol.

Use the molar volume the question gives

Older papers use 22.4 L mol

^{-1}

at STP and newer data 22.7 L mol

^{-1}

. Take whichever is stated; the nearest-integer answer usually survives either.

Ortho-nitro is not the strongest

The intramolecular hydrogen bond between OH and

\mathrm{NO_2}

in o-nitrophenol holds the proton back a little. So p-nitrophenol (7.1) is slightly stronger than o-nitrophenol (7.2), and m-nitrophenol (8.3) is the weakest of the three.

Methoxy at meta makes phenol stronger

Do not treat

\mathrm{OCH_3}

as always electron-releasing. At meta its +R cannot reach the oxygen, so its −I effect makes m-methoxyphenol more acidic than phenol.

Distance beats electronegativity for −I

2-Chlorocyclohexanol is more acidic than 4-fluorocyclohexanol even though F is more electronegative than Cl. The inductive pull dies away within two or three bonds.

Benzyl alcohol is not a phenol

In

\mathrm{C_6H_5CH_2OH}

the OH sits on a

\mathrm{CH_2}

, not on the ring. It behaves as an alcohol: no salt with NaOH and no colour with

\mathrm{FeCl_3}

.

One nitro group is not enough for bicarbonate

m- and p-Nitrophenol are still weaker acids than carbonic acid. It takes two or three nitro groups at ortho and para (2,4-dinitrophenol, picric acid) to release

\mathrm{CO_2}

.

Reactions of Alcohols: Substitution, Dehydration and Rearrangement

Learn this subtopic in the notes

Reactions at the O–H and C–O bonds of alcohols

Molar mass after complete acetylation

M_{\text{product}} = M + 42n \qquad (n = \text{number of OH groups})

Acid dehydration and carbocation shifts

E1 dehydration with a possible shift

\mathrm{ROH} \xrightarrow{\mathrm{H^+}} \mathrm{ROH_2^+} \xrightarrow{-\mathrm{H_2O}} \mathrm{R^+} \xrightarrow{\text{1,2-shift if better}} \mathrm{R'^+} \xrightarrow{-\mathrm{H^+}} \text{most substituted alkene}

Common traps

Ethanol is primary: no quick Lucas turbidity

Only a tertiary alcohol clouds Lucas reagent at once. A primary alcohol such as ethanol stays clear at room temperature.

Hot copper dehydrates a tertiary alcohol

A 3° alcohol has no H on its carbinol carbon, so Cu at 573 K cannot make a ketone from it.

\mathrm{(CH_3)_3COH}

gives 2-methylpropene instead, and that alkene can then react further in acid.

Acrolein is pungent, not fruity

The acrolein test for glycerol works by its sharp, irritating smell. Fruity smells belong to esters.

Check the neighbours before drawing the alkene

If the carbon next to a 2° cation is tertiary or quaternary, expect a shift. The product then has a different carbon skeleton from the alcohol, and the unshifted alkene is always among the options.

A primary alcohol can rearrange too

Cyclohexylmethanol in hot acid does not stop at a primary cation. A 1,2-hydride shift from the ring carbon gives the 3° 1-methylcyclohexyl cation, and 1-methylcyclohexene forms.

Acid on an alcohol is Saytzeff; a bulky base on a halide is Hofmann

1-Methylcyclohexanol with

\mathrm{H_3PO_4}

gives 1-methylcyclohexene (inside the ring). The matching chloride with

\mathrm{(CH_3)_3COK}

gives mostly methylenecyclohexane (outside the ring).

Preparation of Phenols and Named Reactions

Learn this subtopic in the notes

Four ways to make phenol

Starting material	Reagents	Intermediate	Products
Chlorobenzene	NaOH, 623 K, about 300 atm; then $\mathrm{H^+}$	Sodium phenoxide	Phenol
Benzenesulphonic acid (benzene + oleum)	Fused NaOH; then $\mathrm{H^+}$	Sodium phenoxide	Phenol
Aniline	$\mathrm{NaNO_2}$ + HCl at 273–278 K; then warm water	Benzenediazonium chloride, $\mathrm{C_6H_5N_2^+Cl^-}$	Phenol, $\mathrm{N_2}$ and HCl
Cumene (isopropylbenzene)	$\mathrm{O_2}$ (air); then dilute acid	Cumene hydroperoxide, $\mathrm{C_6H_5C(CH_3)_2OOH}$	Phenol and propanone (acetone) The intermediate is a hydroperoxide (O–O–H), not an alcohol or an ester; acetone is the by-product.

Carbolic acid is the old name for phenol.

Named reactions of phenol

Reaction	Reagents	Product	Key point
Reduction by zinc dust	Zn, heat	Benzene	Zinc takes the oxygen as ZnO
Oxidation	$\mathrm{Na_2Cr_2O_7/H_2SO_4}$ (chromic acid)	Benzo-p-quinone	A conjugated diketone, O=C at C-1 and C-4
Reimer–Tiemann	$\mathrm{CHCl_3}$ + aq. NaOH, then $\mathrm{H^+}$	Salicylaldehyde (2-hydroxybenzaldehyde), ortho major	Electrophile $\mathrm{:CCl_2}$ ; intermediate is the o-(dichloromethyl)phenoxide, a substituted benzal chloride
Kolbe	NaOH; $\mathrm{CO_2}$ at 400 K, 4–7 atm; then $\mathrm{H^+}$	Salicylic acid (2-hydroxybenzoic acid)	$\mathrm{CO_2}$ is a weak electrophile, so the phenoxide ion is needed
Acetylation	Acetic anhydride or $\mathrm{CH_3COCl}$	Phenyl acetate; salicylic acid gives aspirin	Reaction at the O–H, not on the ring
Nitration with conc. $\mathrm{HNO_3}$	Conc. $\mathrm{HNO_3}$ (best via phenol-2,4-disulphonic acid)	Picric acid (2,4,6-trinitrophenol)	A yellow, strongly acidic solid
With HCl	Conc. HCl	No reaction: no chlorobenzene	The phenolic C–O bond does not break

CHCl₃ puts in a CHO group, CO₂ puts in a COOH group; both go ortho to the OH.

Common traps

Hydrolysis of benzal chloride gives benzaldehyde

\mathrm{C_6H_5CHCl_2}

with water gives

\mathrm{C_6H_5CHO}

, not phenol. Cumene does not hydrolyse at all; it must be oxidised to the hydroperoxide first.

Multiply step yields, do not add them

Two steps of 80% and 50% give 40% overall, not 65% or 130%.

Reimer–Tiemann gives the ortho aldehyde as the major product

The phenoxide oxygen guides

\mathrm{:CCl_2}

to the ortho carbon. A statement that p-hydroxybenzaldehyde is the major product is false. The two isomers can be separated by steam distillation, because the ortho one is volatile.

Chloroform gives the aldehyde, carbon dioxide gives the acid

\mathrm{CHCl_3}

/NaOH makes salicylaldehyde;

\mathrm{CO_2}

/NaOH makes salicylic acid. Swapping them is the usual wrong option in a match list.

Ring Substitution of Phenols and Phenol Tests

Learn this subtopic in the notes

Bromination of phenol: the solvent decides

Bromine water on phenol

\mathrm{C_6H_5OH + 3Br_2 \xrightarrow{H_2O} C_6H_2Br_3OH\downarrow + 3HBr}

Nitration of phenol and picric acid

Percentage of oxygen in a compound

\%\,\mathrm{O} = \dfrac{16 \times (\text{number of O atoms})}{M} \times 100

The phthalein dye test for phenols

Phenol	Position para to OH	Product with phthalic anhydride	Colour in alkali
Phenol	Free	Phenolphthalein	Pink in dilute NaOH; colourless in acid and in excess strong alkali
o-Cresol	Free	o-Cresolphthalein	Purple-red
p-Cresol	Blocked by $\mathrm{CH_3}$	No phthalein dye	No colour The usual answer to 'which phenol gives no colour'.
Resorcinol (benzene-1,3-diol)	Free	Fluorescein	Yellow-green fluorescence

Look for the ring carbon para to OH: if it carries a substituent, no phthalein forms.

Common traps

CS₂ and CHCl₃ both count as low polarity

Either solvent, at low temperature, gives mainly p-bromophenol. Only water (or another polar medium) gives the tribromo product.

Count bromine molecules, not bromine atoms

Three

\mathrm{Br_2}

(160 g mol

^{-1}

each) are used per phenol. Three Br atoms go onto the ring and three leave as HBr.

Picric acid is a phenol, not TNT

2,4,6-Trinitrotoluene (TNT) has a methyl group. Picric acid has an OH group, which is why it is a strong acid.

Steam carries off the ortho isomer

The hydrogen bond in o-nitrophenol is inside one molecule, so its molecules are held together weakly. p-Nitrophenol, bonded to its neighbours, stays in the flask.

Excess alkali removes the pink colour

Phenolphthalein is pink only in dilute alkali. In a large excess of concentrated NaOH it turns colourless again, so 'colourless' can be the right answer at the end of a sequence.

Ethers: Williamson Synthesis and Cleavage

Learn this subtopic in the notes

Making ethers: Williamson synthesis and acid routes

Williamson synthesis

\mathrm{R{-}O^-Na^+ + R'{-}X \xrightarrow{S_N2} R{-}O{-}R' + NaX} \qquad (\mathrm{R'} = \mathrm{CH_3} \text{ or primary; never aryl})

Cleaving ethers with HI and HBr

Which bond HI breaks

\mathrm{Ar{-}O{-}R + HI \to Ar{-}OH + R{-}I} \qquad \mathrm{R_3C{-}O{-}R' + HI \to R_3C{-}I + R'{-}OH}

Aryl ethers on the ring and in structure proofs

Degree of unsaturation (rings + π bonds)

\text{DU} = \dfrac{2C + 2 - H}{2} \qquad (\text{O atoms do not count})

Common traps

The halide side must be simple

The alkoxide can be as bulky as you like; the halide cannot. A tertiary halide eliminates and an aryl halide does not undergo SN2.

SN2 does not move the double bond

With an allylic bromide, the SN2 product has oxygen on the carbon that carried Br. The product with oxygen on the far end of the old C=C comes from SN1 or SN2′ and is the usual wrong option.

A tertiary group reverses the 'smaller group' rule

With a methyl or primary partner, iodide ends on the smaller group. When one group is tertiary, the iodide ends on the tertiary carbon, because that bond breaks first to give the cation.

Phenol is the end of the line

An aryl alkyl ether gives a phenol and an alkyl halide. Even with excess HI the phenol is not turned into an aryl iodide.

The ring is not a π bond

The degree of unsaturation counts rings and π bonds together. A benzene ring uses four: one for the ring and three for its π bonds.

Para blocked means ortho to the alkoxy group

In 4-nitroanisole the next bromine goes ortho to

\mathrm{OCH_3}

, not ortho to

\mathrm{NO_2}

. The activating group wins the directing contest.

Alcohols, Phenols and Ethers formulas

Classification, Preparation and Physical Properties

Classes, common names and boiling points

Routes that make alcohols, and routes that do not

Common traps

Acidity of Alcohols and Phenols

Why phenol is a stronger acid than an alcohol

Ranking substituted phenols and alcohols by pKa

Solubility and colour screens with NaOH, NaHCO₃ and FeCl₃

Common traps

Reactions of Alcohols: Substitution, Dehydration and Rearrangement

Reactions at the O–H and C–O bonds of alcohols

Acid dehydration and carbocation shifts

Common traps

Preparation of Phenols and Named Reactions

Four ways to make phenol

Named reactions of phenol

Common traps

Ring Substitution of Phenols and Phenol Tests

Bromination of phenol: the solvent decides

Nitration of phenol and picric acid

The phthalein dye test for phenols

Common traps

Ethers: Williamson Synthesis and Cleavage

Making ethers: Williamson synthesis and acid routes

Cleaving ethers with HI and HBr

Aryl ethers on the ring and in structure proofs

Common traps

More JEE Mains Chemistry formula sheets