Why Alcohols Boil So Much Higher Than Similar Hydrocarbons
Classify and name alcohols, phenols and ethers, prepare alcohols from alkenes, carbonyl compounds and Grignard reagents, explain their boiling points and solubility, and use their key reactions with the Lucas test.
What do alcohols, phenols and ethers have in common?
Hand sanitiser, the antiseptic smell of a hospital and many laboratory solvents all involve compounds in which oxygen is bonded to carbon. Alcohols and phenols carry an –OH group, while ethers link two carbon groups through an oxygen atom.
This part covers classification and naming, preparing alcohols, their physical properties, and their key reactions with the Lucas test.
This part covers classification and naming, preparing alcohols, their physical properties, and their key reactions with the Lucas test.
How are alcohols, phenols and ethers classified and named, and what are their structures?
Alcohols and phenols are classified by the number of –OH groups as mono-, di- or polyhydric, and alcohols further as primary, secondary or tertiary by the carbon carrying –OH; IUPAC names use the ending -ol, or an alkoxy prefix for ethers.
Classification:
- Monohydric, dihydric, trihydric — one, two or three –OH groups, as in ethanol, ethane-1,2-diol and propane-1,2,3-triol
- Primary, secondary, tertiary — –OH on a carbon bonded to one, two or three other carbons
- Ethers — simple when both groups match, as in CHOCH, and mixed when they differ, as in CHOCH
Naming:
- CHCH(OH)CH — isopropyl alcohol; IUPAC propan-2-ol
- (CH)COH — tert-butyl alcohol; 2-methylpropan-2-ol
- CHOCH — anisole; methoxybenzene
Structures. In methanol the C–O–H angle is about , slightly below tetrahedral because oxygen's lone pairs repel the bonds. Phenol's C–O bond, pm, is shorter than methanol's pm because the lone pair is partly shared with the ring. In methoxymethane the C–O–C angle widens to about .
Worked example. For CHCHCH(OH)CH(CH), the five-carbon chain puts –OH at C3 from either end, so the tie is broken by giving the methyl the lower number: 2-methylpentan-3-ol.
An everyday example. Glycerol in skin creams is propane-1,2,3-triol, a trihydric alcohol whose three –OH groups hold on to moisture.
The substance. Phenols are not simply aromatic alcohols — their –OH sits directly on an ring carbon, which changes their chemistry sharply.
Classification:
- Monohydric, dihydric, trihydric — one, two or three –OH groups, as in ethanol, ethane-1,2-diol and propane-1,2,3-triol
- Primary, secondary, tertiary — –OH on a carbon bonded to one, two or three other carbons
- Ethers — simple when both groups match, as in CHOCH, and mixed when they differ, as in CHOCH
Naming:
- CHCH(OH)CH — isopropyl alcohol; IUPAC propan-2-ol
- (CH)COH — tert-butyl alcohol; 2-methylpropan-2-ol
- CHOCH — anisole; methoxybenzene
Structures. In methanol the C–O–H angle is about , slightly below tetrahedral because oxygen's lone pairs repel the bonds. Phenol's C–O bond, pm, is shorter than methanol's pm because the lone pair is partly shared with the ring. In methoxymethane the C–O–C angle widens to about .
Worked example. For CHCHCH(OH)CH(CH), the five-carbon chain puts –OH at C3 from either end, so the tie is broken by giving the methyl the lower number: 2-methylpentan-3-ol.
An everyday example. Glycerol in skin creams is propane-1,2,3-triol, a trihydric alcohol whose three –OH groups hold on to moisture.
The substance. Phenols are not simply aromatic alcohols — their –OH sits directly on an ring carbon, which changes their chemistry sharply.
How are alcohols prepared from alkenes, carbonyl compounds and Grignard reagents?
Alcohols are made by acid-catalysed hydration of alkenes, giving the Markovnikov product, by hydroboration-oxidation, giving the anti-Markovnikov product, by reducing aldehydes, ketones or acids, and by adding Grignard reagents to carbonyl compounds.
From alkenes:
- Acid-catalysed hydration — propene gives mainly propan-2-ol
- Hydroboration-oxidation — diborane adds, then HO with NaOH replaces boron by –OH, so propene gives propan-1-ol
From carbonyl compounds:
- Aldehydes give primary and ketones give secondary alcohols with H/Pd, NaBH or LiAlH
- Carboxylic acids need the strong reducing agent LiAlH
From Grignard reagents, followed by hydrolysis:
- Methanal gives a primary alcohol
- Other aldehydes give secondary alcohols
- Ketones give tertiary alcohols
Worked example. To make 2-methylpropan-2-ol, add CHMgBr to propanone and hydrolyse. The Grignard methyl group bonds to the carbonyl carbon, which already carries two methyl groups, so the product carbon holds three methyls and one –OH.
An everyday example. Fuel ethanol blended into petrol in India is made largely by fermenting sugarcane molasses — a biological alternative to hydrating ethene.
The substance. Hydroboration gives the opposite regiochemistry from acid hydration, so the method chosen decides which alcohol you get.
From alkenes:
- Acid-catalysed hydration — propene gives mainly propan-2-ol
- Hydroboration-oxidation — diborane adds, then HO with NaOH replaces boron by –OH, so propene gives propan-1-ol
From carbonyl compounds:
- Aldehydes give primary and ketones give secondary alcohols with H/Pd, NaBH or LiAlH
- Carboxylic acids need the strong reducing agent LiAlH
From Grignard reagents, followed by hydrolysis:
- Methanal gives a primary alcohol
- Other aldehydes give secondary alcohols
- Ketones give tertiary alcohols
Worked example. To make 2-methylpropan-2-ol, add CHMgBr to propanone and hydrolyse. The Grignard methyl group bonds to the carbonyl carbon, which already carries two methyl groups, so the product carbon holds three methyls and one –OH.
An everyday example. Fuel ethanol blended into petrol in India is made largely by fermenting sugarcane molasses — a biological alternative to hydrating ethene.
The substance. Hydroboration gives the opposite regiochemistry from acid hydration, so the method chosen decides which alcohol you get.
Why do alcohols and phenols have high boiling points and dissolve in water?
Alcohols and phenols boil far higher than hydrocarbons, ethers or haloalkanes of similar molar mass because their –OH groups hydrogen-bond to each other, and the smaller members dissolve in water because they also hydrogen-bond with water molecules.
Boiling points:
- Rise with more carbon atoms, as van der Waals forces grow
- Fall with branching, as molecules become more compact
- Much higher than similar-mass compounds without –OH
Solubility:
- Methanol, ethanol and propanol mix with water in all proportions
- Solubility falls as the hydrocarbon part grows, because the large non-polar chain disrupts hydrogen bonding with water
Worked example. Three compounds of similar molar mass:
- Ethanol, g mol — boils at K
- Methoxymethane, g mol — boils at K
- Propane, g mol — boils at K
Ethanol boils K above its ether isomer, entirely because of hydrogen bonding between its molecules.
An everyday example. Surgical spirit mixes completely with water, because its small alcohol molecules hydrogen-bond readily with water.
The substance. Ethers cannot hydrogen-bond with each other because they have no O–H, so their boiling points stay close to those of alkanes.
Boiling points:
- Rise with more carbon atoms, as van der Waals forces grow
- Fall with branching, as molecules become more compact
- Much higher than similar-mass compounds without –OH
Solubility:
- Methanol, ethanol and propanol mix with water in all proportions
- Solubility falls as the hydrocarbon part grows, because the large non-polar chain disrupts hydrogen bonding with water
Worked example. Three compounds of similar molar mass:
- Ethanol, g mol — boils at K
- Methoxymethane, g mol — boils at K
- Propane, g mol — boils at K
Ethanol boils K above its ether isomer, entirely because of hydrogen bonding between its molecules.
An everyday example. Surgical spirit mixes completely with water, because its small alcohol molecules hydrogen-bond readily with water.
The substance. Ethers cannot hydrogen-bond with each other because they have no O–H, so their boiling points stay close to those of alkanes.
What are the main reactions of alcohols, and how does the Lucas test tell primary, secondary and tertiary alcohols apart?
Alcohols react by breaking the O–H bond, as in esterification, or the C–O bond, as with hydrogen halides, phosphorus halides and acid dehydration; they oxidise according to their class, and the Lucas test separates primary, secondary and tertiary alcohols by how fast they turn cloudy.
Esterification — alcohol and carboxylic acid with conc. HSO give an ester and water.
C–O cleavage — with HX (tertiary > secondary > primary), PCl, PCl or SOCl.
Dehydration with conc. HSO at K, for ethanol:
- Step 1 — the –OH is protonated
- Step 2 — water leaves, forming a carbocation; this is the slow step
- Step 3 — a proton is lost, forming ethene
Tertiary alcohols dehydrate most easily because they form the most stable carbocations.
Oxidation:
- Primary — aldehyde, then carboxylic acid; PCC stops at the aldehyde
- Secondary — ketone
- Tertiary — resist oxidation; over hot copper they dehydrate to alkenes
Lucas test — conc. HCl with anhydrous ZnCl:
- Tertiary — turns cloudy immediately
- Secondary — turns cloudy within about five minutes
- Primary — stays clear at room temperature
Worked example. Three isomers of CHO are tested: 2-methylpropan-2-ol turns cloudy at once, butan-2-ol after a few minutes, and butan-1-ol stays clear — so they are tertiary, secondary and primary.
An everyday example. Many fruity food flavourings are esters made by reacting alcohols with carboxylic acids.
The substance. The Lucas test tracks carbocation stability — the more stable the carbocation, the faster the cloudy alkyl chloride forms.
Esterification — alcohol and carboxylic acid with conc. HSO give an ester and water.
C–O cleavage — with HX (tertiary > secondary > primary), PCl, PCl or SOCl.
Dehydration with conc. HSO at K, for ethanol:
- Step 1 — the –OH is protonated
- Step 2 — water leaves, forming a carbocation; this is the slow step
- Step 3 — a proton is lost, forming ethene
Tertiary alcohols dehydrate most easily because they form the most stable carbocations.
Oxidation:
- Primary — aldehyde, then carboxylic acid; PCC stops at the aldehyde
- Secondary — ketone
- Tertiary — resist oxidation; over hot copper they dehydrate to alkenes
Lucas test — conc. HCl with anhydrous ZnCl:
- Tertiary — turns cloudy immediately
- Secondary — turns cloudy within about five minutes
- Primary — stays clear at room temperature
Worked example. Three isomers of CHO are tested: 2-methylpropan-2-ol turns cloudy at once, butan-2-ol after a few minutes, and butan-1-ol stays clear — so they are tertiary, secondary and primary.
An everyday example. Many fruity food flavourings are esters made by reacting alcohols with carboxylic acids.
The substance. The Lucas test tracks carbocation stability — the more stable the carbocation, the faster the cloudy alkyl chloride forms.
Exam tip
What earns full marks on alcohols?
Draw the carbon carrying –OH and count its carbon neighbours before answering any question on reactivity, oxidation or the Lucas test.
- Hydration: acid gives Markovnikov; hydroboration gives anti-Markovnikov
- Grignard: methanal gives primary, other aldehydes secondary, ketones tertiary alcohols
- Boiling point: hydrogen bonding; lower with branching
- Dehydration: protonation, carbocation, loss of H; tertiary easiest
- Lucas test: tertiary immediate, secondary within minutes, primary none
The trap. Writing that tertiary alcohols oxidise to ketones. They resist oxidation, because the –OH carbon has no hydrogen to lose.
- Hydration: acid gives Markovnikov; hydroboration gives anti-Markovnikov
- Grignard: methanal gives primary, other aldehydes secondary, ketones tertiary alcohols
- Boiling point: hydrogen bonding; lower with branching
- Dehydration: protonation, carbocation, loss of H; tertiary easiest
- Lucas test: tertiary immediate, secondary within minutes, primary none
The trap. Writing that tertiary alcohols oxidise to ketones. They resist oxidation, because the –OH carbon has no hydrogen to lose.
Did you know
Why does hand sanitiser feel cold on your skin?
Rub a little alcohol-based sanitiser on your hands and they feel cool almost at once.
The small alcohol molecules escape into the air quickly, and every molecule that evaporates carries energy away from your skin. Alcohols do hydrogen-bond, but ethanol and propan-2-ol form fewer hydrogen bonds per molecule than water, so they evaporate much faster.
The same rapid evaporation is why sanitiser dries within seconds and leaves no wet film behind.
The small alcohol molecules escape into the air quickly, and every molecule that evaporates carries energy away from your skin. Alcohols do hydrogen-bond, but ethanol and propan-2-ol form fewer hydrogen bonds per molecule than water, so they evaporate much faster.
The same rapid evaporation is why sanitiser dries within seconds and leaves no wet film behind.
Exam relevance
How are alcohols tested in JEE Main and NEET?
Alcohols, Phenols and Ethers is a core organic chapter in both JEE Main and NEET Chemistry, and its reactions feed conversion problems across later chapters.
What gets asked. Naming and classifying alcohols, hydration versus hydroboration-oxidation products, which alcohol a Grignard reagent gives, boiling point comparisons, the dehydration mechanism and carbocation stability, oxidation products, and the Lucas test.
Question types. Reaction-sequence and match-the-column questions in both exams, and assertion-reason questions on hydrogen bonding in NEET.
The trap that costs marks. Giving the Markovnikov product for hydroboration-oxidation, which actually gives the anti-Markovnikov alcohol.
What gets asked. Naming and classifying alcohols, hydration versus hydroboration-oxidation products, which alcohol a Grignard reagent gives, boiling point comparisons, the dehydration mechanism and carbocation stability, oxidation products, and the Lucas test.
Question types. Reaction-sequence and match-the-column questions in both exams, and assertion-reason questions on hydrogen bonding in NEET.
The trap that costs marks. Giving the Markovnikov product for hydroboration-oxidation, which actually gives the anti-Markovnikov alcohol.
Key takeaways
What must you be able to do from this part?
- Classification and naming: primary, secondary and tertiary by the carbon holding –OH; CHCHCH(OH)CH(CH) is 2-methylpentan-3-ol
- Preparation: acid hydration gives Markovnikov alcohols, hydroboration-oxidation anti-Markovnikov; Grignard reagents with carbonyls give all three classes
- Physical properties: hydrogen bonding makes ethanol boil at K against K for methoxymethane
- Reactions: esterification, C–O cleavage, dehydration through a carbocation, oxidation by class, and the Lucas test
Which Grignard reagent and carbonyl compound would you combine to make butan-2-ol?
- Preparation: acid hydration gives Markovnikov alcohols, hydroboration-oxidation anti-Markovnikov; Grignard reagents with carbonyls give all three classes
- Physical properties: hydrogen bonding makes ethanol boil at K against K for methoxymethane
- Reactions: esterification, C–O cleavage, dehydration through a carbocation, oxidation by class, and the Lucas test
Which Grignard reagent and carbonyl compound would you combine to make butan-2-ol?