Assertion: Addition of water to but-1-ene in acidic medium yields butan-1-ol. Reason: Addition of water in aci

NCERT Exemplar · Q65

Q. Assertion: Addition of water to but-1-ene in acidic medium yields butan-1-ol. Reason: Addition of water in acidic medium proceeds through the formation of primary carbocation.

(i) Assertion and reason both are correct statements and reason is the correct explanation for assertion.

(ii) Assertion and reason both are correct statements but reason is not the correct explanation for assertion.

(iii) Assertion is correct statement but reason is wrong statement.

(iv) Assertion is wrong statement but reason is correct statement.

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Concept understanding — Boiling Point Trends

Boiling Point Trends (Organic Compounds)

A substance's boiling point is set by how much energy is needed to overcome the attractive forces HOLDING its molecules together in the liquid — the stronger those intermolecular forces, the higher the boiling point.

The Forces, Weakest to Strongest

Van der Waals (London dispersion) forces — present in every molecule, and they grow stronger as the molecule gets bigger (more electrons, larger surface area of contact between neighbouring molecules) and more polarisable.
Dipole–dipole forces — present in polar molecules, add an extra attraction on top of dispersion forces.
Hydrogen bonding — present when H is bonded directly to N, O, or F; much stronger than ordinary dipole–dipole attraction, and it raises the boiling point sharply compared to a similarly-sized molecule without it.

Trend 1: Down a Series of Halogens (Same Alkyl Group)

For a fixed R group, boiling point rises as the halogen gets heavier: $R{-}I > R{-}Br > R{-}Cl > R{-}F$ . This looks surprising at first, since electronegativity (and so bond polarity/dipole moment) actually DECREASES down the group — but boiling point here is dominated by the growing size and polarisability of the halogen atom (stronger dispersion forces), which outweighs the shrinking dipole contribution.

The measured values for the methyl, ethyl and propyl halides show this rise clearly:

Measured boiling points of methyl, ethyl and propyl chlorides, bromides and iodides (NCERT Fig 6.1), rising steadily from R-Cl to R-Br to R-I for every alkyl group

Trend 2: Chain Length and Branching

Longer chains (more carbons) have more surface area for van der Waals contact between neighbouring molecules, so boiling point rises with chain length within a homologous series.
Branching LOWERS boiling point compared to a straight-chain isomer of the same molecular formula — a more compact, spherical shape has less surface-to-surface contact with neighbouring molecules, weakening the dispersion forces. (E.g. neopentane boils well below n-pentane.)

Trend 3: Hydrogen Bonding Beats Molecular Mass …

Why this formula?

Boiling Point Trends: Why They Happen

Boiling point is the temperature at which a liquid's vapor pressure equals the external atmospheric pressure. To understand why boiling points follow certain trends, we must first understand what determines vapor pressure.

The Core Idea: Intermolecular Forces

A liquid boils when its molecules have enough kinetic energy to overcome the intermolecular forces (IMFs) holding them together in the liquid phase. Stronger IMFs → harder to escape → lower vapor pressure at a given temperature → higher boiling point.

There is no single "formula" for boiling point, but the relationship is captured by the Clausius–Clapeyron equation, which links vapor pressure ( $P$ ) to temperature ( $T$ ) and the enthalpy of vaporization ( $\Delta H_{\text{vap}}$ ):

\ln P = -\frac{\Delta H_{\text{vap}}}{R} \cdot \frac{1}{T} + C

Where:

$P$ = vapor pressure
$\Delta H_{\text{vap}}$ = enthalpy of vaporization (energy needed to vaporize 1 mole)
$R$ = gas constant
$T$ = absolute temperature (Kelvin)
$C$ = constant (depends on substance)

Why this formula makes sense

$\Delta H_{\text{vap}}$ is large when IMFs are strong — more energy is needed to separate molecules.
At boiling point, $P = P_{\text{atm}}$ (usually 1 atm). So a substance with larger $\Delta H_{\text{vap}}$ needs a higher $T$ to reach that pressure.

Thus, boiling point ∝ strength of intermolecular forces.

The Four Key Trends (with Reasoning)

1. Trend across a period (e.g., Period 2: CH₄ → NH₃ → H₂O → HF)

Molecule	IMFs present	Boiling point (°C)
CH₄	London dispersion only	-161
NH₃	Dispersion + H-bonding	-33
H₂O	Dispersion + H-bonding (2 per molecule)	100
HF	Dispersion + H-bonding	19

Why?

CH₄ is nonpolar — only weak London dispersion forces.
NH₃, H₂O, HF have hydrogen bonding (strongest IMF).
H₂O forms two H-bonds per molecule (donor + acceptor), while NH₃ forms one and HF forms one — hence H₂O has the highest boiling point.

Key insight: Hydrogen bonding dominates over molecular mass in small molecules.

2. Trend down a group (e.g., Halogens: F₂ → Cl₂ → Br₂ → I₂)

Molecule	Molar mass (g/mol)	Boiling point (°C)
F₂	38	-188
Cl₂	71	-34
Br₂	160	59
I₂	254	184

Why?

All are nonpolar — only London dispersion forces.
Dispersion force strength increases with number of electrons (larger molar mass → more polarizable electron cloud → stronger temporary dipoles).
So boiling point increases down the group.

Key insight: For nonpolar molecules, molar mass (electron count) is the primary factor.

3. Branching in alkanes (e.g., C₅H₁₂ isomers)

Isomer	Boiling point (°C)
n-pentane (straight chain)	36
2-methylbutane (branched)	28
2,2-dimethylpropane (highly branched)	10

Why?

All have same molecular formula — same molar mass. …

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Acid-catalysed hydration of but-1-ene follows Markovnikov's rule: protonation occurs at C1, generating a secondary carbocation at C2, which then reacts with water to give butan-2-ol -- not butan-1-ol, and not through a primary carbocation. Both the assertion and the reason, as printed, are incorrect.

Understanding the reaction

Acid-catalysed hydration of an alkene is a classic electrophilic addition, and its regiochemistry is governed entirely by carbocation stability (Markovnikov's rule), not by which carbon is 'easier to reach'.

Step 1: Protonation

But-1-ene is CH3-CH2-CH=CH2 (double bond between C1 and C2, C1 being the terminal =CH2). Under acid catalysis, H+ adds to C1 -- the carbon that already carries more hydrogens -- because this places the resulting positive charge on C2, which is flanked by an ethyl group and a methyl group:

CH3-CH2-CH=CH2 + H+ -> CH3-CH2-CH(+)-CH3

This is a secondary carbocation. A primary carbocation (positive charge on the terminal carbon) would be far less stable and does not form preferentially here.

Step 2: Water attacks the carbocation

Water attacks the more stable secondary carbocation at C2. After loss of a proton, the product is:

CH3-CH2-CH(OH)-CH3 = butan-2-ol

Watch out

Butan-1-ol (the primary alcohol, CH3CH2CH2CH2OH) is the anti-Markovnikov product. It is not formed by ordinary acid-catalysed hydration -- it requires a different method entirely, such as hydroboration-oxidation.

Step 3: Evaluating the assertion and the reason

Assertion ('yields butan-1-ol'): false -- the real product is butan-2-ol. …

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Concept: Markovnikov’s Rule & Carbocation Stability in Electrophilic Addition

Method: Stability-Based Carbocation Analysis

Step 1: Identify the reaction type

The addition of water ( $H_2O$ ) to an alkene in acidic medium is acid-catalysed hydration — an electrophilic addition reaction.

Step 2: Determine the intermediate carbocation

But-1-ene: $CH_3CH_2CH=CH_2$
Protonation of the double bond can occur at two positions:

Protonation site	Carbocation formed	Type
At C-1 (terminal)	$CH_3CH_2CH^+CH_3$	Secondary (more stable)
At C-2	$CH_3CH_2CH_2CH_2^+$	Primary (less stable)

Step 3: Apply Markovnikov’s rule

The reaction proceeds via the more stable carbocation — the secondary one.

Thus, the major product is butan-2-ol, not butan-1-ol.

Step 4: Evaluate the Assertion

“Addition of water to but-1-ene in acidic medium yields butan-1-ol.”

This is wrong — the major product is butan-2-ol.

Step 5: Evaluate the Reason

“Addition of water in acidic medium proceeds through the formation of primary carbocation.” …

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Common Mistakes & How to Avoid Them

Mistake 1: Assuming the product is butan-1-ol (primary alcohol)

Why students make this mistake:

They see "addition of water" and think the —OH group simply attaches to the first carbon (C-1) of but-1-ene, giving butan-1-ol.

The correct understanding:

In acidic medium, the reaction follows Markovnikov's rule. The proton (H⁺) adds to the less substituted carbon (C-1), forming a secondary carbocation at C-2 (not a primary one). Water then attacks this secondary carbocation, yielding butan-2-ol (a secondary alcohol).

How to avoid:

Always check the carbocation stability order: tertiary > secondary > primary > methyl.
For unsymmetrical alkenes, the proton goes to the carbon with more hydrogens, so the carbocation forms on the more substituted carbon.
Key result: But-1-ene + H₂O/H⁺ → butan-2-ol, not butan-1-ol.

Mistake 2: Believing the reason statement is correct

Why students make this mistake:

They think "primary carbocation" sounds plausible because the proton adds to C-1 (which is primary).

The correct understanding:

The intermediate is actually a secondary carbocation (at C-2), not a primary one. A primary carbocation is highly unstable and does not form under these conditions.

How to avoid:

Memorise the stability trend and apply it to every electrophilic addition.
If a primary carbocation would form, the reaction will rearrange to a more stable carbocation (e.g., via hydride shift).
Key result: The reason statement is wrong — the intermediate is secondary, not primary.

Mistake 3: Choosing option (A) or (B)

Why students make this mistake:

They think both statements are correct, or they think the reason explains the assertion even if the product is wrong.

The correct understanding:

Assertion is wrong (product is butan-2-ol).
Reason is wrong (intermediate is secondary carbocation).
Assertion: wrong (product is butan-2-ol).
Reason: wrong (intermediate is secondary carbocation). So neither statement is strictly correct. But the given options don't have "both wrong". Let's re-read the options:

(D) Assertion is wrong statement but reason is correct statement. …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

Showing the 12 most recent of 14 on this concept.

AP EAPCET 2026Set eng-2026-05-13-AN1 markMCQ
$n-C_4H_9OH$
›Reveal solutionSolution
This tests the classic boiling-point trend among an alcohol, a 1° amine, a 2° amine, and a 3° amine of comparable molecular weight, driven by hydrogen-bonding capacity. The match is A-III, B-IV, C-II, D-I.

Concept and Intuition

For molecules of similar size, boiling point tracks how strongly molecules can hydrogen-bond to each other:

Alcohols (O–H) hydrogen-bond most strongly (O is more electronegative than N, and the O–H bond is highly polarized), so they have the highest boiling points among comparably-sized compounds.

Primary amines have two N–H bonds per molecule available for intermolecular hydrogen bonding — next highest.

Secondary amines have only one N–H bond — weaker hydrogen bonding, lower boiling point than primary amines.

Tertiary amines have no N–H bond at all (nitrogen's lone pair can still accept a hydrogen bond from something else, but the molecule itself cannot donate one), so they rely mainly on weaker dipole–dipole and dispersion forces — lowest boiling point of the four.

Step-by-Step Solution

A. $n$ - $C_4H_9OH$ (n-butanol): a primary alcohol — strongest H-bonding → highest boiling point among the four, 390.3 K → list item III. A-III.

C. $n$ - $C_4H_9NH_2$ (n-butylamine): a primary amine, two N–H bonds → next highest, 350.8 K → list item II. C-II.

B. $(C_2H_5)_2NH$ (diethylamine): a secondary amine, one N–H bond → lower still, 329.3 K → list item IV. B-IV. …
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AP EAPCET 2026Set ap-2026-05-19-FN1 markMCQ
$\underline{A}$
›Reveal solutionSolution
Boiling points of group 15 hydrides dip after ammonia (loss of H-bonding) then rise with increasing molar mass; highest is BiH3, lowest is PH3.

Concept and Intuition

$NH_3$ has strong intermolecular hydrogen bonding (N is small and highly electronegative), giving it an unusually high boiling point for its size. Once H-bonding is lost going to $PH_3$ , boiling point drops sharply because only weak van der Waals (London dispersion) forces operate. As you continue down the group ( $AsH_3\to SbH_3\to BiH_3$ ), molecular size and mass increase steadily, so van der Waals forces strengthen again and boiling point rises — eventually exceeding even $NH_3$ .

Step-by-Step Solution

Approximate boiling points: $NH_3\approx -33°C$ , $PH_3\approx -87.7°C$ , $AsH_3\approx -55°C$ , $SbH_3\approx -17°C$ , $BiH_3\approx +17°C$ .

Lowest of these is $PH_3$ (the H-bonding of $NH_3$ is gone, and molecular mass is still small). …
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AP EAPCET 2025Set eng-2025-05-22-FN1 markMCQ
$Q. The correct order of boiling points of the compounds given below is A) Methoxy ethane B) Propan-1-ol C) Propanal D) Propanone (A) C > B > A > D (B) B > D > C > A (C) B > C > D > A (D) C > A > B > D$
›Reveal solutionSolution
Tests ranking boiling points by intermolecular forces: H-bonding alcohol > dipolar ketone > dipolar aldehyde > weakly-polar ether.

Concept and Intuition

For molecules of similar molar mass, boiling point is set by the strength of intermolecular forces. An –OH group enables strong hydrogen bonding (raising b.p. sharply above similarly-sized non-alcohols). A C=O group gives a fairly strong permanent dipole (ketones/aldehydes), but weaker than H-bonding. An ether has a weaker net dipole (bond dipoles partly oppose) and no H-bond donor, so it boils at the lowest temperature of the four functional classes here.

Step-by-Step Solution

B) Propan-1-ol, $\mathrm{CH_3CH_2CH_2OH}$ : extensive intermolecular H-bonding via $-\mathrm{OH}$ gives it the highest boiling point of the four.

D) Propanone (acetone), $\mathrm{CH_3COCH_3}$ : a symmetric ketone with a strong dipole from C=O but no H-bond donor — boils next highest.

C) Propanal, $\mathrm{CH_3CH_2CHO}$ : also has a polar C=O, but the aldehyde's dipole/packing gives it a slightly lower boiling point than the ketone of the same carbon count. …
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AP EAPCET 2025Set eng-2025-05-24-FN1 markMCQ
$Q. Observe the following substances. Ethanol, acetic acid, ethylamine, trimethylamine, salicylic acid, ethanal. In the above list, the number of substances with H-bonding is (A) 4 (B) 3 (C) 5 (D) 2$
›Reveal solutionSolution
Tests recognizing which functional groups (O–H, N–H) enable hydrogen bonding; 4 of the 6 substances qualify.

Concept and Intuition

Hydrogen bonding needs a hydrogen atom covalently bonded to a small, highly electronegative atom — O, N, or F — so that the H carries a strong partial positive charge able to interact with a lone pair on a neighbouring electronegative atom. A carbonyl oxygen (as in an aldehyde) or a nitrogen with no attached H (as in a fully substituted tertiary amine) cannot act as an H-bond donor themselves.

Step-by-Step Solution

Ethanol ( $\mathrm{C_2H_5OH}$ ): has an O–H group → capable of H-bonding.

Acetic acid ( $\mathrm{CH_3COOH}$ ): has a carboxylic O–H group → capable of H-bonding.

Ethylamine ( $\mathrm{C_2H_5NH_2}$ ): a primary amine with N–H bonds → capable of H-bonding.

Trimethylamine ( $\mathrm{N(CH_3)_3}$ ): a tertiary amine — nitrogen has no attached H, so it cannot donate a hydrogen bond → excluded.

Salicylic acid: has both a carboxylic O–H and a phenolic O–H → capable of H-bonding. …
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AP EAPCET 2023Set ap-2023-05-22-AN1 markMCQ
$Q. What is the correct boiling point order of the following haloalkanes? i) 2-chloro 2-methylpropane ii) 1-Cholobutane iii) 2-Chlorobutane (A) i > ii > iii (B) ii > iii > i (C) i < ii < iii (D) i > iii > ii$
›Reveal solutionSolution
Among isomeric C₄H₉Cl haloalkanes, the straight-chain isomer boils highest and the most branched (tertiary) isomer boils lowest, giving the order ii > iii > i.

Concept and Intuition

For a set of structural isomers with the same molecular formula, boiling point is governed mainly by the strength of intermolecular van der Waals (London dispersion) forces, which depend on the surface area available for molecules to contact each other. A straight (unbranched) chain packs closely and has more surface contact, giving stronger dispersion forces and a higher boiling point. Branching makes the molecule more compact/spherical, reducing surface area and intermolecular contact, and hence lowering the boiling point.

Step-by-Step Solution

Identify the three isomers, all of formula $C_4H_9Cl$ : (i) 2-chloro-2-methylpropane (tert-butyl chloride) — most branched, chlorine on a tertiary carbon; (ii) 1-chlorobutane — straight (unbranched) chain, chlorine on a primary carbon; (iii) 2-chlorobutane — chlorine on a secondary carbon, slightly branched.

Rank by branching (least to most): (ii) unbranched < (iii) one branch point < (i) most branched (quaternary-like carbon skeleton around the C–Cl carbon). …
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AP EAPCET 2023Set ap-2023-05-22-AN1 markMCQ
$Q. Para-nitro phenol has higher boiling point than ortho-nitrophenol. This is due to (A) The presence of intermolecular hydrogen bonding between Para-nitro phenol molecules (B) The presence of intramolecular hydrogen bonding in Para-nitro phenol molecules (C) The presence of intermolecular hydrogen bonding between ortho-nitro phenol molecules (D) The absence of intramolecular hydrogen bonding between ortho-nitro phenol molecules$
›Reveal solutionSolution
Para-nitrophenol boils higher than ortho because ortho forms intramolecular H-bonding (chelation) while para is forced into intermolecular H-bonding, which needs more energy to break.

Concept and Intuition

Boiling point depends on the strength of the forces holding molecules together in the liquid. Ortho-nitrophenol's $\mathrm{-OH}$ and $\mathrm{-NO_2}$ are adjacent, so they hydrogen-bond to each other within the same molecule (a six-membered ring "chelate"). This uses up the $\mathrm{-OH}$ 's hydrogen-bonding capacity internally, so ortho-nitrophenol molecules interact with each other only weakly (via van der Waals forces) — it boils low and is even steam-volatile. In para-nitrophenol the groups are on opposite ends of the ring and cannot reach each other, so the $\mathrm{-OH}$ of one molecule instead hydrogen-bonds to the $\mathrm{-NO_2}$ / $\mathrm{-OH}$ of a neighbouring molecule — building an extended, harder-to-break intermolecular network, hence a higher boiling point.

Step-by-Step Solution

Identify the substitution pattern: ortho places $\mathrm{-OH}$ and $\mathrm{-NO_2}$ next to each other; para places them across the ring.

Ortho: intramolecular H-bond forms a stable ring — no need for the molecule to H-bond with neighbours. …
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AP EAPCET 2023Set eng-2023-05-15-FN1 markMCQ
$Q. Arrange the following in increasing order of their boiling points N-Ethylethanamine - I Butanamine - II N,N-dimethylethanamine - III (A) III > II > I (B) III > I > II (C) II > III > I (D) II > I > III$
›Reveal solutionSolution
Boiling point of amines of the same formula falls as 1° > 2° > 3°, because more N–H bonds mean stronger intermolecular hydrogen bonding.

Concept and Intuition

All three compounds share the molecular formula $C_4H_{11}N$ , so molecular weight/dispersion forces are essentially comparable; the boiling-point differences are governed by hydrogen bonding capacity. A primary amine ( $-NH_2$ ) has two N–H bonds and can form the most extensive intermolecular hydrogen-bond network, giving it the highest boiling point among the three classes for a given carbon count. A secondary amine ( $-NH-$ ) has only one N–H bond, so it hydrogen-bonds less extensively (lower bp than the primary isomer). A tertiary amine has no N–H bond at all, so it cannot hydrogen-bond with itself, relying only on weaker dipole–dipole and dispersion forces, giving it the lowest boiling point.

Step-by-Step Solution

Classify each compound: Butanamine (II) = $\mathrm{CH_3CH_2CH_2CH_2NH_2}$ , a primary amine (2 N–H bonds).

N-Ethylethanamine (I) = diethylamine, $\mathrm{(C_2H_5)_2NH}$ , a secondary amine (1 N–H bond).

N,N-Dimethylethanamine (III) = $\mathrm{CH_3CH_2N(CH_3)_2}$ , a tertiary amine (0 N–H bonds). …
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AP EAPCET 2023Set eng-2023-05-18-AN1 markMCQ
$Q. The correct order of boiling points of following molecules is$
(i) n – Hexane
(ii) 2-methylpentane
(iii) 2,3 – dimethylbutane (A) i > ii > iii (B) iii > ii > i (C) iii > i > ii (D) i > iii > ii
›Reveal solutionSolution
Boiling point falls as branching increases among isomeric alkanes, so n-hexane > 2-methylpentane > 2,3-dimethylbutane.

Concept and Intuition

All three compounds are isomers of hexane ( $C_6H_{14}$ ), so they have identical molecular formula and hence similar total van der Waals attraction potential — but the shape of the molecule matters. A straight, extended chain (n-hexane) has more surface-to-surface contact with neighbouring molecules, maximizing van der Waals (London dispersion) forces. Branching makes the molecule more compact and spherical, reducing effective surface contact and hence the strength of intermolecular attractions, which lowers the boiling point.

Step-by-Step Solution

n-Hexane: a straight, unbranched 6-carbon chain — largest surface area for intermolecular contact — highest boiling point among the three.

2-Methylpentane: one methyl branch — somewhat more compact than n-hexane — intermediate boiling point.

2,3-Dimethylbutane: two methyl branches, the most compact/spherical of the three — smallest surface area for contact — lowest boiling point. …
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AP EAPCET 2023Set eng-2023-05-18-AN1 markMCQ
$Q. At 298 K, if the vapour pressure of pure liquids toluene, benzene, chloroform and dichloromethane are 60, 160, 200 and 415 torr respectively. Then which liquid is having high boiling point? (A) Toluene (B) Benzene (C) Chloroform (D) Dichloromethane$
›Reveal solutionSolution
Boiling point and vapour pressure (at fixed T) are inversely related; toluene's lowest vapour pressure (60 torr) means it has the highest boiling point.

Concept and Intuition

Vapour pressure measures how readily a liquid's molecules escape into the gas phase at a given temperature — it is a direct measure of volatility. Boiling point is the temperature at which vapour pressure equals atmospheric pressure. A liquid that already has a low vapour pressure at a reference temperature needs to be heated more to reach atmospheric pressure, so lower vapour pressure at a fixed T corresponds to a higher boiling point.

Step-by-Step Solution

List the vapour pressures at 298 K: toluene 60 torr, benzene 160 torr, chloroform 200 torr, dichloromethane 415 torr.

Rank from lowest to highest vapour pressure: toluene < benzene < chloroform < dichloromethane. …
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AP EAPCET 2023Set eng-2023-05-18-AN1 markMCQ
$\mathrm{NH_3}$
›Reveal solutionSolution
Boiling points of these hydrides are governed mainly by hydrogen bonding strength/extent, giving the increasing order $HCl<NH_3<HF<H_2O$ .

Concept and Intuition

Among simple hydrides, boiling point is strongly influenced by hydrogen bonding, which occurs when H is bonded to a small, highly electronegative atom (N, O, F). $HCl$ 's Cl is not electronegative/small enough to hydrogen bond significantly, so it relies only on weaker dipole-dipole/dispersion forces and has the lowest boiling point among these four. Among the hydrogen-bonded species, $H_2O$ forms an extensive 3-D hydrogen-bonded network (2 lone pairs and 2 H atoms per molecule, ideal for a 3-D network) giving it the highest boiling point, while $HF$ and $NH_3$ form more limited (chain-like or less networked) hydrogen bonding.

Step-by-Step Solution

$HCl$ : negligible hydrogen bonding (Cl is not electronegative/small enough) — lowest boiling point among the four ( $\approx-85^\circ C$ ).

$NH_3$ : hydrogen bonds via N, but only one lone pair per molecule to hydrogen bond with $\Rightarrow$ boiling point $\approx-33^\circ C$ .

$HF$ : strong hydrogen bonding via a highly electronegative F, but limited to one H and three lone pairs (only one bond forms per molecule in the chain) $\Rightarrow$ boiling point $\approx19.5^\circ C$ . …
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AP EAPCET 2022Set ap-2022-07-11-AN1 markMCQ
$\mathrm{CH_3CH(OH)CH_2CH_3}$
›Reveal solutionSolution
Among an alcohol, a ketone, an alkane, and an ether of comparable size, the alcohol has the highest boiling point because only it can hydrogen-bond between its own molecules.

Concept and Intuition

Boiling point depends on the strength of intermolecular forces that must be overcome to vaporise the liquid. Alcohols (-OH group) can form hydrogen bonds with each other, a strong, directional intermolecular force. Ketones and ethers only have permanent dipole-dipole interactions (no O-H or N-H to hydrogen-bond with each other), which are weaker than hydrogen bonding. Alkanes have only weak, non-polar van der Waals (London dispersion) forces, the weakest of all.

Step-by-Step Solution

Butan-2-ol: contains -OH, capable of strong intermolecular hydrogen bonding $\Rightarrow$ highest boiling point among these four.

Butan-2-one: a ketone, polar C=O but no H-bond donor $\Rightarrow$ moderate boiling point (dipole-dipole), lower than the alcohol.

Ethyl propyl ether: polar C-O-C but no H-bond donor either $\Rightarrow$ boiling point similar to or slightly below the ketone. …
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AP EAPCET 2022Set eng-2022-07-05-FN1 markMCQ
$Q. Arrange the following in decreasing order of their boiling points$
(a) $CH_3CH_2CH_2CH_2OH$ (butan-1-ol)
(b) $CH_3CH_2CH_2CH_2NH_2$ (butan-1-amine, a primary amine)
(c) a tertiary amine, $(CH_2CH_3)$ chain with an N bearing two other alkyl branches (drawn as a small N with two branches, i.e. a trialkylamine)
(d) a secondary amine with an N-H, drawn as two ethyl-type chains joined through an N-H (a secondary amine) (A) a > b > d > c (B) a > c > d > b (C) b > c > d > a (D) c > a > b > d
›Reveal solutionSolution
Boiling point here tracks hydrogen-bonding ability: the alcohol (strongest H-bonding) is highest, then primary amine (two N–H), then secondary amine (one N–H), then tertiary amine (no N–H, weakest): a > b > d > c.

Concept and Intuition

For molecules of comparable molecular weight, boiling point is governed largely by the strength and extent of intermolecular hydrogen bonding. Oxygen is more electronegative than nitrogen, so O–H···O hydrogen bonds are stronger than N–H···N hydrogen bonds — alcohols therefore boil higher than amines of similar size. Among amines themselves, hydrogen bonding requires an N–H bond to donate; a primary amine has two N–H bonds (most extensive hydrogen-bonded network), a secondary amine has only one N–H bond (less association), and a tertiary amine has none (cannot hydrogen-bond to itself at all, only weaker dipole-dipole/van der Waals forces), giving it the lowest boiling point of the three.

Step-by-Step Solution

Butan-1-ol (a): $-\mathrm{OH}$ group, strongest hydrogen bonding of the four compounds → highest boiling point.

Butan-1-amine (b), a primary amine: two N–H bonds, extensive intermolecular hydrogen bonding → next highest. …
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Q.Assertion: Addition of water to but-1-ene in acidic medium yields butan-1-ol. Reason: Addition of water in acidic medium proceeds through the formation of primary carbocation.

Understanding the reaction

Step 1: Protonation

Step 2: Water attacks the carbocation

Step 3: Evaluating the assertion and the reason

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Q.Assertion: Addition of water to but-1-ene in acidic medium yields butan-1-ol.
Reason: Addition of water in acidic medium proceeds through the formation of primary carbocation.

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