Alcohols Phenols and Ethers revision notes

Key Concepts & Definitions

Alcohols

Compounds formed when a hydrogen atom in an aliphatic hydrocarbon is replaced by a hydroxyl (–OH) group.

Phenols

Compounds containing an –OH group directly attached to an sp2sp^2sp2 hybridized carbon atom of an aromatic system. Also known as carbolic acid.

Ethers

Compounds formed by substituting the hydrogen atom of a hydroxyl group in an alcohol or phenol with an alkyl or aryl group (R–O–R' or R–O–Ar).

Monohydric, Dihydric, Trihydric/Polyhydric

Classification based on whether the compound contains one, two, three, or many hydroxyl groups, respectively. Important Polyhydric Names: Ethane-1,2-diol is Ethylene glycol. Propane-1,2,3-triol is Glycerol.

Primary (1∘1^\circ1∘), Secondary (2∘2^\circ2∘), Tertiary (3∘3^\circ3∘) Alcohols

Monohydric alcohols where the –OH group is attached to an sp3sp^3sp3 hybridized primary, secondary, or tertiary carbon atom, respectively.

Allylic Alcohols

The –OH group is attached to an sp3sp^3sp3 hybridized carbon adjacent to a carbon-carbon double bond. Can be 1∘1^\circ1∘, 2∘2^\circ2∘, or 3∘3^\circ3∘.

Benzylic Alcohols

The –OH group is attached to an sp3sp^3sp3 hybridized carbon atom next to an aromatic ring. Can be 1∘1^\circ1∘, 2∘2^\circ2∘, or 3∘3^\circ3∘.

Vinylic Alcohols

The –OH group is bonded directly to a carbon-carbon double bond (sp2sp^2sp2 hybridized vinylic carbon).

Symmetrical (Simple) Ethers

Ethers where the alkyl/aryl groups attached to the oxygen atom are identical (e.g., C2H5OC2H5C_2H_5OC_2H_5C2​H5​OC2​H5​).

Unsymmetrical (Mixed) Ethers

Ethers where the two groups attached to the oxygen are different (e.g., C2H5OCH3C_2H_5OCH_3C2​H5​OCH3​).

Important Common Names

Catechol: 1,2-Benzenediol. Resorcinol: 1,3-Benzenediol. Hydroquinone/Quinol: 1,4-Benzenediol. o-, m-, p-Cresol: 2-Methylphenol, 3-Methylphenol, 4-Methylphenol. Anisole: Methoxybenzene (C6H5OCH3C_6H_5OCH_3C6​H5​OCH3​). Phenetole: Ethoxybenzene (C6H5OC2H5C_6H_5OC_2H_5C6​H5​OC2​H5​).

Commercial Alcohols

Methanol (Wood Spirit): Produced by catalytic hydrogenation of CO at high pressure/temp with ZnO−Cr2O3ZnO-Cr_2O_3ZnO−Cr2​O3​ catalyst.JEE TIPHighly poisonous; it oxidizes in the body to methanal and then methanoic acid, causing blindness or death. Medical antidote is intravenous infusion of diluted ethanol, which swamps the oxidizing enzyme. Ethanol: Obtained commercially by fermentation of sugars using the enzymes invertase (converts sugar to glucose/fructose) and zymase (converts glucose/fructose to ethanol and CO2CO_2CO2​) found in yeast. Oxidation of Fermentation: If air enters the fermentation mixture, oxygen oxidizes ethanol to ethanoic acid, ruining the taste. Denatured Alcohol: Commercial alcohol made unfit for drinking by mixing it with copper sulphate (for color) and pyridine (a foul-smelling liquid).

Important Rules, Laws & Principles

• Markovnikov's Rule: Applies to the acid-catalyzed hydration of unsymmetrical alkenes to form alcohols; the nucleophile (water) attaches to the more substituted carbon.
• Anti-Markovnikov's Addition (Effective): Hydroboration-oxidation of alkenes yields alcohols that look exactly as if water was added in opposition to Markovnikov's rule.JEE TIPNo carbocation is formed here, yielding excellent alcohol production without rearrangements.
• Brønsted Acid-Base Principle: Alcohols and phenols act as Brønsted acids (donating protons to stronger bases). Alcohols also act as Brønsted bases due to unshared electron pairs on oxygen making them proton acceptors.
• Zymase Inhibition Rule: During fermentation, the action of the zymase enzyme is naturally inhibited once the alcohol percentage exceeds 14%.

Structures & Bonding

• Alcohols: The oxygen is attached to carbon via a sigma bond formed by overlapping $sp^3$ hybridized orbitals of carbon and oxygen. The C-O-H bond angle is slightly less than the tetrahedral angle ($109^\circ 28'$) due to repulsion between the unshared electron pairs on oxygen.
• Phenols: The –OH group attaches to an $sp^2$ hybridized carbon. The C-O bond length (136 pm) is slightly less than in methanol due to: (i) partial double bond character from conjugation of oxygen's lone pair with the aromatic ring, and (ii) the $sp^2$ hybridized state of the carbon.
• Ethers: The oxygen is $sp^3$ hybridized. The C-O-C bond angle is slightly greater than the tetrahedral angle due to the repulsive interaction between the two bulky (–R) groups. The C-O bond length (141 pm) is almost the same as in alcohols.

Physical Properties, Trends & Comparisons

• Boiling Points of Alcohols & Phenols:
  • Increase with an increase in the number of carbon atoms (due to increased van der Waals forces).
  • In alcohols, boiling points decrease with an increase in branching because surface area decreases, lowering van der Waals forces.
  • B.P. of alcohols/phenols are exceptionally high compared to hydrocarbons, ethers, and haloalkanes of comparable molecular mass due to extensive intermolecular hydrogen bonding.
• Boiling Points of Ethers: Weak polarity does not appreciably affect their boiling points, making them comparable to alkanes of similar mass and much lower than isomeric alcohols.
• Solubility: Alcohols, phenols, and ethers are soluble in water because their oxygen atoms form hydrogen bonds with water molecules. Solubility decreases as the size of the hydrophobic alkyl/aryl group increases. Ethers and alcohols of comparable molecular mass have almost the exact same solubility in water (e.g., ethoxyethane 7.5g/100mL vs butan-1-ol 9g/100mL).
• Acidity Trend (Phenols vs. Water vs. Alcohols):
  • Phenols > Water > Alcohols.
  • Phenols are stronger acids because the phenoxide ion is stabilized by resonance (delocalization of negative charge), whereas the alkoxide ion is not.JEE TIPWater is a better proton donor than alcohols (except methanol); thus, alkoxides are stronger bases than hydroxides (e.g., sodium ethoxide is a stronger base than NaOH).
  • Acidity within Alcohols: Primary > Secondary > Tertiary. Electron-releasing alkyl groups increase electron density on oxygen, decreasing the polarity of the O–H bond, making $3^\circ$ the weakest acid.
• Substituent Effects on Phenol Acidity:
  • Electron-withdrawing groups (EWG like $-NO_2$) enhance acidity, especially at ortho and para positions due to effective delocalization of the negative charge.
  • Electron-releasing groups (ERG like $-CH_3$) decrease acidity because they do not favor phenoxide formation.
  • Specific pKa Values: Phenol ($pKa$ ~10.0) is roughly a million times more acidic than ethanol ($pKa$ ~15.9). Nitrophenols have $pKa$ ~7.1-8.3. Trend: p-Nitrophenol > o-Nitrophenol > m-Nitrophenol > Phenol > o/m/p-Cresol. o-Nitrophenol is slightly less acidic than p-Nitrophenol due to intramolecular hydrogen bonding.

Preparation of Alcohols

1. From Alkenes (Acid Catalyzed Hydration): Alkenes react with water + acid catalyst following Markovnikov's rule. Mechanism involves protonation to form a carbocation, nucleophilic attack by water, and deprotonation.
2. From Alkenes (Hydroboration-Oxidation): Reaction of alkenes with diborane $(BH_3)_2$ followed by oxidation with $H_2O_2$ in aqueous NaOH. Yields anti-Markovnikov alcohol products in excellent yield.
3. From Carbonyl Compounds (Reduction):
   • Catalytic hydrogenation (using Pt, Pd, or Ni).
   • Treatment with $NaBH_4$ or $LiAlH_4$.
   • Aldehydes yield $1^\circ$ alcohols; ketones yield $2^\circ$ alcohols.
   • Carboxylic acids are reduced to $1^\circ$ alcohols by $LiAlH_4$. Because $LiAlH_4$ is expensive, acids are usually first converted to esters, then catalytically hydrogenated.
4. From Grignard Reagents: Nucleophilic addition of RMgX to carbonyls, followed by hydrolysis.
   • Methanal (Formaldehyde) + RMgX $\rightarrow$ $1^\circ$ Alcohol.
   • Other Aldehydes + RMgX $\rightarrow$ $2^\circ$ Alcohol.
   • Ketones + RMgX $\rightarrow$ $3^\circ$ Alcohol.

Preparation of Phenols

1. From Haloarenes (Dow Process context): Chlorobenzene fused with NaOH at 623 K and 320 atm forms sodium phenoxide, which is acidified to phenol.
2. From Benzenesulphonic acid: Benzene sulfonated with oleum forms benzene sulphonic acid. Heated with molten NaOH to form sodium phenoxide, followed by acidification.
3. From Diazonium salts: Aniline treated with $NaNO_2 + HCl$ at 273-278 K forms benzene diazonium chloride. Warmed with water or treated with dilute acids to yield phenol.
4. From Cumene (Commercial Method): Cumene (isopropylbenzene) is oxidized in air to cumene hydroperoxide. Treated with dilute acid to yield phenol and acetone.JEE TIPAcetone is a highly valuable by-product produced in large quantities here.

Preparation of Ethers

1. By Dehydration of Alcohols: Heating ethanol with conc. $H_2SO_4$ at 413 K yields ethoxyethane (at 443 K, ethene forms via elimination).
   • Mechanism: $S_N2$ attack of an alcohol molecule on a protonated alcohol.
   • Limitations: Only suitable for $1^\circ$ unhindered alkyl groups. For $2^\circ$ and $3^\circ$ alcohols, dehydration to alkenes strongly competes and dominates.
2. Williamson Synthesis: Reaction of an alkyl halide with sodium alkoxide ($R-X + NaOR' \rightarrow R-O-R' + NaX$).
   • Involves an $S_N2$ attack of the alkoxide ion on the primary alkyl halide.
   • Phenols can also be converted to ethers via sodium phenoxide + alkyl halide.

Reactions & Mechanisms (Alcohols & Phenols)

Cleavage of O-H Bond (Acidity and Nucleophilic Behavior)

• Reaction with Metals: Alcohols/phenols react with active metals (Na, K, Al) to form alkoxides/phenoxides and $H_2$ gas.
• Esterification: Reaction with carboxylic acids, acid chlorides, and acid anhydrides forms esters.
  • With acids/anhydrides: Reversible, catalyzed by conc. $H_2SO_4$. Water must be removed continuously.
  • With acid chlorides: Carried out in the presence of a base (pyridine) to neutralize the HCl formed.
  • Acetylation: Introducing an acetyl ($CH_3CO$) group. Acetylation of salicylic acid produces aspirin.

Cleavage of C-O Bond (Alcohols Only)

Phenols only show C-O cleavage when reacting with zinc dust.

• Reaction with Hydrogen Halides (Lucas Test): $ROH + HX \rightarrow RX + H_2O$.
  • Differentiates $1^\circ, 2^\circ, 3^\circ$ alcohols.
  • $3^\circ$ alcohols produce immediate turbidity with Lucas reagent (conc. HCl + $ZnCl_2$). $1^\circ$ alcohols do not produce turbidity at room temp.
• Reaction with Phosphorus Trihalides: $ROH + PBr_3 \rightarrow RBr$.
• Dehydration to Alkenes: Using protic acids (conc. $H_2SO_4, H_3PO_4$) or catalysts (anhydrous $ZnCl_2$, alumina).
  • Ease of dehydration: $3^\circ > 2^\circ > 1^\circ$.
  • Mechanism (Ethanol): 1. Protonation of alcohol. 2. Formation of carbocation (Slowest, Rate Determining Step). 3. Elimination of a proton to form ethene.
• Oxidation (Dehydrogenation):
  • $1^\circ$ alcohols $\rightarrow$ Aldehydes (using $CrO_3$ in anhydrous medium or PCC) $\rightarrow$ Carboxylic acids (using strong agents like acidified $KMnO_4$).
  • $2^\circ$ alcohols $\rightarrow$ Ketones (using $CrO_3$).
  • $3^\circ$ alcohols $\rightarrow$ Do not easily oxidize. Under drastic conditions ($KMnO_4$ + heat), C-C cleavage occurs, yielding acids with fewer carbon atoms.
  • Heated Copper (Cu at 573 K): $1^\circ \rightarrow$ Aldehyde; $2^\circ \rightarrow$ Ketone; $3^\circ \rightarrow$ Alkene (Dehydration occurs instead of oxidation).

Electrophilic Aromatic Substitution of Phenols

The –OH group is strongly activating and ortho/para directing due to resonance.

• Nitration:
  • With dilute $HNO_3$ (298 K): Yields mixture of ortho and para nitrophenol.
  • With conc. $HNO_3$: Yields 2,4,6-trinitrophenol (Picric Acid).JEE TIPModern prep of picric acid: phenol + conc. $H_2SO_4$ $\rightarrow$ phenol-2,4-disulphonic acid, then treated with conc. $HNO_3$.
• Halogenation:
  • In low polarity solvents ($CHCl_3, CS_2$) at low temp: Yields monobromophenols (o- and p-bromophenol).
  • With Bromine Water: Yields 2,4,6-tribromophenol (white precipitate).
• Kolbe's Reaction: Phenoxide ion + $CO_2$ (weak electrophile) + $H^+$ $\rightarrow$ Ortho-hydroxybenzoic acid (Salicylic acid).
• Reimer-Tiemann Reaction: Phenol + $CHCl_3$ + aq. NaOH $\rightarrow$ intermediate benzal chloride $\rightarrow$ hydrolysis yields Salicylaldehyde (–CHO introduced at ortho position).
• Reaction with Zinc Dust: Phenol + Zn (heat) $\rightarrow$ Benzene + ZnO.
• Oxidation of Phenol: With chromic acid ($Na_2Cr_2O_7 / H_2SO_4$), phenol oxidizes to benzoquinone (a conjugated diketone). In air, it slowly forms dark mixtures containing quinones.

Reactions & Mechanisms (Ethers)

• Cleavage of C-O Bond by Hydrogen Halides (HX): $R-O-R + HX \rightarrow RX + ROH$. With excess HX at high temp, $ROH$ also becomes $RX$.
  • Reactivity order: HI > HBr > HCl.
  • Alkyl aryl ethers strictly cleave at the alkyl-oxygen bond (due to the stronger, partial double-bond character of the aryl-oxygen bond). Yields Phenol + Alkyl Halide. Phenol does not react further with HI.
  • Mechanism / Regioselectivity:
    • If alkyl groups are $1^\circ$ or $2^\circ$: Cleavage follows $S_N2$. The halide ion ($I^-$) attacks the less sterically hindered (smaller) alkyl group, forming the smaller alkyl halide.
    • If one alkyl group is $3^\circ$: Cleavage follows $S_N1$. The leaving group departs to form a stable $3^\circ$ carbocation, which is then attacked by the halide. Thus, the tertiary alkyl halide is formed.
• Electrophilic Substitution of Aromatic Ethers (Anisole): The alkoxy (–OR) group is activating and ortho/para directing.
  • Halogenation: Bromination with $Br_2$ in ethanoic acid yields para isomer (90% yield) without needing an iron catalyst.
  • Friedel-Crafts: Alkylation/acylation using alkyl/acyl halides + anhydrous $AlCl_3$ gives o/p products.
  • Nitration: Mixture of conc. $H_2SO_4$ and $HNO_3$ yields o/p nitroanisole.

Formulae & Equations

• Alcohol Hydration: $Alkene + H_2O \xrightarrow{H^+} Alcohol$
• Hydroboration-Oxidation: $Alkene \xrightarrow{(i) (BH_3)_2 \ (ii) H_2O_2/OH^-} Alcohol$
• Grignard Synthesis: $R-Mg-X + C=O \rightarrow R-C-O-MgX \xrightarrow{H_2O} R-C-OH + Mg(OH)X$
• Esterification: $ROH + R'COOH \xrightleftharpoons{H^+} R'COOR + H_2O$
• Lucas Reaction: $ROH + HCl \xrightarrow{ZnCl_2} RCl + H_2O$
• Dehydration: $CH_3CH_2OH \xrightarrow[443 K]{conc. H_2SO_4} CH_2=CH_2 + H_2O$
• Williamson Synthesis: $R-X + Na^+O^--R' \rightarrow R-O-R' + NaX$
• Ether Cleavage: $R-O-R' + HI \rightarrow R-I + R'-OH$

⚠️ EXCEPTIONS & ANOMALIES

• Bond Angle Anomaly: The C-O-C bond angle in ethers ($>109.5^\circ$) is greater than the tetrahedral angle, whereas the C-O-H bond angle in alcohols ($<109.5^\circ$) is less. Why: Ethers experience intense steric repulsion between two bulky alkyl groups, overriding lone-pair repulsion. Alcohols lack the second bulky group, so lone-pair repulsion dominates.
• Boiling Point vs Branching Exception: For isomeric alcohols, as branching increases, boiling point decreases. Why: Branching makes the molecule more spherical, decreasing the surface area and thereby weakening the van der Waals dispersion forces.
• Halogenation Catalyst Anomaly: Bromination of benzene strictly requires a Lewis acid catalyst ($FeBr_3$). Bromination of phenol occurs without any Lewis acid. Why: The –OH group is so strongly activating that it polarizes the bromine molecule directly.
• Ether Cleavage Mechanism Flip (The $3^\circ$ Exception): Cleavage of mixed ethers with HI normally follows $S_N2$ (the halogen attacks the smaller, less hindered alkyl group). However, if one group is tertiary ($3^\circ$), the mechanism entirely flips to $S_N1$, and the halogen attacks the bulky $3^\circ$ group. Why: The stability of the intermediate $3^\circ$ carbocation completely overrides the $S_N2$ steric preference.
• Alkyl Aryl Ether Cleavage Exception: Ethers normally cleave into two alkyl halides with excess HI. Alkyl aryl ethers (like anisole) never cleave at the aryl-oxygen bond, only the alkyl-oxygen bond, yielding Phenol + Alkyl Halide. Why: The aryl-oxygen bond has partial double-bond character due to resonance and $sp^2$ hybridization, making it too strong to break.
• Williamson Synthesis Reagent Reversal: Mixing a $3^\circ$ alkyl halide + $1^\circ$ alkoxide yields $0\%$ ether and $100\%$ alkene. Why: Alkoxides are strong bases. Steric hindrance in the $3^\circ$ halide prevents $S_N2$ substitution, so elimination dominates. To make the ether, you must use a $1^\circ$ halide and a $3^\circ$ alkoxide.
• Temperature-Dependent Dehydration Anomaly: Reacting ethanol with conc. $H_2SO_4$ gives completely different functional groups depending on a slight temperature change. Why: At 413 K, substitution ($S_N2$) occurs forming ethoxyethane (ether). At 443 K, elimination occurs forming ethene (alkene).
• Heated Copper ($3^\circ$ Alcohol) Exception: Passing $1^\circ$ and $2^\circ$ alcohol vapors over Cu at 573 K causes dehydrogenation (oxidation) to aldehydes and ketones. Passing $3^\circ$ alcohols over the same catalyst causes dehydration. Why: $3^\circ$ alcohols do not have an alpha-hydrogen to lose for oxidation, so they lose a water molecule to form an alkene instead.
• Acid vs. Base Anomaly for Alcohols: Alcohols are amphoteric. They act as Brønsted acids (donating a proton to active metals) AND as Brønsted bases (accepting a proton due to lone pairs on oxygen).
• Solubility vs. Boiling Point Disconnect in Ethers: Ethers have very low boiling points (similar to alkanes because they can't H-bond with themselves), but they have high solubility in water (similar to alcohols because they can H-bond with water).

Previous Year JEE Topics

• Acidic Strength Comparisons: Specifically substituent effects on phenol (e.g., p-nitrophenol vs m-nitrophenol vs p-cresol).
• Regioselectivity in Ether Cleavage by HI: Differentiating between $1^\circ/2^\circ$ (forms smaller alkyl iodide) and $3^\circ$ (forms $3^\circ$ alkyl iodide via carbocation).
• Cumene Process: Identifying the intermediates (cumene hydroperoxide) and the economically vital by-product (acetone).
• Lucas Test: Time taken for turbidity appearance ($3^\circ$ immediate, $1^\circ$ none at room temp) to distinguish structural isomers of alcohols.
• Reimer-Tiemann & Kolbe's Reaction: Intermediate formation (benzal chloride in R-T) and specific final products (salicylaldehyde vs. salicylic acid).

Memory Aids & JEE Traps

[JEE TIP] Trap 1 - Ether Cleavage Regioselectivity (The $S_N1$ vs $S_N2$ Trap)

• Misconception: When a mixed ether reacts with HI, the iodide always attacks the more highly substituted (more stable) carbon.
• Correct Understanding: It is primarily an $S_N2$ reaction, so the iodide attacks the smaller, least sterically hindered carbon (e.g., methyl). It ONLY shifts to attacking the more substituted carbon via $S_N1$ if a tertiary ($3^\circ$) alkyl group is present.

[JEE TIP] Trap 2 - Williamson Synthesis Limitation

• Misconception: You can react any alkyl halide with any sodium alkoxide to get the corresponding ether.
• Correct Understanding: Using a secondary ($2^\circ$) or tertiary ($3^\circ$) alkyl halide will result almost exclusively in elimination (forming an alkene) because alkoxides are strong bases. You must use a $1^\circ$ alkyl halide.

[JEE TIP] Trap 3 - Cleavage of Anisole (Alkyl Aryl Ethers)

• Misconception: Heating anisole with excess HI yields iodobenzene and methanol (or methyl iodide).
• Correct Understanding: The $sp^2$ Carbon-Oxygen bond in the benzene ring has partial double bond character and never breaks. The product is ALWAYS phenol and methyl iodide. Furthermore, phenol does not react further with HI.

[JEE TIP] Trap 4 - Dehydration Temperatures

• Misconception: Reacting ethanol with conc. $H_2SO_4$ automatically forms ethene.
• Correct Understanding: The product is strictly temperature-dependent. 443 K yields ethene (elimination), but 413 K yields ethoxyethane (bimolecular substitution).

[JEE TIP] Trap 5 - Oxidation over Heated Copper (Cu/573K)

• Misconception: Passing any alcohol over heated Cu oxidizes it to a carbonyl compound.
• Correct Understanding: While $1^\circ \rightarrow$ aldehyde and $2^\circ \rightarrow$ ketone, passing a $3^\circ$ alcohol over Cu/573K results in dehydration to an alkene, NOT oxidation.

[JEE TIP] Trap 6 - Yield of Picric Acid via Direct Nitration

• Misconception: Reacting phenol with concentrated $HNO_3$ is the best way to prepare picric acid (2,4,6-trinitrophenol).
• Correct Understanding: Direct nitration gives a very poor yield due to the oxidizing nature of conc. nitric acid. High yields require sulfonating phenol first (with conc. $H_2SO_4$) before adding $HNO_3$.

[JEE TIP] Trap 7 - Halogenation Catalysts for Phenol

• Misconception: Just like benzene, you must use a Lewis acid catalyst ($FeBr_3$ or $AlCl_3$) to brominate phenol.
• Correct Understanding: Phenol is so highly activated by the –OH group that it polarizes the halogen molecule directly. No Lewis acid is required.

[JEE TIP] Trap 8 - Base Strength of Alkoxides vs. Hydroxides

• Misconception: Because water is a neutral molecule and alcohols are "acidic", sodium hydroxide is a stronger base than sodium ethoxide.
• Correct Understanding: Alcohols (except methanol) are weaker acids than water. Therefore, their conjugate bases (alkoxides) are stronger bases than the hydroxide ion.

[JEE TIP] Trap 9 - Volatility of Nitrophenols

• Misconception: Para-nitrophenol is more volatile than ortho-nitrophenol because it is more symmetrical.
• Correct Understanding: Ortho-nitrophenol is steam volatile due to intramolecular hydrogen bonding. Para-nitrophenol forms intermolecular H-bonds, causing molecules to associate and drastically raising its boiling point (less volatile).

[JEE TIP] Trap 10 - Carbocation Rearrangements in Hydration

• Misconception: Both Acid-Catalyzed Hydration and Hydroboration-Oxidation give alcohol products strictly based on where the double bond originally was.
• Correct Understanding: Acid-catalyzed hydration passes through a carbocation intermediate, meaning 1,2-hydride or 1,2-methyl shifts will occur to form a more stable carbocation before water attacks. Hydroboration-oxidation does not form a carbocation, so no rearrangements happen.