Hydroxy Compounds

General forumla of alcohols

R-OH

General formula of phenol

-OH DIRECTLY attached to an aromatic ring

Why do alcohols/phenols have higher boiling points than their isoelectronic hydrocarbons?

• Strength of dispersion forces is about the same due to similar size of electron cloud

• More energy is required to overcome intermolecular hydrogen bonding in alcohols

• which are stronger than the disperson forces in alkanes

Note: larger e- cloud —> higher probability of distortion of e- cloud —> stronger and more significant dispersion forces —> difference between bps decreases

boiling point increases/decreases as alkyl chain length increases

• alkyl chain length increases

• electron cloud size increases

• more energy required to overcome stronger dispersion forces between alcohol molecules

Why are alcohols more soluble in water than corresponding alkanes(same carbon skeleton)?

• more energy released from stronger hydrogen bonding formed between alcohol molecules and water molecules

• is more able to overcome

the dispersion forces and intermolecular hydrogen bonding between alcohol molecules +
the hydrogen bonding between water molecules

As alkyl chain length increases, the solubility of alcohols increases/decreases

decreases:

• hydrocarbon chain length increases

• dispersion forces between alcohol molecules increases

• energy released from hydrogen bonding between alcohol molecules and water molecules

• is less able to overcome the stronger dispersion forces between alcohol molecules and hydrogen bonding between water molecules

Why are phenols moderately soluble in water?

• large alkyl groups increase strength of dispersion forces

• energy released from hydrogen bonding between alcohol molecules and water molecules

• is less able to overcome the stronger dispersion forces between alcohol molecules and hydrogen bonding between water molecules

Alcohols can act as Bronsted acids or Bronsted bases because…

presence of OH group

O-H bond can break to give H+ and R-O-)alkoxide ion

• electron-deficient hydrogen that can be removed in the presence of a sufficiently strong base (Bronsted acid: proton donor)

• electron-rich oxygen in that can be protonated in the presence of a sufficiently strong acid (Bronsted base: proton acceptor)

Why are alcohols weaker acids than water?

• alkyl groups in alcohol are inductively electron-donating

• intensifying the negative charge on the alkoxide ion, destabilising it

• deprotonation is able to take place less easily

Note: length of alkly group does not have significant impact (focus on stability of alkoxide ion)

Effect of electron-withdrawing groups((e.g. −NO2, –F, –Cl, –Br, –I, –COCH3, –CO2H, –CN, −CO2R, –NH2, –OH, –OCH3) on acidity of alcohol

• electron-wtihdrawing groups disperse the negative charge on the alkoxide ion/phenoxide

• stabilising it and promoting ionisation of alcohol

• increasing the acidity of the alcohol

• higher Ka, lower pKa

Effect of electron-donating groups(e.g. –CH3) on acidity of the alochol

• electron-withdrawing group intensifies the negative charge on the alkoxide/phenoxide

• destabilising it, making it less easy for the alcohol to be deprotonated

• lowering the acidity of the alcohol

• lower Ka, higher pKa

Effect of distance of electron-withdrawing group from the negatively oxygen of alkoxide ion on acidity of the alcohol

• as distance increases

• inductive effect decreases [inductive effect does NOT apply to phenols]

• negative charge is less dispersed, alkoxide is less stable and less likely to be deprotonated

• lower acidity, lower Ka, higher pKa

Why are phenols more acidic than alcohols?

• the negative charge of the oxygen an be delocalised into the benzene ring

• making the alkoxide more stable and more likely to deprotonate

• more acidic, higher Ka, lower pKa

Combustion of alcohols

acid-metal reaction with sodium for alcohol

RnCs: Na(s0, room temperature

Observations: slow effervescence of hydrogen gas

acid-metal reaction with sodium for phenol

RnCs: Na(s), room temperature

Observation: rapid effercsence of hydrogen gas

acid-base reaction for sodium hydroxide with alcohol

• not possible

• alcohols are not acidic enough to react with NaOH(aq)

acid-base reaction for sodium hydroxide phenol

RnCs: NaOH (aq), room temperature

Observation: coudy mixture dissolves to form a colourless homogenous solution

Nucleophilic substituition with phosphorus halides

RnCs: PCl5 (phosphorus(V) chloride)

Observation: dense white fumes of HCl produced

Note: good distinguishing test for alcohol group (given carboxylic acid is absent)

nucleophilic substitution of tertiary alcohol with hydrogen chloride

RnCs: concentrated HCl

Observation: solution turns cloudy

(faster rate than with primary or secondary alcohol)

nucleophilic substitution of primary or secondary alcohol with hydrogen chloride

RnCs: concentrated HCl, ZnCl2(catalyst), heat

Observation: solution turns cloudy

nucleic substitution with hydrogen bromide

RnCs: concentrated HBr (aka hydrobromic acid)

or NaBr, conc, H2So4 (to generate HBr), heat

nucleic substituition with hydrogen iodide

RnC: concentrated HI (aka hydroiodic acid)

Why don’t phenols react in nucleic substitution reactions?

• lone pair on oxygen atom delocaliseses into the benzene ring, resulting in a partial double bond character in the C-O bond. The C-O bond is stronger and harder to break

• nucleophile approaching the C-O bond from the rear side is hindered by the benzene ring. electron cloud of benzene ring repels the lone pair of electrons on nucleophile

dehydration/elmitation of alcohol

RnCs: concentrated H3Po4 catalysst (Phosphoric acid), heat

or excess concentrated H2So4, heat (but not preferred as it is an OA and can produce unwanted side products)

or Al2O3, heat

Note:

• alcohol must ave a H atom bonded to the C atom adjacent to another C atom bearin g the hydroxyl group

Why doesn’t phenol react in dehydration?

• strong C-O bond that is not easily broken

• carbon atom adjacent to C atom attached to hydroxy group is not attached to any H atom

condenstation of alcohol with carboxylic acid

RnCs: carboxylic acid, few drops of concentrated H2So4, heat

Note: reaction is slow and reversible

Why can’t phenols react with carboxylic acids?

• lone pair of electrons on oxygen atom are less availale to attack carboxyl carbon of carboxylic acid due to delocalisation of the lone pair into the benezene ring

Condensation of alcohol with acyl chloride

RnC: acyl chloride, room temperature

observation: white fumes of HCl

note:

• acyl chloride reacts more readily with alcohol, hence no catalysts required

• reaction goes to completion and is irreversible

Condenstaion of phenol with acyl chloride

RnC: acyl chloride, room temperature

obersvation: whtie fumes of HCl

Note:

• phenol can only react with acyl chloride for condensation as it is a weaker nucleophile

How to increase yield of condensation of phenol with acyl chloride?

• reaction is conducted under basic conditions where Na or NaOH is present to generate a phenoxide ion

• ion has a negative charge and is a stronger nucleophile

why can’t oxidation using K2Cr2O7 or KMnO4 occur for tertiary alcohols?

• there must be at least one hydrogen atom bonded to carbon atom bearing the hydroxy group for oxidation to occur using the above reagents

oxidation of primary alcohol to aldehyde

RnCs: K₂Cr₂O₇, H2SO4(aq), heat with immediate distillation

Observation: orange(Cr₂O₇) solution turns green (Cr³⁺)

Note:

• distillation is to prevent further oxidation to form carboxylic acid by distilling aldehyde away since it has a lower bp

• KMnO4 cannot be used because it is stronger as a OA and wil oxidsie the alcohol directly into carboxylic acid

• only K₂Cr₂O₇(potassium dichromate) is selective enough

oxidation of primary alcohol to carboxylic acid

RnCs: KMnO₄, H₂SO₄, heat under reflux

or K₂Cr₂O₇, H₂SO₄, heat under reflux

Observations: purple(MnO₄^-) solution turns colourless (Mn²⁺)

orange(Cr₂O₇) solution turns green (Cr³⁺)

Oxidation of secondary alcohol to ketone

RnCs: KMnO_4/K₂Cr₂O₇
+H₂SO₄, heat under reflux

Observations: purple(MnO₄^-) solution turns colourless (Mn²⁺) OR

orange(Cr₂O₇^2-) solution turns green (Cr³⁺)

Using oxidation as a distinguishing test, what are the expected observations for primary, secondary and tertiary alcohols respectively?

primary and secondary: colour change (either orange to green OR purple to colourless depending on AI)

tertiary: no colour change

tri-iodomethane (iodoform) test

RnCs: I₂(aq), NaOH(aq), warm

Observations: Yellow ppt(CHI₃) formed

Why is the tri-iodomethane test known as a step-down reaction?

• the reaction involves the breaking of the C-C bond to remove the methane group

• this shortens the carbon chain by a single carbon

Electrophilic substitution with nitric acid

RnCs: dilute HNO₃(aq)

Observation: pale yellow liquid formed

electrophilic substituition with bromine (bromination)

RnCs: Br₂(aq) (bromine water), room temperature

observation: yellow-orange solution(bromine water) decolourised, white ppt formed

complex formation with iron(III) chloride

RnCs: neutral FeCl₃ (iron(III) chloride), room temperature

conditions: violet colouration