4.3 - Alcohols and phenols

How can alcohols be classified?

• Primary

• Secondary

• Tertiary

What are primary alcohols?

The carbon which carries the -OH group is only attached to one alkyl group

• general formula = RCH₂OH

What are secondary alcohols?

The carbon with the -OH group attached is joined directly to two alkyl groups which may be the same or different

• general formula = R₂CHOH

What are tertiary alcohols? 

The carbon with the -OH group attached is joined directly to three alkyl groups which may be the same or different 

• general formula = R₃COH

What are the 2 methods of forming primary and secondary alcohols?

• From halogenoalkanes by a substitution reaction

• By reducing aldehydes, ketones or carboxylic acids 

How do you form alcohols from hydrolysis of a halogenoalkane (nucleophilic substitution)?

• covalent bond is broke by reaction with water- NaOH is used instead as it reacts more quickly than water

What are the conditions for nucleophilic substitution to form an alcohol? 

Heat 1 bromobutane under reflux with aqueous alkali - NaOH

What is a nucelophile?

An electron pair donor

• must have at least lone pair of electrons

How is the rate of reaction of hydrolysis of halogenoalkanes affected by different halogens?

The weaker the C-X bond, the faster the reaction

How do you form primary and secondary alcohols from reducing aldehydes and ketones? 

Using an aqueous solution of the reducing agent sodium tetrahydridoborate (III) (NaBH₄) 

Aldehyde —> primary alcohol

• R-CHO + 2 [H] → R-CH₂OH

• CH₃-CHO + 2[H] → CH₃-CH₂OH

Ketone —> secondary alcohol

• R₁-CO-R₂ + 2[H] → R₁-CH(OH)-R₂

• CH₃-CO-CH₃ + 2[H] → CH₃-CH(O)-CH₃

What are the conditions for reducing aldehydes and ketones into primary and secondary alcohols? 

• NaBH₄ in aqueous methanol

• Room temperature 

How do you reduce a carboxylic acid into an alcohol?

• Have to use a stronger reducing agent

• excess lithium tetrahydridoaluminate (III) (lithium aluminium hydride or lithal) dissolved in ethoxyethane

CH₃ + 4 [H] → CH₃CH₂OH + H₂O

When reducing carboxylic acids into alcohols, why must the reaction mixture be removed?  

• excess reducing agent is used

• so reaction mixture must be removed by adding a dilute acid

• alcohol then needs to be separated from mixture using separating funnel

• if the alcohol dissolves in the aqueous solution it can be extracted using another organic solvent like ethoxyethane (solvent extraction) 

What are the reactions of primary and secondary alcohols?

• alcohols with hydrogen halides (chlorination, bromination, iodination)

• esterification

What is chlorination of alcohols?

• HCl gas passed into alcohol in presence of anhydrous zinc chloride - acts as a catalyst

• CH₃CH₂OH + HCl → CH₃CH₂Cl + H₂O

What is a possible issue with the method of chlorination of alcohols?

Separation

How do you prepare Hydrogen bromide for bromination of alcohols?

• HBr is prepared in situ by reacting concentrated sulfuric (VI) acid with potassium bromide 

• H₂SO₄ + KBr → KHSO₄ + HBr

What is the general equation of bromination of alcohols?

ROH + HBr → RBr + H₂O

What is the method for the bromination of alcohols?

• Concentrated sulfuric acid is added carefully to the alcohol with continuous cooling

• Potassium halide is added and mixture is heated in a distillation apparatus

• the halogeno compound distils over

e.g. CH₃CH₂OH + KBr + H₂SO₄ → CH₃CH₂Br + KHSO₄ + H₂O

What is the iodination of alcohols?

• Refluxing the alcohol with excess hydroiodic acid 

e.g. CH₃CH₂OH + HI → CH₃CH₂I + H₂O

Describe esters

• naturally occuring

• found in sweet smelling flowers and fruits

• important in cosmetic industry as components of perfume

• also used as solvents e.g. nail varnish remover

• can be made synthetically 

How can esters be made synthetically?

• Alcohol with carboxylic acids

• Alcohol with ethanoyl chloride 

What is the reaction of alcohols with carboxylic acids?

• acid + alcohol ⇋ ester + water

RCOOH + ROH ⇋ RCOOR + H₂O

e.g. CH₃COOH   +  CH₃OH     CH₃COOCH₃  +  H₂O

  ethanoic acid     +   methanol         methyl ethanoate   +  water

What are the reaction conditions for alcohols with carboxylic acids?

• Acid and alcohol refluxed for 10 mins with concentrated sulfuric (VI) acid as catalyst

• Sweet smelling ester formed is then distilled off 

What is the reaction of alcohols with ethanoyl chloride?

• ethanoyl chloride + alcohol → ester + hydrogen chloride 

CH₃COCl + ROH → CH₃COOR + HCl

e.g. CH₃COCl  +  CH₃OH → CH₃COOCH₃   +  HCl

ethanoyl chloride  + methanol → methyl ethanoate   + hydrogen chloride

What condition does ethanoyl chloride need to be in to form an ester?

• Has to be completely dry (no water)

• or it will turn back into carboxylic acid

What is a phenol?

• Aromatic alcohol

• Contains OH group directly bonded to a carbon in a benzene ring 

• White solid 

• Hydrogen bonding makes boiling point higher 

• It’s a compound and also a functional group 

What is the formula of phenol?

C₆H₅OH

What type of interaction occurs between the delocalised electrons in the benzene ring and the lone pairs on the oxygen atom in phenol? 

Interaction between the oxygen’s lone pair (in a p orbital) and the delocalised pi electrons of the benzene ring

Which oxygen lone pair interacts with the benzene ring in phenol? 

The lone pair in the p orbital (P₂) that can overlap sideways with the ring’s pi system 

What does the sideways overlap of p orbitals between oxygen and benzene carbons produce?

A delocalised pi-electron cloud including the oxygen atom 

What happens due to the donation of the oxygen’s lone pair into the benzene ring?

Lone pair is now delocalised with the ring pi-electrons, increasing electron density around the ring 

How does the donation of the oxygen’s lone pair affect the reactivity of phenol?

It increases electron density in the ring making it easier for phenol to be attacked by an electrophile

How does the donation of the oxygen’s lone pair affect the acidity of phenol compared to alcohols? 

The -OH groups How hydrogen becomes more acidic (δ+) than in aliphatic alcohols because the oxygen’s lone pair is party delocalised

How does the donation of the oxygen’s lone pair affect the C-O bond in phenol compared to alcohols?

The C-O bond becomes shorter and stronger than in alcohols

What is the solubility of phenol? 

Not very soluble in water due to large non polar aromatic ring

What happens when phenol dissolves in water?

It partially dissociates to form a phenoxide ion and a proton

• makes it a weak acid

• weaker than carboxylic acids

• makes it unlike alcohols which do not form acidic solutions 

What are the reactions of phenol? 

• Phenols with water - acts as an acid

• Phenols with hydroxides

• Bromination of phenol 

• nitration of phenol

What functional group in phenol makes it reactive and allows it to act as an acid?

The polar -OH group 

Why can phenol act as an acid?

Because the lone pair on the oxygen atom is partially drawn into the benzene ring, making the O-H bond weaker and the hydrogen atoms more δ+ than aliphatic alcohols

What type of reactions can phenol undergo on the benzene ring?

electrophilic substitution reactions

What happens when phenol reacts with water?

• It forms an acidic solution 

• C₆H₅OH + H₂O → C₆H₅O^- + H₃O⁺

What ion is produced when phenol loses a hydrogen ion? (H⁺)

Phenoxide ion (C₆H₅O^-)

How does phenol react with alkalis?

Reacts to form salts 

How do phenols react with hydroxides?

• Acidic enough to react with strong bases e.g. NaOH

• It is not acidic enough to react with weak bases eg. sodium carbonate

• C₆H₅OH + NaOH → C₆H₅O^-Na⁺ + H₂O

produces sodium phenoxide 

What type of reaction does phenol undergo with bromine water?

Electrophilic substitution (bromination)

Why is the benzene ring in phenol more reactive than in benzene itself?

• cos of the hydroxyl group attached to it

• The donation of the oxygen’s lone pair into the ring increases the electron density around the ring making it easier for phenol to attract an electrophile 

What happens when phenol reacts with bromine water?

• Decolourises the bromine water 

• forms a white precipitate of 2,4,6 - tribromophenol in a solution of hydrogen bromide 

What happens when phenol reacts with dilute nitric acid (nitrification)?

• reacts readily at room temperature

• forms a mixture of 2-nitrophenol and 4-nitrophenol

Why is phenol more reactive than benzene?

The -OH group donates a lone pair of electrons into the ring, activating it and making it more attractive to electrophiles

What do activating groups do?

they increase the electron density of the aromatic ring making it more reactive towards electrophiles

What are other activating groups apart from -OH?

-NH₂ and -CH₃

What do deactivating groups do?

They withdraw electron density away from the ring making it less attractive to electrophiles and therefore less reactive

What are some examples of deactivating groups?

-NO₂, -CHO, -CN

How does the presence of an activating group affect the reaction conditions? 

The aromatic ring reacts rapidly with electrophiles without needing a catalyst or a high temperature 

How does the presence of a deactivating group affect the reaction conditions? 

The aromatic ring reacts slowly with an electrophile and requires high temperature and a catalyst 

What do directing effects determine?

The position of the second substituent on the benzene ring

What positions do -OH and -NH₂ groups direct new substituents to?

Positions 2 or 4

Are positions 2,4 - directing groups activating or deactivating groups?

They are activating groups (except halogens which are deactivating but still 2,4 - directing) 

What position does a -NO₂ group direct a new substituent to?

Position 3 

Are 3-directing groups activating or deactivating?

Deactivating

Why are directing effects important in synthesis?

To ensure substitution occurs at the desired carbon in the ring

What is the reaction of phenol with an acid chloride e.g. ethanoyl chloride? 

• Forms esters

Phenol + ethanoyl chloride → phenyl ethanoate + hydrogen chloride   

What is the equation for the reaction of a phenol with ethanoyl chloride?

Phenol + ethanoyl chloride → phenyl ethanoate + hydrogen chloride 

• C₆H₅OH + CH₃COCl → CH₃COOC₆H₅ + HCl

How is reacting phenol with ethanoyl chloride different from the esterification of alcohols? 

Alcohols form esters with carboxylic acids and acid chlorides, phenols only form esters with acid chlorides 

What is the reaction of phenol ethanoyl chloride called?

acylation (specifically ethanoylation)

What other functional group undergoes ethanoylation reactions?

Aminnes (RNH₂)

• so if a compound undergoes an ethanoylation reaction we know it is an alcohol, phenol, or an amine 

What happens when phenol reacts with a solution of iron (III) chloride, FeCl₃?

• forms a purple (violet) complex

• and some other colours 

What is the reaction of phenol and iron chloride used for?

To distinguish phenols from alcohols (alcohols do not react)

Why was phenol historically significant?

It was used by Lister as an antiseptic

Why isn’t pure phenol used now?

It is toxic and causes burns

What substituted phenol is used instead of pure phenol? 

2,4,6-trichlorophenol (in TCP and Dettol)

What are some other uses of phenol apart from antiseptic?

As the starting material in the manufacture of the analgesics (pain killers) paracetamol and aspirin