4.3 Alcohols and Phenols

Nucleophiles

ions or compounds possessing a lone pair of electrons that can seek out a relatively positives site

donates a pair

Eg. -OH, -CN, NH2

Primary, secondary and teritary alcohols

P- Only one alkyl group attached to the carbon hydroxide group

S- 2

T- 3

Electronegativity

Ability of an atom in a chemical bond to polarise electron density towards itself

Difference in boiling points between alcohols and alkanes

• In alcohol there are hydrogen bonds set up between the slightly positive hydrogen and lone pairs on oxygen in other molecules

• in alkanes the only intermolecular forces are vander waals dispersion forces hydrogen bonds are much stronger and take more energy to separate alcohol molecules

Forming primary and secondary alcohols from halagenoalkanes

refluxing

• Carried out by refluxing together the halogenoalkane and an aqueous solution of an alkali (Na or KOH)

• during reflux the liquid is evaporated, cooled and condensed to continue the reaction

• separated at the end of the reaction by fractional distillation from diff boiling points

Forming a primary alcohol Eg. Butan-1-ol from 1-bromobutane

substitution

CH3CH2CH2CH2Br + NaOH → CH3CH2CH2CH2OH + NaBr

nucleophilic substitution where the hydroxide ion acts as the nucleophile and attacks the relatively positive carbon atom on the C-Br bond

secondary alcohol formation example

Rate of hydrolysis for alcohols from halogenoalkanes

C—I > C—Br > C—Cl

yield of primary alcohols is usually higher than the yield of secondary alcohols

when producing secondary alcohols a alkene can be formed as a side product

greater yields of alkenes are produced if high concentrations of the alkali are used or if an ethanolic solution of the alkali

Formation of alcohols from the reduction of a carbonyl compound

Both aldehydes and ketones can be reduced to primary and secondary alcohols by using an aqueous solution of sodium tetrahydridoboarate (NaBH4)

use [H] to represent a reducing agent

Reducing carboxylic acids

dissolved in

Sodium tetrahydridoboarate is not powerful enough to reduce carboxylic acids and the stronger reducing agent lithiumtetrahydridoaluminate (LiAlH4)

Dissolved in ethoxyethane

Reactions with the reducing agents

Sodium tetrahydridor borate is a much safer reaction as an excess of lithium tetrahydridaluminate is difficult to dispose and the solvent ethoxyethane is very flammable

To obtain a good yield of alcohol

the organic mixture

An access of the reducing agent is used

the excess reducing agent can be removed by adding a dilute acid

the organic products needs to be separated from the aqueous mixture

carried out using a separating funnel if the alcohol produced is immisciple with water or separated by adding an organic solvent such as ethoxyethane by the technique of solvent extraction

Reaction of primary and secondary alcohols with hydrogen halides

Halogenoalkanes are produced by reacting the alcohols with hydrogen halides

the reactions are generally slow and reversible often giving poor yields

Chlorination

Passes hydrogen chloride gas into the alcohol in the presence of anhydrous zinc chloride, acting as a catalyst

Chlorination using phosphorus chloride (V)

Room temperature

one problem with this reaction is that phosphorus oxide trichloride (POCl3) is a liquid and needs to be removed from the reaction mixture. if the halogenoalkane has a similar boiling temperature to POCl3 then separation becomes difficult

Chlorination using sulphur (VI) oxide dichloride SOCl2

An advantage to this method is that the co products sulphur oxide and hydrogen chloride are both gaseous and are easily lost from the reaction mixture, ensuring easier separation

Bromination

‘in situ’ reaction: potassium bromide and 50% sulfuric (VI) acid is heated

the sulfuric acid protonates the alcohol and this then reacts with the bromide ions from the potassium bromide

CH3CH2CH2CH2OH + KBr + H2SO4 → CH3CH2CH2CH2Br + KHSO4 + H2O

Iodination

Warm dump red phosphorus and iodine together to form phosphorus iodide (III) PI3

2P + 3I2 → 2Pl3

3CH3CH2CH2OH + PI3→ 3CH3CH2CH2I + H3PO3

Reaction with carboxylic acids

Primary and secondary alcohols react with carboxylic acids to give esters

Alcohol + COOH → ester + H2O

the reaction is reversible and eventually the mixture will reach a position of equilibrium

To increase the yield of Esters

Little concentrated sulfuric (VI) acid is added to the mixture of the alcohol and the carboxylic acid, the mixture is heated under reflux

the products can then be distilled and the Ester collected at its boiling temperature

the distillate generally consists of the Ester and water together with a little unreacted alcohol and carboxylic acid. Many esters are immiscible with water and the distillate often consists of two layers

Separating funnel used to extract the Ester

Then shaken with sodium hydrogen carbonate solution to remove any remaining carboxylic acid

Ester is then dried with anhydrous calcium chloride which reacts with any remaining alcohols

it can then be redistilled to give a pure product

Reactions with ethanoyl chloride

Alcohol reacts rapidly with ethanyol chloride giving an ester. During this reaction misty fumes of hydrogen chloride are seen.

Gives a better yield of an Ester than using a carboxylic acid as the reaction is not reversible

however the cost of acid chloride means this is not a cost efficient process in industry

Reactivity of phenols compared to alcohols

lone pair overlap

extended delo

Reactivity is very different

One of the lone pairs of oxygen atoms overlap with the delocalized Pi system to form a more extended delocalized system.

As a result the C—O bonds in phenols is shorter and stronger than in alcohols

making C—O bond fission in phenols harder than co-bond in alcohols

the extended delocalization creates a higher electron density in the Ring and makes the ring structure more susceptible to attack by electrophiles

Acidity of phenol

Phenol is a very weak acid and the position of the equilibrium lies to the left

phenol can lose a hydrogen ion because the phenoxide ion formed is stabilized to some extent. The negative charge on the oxygen atom is delocalized around the ring

The more stable the ion the more likely it is to form

Why is phenol only a weak acid

One of the lone pairs on the oxygen atom overlap with the delocalized electrons on the benzene ring

this overlap leads to a delocalization which extends from then ring out over the oxygen atom resulting in the negative charge no longer entirely localised on the oxygen, but is spread out around the whole ion

spreading the charge around makes the ion more stable

oxygen is the most electronegative element in the ion and the delocalized electron will be drawn towards it. This means that they will be still a lot of charge around the oxygen

which will tend to attract the hydrogen ions back again

Phenol with the sodium hydroxide

Gives a colourless solution containing sodium phenoxide- showing it must be acidic

hydrogen ion has been removed by the strongly basic hydroxide ion in the solution

this shows phenol reacting as an acid losing a proton to the aqueous hydroxide ion

The presence of substituents on the benzene ring affects the acidity of phenol

phenol tion with sodium carbonate/ sodium hydrogen carbonate

Phenol is not strong enough to react to produce carbon dioxide. No bubbles are produced

shows it's only a weak acid

Phenol with metallic sodium

Slow reaction because phenol is such a weak acid

phenol is warmed in a dry tube until it is molten and a small piece of sodium is added there is some fizzing as hydrogen gases given off

mixture is left in the tube will contain sodium phenoxide

Directing effect of the -OH group

Has more activating effect on some positions around the ring than others

incoming groups will go into these positions much faster

has a 2,4,6 directing effect, incoming groups will tend to go into the 2, 4 or 6 position

Phenol with bromine

Presence of an OH group bonded directly to a benzene ring will activate the ring to attack by electrophiles causing directing effect

when female reacts with bromine the increased electron density in the ring polarises the bromine molecules

Aqueous bromine reacts with phenol to produce a white precipitate of 2,4,6- tribromophenol

Colour change in the bromine test

Aqueous bromine= orange

products= colourless solution and a white precipitate

this means the orange colour disappears since the bromine is decolorized

How is reaction with bromine with phenol different to an alkene

In an alkene reaction, it will result in the decolorization of bromine without the additional formation of a white precipitate

mechanism for this reaction is electrophilic substitution. Electrophile is Br+

Bromine reaction with water

gives HOBr and HBr

Br2 + H2O = H-O-Br + HBr

concentration of Br+ from HOBr is very small as a position of the equilibrium lies well to the left

Reaction of Phenols and ethanoyl chloride

Alcohols and phenols can react as nucleophiles by the use of their oxygen ion pairs

the delocalisation of an electron pair from the oxygen atom in a phenol means it is more difficult for a phenol to react as a nucleophile

as a result carboxylic acids are not suitable reagents to make an ester with phenol

Phenol and ethanoyl chloride

Slow at room temp

Phenol and a pyridine ( a base)

Can be added to speed up the reaction

the pyrdine react with the hydrogen chloride co-product to give pyridinium chloride (C5H5NH+Cl-)

Phenol and ethanoic anhydride

Ethanoyl chloride is an expensive reagent and ethanoic anhydride is used in preference

Phenol and acyl chloride

Less reactive

aqueous conditions can be used as the acyl chloride is only slowly hydrolysed by water

Test for phenols

Phenol will react with iron (lll) chloride to produce a purple color in the aqueous solution

the colour is produced by a complex being formed between the two reagents