ALCOHOL

ALCOHOL

Chemistry of -OH

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Alcohols

Butan - 1, 4 - diol

Butan- 1- ol

Propan- 2- ol

Propan- 1- ol

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These all have the formula C4H9OH

butan-1-ol butan-2-ol

2-methylpropan-1-ol2-methylpropan-2-ol

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Bond angles in alcohol groups

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Solubility in water

The alcohol groups form hydrogen bonding which makes the short chain molecules soluble in water.

The solubility in water decreases as the chain length increases.

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Low-mass alcohols are soluble in water (because they hydrogen bond with water).

As the hydrocarbon chain lengthens, the solubility decreases.

Ethanol Propan-1-ol Butan-1-ol

This photo shows ethanol, propan-1-ol and butan-1-ol in water. The first two are completely miscible in water, while butan-1-ol is not miscible in water.

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Boiling Points of Alcohols

Increases with molecular size due to increased instantaneous dipoles

• Alcohols have higher boiling points than similar molecular mass alkanes

•This is due to the added presence of inter-molecular hydrogen bonding

• More energy is required to separate the molecules

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CLASSIFICATION OF ALCOHOLSCLASSIFICATION OF ALCOHOLS

Aliphatic • general formula CnH2n+1OH - provided there are no rings

• the OH replaces an H in a basic hydrocarbon skeleton

Structuraldifferences • Alcohols are classified according to the environment of

the OH group

• Chemical behaviour, e.g oxidation, often depends on the

structural type

NB. Aliphatic - straight chain molecule (not a ring / cyclic)

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PRIMARY 1° SECONDARY 2° TERTIARY 3°

Luca’s Reagent

• Solution of anhydrous zinc chloride in concentrated hydrochloric acid

• This solution is used to classify alcohols of low molecular weight.

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Distinguishing alcohols

Lucas reagent can be used to distinguish between low mass primary, secondary and tertiary alcohols.

Lucas reagent contains anhydrous zinc chloride dissolved in concentrated hydrochloric acid. It contains a very high concentration of chloride ions and the Zn2+ ion acts as a catalyst.

Take 1–2 mL of Lucas reagent in a dry test tube, add a few drops of the alcohol and shake. If there is no reaction, place the test tube in a beaker of boiling water for a few minutes.

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Distinguishing alcohols - Lucas test

Primary alcohol - remain unchanged

tertiary alcohol - turns cloudy immediately

Secondary alcohol - will turn cloudy but takes a bit of time

Lucas reagent = conc. HCl and ZnCl2

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Tertiary alcohols turn cloudy immediately.

Once heated, the secondary alcohol quickly turned cloudy.

The primary alcohol tube is unchanged.

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OXIDATION OF PRIMARY ALCOHOLSOXIDATION OF PRIMARY ALCOHOLS

Primary alcohols are easily oxidised to aldehydes

e.g. CH3CH2OH(l) + [O] ——> CH3CHO(l) + H2O(l)

ethanol ethanal

it is essential to distil off the aldehyde before it gets oxidised to the acid

CH3CHO(l) + [O] ——> CH3COOH(l)

ethanal ethanoic acid

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Practical details• The alcohol is dripped into a warm solution of acidified k2cr2o7

• Aldehydes have low boiling points - no hydrogen bonding - they distil off immediately

• If it didn’t distil off it would be oxidised to the equivalent carboxylic acid

• To oxidize an alcohol straight to the acid, reflux the mixture

• Compound Formula Intermolecular bonding boiling point

• Ethanol C2H5OH hydrogen bonding 78°C

• Ethanal CH3CHO dipole-dipole 23°C

• Ethanoic acid CH3COOH hydrogen bonding 118°C

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Oxidising a primary alcohol to an aldehyde

Full oxidation is not wanted:

use dilute acid and less dichromate. The reaction mixture is heated gently,

ethanal vapourises (21°C) as soon as it is formed and distils over. This stops it being oxidised further to ethanoic acid.

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Apparatus for the oxidation of ethanol to ethanoic acid

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Oxidising a primary alcohol to a carboxylic acid

reflux Distil to separate

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Oxidising a secondary alcohol to a ketone

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Oxidation of alcohols

Primary and secondary alcohols are oxidised by acidified potassium dichromate.

A beaker of hot water speeds up the reaction.

There is no reaction with tertiary alcohols.

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Oxidation of alcohols

Primary alcohols

tertiary alcohol

Secondary alcohol

aldehydes

Carboxylic acid

Don’t oxidise

Ketones

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Formation of ethanol by fermentation

Conditions yeastwarm, but no higher than 37°C (optimum temp. for

yeast)

Advantages LOW ENERGY PROCESSUSES RENEWABLE RESOURCES - PLANT MATERIALSIMPLE EQUIPMENT

Disadvantages SLOWPRODUCES IMPURE ETHANOL - will need distilling to purifyBATCH PROCESS

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Formation of haloalkane

Ethanol and PCl5

C2H5OH(l) + PCl5(s) C2H5Cl(g) + POCl3(l) + HCl(g)fumes

solid

Ethanol and SOCl2

C2H5OH(l) + SOCl2(l) C2H5Cl(g) + SO2(g) + HCl(g)

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Phosphoryl chloride

Thionyl chloride

Formation of ethanol from ethene

Advantages FastPure ethanol producedContinuous process

Disadvantages high energy processExpensive plant requiredUses non-renewable fossil fuels to make ethene

Uses of ethanol alcoholic drinksSOLVENT - industrial alcohol / methylated spiritsFUEL - petrol substitute in countries with limited oil reserves

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Dehydration of alcohols

Reagent: concentrated sulphuric acid

or passing the alcohol over aluminium oxide

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Reaction with sodium

The reaction is similar to the reaction of alkali metals with water, but less vigorous.

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Esterification

Catalyst: concentrated H2SO4 (dehydrating agent - it removes water causing the equilibrium to move to the right and increases the yield

Conditions: reflux

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Uses of esters Esters are fairly unreactive but that doesn’t make them useless

Used as flavourings

Naming esters Named from the alcohol and carboxylic acid which made them...

CH3OH + CH3COOH CH3COOCH3 + H2O

from ethanoic acid CH3COOCH3 from methanol

METHYL ETHANOATE

Esters

Ethanoate Methyl

Methyl Ethanoate

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