Alcohols

Butan - 1, 4 - diol

Butan- 1- ol

Propan- 2- ol

Propan- 1- ol

These all have the formula C4H9OH

butan-1-ol butan-2-ol

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

Bond angles in alcohol groups

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.

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.

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

Mr bp / °C

propane C3H8 44 -42 just instantaneous dipoles

ethanol C2H5OH 46 +78 instantaneous dipoles + hydrogen bonding

Boiling point is higher for “straight” chain isomers. bp / °C

butan-1-ol CH3CH2CH2CH2OH 118

butan-2-ol CH3CH2CH(OH)CH3 100

2-methylbutan-2-ol (CH3)3COH 83

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, eg oxidation, often depends on the

structural type

PRIMARY 1° SECONDARY 2° TERTIARY 3°

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

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.

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

Tertiary alcohols turn cloudy immediately.

Once heated, the secondary alcohol quickly turned cloudy.

The primary alcohol tube is unchanged.

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

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 oxidise 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

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.

Apparatus for the oxidation of ethanol to ethanoic acid

Oxidising a primary alcohol to a carboxylic acid

reflux Distil to separate

Oxidising a secondary alcohol to a ketone

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.

Oxidation of alcohols

Primary alcohols

tertiary alcohol

Secondary alcohol

aldehydes

Carboxylic acid

Don’t oxidise

Ketones

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

Formation of haloalkane

Ethanol and PCl5

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

Ethanol and SOCl2

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

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

Dehydration of alcohols

Reagent: concentrated sulphuric acid

or passing the alcohol over aluminium oxide

Reaction with sodium

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

Esterification

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

Conditions: reflux

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