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4.2.1 Alcohols

General formula alcohols CnH2n+1OH The alcohols have relatively low volatility due to their ability to form hydrogen bond between alcohol molecules.

δ - + - δ δ δ + The smaller alcohols (up to 3 carbons) are O H O H soluble in water because they can form H hydrogen bonds with water. The longer the C Hδ + hydrocarbon chain the less soluble the H H alcohol. C H H

Uses of alcohols is ‘alcohol’ in alcoholic drinks. Ethanol is commonly used as a in the form of methylated spirits. Methanol is used as a petrol additive to improve combustion and is increasing important as a feedstock in the production of organic chemicals;

Classifying Alcohols H

H H H H H H O H H

H C C C H H C C C O H H C C C H

H O H H H H H H H C H Propan-1-ol H Primary Propan-2-ol H Secondary methylpropan-2-ol Tertiary Primary alcohols are alcohols Secondary alcohols are alcohols where 1 carbon is attached to where 2 carbon are attached to the Tertiary alcohols are alcohols the carbon adjoining the carbon adjoining the oxygen where 3 carbon are attached to oxygen the carbon adjoining the oxygen

Reactions of alcohols

Complete Combustion In excess oxygen alcohols will burn with complete combustion

The products of complete combustion are CO2 and H2O.

CH3CH2OH (l) + 3 O2(g)  2CO2(g) + 3 H2O(l)

N Goalby chemrevise.org 1 Oxidation reactions of the alcohols

The exact reaction, however, depends on the Potassium dichromate K2Cr2O7 is an oxidising agent type of alcohol, i.e. whether it is primary, that causes alcohols to oxidise. secondary, or tertiary, and on the conditions.

Partial Oxidation of Primary Alcohols H O An aldehyde’s name ends Reaction: primary alcohol  aldehyde in –al Reagent: potassium dichromate (VI) solution and dilute H C C It always has the C=O bond sulphuric acid. on the first carbon of the H Conditions: (use a limited amount of dichromate) warm H chain so it does not need gently and distil out the aldehyde as it forms: Ethanal an extra number

H H Observation: the H H H O orange dichromate ion + [O] H C C C + H2O 2- H C C C O H (Cr2O7 ) reduces to H the green Cr 3+ ion H H H H H O OH + [O] + H2O

CH3CH2CH2OH + [O]  CH3CH2CHO + H2O propan-1-ol propanal Write the oxidation equations in a simplified form using [O] which represents O from the oxidising agent

When writing the formulae of aldehydes in a condensed way write CHO and not COH e.g.CH3CH2CHO

Distillation It’s important to be able to draw and label this In general used as separation technique to apparatus accurately. Don’t draw lines between separate an organic product from its reacting flask, adaptor and mixture. Need to collect the distillate of the . approximate range of the desired .

Note the bulb of the thermometer should be at the T junction connecting to the condenser to measure the correct boiling point

Note the water goes in the bottom of the Water condenser to go against gravity. This allows out more efficient cooling and prevents back flow of water. Round Water in bottomed flask Heat

N Goalby chemrevise.org 2 Full Oxidation of Primary Alcohols

H H O Reaction: primary alcohol  carboxylic acid

Reagent: potassium dichromate(VI) solution and dilute H C C C sulphuric acid

: use an excess of dichromate, and O H Conditions heat H H under : (distill off product after the reaction has finished) Propanoic acid

H H H H H O + 2 [O] + H O H C C C O H H C C C 2 Observation: the orange dichromate H H H H H O H 2- ion (Cr2O7 ) OH + 2[O] OH reduces to the + H2O green Cr 3+ ion O

CH3CH2CH2OH + 2[O]  CH3CH2COOH + H2O propan-1-ol Propanoic acid

Reflux

Reflux is used when heating organic reaction mixtures for long Water out periods. The condenser prevents organic vapours from escaping by condensing them back to .

condenser Never seal the end of the condenser as the build up of pressure could cause the apparatus to explode. This is true of any apparatus where volatile liquids are heated including the set up

Water in Anti-bumping granules are added to the flask in both distillation and reflux to prevent vigorous, uneven boiling by making small bubbles form instead of large bubbles Round bottomed flask

Heat

N Goalby chemrevise.org 3 Oxidation of Secondary Alcohols Ketones end in -one H O H Reaction: secondary alcohol  ketone When ketones have 5C’s Reagent: potassium dichromate(VI) solution and H C C C H or more in a chain then it dilute sulphuric acid. H H needs a number to show Conditions: heat under reflux the position of the double Propanone bond. E.g. pentan-2-one

H H H H O H H C C C H + [O] Observation: the H C C C H + H O orange dichromate 2 2- H O H ion (Cr2O7 ) reduces 3+ H H to the green Cr ion H propan-2-ol Propanone

There is no further oxidation of the ketone under these conditions.

Tertiary alcohols cannot be oxidised at all by potassium dichromate: This is because there is no hydrogen atom bonded to the carbon with the OH group

Reaction of alcohols with Dehydrating agents

Reaction: Alcohol  Alkene Dehydration Reaction: removal of a water molecule from a molecule Reagents: Concentrated Sulphuric or Phosphoric acids Conditions: warm (under reflux) Role of reagent: dehydrating agent/catalyst Type of reaction: acid catalysed elimination

H H H H H H C

H C C C O H H C C + H2O

H H H H H propan-1-ol Propene

Some 2o and 3o H

alcohols can give H H H C more than one Butan-2-ol But-1-ene product, when the H H H H C C C H double bond

forms between H C C C C H H H H different carbon H H atoms H H O H H H

C C H H H C C Butan-2-ol can form both alkenes But-2-ene although more but-2-ene would be H H formed But-2-ene could also exist as E and Z isomers

N Goalby chemrevise.org 4 Substitution reactions of Alcohols to form Haloalkanes

A mixture of a halide ions with concentrated acid NaCl + Various other halogenating compounds H2SO4 can used for substituting a halogen on to an alcohol can be used to substitute the –OH group for a halogen

Reaction: Alcohol  Haloalkane

Reagents: Concentrated Sulphuric and sodium halide

NaCl + H2SO4  NaHSO4 + HCl

CH3CH2OH + HCl  CH3CH2Cl + H2O

N Goalby chemrevise.org 5