Chapter 14 Alcohols Flashcards
Alcohol homologous series
They all contain the -OH group (known as the hydroxyl group) which is responsible for both the physical and chemical properties of alcohols
Advantages of methanol
CH3OH - used as high-performance fuel because of its efficient combustion ; it is also an important chemical feedstock (starting material in many industrial syntheses) - can be converted into polymers, paints, solvents, adhesives and insulation
Where is ethanol used?
Alcoholic drinks and as a fuel ; homologous group increase by CH2
Draw 2-methylbutane-2,3-diol
DRAWN pg 222
What to keep in mind when comparing?
Physical properties of alcohols with alkanes with the same number of carbon atoms
Difference between alcohol and alkanes
Alcohols are less volatile and have higher melting point as well as greater water solubility than the corresponding alkanes - THESE DIFFERENCES BECOME SMALLER AS THE LENGTH OF CARBON CHAIN INCREASES
Why are there these differences?
The polarity of the bonds in both alkanes and alcohols and strength of these intermolecular forces
Alkanes Polarity and implications
Alkanes have non polar bonds because the electronegativity of hydrogen and carbon are very similar therefore alkanes molecules are non-polar THEREFORE ONLY WEAK LONDON FORCES
Alcohol IM and implications
Have a polar OH bond because of the difference in electronegativity of oxygen and hydrogen therefore they are polar and can take part in much stronger hydrogen bonds between these polar groups - weak London forces AND ALSO much stronger hydrogen bonds
As chain length increases
Contribution of OH groups decreases and therefore the alcohols resemble the alkanes more closely
Volatility of Alcohols/Alkenes
In liquid state - intermolecular hydrogen bonds hold the alcohol molecules together and thus requires a lot more energy to overcome them and turn the alcohols into gas ; more energy is required than overcoming the weak London forces in alkanes so alcohols have a lower volatility than the alkanes with the same number of carbon atoms
Higher the boiling point
Lower the volatility
Solubility of alcohols in water
Alkanes are non-polar therefore they cannot form hydrogen bonds with water. Alcohols like methanol and ethanol are completely soluble in water as hydrogen bonds form between the OH group and the water molecules. As hydrocarbon chain increases in size, influence of OH becomes smaller and solubility of longer chain becomes more like that of alkanes - solubility decrease
3 classifications of alcohols
Primary
Secondary
Tertiary
Primary alcohols
OH group is attached to a carbon atom that is attached to two hydrogen atoms and one alkyl group
What is methanol classified as?
STILL A PRIMARY EVEN THOUGH 3 HYDROGENS
Secondary alcohol
The OH group is attached to a carbon atom that is attached to two alkyl groups and 1 hydrogen atom
Tertiary alcohols
OH group is attached to a carbon that is attached to no hydrogen atoms and 3 alkyl groups
Why is it important to recognise the 3 different classes of alcohol?
To predict how the alcohol will react with oxidising agents
Alcohol combustion
Burn completely in a plentiful supply of oxygen to produce CO2 and H2O - reaction is exothermic, releasing a lot of energy in the form of heat. Number of carbon atoms in the alcohol chain increases, the quantity of heat released per mole also increases
Usual oxidising mixture
Solution of potassium dichromate (VI) K2Cr2O7 acidified with dilute H2SO4
If alcohol is oxidised then
Orange solution containing dichromate ions is reduced to green solution containing chromium(iii) ions
Equation for reduction of dichromate ions
Cr2O7(2-) -> Cr(3+)
Oxidation of primary alcohols
Oxidised to aldehydes or Cabo your acids - product of oxidation depends on reaction conditions because aldehydes oxidise themselves to COOH
Preparation of aldehydes
Gentle heating of primary alcohols with acidified potassium dichromate forms an aldehyde - to ensure that the aldehyde is prepared rather than the carboxylic acid, the aldehyde is distilled out of the reaction mixture as it forms. This prevents any further reactions with the oxidising agent
Butan-1-ol + [O]
Butanal + water
Preparation of carboxylic acids
If a primary alcohol is heated strongly under reflux with an excess of acidified potassium dichromate (VI) - a carboxylic acid is formed. Excess ensures all the alcohol is oxidised and heating under reflux means any aldehyde initially formed also undergoes oxidation to the carboxylic acid
Equation of oxidation of Butan-1-ol to Butanoic acid
But an-1-ol + 2[O] -> Butanoic acid + H2O
How does reagent and conditions influence product formed
Aldehyde = distillation to remove aldehyde
Carboxylic acid = heat under reflux
Oxidation of secondary alcohols
Using acidified dichromate ions - they are oxidised to Ketones - heated under reflux with oxidising mixture and colour change again from organs to green.
Oxidation of Propan-2-ol
DRAW Equation
Oxidation of tertiary
Do not undergo oxidation and remain orange
Dehydration reaction
WATER MOLECULE IS REMOVED - alcohol heated under reflux in presence of an acid catalyst such as H2SO4/H3PO4 ; PRODUCES AN ALKENE
What type of reaction is dehydration
Elimination (saturated to unsaturated)
Equation of dehydration of cyclohexanol
Drawn - cyclohexene + water
Where are atoms taken away from in dehydration
Adjacent carbon atoms - OH and H
2 steps to substitution reaction of alcohols
1) Sodium halide + H2SO4 react to produce hydrogen halide and salt
2) hydrogen halide reacts with alcohol to produce the haloalkane - SUBSTITUTION
Overall equation for propan-2-ol substitution
Starting with Soidum halide and acid
DRAWN AND TOTAL EQUATION
