Chapter 14 Alcohols Flashcards
(38 cards)
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
