2.7- Alcohols and Carboxylic Acids Flashcards
(a)
Industrial preparation of ethanol from ethene
Hydration of ethene
Ethene, obtained on a large scale by cracking hydrocarbons produced from petroleum, reacts
with steam to produce ethanol.
CH2=CH2(g) + H2O(g) ⇌ CH3CH2OH(g)
(a)
Conditions of hydration of ethene
The conditions used are a temperature of about 300˚C, a pressure of 60-70 atm and a catalyst of phosphoric acid.
(b)
Preparation of ethanol and other alcohols by fermentation
Sugars are converted into ethanol. Sugar, e.g. glucose, is dissolved in water, yeast is added, and the mixture is left in a warm place. Yeast contains enzymes that catalyse the reaction. CO2 escapes as a gas but the ethanol has to be separated from the remaining liquid mixture by distillation.
C6H12O6 → 2C2H5OH + 2CO2
(b)
Biofuels
These are fuels that are produced from living organisms. The two main types are bioethanol, obtained from sugars, and biodiesel, obtained from oils and fats.
(b)
Advantages of biofuels
They are renewable. Plants can be grown each year and biofuels can also be produced using waste materials from animals.
They are carbon neutral.
(b)
Disadvantage of biofuels
Land use. Land that is used to produce plants for biofuels cannot be used to produce food. Some environmentally significant areas, such as forests, are being destroyed to create land for biofuel production.
Use of resources. Growing crops suitable for biofuels needs large quantities of water and fertilisers. The use of water can strain local resources and the use of large quantities of fertilisers can cause water pollution.
(c)
Dehydration reactions of alcohols
Many alcohols can be dehydrated to form alkenes, e.g.
CH3CH2CH2OH → CH3CH=CH2 + H2O
(c)
Dehydrating agents
The most common dehydrating agents are concentrated sulfuric acid or heated aluminium
oxide.
(d)
classification of alcohols as primary, secondary and tertiary
In 1˚ alcohols the carbon attached to the –OH is joined to one alkyl group. In 2˚ alcohols it’s
joined to two alkyl groups. In 3˚ alcohols it’s joined to three alkyl groups.
(e)
Oxidation
Primary alcohols are oxidised to aldehydes and then to carboxylic acids. Secondary alcohols are
oxidised to ketones only. Tertiary alcohols are not oxidised.
(e)
Oxidation conditions
Acidified potassium dichromate(VI) (K2Cr2O7/H2SO4) is used as the oxidising agent.
(e)
Oxidation of primary alcohols to aldehydes/carboxylic acids
Gently heating a 1° alcohol with potassium dichromate(VI) solution
and sulfuric acid should produce an aldehyde. If the alcohol is mixed with excess oxidising agent and heated under reflux, the carboxylic acid forms.
(e)
Oxidation of secondary
alcohols to ketones
Refluxing with Cr2O72–/H+ will produce a ketone.
(f)
Test for primary/secondary alcohols
Acidified potassium dichromate(VI)
The dichromate(VI) ion, Cr2O72–, changes colour from orange to green. This colour change can be used as a test to distinguish tertiary alcohols from primary and secondary alcohols.
(f)
Test for carboxylic acids
Sodium
hydrogencarbonate
