AlcoholsL.O: To understand the structure of alcohols and describe some reactions involving alcohols

Friday, April 21, 2023

Do you remember making wine in year 8? Alcoholic drinks contain the alcohol

ethanol. The drink is produced by fermenting plant sugars

What is the combustion equation for ethanol?

Why does beer / wine smell and taste of vinegar when it is left out? Ethanol is oxidised into ethanoic acid

by microbes which live in the air

Ethanol can also be oxidised by some oxidising agents like potassium dichromate

Alcohols Alcohols (which are organic compounds as they contain carbon) have an –OH

functional group, with the general formula

CnH2n+1OH

Methanol – CH3OH

Ethanol – C2H5OH

Propanol – C3H7OH

Different alcohols are part of the same homologous series as they have the same general formula

What do you observe when sodium is reacted with waterhttp://www.youtube.com/watch?v=aGaWwnMyKVE

Alcohol As Solvents

Alcohols such as methanol and ethanol can dissolve most compounds that water dissolves but they can also dissolve substances that water cannot, such as oils, fats and hydrocarbons

Ethanol is the solvent for perfumes and aftershaves (it can mix with the oils (giving the smell) and the water)

Methylated spirits (meths) is ethanol with other chemicals added to it – meths are used as paint cleaners and fuels (though they are poisonous to drink so are dyed)

Ethanol

Ethanol is also useful as a fuel – for use in cars and other vehicles, it is usually mixed with petrol

Producing Ethanol – Hydration of Ethene

Ethanol can be produced in 3 major ways. You need to know the advantages and disadvantages of each method

Ethene, which is obtained from crude oil, can be reacted with steam under high temperature and pressure to form pure ethanol.

Ethene + Steam Ethanol

C2H4 + H2O C2H5OHConditions = 300oc, 60-70atmospheres

Advantages•100% atom economy (See 7.1)•95% yield•Produces 96% pure ethanol

Disadvantages•Feedstock is non-renewable.•Ethene must be produced by cracking which requires energy.•Process requires a lot of energy.

Producing Ethanol – Fermentation of Sugar

For thousands of years humans have been producing ethanol by fermentation of sugar. Common feed stocks are maize, corn and sugar cane.

Glucose Ethanol + Carbon Dioxide

C6H12O6 2C2H5OH + 2CO2

Conditions = 25-37oc, 1atmosphere

Advantages•Uses renewable feedstock•Operates at low temperatures and pressures•Carbon neutral fuel source

Disadvantages•Produces impure ethanol•Atom economy is 51%•Still requires fossil fuels for purification•Feed stock needs to be grown which takes land which could be used to grow food instead.

However the product is impure as concentrations higher than 15% denature the enzymes killing the yeast. To purify it distillation is used, which uses energy.

Exam Questions

Method 1 is the least sustainable as it

uses ethene which comes from crude oil

and is not renewable. Methods 2 and 3

use renewable resources which can be

regrown.

Atom economy. A process which

produces less waste is more sustainable.

If waste products can be used elsewhere

the process is more sustainable.

Exam Questions

Sodium fizzes slowly

Sodium fizzes violently on surface of water

No reaction

Ethanol and water both contain O-H groups

so react similarly. However hexane only

contains C-H and C-C bonds so is unreactive.

Exam QuestionsHigh concentrations of ethanol denature the

enzymes in yeast killing it.

Fractional Distillation

The mixture of ethanol and water is

heated. Ethanol has a lower boiling

point than water so evaporates and is

collected first.

Exam Questions

High concentrations of ethanol denature the

enzymes in yeast killing it and the reaction

does not complete. Also other reactions take

place using the glucose.

Exam Questions

At high temperatures the enzymes in the

yeast denature killing it. However at low

temperatures there is little energy for the

reaction to take place and the reaction is too

slow.

Exam Questions

Kerboodle alcohol ‘on your marks’

Ethanol

Ethanol can be manufactured by reacting ethene (from cracking crude oil) with steam –phosphoric acid is used as a catalyst: -

Ethene + Steam Ethanol

C2H4+ H2O C2H5OH

In the reaction ethanol is the only product – the process is continuous as long as ethene and steam are fed into one end of the reaction vessel, ethanol will be produced

These features make it an efficient process, but as ethene is made from crude oil, which is a non-renewable resource, it cannot be replaced once it is used up and it will run out one day

Ethanol & Fermentation

Ethanol can also be made via fermentation – sugar from plant material is converted into ethanol and carbon dioxide…

Enzymes found in single-celled fungi (yeast) are the natural catalysts that can make this process happen (this is a renewable resource): -

C6H12O6 2C2H5OH + 2CO2

More than 90% of the world’s ethanol is made by fermentation