#Alcohols

#OrganicChemistry

#Chemistry

Introduction

• All alcohols contain the hydroxyl (-OH) functional group which is the part of alcohol molecules that is responsible for their characteristic reactions

• Alcohols are a homologous series of compounds that have the general formula CnH2n+1OH

• They differ by one -CH2 in the molecular formulae from one member to the next

• Ethanol (C2H5OH) is one of the most important alcohols

  • Ethanol can also be represented by its structural formula CH3CH2OH

• It is the type of alcohol found in alcoholic drinks such as wine and beer

• It is also used as fuel for cars and as a solvent

• Ethanol will undergo complete combustion and burn in excess oxygen to produce carbon dioxide and water 

C2H5OH (l) + 3O2 (g) → 2CO2 (g) + 3H20 (l)

The manufacture of ethanol

• There are two methods used to manufacture ethanol:

  • The hydration of ethene with steam

  • The fermentation of glucose

• Both methods have advantages and disadvantages which are considered

Fermentation of glucose

• Sugar or starch is dissolved in water and yeast is added

• The mixture is then fermented between 25 and 35 °C with the absence of oxygen for a few days

• Yeast contains enzymes that catalyse the break down of starch or sugar to glucose

• If the temperature is too low the reaction rate will be too slow and if it is too high the enzymes will become denatured

• The yeast respire anaerobically using the glucose to form ethanol and carbon dioxide:

C6H12O6 → 2CO2 + 2C2H5OH

• The yeast are killed off once the concentration of alcohol reaches around 15%, so the reaction vessel is emptied and the process is started again

• Ethanol production by fermentation is therefore a batch process

Hydration of ethene

• A mixture of ethene and steam is passed over a hot catalyst of phosphoric acid at a temperature of approximately 300 °C

• The pressure used is 60 atmospheres (6000kPa)

• The gaseous ethanol is then condensed into a liquid for use

Advantages and Disadvantages

Equipment

H = complex set up required

F = simple equipment needed

Raw materials

H = uses non- renewable resources (crude oil)

F = uses renewable resources (sugar cane)

Type of process

H = continuous process- a steam of reactants is constantly passed over a catalyst

F = batch process- everything is mixed together in a reaction vessel and left for several days and the process repeated again

Rate of reaction

H = fast

F = very slow (several days)

Quality of product

H = produces pure ethanol

F = produces a dilute solution requiring further processing

Atmospheric effects

H = no greenhouse gases produced but pollutants are formed from the burning of fossil fuels to maintain high temperatures

F = carbon dioxide produced which is a greenhouse gas

Reaction conditions

H = high temperatures and pressures required increasing the energy input and cost

F = low temperatures required