Alcohols

How are alcohols are produced?

• Produced industrially by hydration of alkenes in the presence of an acid catalyst

How is ethanol produced and what’s the formula?

• By the fermentation of glucose

• Glucose→Ethanol+Carbon dioxide

• C6H12O6 → C2H5OH+CO2

How can ethanol produced by fermentation be seperated?

• Fraction distillation

When the ethanol is separated what can it be used as?

• Biofuel

Define biofuel

• A fuel derived from organic matter like plants

• Can be considered a renewable source of energy

What are the conditions needed for fermentation?

• Temperatures around 30-40 degrees

• Yeast (which produces an enzyme)

• No air (presence of oxygen would lead to aerobic respiration instead of fermentation)

Explain why the optimum temperature for fermentation is 38 degrees

• At lower temperatures the reaction is too slow

• At high temperatures the enzymes will denature

Why is fermentation done in the absence of air

• Oxygen can oxidise the ethanol

• producing ethanoic acid (Vinegar) instead

Advantages of fermentation

• Sugar (glucose) is a renewable source

• Production uses cheap equipment

Disadvantages of fermentation

• Uses up land that could be used for food

• Batch process is slow and raises production costs

• Ethanol made is not pure and needs purifying by fractional distillation

Why is a batch process used in fermentation?

• Allows for a high ethanol yield

• Fermentation stops when ethanol concentration becomes toxic to yeast

• Batch process ensures fermentation occurs for an optimal time before the yeast becomes inactive

Define the term carbon neutral

• An activity that has no net carbon emissions to the atmosphere

Why is the production of ethanol considered carbon neutral

• Its a biofuel formed from fermentation

• Any carbon dioxide given off when the biofuel is burned would have been extracted from the air via photosynthesis by the plants grown

• Meaning there is no net carbon emission

Equations showing that the production of ethanol is carbon neutral

Removal of CO2 by photosynthesis

• 6CO2 + 6H2O → C6H12O6 + 6o2

Production of CO2 by fermentation and combustion

• C6H1206 → C2H5OH + 2CO2 (fermentation)

• C2H5OH + 6O2 → 4CO2 + 6H2O (Combustion)

For every 6 molecules of carbon dioxide absorbed, 6 molecules of carbon dioxide are emitted

Why may the production of ethanol not be considered carbon neutral?

• The equations that show it being a carbon neutral process do not take into consideration

  • Energy required to fractionally distil the ethanol

  • Energy required to irrigate( water) the plants.- Water pumps are powered by fossil fuels

What does dehydrating an alcohol do?

• Removes a water molecule to form an alkene and water

Conditions for dehydration

• Alcohol is heated under reflux (heating a reaction without losing the volatile substances)

• Heated in the presence of concentrated sulfuric acid or phosphoric (v) acid

• Acid act as a catalyst

Dehydration of alcohols is an example of an…

• Elimination reaction

Products of dehydration of alcohols

• Alkene

• H2o

First stage of dehydration of alcohols

• Lone pair of electrons on oxygen atom will be attracted to the positive hydrogen ion from the acid catalyst

• Lone pair forms a covalent bond with the hydrogen ion

• Forming an intermediate molecule with a positive oxygen atom

Second stage of dehydration of alcohols

• Pair of electrons in the covalent bond between the C-O move onto the oxygen

• This releases the molecule of water to be released

• At the same time the pair of electrons between a C-H bond move between 2 carbon atoms

• Hydrogen is now released as H⁺ (regenerating the acid catalyst

What can alkenes produced from dehydration of alcohols be used for ?

• used to produce addition polymers (without using monomers derived from crude oil)

Describe a primary alcohol

• Carbon atom bonded to the hydroxyl group (OH) is bonded to one more carbon atom

What alcohol is considered a primary alcohol despite not fitting the definition?

• Methanol

Describe a secondary alcohol

• The carbon atom bonded to the hydroxyl group is bonded to 2 other carbon atoms

Describe a tertiary alcohol

• The carbon atom bonded to the hydroxyl group is bonded to 3 other carbon atoms

What does volatility of a molecule tell you?

• How readily a molecule turns into a gas

Why are alcohols highly soluble in water

• Alcohol functional group can form hydrogen bonds with water molecules

Why do long carbon chain alcohols become less soluble in water?

• The non polar carbon chain cannot form hydrogen bonds

• So as carbon chain is increases a larger part of the molecule is unable to form hydrogen bonds with water molecules

What can primary alcohols be oxidised to make?

• Aldehyde

• (and a molecule of water is produced)

What is the chemical needed for oxidation called and give an example?

• Oxidising agent

• Acidified potassium dichromate K₂Cr₂O₇/H⁺

What happens in the oxidation of primary alcohols?

• The oxidising agent is reduced from the dichromate(VI) ion which is orange in colour to the Chromium (III) ion which is green

• oxidation in terms of hydrogen means a loss of hydrogen (alcohol is losing 2 hydrogens)

What is the problem with the oxidation of primary alcohols?

• Aldehydes are extremely easy to oxidise further

• So if only aldehyde is wanted from this reaction it must be removed as soon as it forms.

How and why can you remove the aldehyde from the reaction?

• Through distillation

• Aldehydes have low boiling points

Explain the method of distillation

• By gently heating the alcohol and the oxidising agent we produce the aldehyde

• The aldehyde evaporates and passes through the condenser

• Where it condenses back to a liquid and is removed

Why does an excess of alcohol and a limiting oxidising agent favour the production of an aldehyde?

• If there’s too much oxidising agent the aldehyde will get oxidised further into a carboxylic acid.

What happens when aldehydes are oxidised?

• They make a carboxylic acid

How do you get from a primary alcohol to a carboxylic acid?

• Using 2 molecules of oxidising agent

How can you ensure that all the aldehyde produced is then oxidised to a carboxylic acid?

• Use excess oxidising agent

• Heat the reaction under reflux

• Use concentrated sulfuric acid instead of dilute in acidified potassium dichromate (oxidising agent)

What happens when you heat the reaction under reflux?

• When you heat a reaction under reflux any volatile products are condensed and return to the reaction mix

• The aldehyde will keep condensing back down until it forms a carboxylic acid

What will be obtained in the end after heating the reaction under reflux ?

• A mixture of chemicals containing carboxylic acid and any unreacted alcohol and aldehyde '

• Carboxylic acids have higher boiling points than aldehydes so they can be separated using distillation

What does the oxidation of secondary alcohols produce?

• Ketone

• Molecule of water

Why cant you oxidise ketones further?

• The carbon atom bonded to the oxygen is not bonded to any hydrogen atoms

Why cant a tertiary alcohol be oxidised?

• The carbon atom bonded to the hydroxyl group is not bonded to any hydrogen atoms

How do you use tollens reagent to distinguish between aldehydes and ketones?

• Tollens reagent is formed by mixing aqueous ammonia and silver nitrate. Forming a complex ion Ag(NH₃)₂⁺

• Only aldehydes are oxidised by tollens reagent into a carboxylic acid. Silver ions are reduced to silver atoms

• With aldehydes a silver mirror forms inside the test tube. Ketones result in no visible change

How do you use Fehling’s solution to distinguish aldehydes and ketones?

• Fehling’s solution contains blue Cu²⁺ ions

• Only aldehydes are oxidised by Fehling’s solution into carboxylic acid. Copper ions are reduced to copper(I) oxide

• With aldehydes blue Cu2+ ions in solution change to a red precipitate of Cu2O

• Ketones do not react