Module 4: Chapter 14 - Alcohols Flashcards
What is the density of water?
1 g cm⁻³
What causes the differences in physical properties between alcohols and alkanes of equal chain length?
All alkanes have non-polar bonds as the electronegativity of hydrogen and carbon are very similar. The alkane molecules are therefore non-polar. The intermolecular forces between non-polar molecules and only very weak London forces. Alcohols have a polar O-H bond due to the large difference in electronegativity between the oxygen and hydrogen atoms, alcohol molecules are therefore polar. The intermolecular forces will be very weak London forces byt there will also be much stronger hydrogen bonds between the polar O-H groups
How does the volatility and boiling points of alcohols compare to relative alkanes?
Alcohols are less volatile and have higher boiling points compared to relative alkanes
Explain why alcohols are less volatile and have higher boiling points than alkanes
Alkanes are non-polar and therefore only contain weak London forces, however, alcohols contain the polar O-H group and are therefore polar, this means that alcohols contain both weaker London forces and much stronger hydrogen bonds which take much more energy to break than just weak London forces. As a result alcohols are less volatile and have higher boiling points
Why does the difference in boiling points between alcohols and alkanes decrease as chain length increases?
As chain length increases, the strength of the London forces increases in both alkanes and alcohols and therefore the contribution of the -OH group decreases. This means that their boiling points are more similar
How does the solubility in water of alcohols and alkanes compare?
Alcohols are much more soluble in water than alkanes
Why are alcohols are much more soluble in water than alkanes?
Alkanes are non-polar molecules and therefore do not interact strongly with the polar water molecules. As a result they cannot break the hydrogen bonds present in water and dissolve. However, alcohols are polar molecules and therefore strongly interact with the polar water molecules, forming hydrogen bonds between water molecules and alcohol molecules. As a result they interact strongly enough to break some of the hydrogen bonds in water and dissolve.
Why do alcohols het less soluble as chain length increases?
As the hydrocarbon chain increases in length, the influence of the -OH group becomes relatively smaller compared to the non-polar carbon chain. Therefore the solubility of longer-chain alcohols decreases
What is the word equation for the combustion of alcohols?
Alcohol + oxygen -> carbon dioxide + water
How does the energy released in combustion of alcohols change with chain length?
As the chain length increases, the quantity of heat released per mole also increases
What oxidising agent is most commonly used?
Potassium dichromate (VI) solution acidified with dilute sulfuric acid
K₂Cr₂O₇(aq) + H₂SO₄(aq)
What colour is acidified potassium dichromate solution?
orange
What colour is a solution containing chromium (III) ions?
green
How can you tell if an alcohol has been oxidised by acidified potassium dichromate?
The solution should turn form orange to green
Which types of alcohol can be oxidised?
Primary and secondary
What are primary alcohols oxidised to?
aldehydes or carboxylic acids (depending on the conditions)
How do you prepare an aldehyde from a primary alcohol?
Gentle heating of primary alcohols with acidified potassium dichromate forms an aldehyde. To ensure than an aldehyde is prepared rather than a carboxylic acid, the aldehyde is distilled out of the reaction mixture as it forms. This prevents any further reaction with the oxidising agent
How do you mark an oxidising agent (such as acidified potassium dichromate) in a chemical equation?
[O]
What is the word equation for the formation of an aldehyde from and alcohol?
alcohol + oxidising agent -> aldehyde + water
How do you prepare a carboxylic acid from a primary alcohol?
If a primary alcohol is heated strongly under reflux, with an excess of acidified potassium dichromate(VI), a carboxylic acid is formed. Use of an excess acidified potassium dichromate ensures all of the alcohol is oxidised and heating under reflux ensures that any aldehyde initially formed in the reaction also undergoes oxidation to form a carboxylic acid
What is the word equation for the formation of a carboxylic acid from a primary alcohol?
Alcohol + 2 oxidising agent -> carboxylic acid + water
What does an aldehyde oxidise to give?
carboxylic acid
What do secondary alcohols oxidise to give?
ketones
How do you prepare a ketone from a secondary alcohol/
A secondary alcohol is heated under reflux (to ensure the reaction goes to completion) with acidified potassium dichromate
What do tertiary alcohols oxidise to give?
Tertiary alcohols do not undergo oxidation reactions
What would the observation be if tertiary alcohols are refluxed with acidified potassium dichromate?
The solution would remain orange, it would not turn green
Why do tertiary alcohols not undergo oxidation reactions?
For an alcohol to oxidise, it must have a hydrogen atom attached to the carbon the hydroxyl group is bonded to. As tertiary alcohols have a hydroxy group bonded to a carbon atom which is then bonded to 3 other carbon atoms, it does not contain this hydrogen atom and therefore cannot undergo oxidation reactions
What is a dehydration reaction?
A reaction in which a water molecule is removed from the starting material
What is the word equation for the dehydration of an alcohol?
Alcohol + acid catalyst -> alkene + water
What are the conditions for the dehydration of an alcohol?
- Heated under reflux
- Acid catalyst (such as concentrated sulfuric/phosphoric acid)
What type of reaction is the dehydration of an alcohol?
Elimination reaction
How do you produce a haloalkane from an alcohol?
React the alcohol with a sodium halide, heating it under reflux with sulfuric acid
What is the word equation for the preparation of a haloalkane from an alcohol?
Alcohol + sodium halide + sulfuric acid -> haloalkane + sodium hydrogen sulfate + water
Explain the reaction for the preparation of a haloalkane from an alcohol
The sodium halide reacts with the sulfuric acid to produce sodium hydrogen sulfate and a hydrogen halide. The hydrogen halide formed then reacts with the alcohol to produce the haloalkane and water
