Chapter 14 - Alcohols Flashcards
What is the functional group of alcohols ?
- OH
Hydroxyl group
Are alcohols polar and why ?
- Alcohols are polar due to the presence of the O-H bond
- The OH bond is polar because of a difference in electronegativity of O and H
What is a primary alcohol ?
The C atom that is attached to the OH is attached to 1 alkyl group and 2 H atoms
What is a secondary alcohol ?
The C atom that is attached to the OH is attached to 2 alkyl groups and 1 H atom
What is a tertiary alcohol ?
The C atom that is attached to the OH is attached to 3 alkyl groups and 0 H atoms
What kind of alcohol is methanol (exception) ?
It is a primary alcohol, it is the only exception and is quite misleading
Why do alcohols have a higher boiling point/melting point ?
- They have a higher boiling point due to the hydrogen bond that can be formed because of the OH group.
- This requires more energy to overcome and therefore increases its boiling point and melting point
Why do alcohols with few carbon atoms have much higher boiling points than alkanes with the same number of carbon atoms ?
- Less carbon atoms means that the OH bond has a stronger influence as it requires the most energy to break compared to the carbon bonds
- More carbon atoms means that the bond enthalpy for the entire thing is more shared out and the OH bond has less of an influence
What is meant by the volatility ?
How easily it can change into a gas
How do alcohols and alkanes compare in volatility ?
Alcohols with the same number of carbon atoms as an alkane will be less volatile
Why are alcohols less volatile than alkanes, when they have the same number of carbon atoms ?
- In the liquid state, the hydrogen bonds hold the alcohol molecules together. These bonds must be broken in order to change the liquid alcohol into gas.
- This requires more energy compared to alkanes therefore they have a lower volatility
Why are alcohols soluble in water ?
The polar OH bonds forms hydrogen bonds with the water molecules
As hydrocarbon chain increases, how does solubility of alcohols change ?
It decreases as the OH groups has a smaller influence and solubility shifts to that of the alkanes
What are the products of the combustion of alcohol ?
- Carbon dioxide
- Water
What is used to oxidise alcohols ?
Potassium dichromate
What is the formula for potassium dichromate ?
K2Cr2O7
What is acidified potassium dichromate ?
- K2Cr2O7/H2SO4
- Potassium dichromate that has been put into acid, leading to it being acidified
What are the two forms of chromium in oxidation reactions ?
- Dichromate(VI) ions that are orange, Cr2O7 (2-)
- Chromium(III) ions that are green, Cr (3+)
What indicates that the alcohol has been oxidised ?
The change in colour of potassium dichromate from orange to green
Which two products can be formed from the oxidation of primary alcohols ?
- Aldehydes
- Carboxylic acids
How are aldehydes prepared from the oxidation of primary alcohols ?
Gentle heating of primary alcohols with acidified potassium dichromate
What measure is taken to ensure no carboxylic acid is made ?
Aldehyde is distilled out of the mixture as it forms to prevent any further reaction
How are carboxylic acids prepared from the oxidation of primary alcohols ?
Primary alcohol is heated strongly under reflux with an excess of acidified potassium dichromate
Why is it under reflux for carboxylic acid preparation ?
- To ensure any aldehyde initially formed also undergoes oxidation to the carboxylic acid
- Any aldehyde formed will drip backdown into the flask and it will carry on undergoing oxidation
What product is formed from the oxidation of secondary alcohols ?
Ketones
How are ketones prepared from the oxidation of secondary alcohols ?
Heated under reflux with acidified potassium dichromate
What product is formed from the oxidation of tertiary alcohols ?
Tertiary alcohols do not undergo oxidation reactions
What is dehydration ?
Any reaction in which a water molecule is removed from the starting material
What are the conditions of a dehydration reaction ?
Alcohol is heated under reflux in the presence of concentrated sulphuric acid or phosphoric acid (H3PO4)
What are the products of the dehydration of an alcohol ?
An alkene and water
Alcohols react with hydrogen halides to form what ?
Haloalkanes and water
What are the conditions of a halogen substitution reaction of an alcohol ?
- Alcohol is heated under reflux with sulphuric acid (H2SO4) and a sodium halide, the hydrogen halide is formed in situ
- The hydrogen halide then reacts to form the haloalkane and water
