14.1(4.2.1) - Alcohols Flashcards
Functional group of alcohols
-OH
What is the functional group called and what does it effect?
Known as hydroxyl group
Responsible for both the physical and chemical properties of the alcohols
Suffix for alcohols
-ol added to some name of longest carbon carbon chain
Physical properties of alcohols
Have a polar O-H bond because of difference in electronegativity of oxygen and hydrogen atoms
Alcohol molecules are therefor polar
Intermolecular forces will be very weak London Forces
Also much stronger hydrogen bonds between polar O-H groups
Boiling point and volatility of alcohols
In liquid state, intermolecular hydrogen bonds hold alcohol molecules together.
These bonds must be broken in order to change liquid alcohol into a gas.
This requires more energy
So alcohols have a lower volatility(evaporate less easily)
BP and volatility of alcohols compared to alkanes
Alcohols require more energy to break strong hydrogen bonds
More energy than overcoming weaker London forces in alkanes
Therefore alkanes have a higher volatility than alcohols with same no. of carbon atoms
Solubility of alcohols in water
Alcohols are polar molecules so can form bonds with water molecules by hydrogen bonding
Hydrogen bond forms from polar -OH group of alcohol and water molecules
Solubility of alkanes in water
Alkanes are non-polar molecules
So cannot from hydrogen bonds with water
Solubility of alcohols with increasing chain length
As alcohols hydrocarbon chains become larger
Influence of the -OH group becomes relatively smaller
Solubility decreases as hydrocarbon chains increase
Three classifications for alcohols
Primary
Secondary
Tertiary
How to classify primary alcohols
When the -OH group is attached to a carbon attached to one other carbon
E.g. methanol
Classifying secondary alcohols
When the alkyl(-OH) group is attached to a carbon attached to two other carbons
E.g. propan-2-ol
Classifying tertiary alcohols
When the -OH group is attached to a carbon atom which is a attached to three other carbon atoms
E.g. 2-methylpropan-2-ol
Combustion of alcohols
Alcohols burn completely in a plentiful supply of oxygen to produce:
Carbon dioxide
Water
Reaction is exothermic - releases lots of energy in form of heat
As chain length of alcohol increases, so does amount of energy released
Oxidising agent for oxidation of alcohols
Acidified Potassium dichromate,
K2Cr2O7.H2SO4
What is an oxidising agent?
A substance that tends to bring about oxidation by being reduced and gaining electrons
Different forms alcohols go to when oxidised
Alcohol - e.g. butan-1-ol
Aldehyde - butanal
Carboxylic acid - butanoic acid
Prep of aldehydes from alcohols
Gentle heating of primary alcohols with acidified potassium dichromate
Aldehyde is distilled out of reaction mixture as it forms to ensure no further reactions occur with oxidising agent
Dichromate ions change colour from orange to green
Preparation of carboxylic acids
Heating primary alcohol strongly under reflux, with excess of acidified potassium dichromate
Use of excess ensures all alcohol is oxidised
Heating under reflux ensures any aldehyde formed in reaction also undergoes oxidation to carboxylic acid
What signifies oxidation in an equation?
When an arrow product arrow has ‘[O]’ on it
Conditions for different oxidations of alcohols
When preparing aldehyde use distillation to remove the aldehyde from reaction mixture
When preparing carboxylic acid, heat alcohol under reflux
Products from oxidation of secondary alcohols
Secondary alcohols are oxidised to ketones
Oxidising secondary alcohols process
To ensure reaction goes to completion, secondary alcohol is heated under reflux with oxidising mixture
Dichromate ions change colour from orange to green
Example of formation of ketone from secondary alcohol
Propan-2-ol + [O] —> propanone
