Chapter 14 - Alcohols Flashcards
What is the functional group of alcohols?
-OH or hydroxyl
What is methanol used for and why?
A high-performance fuel because of its efficient combustion Methanol is also an important chemical feedstock - the starting material in many industrial syntheses
What can methanol be converted into?
Polymers, paints, solvents, insulation, adhesives and many other useful products
What is ethanol used in?
Primarily in alcoholic drinks and as a fuel It also finds use as a solvent and a feedstock
How do you name alcohols?
- add the suffic -ol to the stem name of the longest carbon chain - the position of the alcohol functional group in the chain is indicated using a number
What is the difference between the physical properties of alochols and alkanes with the same number of carbon atoms?
alcohols are less volatile, have higher melting points and greater water solubility
How does the difference in physical properties between alcohols and alkanes change as the length of the carbon chain increases?
The differences become much smaller as the length of the carbon chain increases
Why is there a difference between alcohols and alkanes with the same number of carbon atoms?
- the alkanes have non-polar bonds because the electronegativity of carbon and hydrogen are very similar - the alkane molecules are therefore non-polar - the intermolecular forces between non-polar molecules are very weak London forces - alcohols have a polar O-H bond because of the difference in electronegativity of the oxygen and hydrogen atoms - alcohol molecules are therefore polar - the intermolecular forces will be very weak London forces but there will also be much stronger hydrogen bonds between the polar O-H groups
Why are alcohols and alkanes volatilities different?
- in the liquid state, intermolecular hydrogen bonds hold the alcohol molecules together - these bonds must be broken in order to change the liquid alcohol to a gas - this requires more energy than overcoming the weaker london forces in alkanes, so alcohols have a lower volatility than the alkanes with the same number of carbon atoms
Why are alcohols and alkanes solubility different?
- a compound that can form hydrogen bonds with water is far more water-soluble than a compound that cannot - alkanes are non-polar molecules and cannot form hydrogen bonds with water - alcohols such as methanol and ethanol are completely soluble in water, as hydrogen bonds form between the polar -OH group of the alcohol and the water molecules - as the hydrocarbon chain increases in size, the influence of the -OH group becomes relatively smaller, and the solubility of longer-chain alcohols becomes more like that of hydrocarbons (solubility decreases)
How can alcohols be classified?`
As primary, secondary and tertiary
What does the classification of alcohols depend on?
the number of hydrogen atoms and alkyl groups attached to the carbon atom that contains the alcohol functional group
What is a primary alcohol?
in a primary alcohol, the -OH group is attached to a carbon atom that is attached to two hydrogen atoms and one alkyl group
What is a secondary alcohol?
The -OH group is attached to a carbon atom that is attached to one hydrogen atom and two alkyl groups.
What is a tertiary alcohol?
In a tertiary alcohol the -OH group is attached to a carbon atom that is attached to no hydrogen atoms and three alkyl groups
What happens when alcohols combust?
- alcohols burn completely in a plentiful supply of oxygen to produce carbon dioxide and water - the reaction is exothermic, releasing a large quantity of energy in the form of heat - as the number of carbon atoms in the alcohol chain increases the quantity of heat released per mole also increases
Can alcohols be oxidised?
primary and secondary alcohols can be oxidised by an oxidising agent
What is the oxidising agent for the oxidation of alcohol?
A solution of potassium dichromate, acidified with dilute sulfuric acid
What is the colour change that occurs when alcohols are oxidised?
The orange solution containing dichromate(VI) ions is reduced to a green solution containing chromium (III) ions
What is formed when primary alcohols are oxidised?
Primary alcohols can be oxidised to either aldehydes or carboxylic acids The product of the oxidation depends on the reaction conditions used becuase aldehydes are themselves oxidised to carboxylic acids
How are aldehydes prepared from alcohols?
- on gentle heating of primary alcohols with acidified potassium dichromate, an aldehyde is formed - to ensure that the aldehyde is prepared rather than the carboxylic acid, the aldehyde is distilled out of the reaction mixture as it forms - this prevents any further reaction with the oxidising agent - the dichromate ions change colour from orange to green
How are carboxylic acids formed from alcohols?
- if a primary alcohol is heated strongly under reflux, with an excess of acidified potassium dichromate, a carboxylic acid is formed - use of an excess of the acidified potassium dichromate ensures that all of the alcohol is oxidised - heating under reflux ensure that any aldehyde formed initially in the reaction also undergoes oxidation to the carboxylic acid
What are the conditions needed for a primary alcohol to oxidise to an aldehyde?
Using distillation to remove the aldehyde from the reaction mixture
What are the conditions needed for a primary alcohol to oxidise to a carboxylic acid?
Heating the alcohol under reflux
What do secondary alcohols oxidise to?
Secondary alcohols only oxidise to ketones Ketones do not oxidise further using acidified dichromate ions
How are ketones formed from secondary alcohols?
To ensure that the reaction goes to completion, the secondary alcohol is heated under reflux with the oxidising mixture The dichromate ions once again change colour from orange to green
What do tertiary alcohols oxidise to?
Tertiary alcohols do not undergo oxidation reactions The acidified dichromate remains orange when added to a tertiary alcohol
What is dehydration?
Any reaction in which a water molecule is removed from the starting material
What is needed for an alcohol dehydration reaction to occur?
An alcohol is heated under reflux in the presence of an acid catalyst such as concentrated sulfuric acid or concentrated phosphoric acid
What is the product of an alcohol dehydration reaction?
An alkene
What type of reaction is the dehydration of an alcohol?
an elimination reaction
What happens when alcohols react with hydrogen halides?
Haloalkanes are formed
What conditions are needed for a haloalkane to be formed?
The alcohol is heated under reflux with sulfuric acid and a sodium halide. The hydrogen bromide is formed in situ. The HBr formed reacts with the alcohol to produce the haloalkane.
