4.2.1 Alcohols Flashcards
What is the general formula for alcohols
Cn H₂n₊₁ OH
What functional group do alcohols contain
-OH
Known as the hydroxyl group
What are the first 2 alcohols in the homologous series
methanol and ethanol
List some uses of methanol
know 3
- chemical feedstock
- fuel
- converted into:
polymers
paints
adhesives
solvents
insulation
ect..
List some uses of ethanol
know 3
- used in alcoholic drinks
- fuel
- solvent
- feedstock
Describe how to name an alcohol
- identify the longest continuous chain of carbon atoms
- identify the number of -OH functional groups (e.g. is it 1, or does it need di- or tri-)
- identify the location of the -OH functional groups
- identify any other functional groups
i.e. 2-methylbutane-2,3-diol
What are the classifications of alcohols
- primary
- secondary
- tertiary
What are primary alcohols
In a primary alcohol the -OH group is part of CH₂OH
e.g. Butan-1-ol
Def. An alcohol in which the OH group is attached to a carbon atom that is attached to 2 or 3 hydrogen atoms
What are secondary alcohols
In a secondary alcohol, the -OH is part of CHOH
e.g. Butan-2-ol
Def. An alcohol in which the OH group is attached to a carbon atom that is attached to two carbon chains and one hydrogen atom
What are tertiary alcohols
In a tertiary alcohol, there are no hydrogen atoms on the COH
e.g. 2-methylpropan-2-ol
Def. An alcohol in which the OH group is attached to a carbon atom that is attatched to three carbon atoms and no hydrogen atoms
What is the difference of physical properties of alcohols with alkanes of the same number of carbon atoms
Alcohols are less volatile, have higher melting points and greater water solubility
The differences become much smaller as the length of the carbon chain increases
define volatile
Volatility - the ease at which a liquid turns into a gas.
Volatility increases as boiling point decreases
Why do the physical properties of alcohols and alkanes decrease in differentiation as the length of carbon chains increase
- 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 th epolar O-H groups.
describe the relationship between boiling points and volatility
compounds with low boiling points are volatile
the higher the boiling point the lower the volatility
Where is the hydrogen bond present for alcohols
Between the H from an OH group to the electrons on an oxygen from another alcohol or water
What are the dipoles for a hydrogen bond with alcohols
a 𝛿⁻ is on the oxygen atom and a 𝛿⁺ is on the hydrogen ( a dotten line is shown from the hydrogen to the : (showing electrons) on the oxygen on a different compound which has 𝛿⁻ sign.
Explain why alcohols have higher boiling points than alkanes with similar a Mr
There are hydeogen bonds present between the H in the OH group on one alcohol and the electrons on the O in an OH group on a seperate alcohol.
The hydrogen bonds are stronger that London forces (present between alkanes) so more energy is required to break them hence a higher b.p.
Explain why alcohols are more water soluble than alkanes with similar a Mr
Alcohols can form hydrogen bonds with water so are fully miscible with it
miscible = mix fully.
the hydrogen bond is between the H in the OH group on the alcohol and the electrons of the O in the water molecule
why are complex alcohols less water soluble than simple alcohols
As the hydrocarbon chain increases in size, the influence of the OH group becaomes relatively smaller.
The hydrogen part of the alcohol is unable to form hydrogen bonds if not part of a polar group like the OH functional group. It can only form London forces therefore the more carbons in the chain the less soluble the molecule.
What are the reactions of alcohols
- combustion
- oxidation
- elimination (of water)
- substitution (with halide ions)
Explain combustion as a reaction of alcohols
- alcohols burn completely when there is a plentiful supply of oxygen, complete combustion produces carbon dioxide and water.
- Alcohols are used as fuels as their combustions produces an exothermic reaction.
- The number of carbons atoms in the alcohol chain increases the quantity of heat released per mole also increases.
Explain oxidation as a reaction of alcohols
Alcohols can be oxidised using acidified dichromate (H⁺, Cr₂O₇²⁻). When this reacts it changes from orange to green.
Acidified dichromate (H⁺, Cr₂O₇²⁻) can be represented as [O] when writing balanced equations.
The functional group formed by the oxidation of an alcohol depends on the structure of the alcohol and the conditions.
What happens when tertiary alcohols are reacted with acidified dichromate
(H⁺, Cr₂O₇²⁻)
Tertiary alcohols do not react.
The acidified dichromate remains orange when added to tertiary alcohols
how is acidified dichromate (H⁺, Cr₂O₇²⁻) formed
A solution of potassium dichromate (VI), K₂Cr₂O₇, is acidified with dilute sulfuric acid, H₂SO₄
What happens when secondary alcohols are reacted with acidified dichromate
(H⁺, Cr₂O₇²⁻)
Secondary alcohols are oxidised to ketones.
To ensure that the reaction goes to completion, the secondary alcohol is heated under reflux with oxidising mixture.
The dichromate ions change colour from orange to green.
Describe the aparatus to carry out reflux
A “heat” arrow pointing to a roundbottomed (or pearshaped) flask. Flask is linked to an upright condenser. Water goes in the bottom and out the top so that it fills it. The condenser MUST be shown to be OPEN at the top. It is connected via a bung to the flask.
Describe and explain reflux for oxidising alcohols
The mixture is evapourated, the vapours condense and return to the reaction flask.
The aldehyde reacts with more (H⁺, Cr₂O₇²⁻) to give the carboxylic acid (ONLY WITH PRIMARY ALCOHOLS)
Secondary alcohols are heated using this aparatus to ensure the alcohol has all reacted.
Give the boiling points of the 2nd of the homologuos series of alcohol, aldehydes and carboxylic acid
Ethanol = 78 ℃
Ethanal = 21 ℃
Ethanoic acid = 118 ℃
what are the ways of heating alcohols for oxidisation
Distillation
Reflux
What happens when primary alcohols are reacted with acidified dichromate
(H⁺, Cr₂O₇²⁻)
Primary alcohols give one of two possible products depending on the conditions used:
Distillation gives the aldehyde
and Reflux gives the carboxylic acid (primary only)
What is the order of oxidation for the primary alcohols
- Aldehydes
- carboxylic acid
Describe and explain distillation for oxidising alcohols
the aldehyde has the lowest boiling point so it is removed form the reaction as it is formed. This is to ensure aldehyde is formed rather than a carboxylic acid. This also prevents any further reaction with the oxidising agent. The dichromate ions change from ornage to green
Describe the aparatus to carry out distillation for oxidising alcohols
A “heat” arrow pointing to a roundbottomed (or pearshaped) flask. Flask is linked to a condenser at a downward angle. At the join a thermometer is place upright. The water goes in at the lower end and out at the higher end to ensure the condenser works properly. The content that the condenser condenses is collected in a flask.
What happens when Primary alcohols are reacted with acidified dichromate
(H⁺, Cr₂O₇²⁻) under distillation
An aldehyde is formed and removed from the reaction
What happens when primary alcohols are reacted with acidified dichromate
(H⁺, Cr₂O₇²⁻) under reflux
The aldehyde reacts with more H⁺, Cr₂O₇²⁻ to give the carboxylic acid.
What do each alcohol produce when it has undergone oxidation
₄₊₂- primary alcohol
→ aldehyde
→ carboxylic acid
- secondary alcohol
→ ketone
- Tertiary alcohol
→ no reaction
Explain elimination as a reaction of alcohols
- Also known as elimination of water, or dehydration of alcohols.
- When an alcohol is heated under reflux in the presence of an acid catalyst such as H₃PO₄ or H₂SO₄ then an alkene is made and water is eliminated.
The acid catalyst plays the role of a reagent here
This is the opposite of adding H₂O to an alkene
What are the rules surrounding which hydrogens can be taken in addition to the OH group to form the water eliminated
for reaction of alcohols - elimination of water.
There MUST be the correct number of bonds for each atom after the removal
e.g. if you take an OH group off of a CH₂OH then the second H must be from the CH₂ group before
e.g. if the OH group you take is in the middle of a chain then the only H you cannot remove is the one on the other side of the C atom.
Some alcohols can give two different alkene products.
Define reagent
a reactant that changes the organic compound into something else
Explain substitution as a reaction of alcohols
Alcohols react with halide ions to form haloalkanes when heated under reflux in the presence of acid.
The acid and halide ions are obtained from sulfuric acid and sodium halide which reacts to produce NaHSO₄ and HX
The HX formed reacts with the alcohol to produce the haloalkane
with X representing a halide ion such as Br⁻, Cl⁻ or I⁻
