Alcohols Flashcards
What is the general formula for the homologous series of aliphatic alcohols?
Cn H2n+1 OH
What groups do alcohol’s have?
Hydroxyl -OH group
If an alcohol has three or more carbon atoms…
The position of the hydroxyl group is indicated by a number.
E.g CH3CH2CH2OH is propan-1-ol
Why is the boiling point of an alcohol higher than an alkane?
Because alcohols have relatively strong hydrogen bonds.
The more hydrogen bonds present in a molecule, the lower the volatility and the higher the boiling point.
Why do alcohols dissolve in water?
Due to the hydroxyl’s ability to create hydrogen bonds between the alcohol and water.
The greater the number of carbons in a molecule…
The greater the boiling points and the higher the enthalpy change of combustion.
What is miscibility?
A measure of how easily a liquid mixes.
What happens when water and ethanol mix?
Some of the hydrogen bonds between the molecules in the separate liquids are broken and replaced by hydrogen bonds between water molecules and ethanol molecules.
There is no significant gain or loss in energy.
Why does the miscibility of alcohols in water decrease as the hydrocarbon chain length increases?
Although the hydroxyl group can still form hydrogen bonds to water, the long hydrocarbon chain disrupts hydrogen bonding between other water molecules.
Why is it the hydrocarbon chains do not form strong intermolecular bonds with water molecules?
Because the hydrocarbon chains are essentially non-polar and can only form weak van der Waals’ forces.
What does hydrogen bonding between alcohol molecules due to their volatility?
It reduces it significantly.
The intermolecular forces are significantly stronger in alcohol’s than in alkanes.
What is volatility?
The ease with which a liquid turns to vapour.
Explain why ethanol is high miscible with water.
Ethanol molecules and water molecules can form hydrogen bonds with each other.
Explain why pentan-1-ol is miscible with water, but less so than ethanol.
Pentan-1-ol molecules and water molecules can form hydrogen bonds with each other but the C5 H11 sections in the pentan-1-ol molecules disrupt hydrogen bonding between other water molecules.
Explain why ethanol has a higher boiling point than propane.
Ethanol molecule can form hydrogen bonds with each other, propane molecule can’t.
Explain why propan-1-ol has a higher boiling point than ethanol.
Propan-1-ol molecules have a larger hydrocarbon portion than ethanol molecules, so they form stronger van der Waals’ forces in addition to hydrogen bonding.
Monohydric alcohols contain…
only one –OH group per molecule.
Polyhydric alcohols contain…
more than one –OH group per molecule.
What has a higher boiling point polyhydric or monohydric alcohols and why.
Polyhydric alcohols are more viscous and have higher
boiling points than monohydric alcohols because the degree of hydrogen bonding is much greater.
How are hydrogen bonds set up in alcohols?
Hydrogen bonds set up between the slightly positive hydrogen (δ+) atoms and lone pair of electrons on oxygen (δ-) in other molecules.
The hydrogen atoms are slightly positive because the bonding electrons are pulled away from them towards the very electronegative oxygen atoms.
What does the hydroxyl, -OH, functional group in alcohols enables them to form?
The hydroxyl, -OH, functional group in alcohols enables them to form hydrogen bonds i.e. the hydrogen is attached to an electronegative atom (O) that has two lone pairs of electrons. Alcohols, consequently, can form hydrogen bonds with themselves and with water.
Why are the boiling points of alcohols less than that in water?
In water the two hydrogen atoms in a water molecule can form two hydrogen bonds to two oxygen atoms in two other water molecules.
However, in alcohols there is only one hydrogen atom able to form hydrogen bonds and so there is less hydrogen bonding in alcohols.
What is the trend in volatility with alcohols?
The volatility decreases with increasing carbon chain length.
Why are alcohols less volatile than alkanes?
Alcohols are less volatile than alkanes due to alcohols having hydrogen bonds between the molecules whereas alkanes have weaker van der Waals interactions. Hydrogen bonds are stronger intermolecular bonds than van der Waals resulting in more energy required to separate the alcohol molecules from each other.
What is the trend in boiling point with alcohols?
The boiling point of alcohols increases with increasing an increase in carbon chain length.
Explain the trend in boiling point with alcohols?
As the carbon chain length increases there is the same amount of hydrogen bonding but an increase in the amount van der Waals interaction due to an increased surface area of the molecules.
What are the advantages of using fermentation to produce ethanol?
- it is a low-technology process, which means it can be used anywhere
- it does not use much energy
- it uses sugar cane as a raw material, which is a renewable resource
What are the disadvantages of using fermentation to produce ethanol?
- it is a batch process, which means that once the reaction has finished the vessel needs to emptied before the reaction can be started again
- it is a relatively slow process
- it produces fairly impure ethanol
What are some uses of ethanol?
- Alcoholic drinks
- As a fuel
- As a solvent
- In cosmetics manufacture
- As an intermediate in the manufacture of many other chemicals
Alcohols can be divided into three classes depending on the number of alkyl groups linked to the carbon that is attached to the O-H group. What are these classes?
The three classes are called primary, secondary and tertiary alcohols.
What are primary alcohols?
Primary alcohols contain the –CH2OH group. They have a maximum of one other alkyl group (R) linked to the carbon attached to the –OH group.
What are secondary alcohols?
Secondary alcohols contain the CHOH group. They have two alkyl groups linked to the carbon atom that is attached to the –OH group.
What are tertiary alcohols?
Tertiary alcohols contain the -COH group. They have three alkyl groups linked to the carbon atom that is attached to the –OH group.
In a plentiful supply of oxygen alcohols burn very readily to form…
carbon dioxide and water.
In a limited amount of oxygen alcohols combust to form…
carbon monoxide and water.
What is the oxidising agent used to oxidise primary and secondary alcohols?
Potassium dichromate (K2Cr2O7) in acidic conditions (H+) The acid used to acidify potassium dichromate is sulfuric acid (H2SO4). During oxidation of alcohols. Potassium dichromate is an orange colour and changes to a green colour during the reaction.
Oxidation of primary alcohols produces what?
An aldehyde, then further oxidation will produce a carboxylic acid.
Define reflux.
Reflux: The continual boiling and condensing of a reaction mixture to ensure the reaction goes to completion without the mixture billing dry.
Define distillation.
The process of separating a mixture of liquids due to the difference in boiling points.
Oxidation of a secondary alcohol produces what?
A ketone.
Can tertiary alcohol be oxidised, if so, with what?
Tertiary alcohols cannot be oxidised. No reaction occurs with acidified potassium dichromate; the oxidising agent remains the same colour, orange.
What is the general equation for esterification?
Carboxylic Acid + Alcohol → Ester + Water
What is esterification?
When an alcohol reacts with a carboxylic acid in the presence of a concentrated acid catalyst an ester is formed.
The reaction is called esterification.
Esters have the functional group –COO-
What is the acid catalyst used during esterification?
The acid catalyst used during esterification is concentrated
sulfuric acid (H2SO4) which can be abbreviated to H+.
During esterification water is produced from the -OH of the carboxylic acid and the -H from the alcohol. Esters have a fruity aroma and are used for flavourings and fragrances in food and perfumes.
They are also are used as adhesives and solvents in the chemical industry.
What is elimination?
Alcohols can be dehydrated (lose water) to form an alkene by heating them with a catalyst. The reaction mechanism for this reaction is called elimination.
What is the catalyst used in elimination?
The catalyst used in concentrated phosphoric acid (H3PO4) or concentrated sulfuric acid (H2SO4). The alcohol is heated under reflux to produce an alkene.
Why is ethane-1,2-diol very soluble in water?
Has O-H bonds.
Forms hydrogen bonds with water molecules.
What is esterification?
When an alcohol reacts with a carboxylic acid in the presence of a concentrated acid catalyst an ester is formed. The reaction is called esterification.
Primary alcohols are oxidised to form what?
Aldehydes and then to carboxylic acids
Secondary alcohols are oxidised to form what?
Ketones.
Tertiary alcohols are oxidised to form what?
Won’t be oxidised.
Equation for fermentation of sugar to produce ethanol
C6H12O6 -> 2C2H5OH + 2CO2
Equation for hydration of ethene to produce ethanol
C2H4 + H2O -> C2H5OH
Explain why 1-iodopropane is hydrolysed faster than 1 bromopropane.
C–I bonds broken more easily
C–I bonds are weaker
State two features of molecules that enable them to show E/Z isomerism.
Carbon-carbon double bond
Each carbon atom in the double bond is attached to different groups/atoms.
