4.2 Alcohols, Haloalkanes & Analysis Flashcards
What is a primary alcohol?
The OH functional group is attached to a carbon atom with no more than one alkyl group
What is a secondary alcohol?
The OH functional group is attached to a carbon atom with two alkyl groups
What is a tertiary alcohol?
The OH functional group is attached to a carbon atom attached to three alkyl groups
Why does the boiling point for alcohols increase as the chain gets longer?
There are more surface area contacts so more London forces
More energy is required to overcome these forces
Why are the boiling points for alcohols higher than corresponding alkanes?
There are hydrogen bonds between the alcohol molecules - strongest type of intermolecular force of attraction
More energy is required to overcome this force so higher boiling point
Why are alcohols less volatile than corresponding alkanes?
A substance that is volatile evaporates easily at RTP
Volatility increases as boiling point decreases
Hydrogen bonds make alcohols less volatile than corresponding alkanes
Why are alcohols soluble?
Water molecules are polar and so is the OH functional group in alcohols
The alcohol can form hydrogen bonds with water
As alkyl chain length increases, the solubility of the alcohol decreases because the aliphatic chain cannot form hydrogen bonds
What is reflux?
The constant boiling and condensing of a reaction mixture to ensure that the reaction goes to completion without losing reactants or products as vapour to be air
What is the general formula for alcohols?
CnH2n+1OH
Why must the aldehyde be distilled immediately after being made by oxidation of a primary alcohol?
To prevent any further reaction like making a carboxylic acid
What are the conditions for oxidising primary alcohols?
Gentle heating = aldehyde and water (colour change from orange to green) [O] DISTIL
Stronger heating under reflux with excess potassium dichromate = carboxylic acid and water (colour change from orange to green) 2[O]
What is dehydration?
The chemical reaction in which water is lost from an organic compound - a type of elimination reaction
When water is heated with a strong acid (concentrated sulfuric acid/phosphoric acid - catalyst) water is eliminated to make an alkene from an alcohol
OH group and H atom are lost to form water and pi bond forms between two C atoms
What is halide substitution?
When halide ions react with alcohols and substitutes the alcohol group to form a haloalkane
ROH + HX –> RX + H2O
What are the conditions for oxidising a tertiary alcohol?
No reaction
A tertiary alcohol is very resistant to oxidation and are not oxidised by common oxidising agents like potassium dichromate
What are the conditions for oxidising a secondary alcohol?
Heating under reflux = ketone and water (colour change from orange to green) [O] HEAT
What is nucleophilic substitution?
Atoms or group of atoms is exchanged for a nucleophile
Curly arrow from nucleophile to slightly positive carbon atom donating its lone pairs
Carbon-halogen bond breaks by heterolytic fission as two electrons move to halogen forming X-
What are nucleophiles?
Species that are electron pair donors
How can you measure the rate of hydrolysis (a nucleophilic substitution reaction)?
By measuring how long it takes for a coloured precipitate to form (halogen ion and silver nitrate)
IV - halogen present in haloalkane
DV - time taken for precipitate to form
CV - length of alkyl chain, moles of haloalkane used, concentrations of reactants, volume of reactants, temperature of water bath
What are the trends in the rates of hydrolysis?
Bond enthalpy is the most important factor in the hydrolysis of primary haloalkanes
Out of Cl, Br and I, C-Cl has the highest polarity however it has the longest rate of hydrolysis as it has a higher bond enthalpy
The least polar has the highest rate of hydrolysis because the bond is weaker and easier to break
What happens when a haloalkane is heated under reflux by an aqueous solution containing hydroxide ions?
An alcohol and halogen anion is made
Rate of reaction is faster than using water
What are CFCs?
Chlorofluorocarbons - class of organic compounds that contain chlorine and fluorine atoms
CFCs are gas at room temperature and after use they disperse into the atmosphere
As gases diffuse up layers of the atmosphere, they eventually become exposed to UV radiation in the stratosphere
C-Cl bonds in C2F2Cl2 undergo homolytic fission due to the UV light - C2F2Cl2 –> C2F2Cl’ + Cl’
What do CFCs do to the ozone layer?
The ozone layer is in the stratosphere which is a layer of O3 molecules - protects us from harmful UV radiation and stabilises temperature
The chlorine radicals from CFCs can catalyse the decomposition of ozone to make diatomic oxygen
Cl’ + O3 –> ClO’ + O2
ClO’ + O3 –> Cl’ + O2
(Overall - 2O3 –> 3O2)
Also, nitrogen monoxides can decompose ozone (from lightning, combustion engines)
NO’ + O3 –> NO2’ + O2
NO2’ + O –> NO’ + O2
(Overall - O3 + O –> 2O2)
What quickfit apparatus is needed for distillation?
Round bottomed flask - pour organic mixture with a few anti bumping granules to ensure smooth boiling
Heating mantle/Bunsen burner/Water bath
Thermometer holder
Thermometer
Condenser - connect to water supply
Connector
Collecting flask
What quickfit apparatus is needed for reflux?
Pear shaped flask - pour the reaction mixture with anti bumping granules to ensure smooth boiling
Heating mantle
Condenser
How do you use a separating funnel to remove an organic layer from an aqueous layer?
- put separating funnel in a clamp stand and remove stopper, ensuring tap is closed at the bottom
- pour in the mixture so it is half full, wash out reaction vessel and add this, then putting the stopper back
- take the funnel out of the ring and invert it, open tap to equalise pressure then close tap - repeat until you no longer hear a whistle
- replace funnel in iron ring and give mixture time to separate, remove the stopper, put a beaker under spout and open tap
- collect lower water layer in a beaker, turn the tap off then use a clean beaker to collect organic upper layer and then shake liquid with a drying agent
What is the purpose of drying the organic liquid with a an anhydrous salt?
These absorb water slowly to become hydrated so can be used to dry other chemicals
If drying agent clumps tiger add more
When drying agent remains free moving, the organic product is dry
Use gravity filtration to collect the filtrate which is the dry organic product
What is redistillation?
The purification of a liquid by performing multiple distillations
What is the test used to identify an unsaturated hydrocarbon?
Add a few drops of bromine water to the sample and shake
Positive test - bromine water decolourises
What is the test used to identify a haloalkane?
Add silver nitrate, ethanol and water
White precipitate - chloro-
Cream precipitate - bromo-
Yellow precipitate - iodo-
What is the test used to identify a carbonyl?
Acidified potassium dichromate; Ketones - no change/Aldehyde - turns green
Fehling’s solution; Ketones - no change/Aldehydes - dark red precipitate
Tollens’ reagent; Ketones - no silver mirror/Aldehydes - silver mirror
What is the test used to identify a carboxylic acid?
Universal indicator or pH probe - pH of a weak acid
Reactive metal - effervescence (hydrogen)
Metal carbonate - effervescence (carbon dioxide)
What is the test used to identify an alcohol?
Warm with an equal volume of carboxylic acid and a few drops of sulfuric acid
Positive test - sweet smell of an ester after a short time
