6.4 [Alcohols] Flashcards
What are alcohols?
Alcohols are a homologues series of compounds with general formula CnH2n+1OH replace a hydrogen atom in a hydrocarbon with a hydroxyl group
What are the 3 reactions that alcohols undergo?
Combustion
Conversion to halogenoalkanes
Dehydration to alkenes
How do alcohols undergo combustion?
Alcohols are used as biofuels. If combustion is complete then the products are carbon dioxide and water.
C2H5OH > CO2 + H20
How are alcohols converted into halogenoalkanes?
These involve replacing the hydroxyl group in an alcohol molecule with a halogen atom. Known as halogenation. Different method other than addition
How does Chlorination occur converting alcohols into halogenoalkanes?
Chlorination is carried out using phosphorus (V) Chloride (white solid). Reaction is very vigerous at room temperature. Alcohol and phosphorus chloride reaction mixture does not need heating. 2 Inorganic products, phosphorus oxychloride and hydrogen chloride.
POCl3 and HCl.
How is Chlorination done with tertiary alcohols?
Done in a very different way. Alcohol needs to be mixed by (shaking) with concentrated HCl at room temp.
How is Bromination done with alcohols?
Bromination is carried out using a mixture of potassium bromide and 50% conc H2SO4 (HBr). Reaction mixture is warmed with the alcohol. Inorganic products formed at potassium bromide and potassium hydrogensulfate.
How is iodination carried out with alcohols?
Iodination is carried out using a mixture of red phosphorus and iodine (PI3). The reaction mixture including the alcohol is heated under reflux. Bette r to write 2 equations. Inorganic products are phosphoric acid. (H3PO3)
How is dehydration of alcohols to alkenes done?
Done by heating the alcohol with concentrated phosphoric acid (H3PO3). Reaction is similar to elimination reaction for halogenoalkane. OH group and a hydrogen atom fromm an adjacent carbon atom being removed and a C=C double bond formed in the carbon chain.
Water is the only inorganic product with Butan2-ol it forms Butan-1-ene and 2 since but-2ene exists as a pair of E-Z isomers 3 products.
How does oxidation occur with alcohols?
oxidation is the loss of hydrogen from an alcohol molecule. Oxidation only affects one carbon atom. Atoms removed from an alcohol molecule are the hydrogen of the OH group and a hydrogen atom from the carbon atom joined to the OH group.
Organic product contains a C=O group known as a carbonyl group.
Only primary and secondary alcohols can be oxidised not tertiary in this way because need to have a hydrogen atom on the C of the C-OH group.
What is one of the products of oxidation?
Ketones - When a secondary alcohol is oxidised the organic product belongs to a homologous series called ketones. Formula RCOR 2 alkyl group can be same or different.
What is another product of oxidation?
Aldehydes and carboxylic acids
When a primary alcohol is oxidised, the organic products belong to a homologous series called to aldehydes Formula RCHO.
Complication with Aldehydes that is not with Ketones Aldehydes are more easily oxidised than alcohols so when a primary alcohol is oxidised, the aldehyde formed may be oxidised further. process may gain of an oxygen atom not loss of hydrogen. Oxygen atom gained goes between the C and H of CHO group. organic product belongs to homologous series called carboxylic acids. formula RCOOH
How can the reactions be summarised?
Secondary alcohol > Ketone
Primary alcohol > aldehyde > carboxylic acid
What are the conditions for these oxidation reactions?
Usual reagent for these oxidation reactions is a mixture of potassium dichromate (IV) and sulfuric acid.
Unlike with the inorganic reagents equations involved with this are written as [O] symbol represents the oxygen in the oxidising agent.. Whenever this oxidising agent is used there is a colour change from orange to green.
For example Propan-1-ol to propanal:
CH3CH2CH2OH + [O] > CH3CH2CHO + H20
Propanal to Propanoic acid:
CH3CH2CHO + [O] > CH3CH2COOH
Propan-2-ol to propanone:
CH3CH(OH)CH3 + [O] > CH3COCH3 + H20
What are the different practical techniques needed for oxidation of alcohols or aldehydes?
heating under reflux
distillation with addition
How is a carboxylic acid obtained from a aldehyde when heating under reflux?
When the oxidation is intended to be completed (to obtain a ketone or carboxylic acid) the apparatus used is heating under reflux .
In this apparatus, the products of oxidation stay in the reaction mixture, because if they do boil off, they condense in the vertical condenser and return to the heating flask
A separating funnel is used to separate liquids with different densities. Mixture is added to the flask and liquids separate. denser liquids in 2 separate flasks.
How is the distillation with addition done?
When the oxidation is intended to be incomplete ( to obtain a aldehyde and not a carboxylic acid) the apparatus used is distillation with addition. Only the oxidising agent is heated and the alcohol is slowly added to the oxidising agent. When the aldehyde is formed it immediately distils off, collected to the receiver.
Before purification what could organic compounds be contaminated with?
Unreacted starting materials
Other organic products
The inorganic reagent used or the inorganic product formed from them
water
What are the techniques needed for purifying an organic liquid?
Simple distillation
Fractional distillation
Solvent extraction
drying
What are the different apparatus needed for the purifying of organic liquids
Pear shaped flask
Receiver round bottom flask
Still head
Condenser
Receiver adaptor
Thermometer + adaptor
Separating funnel
fractionating column
Using all these apparatus can create a wide variety of experimental set ups
Heating is needed in many experiments can do this using a Bunsen burner often an electric mantle or a hot water bath is preferred for saftey reasons
What occurs in simple distillation?
Distillation of an impure liquid involves heating it in a flask connected to a condenser
The liquid with the lowest boiling temperature evaporates or boils of first and passes into the condenser first this means it can be collected in the receiver separately from any other liquid that evaporates later.
The purpose of the thermometer is to monitor the temperature of the vapour as it passes over the condenser. If the temperature remains steady; this is an indication that one compound is distilling over. If, after a while, the temperature begins to rise, this indicates a different compound is distilling over.
What are the advantages and disadvantages of simple distillation?
Advantages: Rather than fractional distillation is that it is easier and quicker
Disadvantages: It does not separate the liquids as well as fractional distillation. Should only be used if the boiling temperatures of the liquid being purified is very different from the other liquid in the mixture, ideal difference more than 25
What occurs in fractional distillation?
Fractional distillation uses the same apparatus as simpl++e distillation but with a fractionating column between the heating flask and the still head.
The column is usually filled with glass beads, which acts as a surface on which the vapour leaving the column can condense, and then be evaporated again as more hot vapour passes through the column.
Effectively the vapour undergoes several repeated distillations as it passes up the column providing better separation.
takes longer than simple distillation and is best used when the different in boiling temp is small and a lot of compounds need to be separated
What occurs in solvent extraction?
Method involves using a solvent to remove the desired organic products from the other substances in a reaction mixture. There are several solvents that can be used but main have these features
Solvent added should be immiscible with the solvent containing the desired product
The desired organic product should be much more soluble in the solvent added than in the reaction mixture
Place the reaction mixture in a separating funnel, and then add the chosen solvent it should form a separate layer.
Place the stopper into the neck of the funnel and gently shake the contents of the funnel
Remove the stopper and open the the tap and allow the layer to drain into the flask. Pour the upper layer into a separate flask.
If a suitable solvent is used and the method is followed correctly most of the desired organic product will have moved into the added solvent. Better to use solvent in small proportions than in single large volume as it is more efficient, using more portions of solvent with same total volume removes desired organic product.
Desired organic product has been removed from the reaction mixture but now is mixed with the added solvent so simple distillation has to be used to separate the desired organic product from solvent
What occurs in the process of drying?
A liquid organic product may be partially or completely dissolved in water, so water may be an impurity that needs to be removed by a drying agent. One important feature of drying agents is that it does not react with the organic liquid.
Most common drying agents are anhydrous metal salts, often calcium sulfate, magnesium sulfate and sodium. What these compounds have in common is they form hydrated salts. Come into contact with water in an organic liquid they absorb the water as water of crystallisation. Anhydrous calcium chloride can also be used for some organic compounds.
What process occurs with drying?
The drying agent is added to the organic liquid and the mixture is swirled or shaken then left for a period of time.
Before use a drying agent is powdery but after absorbing water it looks more crystalline
If a bit more drying agent is added then it remained powdery this is an indication that the liquid is dry
The drying agent is removed by decantation pr by filtration.
What are the tests for purity?
For liquids simple way measure its boiling temperatures impurities raise boiling temperatures.
You can compare the boiling temperature of the pure organic compound with the organic compound you have to make the decision about how pure it is.
Apparatus used depends on the volume of liquid available whether it is toxic or flammable. The apparatus for simple distillation can be used.
Test may not be conclusive as thermometer may not read that high or low.
What is the test for Aldehydes?
Aldehydes are tested using Fehling’s solution
A few drops of Fehling’s solution are added and the test tube is gently warmed. If an aldehyde is present a red precipitate will form.
If no aldehyde is present solution remains blue
Ketones will not give a positive result when added to Fehling’s solution
What is the test for oxidation of alcohols?
Potassium dichromate is used in oxidation of alcohols as the oxidising agent. it is reduced as the alcohol is oxidised a colour change from orange the green is observed when the alcohol is oxidised with potassium dichromate.
What does the term heating under reflux mean?
Heating under reflux involves heating a reaction mixture with a condenser fitted vertically.
