6. Organic molecules (ALCOHOLS)
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Give the 3 types of alcohols you can form
Primary - 1 carbon attached to the carbon adjoining the O-H Secondary - 2 carbons attached to the carbon adjoining the O-H Tertiary - 3 carbons attached to the adjoining the O-H
Give the oxidation agent used for oxidising alcohols
Acidified Potassium or Sodium Dichromate
Give the symbol for an oxidising agent
[O]
What are primary alcohols oxidised to
Aldehydes (after immediate distillation) , then carboxylic acids (under reflux) (remember to add H2O in the products for both)
Give the colour change for the oxidation of primary alcohols
Orange to green
How many oxidising agents do you need for reflux of a primary alchohol straight into a carboxylic acids
2 [O]
What are secondary alcohols oxidised to
Ketones
What are tertiary alcohols oxidised into
Nothing, they cannot be oxidised
Describe Fehling’s test in order to distinguish between a aldehyde ( O double bonded to C, single bonded to H) or a ketone (O double bonded to C in the middle of the chain)
1) mix equal volume Fehlings solution and test sample togethor in a test tube 2) if an aldehyde is present a brick red precipitate forms 3) ketones don’t give a positive result
What is the condition required and product of dehydrating an alcohol
Produces an alkene –> The alcohol is heated with hot concentrated phosphoric acid- H3PO4 Elimination reaction Reaction removes a molecule of water Dehydration of primary alchol produces a single alkene Dehydration of secondary alchol produces a mixture of alkenes
Describe the process of distillation
1) heat liquids in a pear shaped flask and lowest boiling point will evaporate first 2) rises out of flask and into a condenser 3) liquid condensed and is collected in a separate flask
Which way does the water come in from in a condenser
Top - out Bottom - in –> this ensures water fills up the condensor so it is cooling as efficiently as possible
Describe how to produce chloroalkanes from alcohols
React the alcohol with PCl5 - HCl and POCl3 are also formed
Describe how to produce bromoalkanes from alcohols
React the alcohol with HBr (formed by reacting 50% conc sulfuric acid with KBr) - also forms H2O
Describe how to produce idoalkanes from alcohols
React alcohol with PI3 (formed by reacting red phosphorous with Iodine, I2) - also forms H3PO3
Using the apparatus for distillation instead of reflux is not an efficient way to produce ethanoic acid from ethanol. Explain why.
Because ethanol will be oxidised into ethanal as that has a lower boiling point than ethanoic acid
Explain the purpose of anti bumping crystals in distillation or reflux
Promotes smooth boiling by providing a rough surface for smaller bubbles to form
Describe important features of a diagram showing reflux or distillation
Have to show the separate apparatus being joined Distillation: Still head Bung Thermometer - Thermometer in distillation must be opposite the entrance of the lieburg condenser during distillation as we are measuring the temperature of the gas Lieburg condenser Collecting vessel Reflux: Lieburg condenser Should not be a bung on reflux as pressure builds up, no thermometer either
What is reflux
continuous evaporating and condensation
Propanal can be produced from the oxidation of propan‑1‑ol. A student set up reflux apparatus for this. Explain why the use of this apparatus would give a very low yield of propanal.
Reflux apparatus is used which involves continuous evaporation and condensing. Propanol would be oxidised to propanal which evaporates then condenses back into the pear shaped flask and is in contact with oxidising agent again, so propanal is further oxidised to propanoic acid
Why is it possible to form multiple alkenes when dehydrating an alchohol
When you dehydrate an alcohol, you remove the -OH group, and a hydrogen atom from the next carbon atom in the chain. With molecules like butan-2-ol, there are two possibilities when that happens. The products are but-1-ene, CH2=CHCH2CH3, and but-2-ene, CH3CH=CHCH3. These can also exhibit E/Z isomers
• Add acidified potassium dichromate (oxidising agent) into a pear shaped flask. Cool using ice water. • Add anti bumping crystals (prevents the formation of large bubbles that causes violent bubbling). Set up apparatus for reflux. • Using a pipette slowly add the ethanol drop by drop to the pear shaped flask and stir (form complete mixture) • When the ethanol has been added, cool the flask to room temperature • Heat using Bunsen burner for 30 mins. • Distil the product using distillation apparatus to purify the carboxyllic acid product Why is the mixture cooled and the ethanol added slowly and drop in this method? What is the purpose of heating the flask? How could you prove you made a carboxylic acid?
Because the reaction is very exothermic To evaporate all the ethanol Add a carbonate —> effervescence is observed as CO2 gas (CO2 gas is confirmed by bubbling through limewater which turns cloudy) Confirmation of carboxylic acid Add a metal : 2CH3COOH + Mg—> (CH3COO)2Mg + H2—> hydrogen gas (H2 gas is confirmed by putting a lit splint in test tube and heading a squeaky pop) Add Fehling’s solution (an oxidising agent) —> no further oxidation - Cu2+ (blue solution) is not reduced to Cu+ (brick red), so solution stays blue pH meter - pH will be under 7
Describe the steps to produce 2-methyl, 2-chloropropane from 2-methylpropane-2-ol (Note, this is different from adding PCl5, as that is how you make chloroalkanes from primary and secondary alcohols. In this practical, we use tertiary alcohols)
• Pour concentrated HCL and 2-methylpropane-2-ol in a conical flask. Swirl gently. • Place bung on conical flask and swirl again. Remove the bung (to release pressure and volatile compounds - HCL fumes are produced. This ensures flask does not break or explode) Repeat this regularly over 20 minutes to allow reaction to occur - its slow. 2 layers will form, with the top layer being the crude layer. Crude layer mainly has product—> london forces, bottom aqueous layer has CL, water, alchohol, ions hydrogen bonds. Crude layer is less dense than water • Add some anhydrous calcium chloride and shake until dissolved (will absorb unreacted alchohol so goes into bottom aqueous layer and not top organic layer. This helps with better separation as you are increasing the density of the aqueous layer. Alcohols can form both hydrogen bonds and London forces, but we need to ensure alcohol is in the aquous layer not organic, as its left over reactant that we don’t need ) • Transfer contents of conical flask to separating funnel and allow it to settle into 2 layers (no bung) • Discard the lower aqueous layer • Add a sodium hydrogen carbonate solution to the organic layer in the separating funnel (removes unreacted HCL. Sodium hydrogen carbonate is also a weaker base than say sodium hydroxide which would cause hydrolysis of the halogenoalkane). Invert the separating funnel and open the tap to release pressure - NaHCO₃ + HCl → NaCl + H₂O + CO₂ - we need to release CO2 gas • Remove and discard aqueous layer again. • Add anhydrous sodium sulphate (drying agent which removes water.). Leave until mixture goes from cloudy to clear • Decant (pour) the liquid (or filter) into a pear shaped flask. Set up for distillation to purify the product (removes volatile compounds) • Collect the fraction boiling between 50 and 52 degrees Celsius