CH6 : Alcohols Flashcards
General formula for alcohols?
CnH2n+1OH
What are primary,secondary and tertiary Alcohols?
Primary - when Carbon atom bonded to -OH group is attached to one other carbon
Secondary - the carbon atom bonded to the -OH group is attached to two other carbon atoms (or alkyl groups)
Tertiary - the carbon atom bonded to the -OH group is attached to three other carbon atoms (or alkyl groups)
Combustion of alcohols ?
Alcohol + oxygen —> CO2 + water
How do alcohols form chloroalkanes ? How can this reaction be used as a test for OH groups?
Reacting alcohols with PCl5 (phosphorus pentachloride) or HCl (chlorination)
Adding PCL5 = phosphoryl chloride (POCL3) / HCl /chloroalkane made
- adding PCl5 to unknown liquid is used as TEST - if HCl steamy fumes present , -OH group is present
Adding HCl - chloroalkane + water made
How can alcohols form bomoalkanes?
Bromination - react with compounds containing Br ions in substitution reaction (KBr)
- OH group replaced by Bromide
Requires catalyst (50% concentrate H2SO4- sulfuric acid)
Forms bromoalkane + water
How do alcohols form iodoalknes ?
React alcohol with phosphorous triiodide (PI3)
- PI3 is made in situ (within reaction mixture) by refluxing alcohol with ‘red phosphorus’ and iodine
Iodination - phosphoric acid (H3PO3) + iodoalkanes
Dehydration of Alcohols into alkenes?
Elimination reaction - water is eliminated
Heat alcohol with concentrated phosphoric acid (acid catalyst)
What oxidising agent is needed for the oxidation of alcohols?
- use oxidising agent (acidified potassium dichromate) - ORANGE
For potassium dichromate(VI) to act as an oxidising agent, it needs to be reduced
- reduction requires hydrogen (H+) ions which are provided by the acidic medium
When alcohols are oxidised the orange dichromate ions (Cr2O72-) are reduced to green Cr3+ ions
Oxidation of primary alcohols ?
Oxidised to form :
ALDEHYDES , which further oxidise into carboxylic acids
- use acidified potassium dichromate
ALDEHYDE - use excess alcohol with oxidising agent - aldehyde has lower BP than alcohol reactant so distilled off as soon as its formed
CARBOXYLIC ACID - alcohol is mixed with excess oxidising agent/heated under reflux - refluxed further
General formula and functional group of aldehydes and ketones?
Test for them?
Functional group - C=O
General formula - CnH2nO
Test: USE BENEDICT’S SOLUTION
if aldehyde : the blue copper ions reduced to brick red precipitate of copper oxide
If ketone : nothing happens
FEHLING’S SOLUTION
- contains copper ions dissolved in NaOH - act as oxidising agent
If aldehyde : aldehyde is oxidised to carboxylic acid
Colour change from BLUE TO RED
If ketone: nothing
Oxidation of sencodary alchols?
Ketones
- alcohol is heated under reflux with oxidising agent
Can’t be further oxidised so doesn’t need to be distilled straight after forming
Oxidation of tertiary alcohols?
Can’t be oxidised - only oxidised by burning them
- doesn’t react with potassium dichromate - stay ORANGE
Difference in structure between aldehydes and ketones?
Aldehyde : have a HYDROGEN and one ALKYL GROUP attached to carbonyl group (C=O)
Ketone: have 2 ALKYL GROUPS attached to carbonyl group
What is the purpose of heating under reflux?
When we want full oxidation
E.g Carboxylic acid from primary alcohol
Ketone from secondary alcohol
Any products of oxidation remain in reaction mixture - products boil off and condense /return to heating flask
What is difference between normal distillation and fractional distillation?
SIMPLE DISTILLATION: separate liquid and soluble solid - heated to BP of liquid
FRACTIONAL DISTILLATION: separate 2 or more liquids that are miscible - heated to BP of liquid with lower BP
How to remove trace of water from organic layer? (Purifying organic layer)
Add anhydrous salt (magnesium sulfate ) or calcium chloride
- drying agent
- keep adding till doesn’t clump tgt
How can you use BP to determine purity of a substance?
Use distillation apparatus to see what temp the product evaporates at
- compare this value with one in the data booklet
(Note : thermometer should bee above level of surface of boiling liquid + measuring temp of saturated vapour)
Also note the pressure as changing pressure can change BP of liquid
- if sample has impurities, value will be higher than one in data booklet
How do anti bumping granules work?
Prevents uncontrolled boiling by distributing heat more evenly
Reason for adding sodium hydrogen carbonate, before adding drying agent, when purifying organic product?
Neutralises HCl in the mixture if was added to remove unreacted substance
- reacts with H+ ions
Prevent build up in pressure
Safety measures taken when distilling?
Add antibumping granules - prevent violent boiling/large bubbles
Thermometer should be opposite entrance of condenser
Leave a gap between condenser and receiver
Application : how to obtain a pure dry sample of cyclohexene from the distillate ?
(Obtaining organic product from reaction mixture and get pure sample )
- Put into separating funnel and add water
(add sodium hydrogen carbonate to organic layer here) - Allow layers to separate in funnel and run off aqueous layer
Run off organic layer into conical flask - Add drying agent to remove traces of water
- Decant organic liquid into distillation flask . Separate cyclohexene from drying agent by REDISTILLING and collect pure cyclohexene
How to test for an acid ?
Add Mg ribbon and hydrogen should be given off
- test for H2 by testing with lit splint and should make squeaky pop
CP 5 : oxidation of ethanol
- add 20cm3 of acidified potassium dichromate to 50cm3 pear shaped flask/cool flask in inched water bath
- set up reflux apparatus
- add anti-bumping granules to pear shaped flask
- measure out 1 cm3 of ethanol/ add dropwise to reflux condenser
- after all ethanol added, remove from ice bath and allow to warm to room temp
- place flask in water bath/heat for 20 mins - ethanol is flammable so naked flame should not be used
- purify product using distillation apparatus
CP 6 : chlorination of 2 methylpropan-2-ol
- Measure 8 cm3 of 2-methylpropan-2-ol in a measuring cylinder /Pour into a separating funnel
- Using a fume hood, add 20 cm3 of concentrated hydrochloric acid to funnel in small portions (make sure to release the pressure by opening the stopper after each addition)
- Leave the separating funnel to stand in the fume hood for 20 minutes and gently shake the separating funnel at 2 minute intervals
- Allow the layers to separate and dispose of the lower aqueous layer by opening the tap
- Add add sodium hydrogen carbonate solution in small portions to the funnel and gently shake the funnel, again, release the pressure at regular intervals- removes acidic impurities
- open the tap once more and dispose of the aqueous layer
- Pour the organic layer into conical flask and add two spatulas of magnesium sulfate - remove water
- Once clear, decant the liquid into distillation apparatus - to separate from drying agent
- Distill the liquid and collect the distillate in the range of 47 - 53oC
Reaction of alcohol with sodium?
2CH3CH2OH + 2Na —> 2CH3CH2O-Na+ + H2
Used to test for alcohols
Observations :
Effervescence
Mixture gets hot
Sodium dissolves
White solid produced
Why is boiling point not the most accurate method of identifying a substance ?
Several substance can have same BP
