CH6 : Alcohols

Question 1

General formula for alcohols?

Answer 1

CnH2n+1OH

Question 2

What are primary,secondary and tertiary Alcohols?

Answer 2

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)

Question 3

Combustion of alcohols ?

Answer 3

Alcohol + oxygen —> CO2 + water

Question 4

How do alcohols form chloroalkanes ? How can this reaction be used as a test for OH groups?

Answer 4

Reacting alcohols with PCl5 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

Question 5

How can alcohols form bomoalkanes?

Answer 5

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

Question 6

How do alcohols form iodoalknes ?

Answer 6

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

Question 7

Dehydration of Alcohols into alkenes?

Answer 7

Elimination reaction - water is eliminated

Heat alcohol with concentrated phosphoric acid (acid catalyst)

Question 8

What oxidising agent is needed for the oxidation of alcohols?

Answer 8

• 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

Question 9

Oxidation of primary alcohols ?

Answer 9

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

Question 10

General formula and functional group of aldehydes and ketones?
Test for them?

Answer 10

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

Question 11

Oxidation of sencodary alchols?

Answer 11

Ketones
- alcohol is heated under reflux with oxidising agent
Can’t be further oxidised so doesn’t need to be distilled straight after forming

Question 12

Oxidation of tertiary alcohols?

Answer 12

Can’t be oxidised - only oxidised by burning them
- doesn’t react with potassium dichromate - stay ORANGE

Question 13

Difference in structure between aldehydes and ketones?

Answer 13

Aldehyde : have a HYDROGEN and one ALKYL GROUP attached to carbonyl group (C=O)

Ketone: have 2 ALKYL GROUPS attached to carbonyl group

Question 14

What is the purpose of heating under reflux?

Answer 14

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

Question 15

What is difference between normal distillation and fractional distillation?

Answer 15

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

Question 16

How to remove a desired organic product from a reaction mixture? (Separation )

Answer 16

• pour mixture into separating funnel and ADD WATER
• shake funnel/allow to settle - organic layer and Aqueous layer (contains impurities) are immiscible - separate into 2 distinct layers
  The layer with higher density will be the lower layer
• open tap and run off each layer into separate containers

Question 17

How to remove trace of water from organic layer? (Purifying organic layer)

Answer 17

Add anhydrous salt (magnesium sulfate ) or calcium chloride
- drying agent
- keep adding till doesn’t clump tgt

Question 18

How can you use BP to determine purity of a substance?

Answer 18

Use distillation apparatus to see what temp the product evaporates at
- compare this value with one in the data booklet

• if sample has impurities, value will be higher than one in data booklet

Question 19

How do anti bumping granules work?

Answer 19

Prevents uncontrolled boiling by distributing heat more evenly

Question 20

Reason for adding sodium hydrogen carbonate, before adding drying agent, when purifying organic product?

Answer 20

Neutralises HCl in the mixture if was added to remove unreacted substance
- reacts with H+ ions
Prevent build up in pressure

Question 21

Safety measures taken when distilling?

Answer 21

Add antibumping granules - prevent violent boiling/large bubbles
Thermometer should be opposite entrance of condenser
Leave a gap between condenser and receiver

Question 22

Application : how to obtain a pure dry sample of cyclohexene from the distillate ?
(Obtaining organic product from reaction mixture and get pure sample )

Answer 22

1. Put into separating funnel and add water
2. Run off lower aqueous layer as higher density
   (May need to add sodium hydrogen carbonate to organic layer here)
3. Add drying agent to remove traces of water
4. Separate cyclohexene from drying agent by REDISTILLING and collect pure cyclohexene

Question 23

How to test for an acid ?

Answer 23

Add Mg ribbon and hydrogen should be given off
- test for H2 by testing with lit splint and should make squeaky pop

Question 24

CP 5 : oxidation of ethanol

Answer 24

• 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

Question 25

CP 6 : chlorination of 2 methylpropan-2-ol

Answer 25

• 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 organic compound from oxidising agent
• Distill the liquid and collect the distillate in the range of 47 - 53oC