Chapter 28 Further Practical Techniques

Question 1

What equipment do you need for filtration under reduced pressure?

Answer 1

Buchner Flask and Funnel
Pressure tubing
Vacuum pump (tap)
Filter Paper

Question 2

How does solubility change with temperature?

Answer 2

High temperatures allow solutes to dissolve more easily

Question 3

How do impurities affect melting point?

Answer 3

They lower the melting point of the substance
The compound will melt over a range

Question 4

Why does recrystallisation work?

Answer 4

Both the impurities and compound dissolve in the hot solvent
As the solution cools, the solubility of the compound decreases until it becomes solid
The impurities will stay dissolved in the cool solution, leaving behind purer crystals when filtered.

Question 5

Why do you add the minimum amount of solvent when recrystallising?

Answer 5

To ensure the solution is saturated thus allowing crystals to form (too dilute and they cannot form)

Question 6

Why do you need to cool to room temperature then with ice?

Answer 6

So the yield does not decrease, decreases when warm
Crystals will form too quickly if straight to ice
Impurities will still remain trapped within the crystals, and small crystals will form

Question 7

Why do you wash the crystals with cold water after filtration under reduced pressure?

Answer 7

This removes any soluble impurities that may remain

Question 8

What should you do if crystals do not begin to form?

Answer 8

Scratch the surface with a glass rod to start the process

Question 9

Why should crystals be compressed in the funnel with filtration under reduced pressure?

Answer 9

Air passes through the sample and not just around it, so better drying

Question 10

How can you check the purity of an organic solid?

Answer 10

Melting point- if pure, distinct sharp melting point at data value, impure melts over a lower range
Spectroscopy- Run NMR/IR, if pure, peaks will match, no other peaks
TLC- Check the Rf value, if pure, will be similar to the data book

Question 11

Diagram for filtration under reduced pressure

Question 12

How can you increase the chain length of an aromatic compound?

Answer 12

Alkylation - react a haloalkane with a halogen carrier
Acylation- react an acyl chloride with a halogen carrier

Question 13

How can you increase the chain length of a carbonyl?

Answer 13

React with KCN/H2SO4, forms HCN in situ
Forms a Cyanohydrin/Hydroxynitrile via nucleophilic addition
For acyl chlorides addition elimination

Question 14

How do you increase the chain length of haloalkanes?

Answer 14

NaCN in ethanol solvent
Forms a nitrile and NaX
Nucleophilic substitution

Question 15

How do you reduce nitriles with an equation?

Answer 15

React with H2 in the presence of a nickel catalyst
R- C///N + 2H2 —/ R-CH2NH2

Or reduction with LiAlH4

Question 16

How do you oxidise nitriles?

Answer 16

React with aqueous Hydrochloric acid, and heat

R- C///N + 2H2O + HCl —/ R-COOH + NH4Cl

Question 17

How do you form Grignards reagent? Why is this not used often?

Answer 17

Mg + Haloalkane —/ R-MgBr
Very reactive, needs a nitrogen environment

Question 18

How can you add a COOH to an alkyl group?

Answer 18

First add a Br via UV and Br2
Then in situ, react with Mg and CO2
Grignards reagent formed, nucleophilic addition to the O=C=O
New step, Protonate with an acid

Question 19

What is the whole process of purifying an organic solid?

Answer 19

If hot, allow the solid to cool
Filter under reduced pressure and let dry
Heat up the chosen solvent
Add the minimum amount of hot solvent to the solid to allow it to dissolve fully
Allow to cool to room temperature, then using an ice bath until crystals form
Filter under reduced pressure again
Run with cool solvent
Allow to air dry
Check purity by comparing melting point to a data base or TLC and recrystallise if necessary

Question 20

What are specific details of recrystallisation?

Answer 20

Heat up the solvent
Add the minimum amount to the solid to allow it to fully dissolve
Cool to room temperature then with an ice bath. If crystals do not reform, scratch with a glass rod.
Filter under reduced pressure.
Wash with cold solvent
Allow to dry

Question 21

How do you add the diazonium ion to benzene? Why is it useful when not intended for the final product?

Answer 21

Nitration with HNO3/H2SO4
Reduction with SN/HCl then NaOH
React with HCl/NaNO2 to form diazonium salt
NH2 a good ortho/para director, and can be removed at the end

Question 22

How do you replace the diazonium ion?

Answer 22

React with CuX to replace with a halogen
React with H3PO2 for replace with hydrogen
React with H2SO4/H2O to replace with O-H

Question 23

How can running TLC/ spectroscopy help determine purity?

Answer 23

Run TLC, compare the Rf value of the sample to the database, pure will have a very similar Rf. Also no other spots e.g starting material or catalyst
Run NMR/IR, compare to a spectral database, and the pure sample will contain the same peaks as the database

Question 24

How do you check the melting point of a substance, what are the steps?

Answer 24

Take a small amount of solid and place into a capillary tube
Tap/flick the capillary tube so the solid falls to the bottom
Place into the melting point apparatus and heat, observe the temperature when the solid melts
Take out the machine to allow the liquid to solidify, repeat, taking care close to the recorded melting points