Topic 18 - Organic Chemistry 3

Question 1

What are arenes?

Answer 1

compounds containing a benzene ring

eg. phenol, nitrobenzene, etc

Question 2

What is the molecular formula of benzene?

Answer 2

C6H6

Question 3

What is the structure of benzene according to the Kekulé model?

Answer 3

6 carbon atoms in a ring with alternating single and double carbon-carbon bonds

Question 4

What were the issues with the Kekulé model of benzene?

Answer 4

• x-ray diffraction shows that benzene only has 1 carbon-carbon bond length but with the Kekulé model, you would expect two
• IR spectrum has fewer peaks than expected for the Kekulé model
• benzene does not undego electrophilic addition reactions, which you would expect it to if it had C=C bonds
• enthalpy change of hydrogenation is much lower than expected for the Kekulé model (benzene is more stable)

Question 5

What is the structure of benzene according to the delocalised model?

Answer 5

• 6 carbon atoms in a ring where each carbon atom forms a sigma bond with its hydrogen and each neighbouring carbon and the remaining p-orbital forms a ring of delocalised electrons
• p orbitals overlap above and below the molecule, forming a delocalised pi-electron system

Question 6

benzene + oxygen →

Answer 6

carbon dioxide + water

Question 7

benzene + bromine →

AlCl3/FeBr3 catalyst present

Answer 7

bromobenzene

Question 8

What are the conditions for the halogenation of benzene?

Answer 8

• dark

- AlCl3/FeBr3 catalyst (halogen carrier)

Question 9

benzene + nitric acid →

sulfuric acid catalyst present

Answer 9

nitrobenzene + water

Question 10

What is in a nitrating mixture?

Answer 10

conc nitric acid and conc sulfuric acid

Question 11

What are the conditions for the nitration of benzene?

Answer 11

• conc sulfuric acid catalyst

- 55°C

Question 12

Why should the nitration of benzene be at 55°C?

Answer 12

to avoid multiple substitution of benzene

Question 13

What is the purpose of using a nitrating mixture for the nitration of benzene?

Answer 13

it produces a stronger electrophile

-nitronium ion

Question 14

What reaction happens in the nitrating mixture?

Answer 14

HNO3 + H2SO4 → NO2+ + HSO4- + 2H2O

Question 15

What reaction happens to reform the catalyst after the nitration of benzene?

Answer 15

H+ + HSO4- → H2SO4

Question 16

benzene + halogenoalkane →

AlCl3 catalyst

Answer 16

alkyl benzene + hydrogen halide

Question 17

What conditions are needed for Friedel-Crafts alkylation (benzene and halogenoalkane)?

Answer 17

• aluminium chloride catalyst

- heat under reflux

Question 18

benzene + acyl chloride →

Answer 18

phenyl ketone + hydrogen chloride

Question 19

What conditions are needed for Friedel-Crafts acylation (benzene and acylchloride)?

Answer 19

• aluminium chloride catalyst

- heat under reflux

Question 20

What is the molecular formula of phenol?

Answer 20

C6H6OH

Question 21

What are the properties of phenol?

Answer 21

• slightly acidic
• not soluble
• solid at room temp
• more reactive than benzene

Question 22

Why is phenol more reactive than benzene?

Answer 22

-phenol’s oxygen activates the benzene ring
-oxygen’s lone pair of e- are incorporated into the delocalised pi-electron system
-this increases electron density in the ring
-it can attract electrophiles more easily
(catalysts/halogen carriers not needed)

Question 23

phenol + bromine water →

Answer 23

2,4,6 tribromophenol + hydrogen bromide

Question 24

What are the properties of amines?

Answer 24

• fishy smell
• generally soluble (can H-bond with water)
• have higher mp/bp than respective hydrocarbons
• have basic nature (nitrogen’s lone pair of e- can accept H+)

Question 25

How does ammonia, primary aliphatic and primary aromatic amine's basicity compare?

Answer 25

primary aliphatic amines > ammonia > primary aromatic amines

Question 26

Why are primary aliphatic amines more basic than ammonia and primary aromatic amines?

Answer 26

- alkyl groups are e- pushing groups - this increases the nitrogen's electron density - nitrogen can accept H+ more easily

Question 27

Why are primary aromatic amines less basic than primary aliphatic amines and ammonia?

Answer 27

- NH2 is an activating group - nitrogen's lone pair of e- are incorporated into the delocalised pi-electron system - nitrogen's lone pair of e- are less accessible to accept H+

Question 28

amine + water →

Answer 28

ammonium salt + hydroxide ion | alkali soln

Question 29

amine + acid →

Answer 29

salt

Question 30

amine + acyl chloride →

Answer 30

N-substituted amide + hydrogen chloride

Question 31

primary amine + halogenoalkane →

Answer 31

secondary amine + ammonium halide | secondary amine can go on to react with halogenoalkane to form tertiary amine

Question 32

amine + copper(II) ions →

Answer 32

complex ions [Cu(RNH2)4)]4+ (aq) royal blue soln

Question 33

How can primary aliphatic amines be prepared?

Answer 33

- halogenoalkane and excess ammonia + reflux | - reducing nitriles using lithium aluminium hydride in dry ether or hydrogen gas with a nickel catalyst

Question 34

How can primary aromatic amines be produced?

Answer 34

reducing nitrobenzene using tin and conc hydrochloric acid + reflux

Question 35

How are polyamides produced?

Answer 35

condensation polymerisation | dicarboxylic acid and diamine

Question 36

What is the general structure of an amino acid?

Answer 36

- amine group (NH2) - carboxyl group (COOH) - variable region

Question 37

Why are amino acids optically active?

Answer 37

they have a chiral centre | except glycine

Question 38

What is a zwitterion?

Answer 38

when an amino acid's amine group has been protonated and carboxy group has been deprotonated at the same time (at specific pH) - neutral (charges cancel) - solid at room temp (can form giant ionic lattices due to charges)

Question 39

How can amino acids be separated?

Answer 39

in chromatography

Question 40

How are dipeptides produced?

Answer 40

condensation reaction between two amino acids -peptide bond formed -water produced (if dipeptides are joined, polypeptide if produced)

Question 41

How can a carbon chain length be increased?

Answer 41

- using cyanide ions (can add one carbon) | - using Grignard reagents (can add multiple carbons)

Question 42

How are Grignard reagents prepared?

Answer 42

magnesium and halogenoalkane | in dry ether

Question 43

How are Grignard reagents used to produce an alcohol?

Answer 43

Grignard reagent is reacted with a carbonyl compound - aldehyde for primary/secondary alcohol - ketone for tertiary alcohol

Question 44

How are Grignard reagents used to produce a carboxylic acid?

Answer 44

Grignard reagent is reacted with carbon dioxide

Question 45

How are Grignard reagents used to produce an alkane?

Answer 45

Grignard reagent is reacted with water

Question 46

How can a carbon chain length be decreased?

Answer 46

using triodomethane reactions (iodine in alkali with strong acid)

Question 47

How is recrystallisation carried out?

Answer 47

- compound is dissolved in minimum volume of hot solvent - it is then filtered hot and allowed to cool (in ice bath) - it is then filtered and washed with a small amount of cold solvent - crystals are dried between filter papers/using a Buchner funnel

Question 48

What is the purpose of recrystallisation?

Answer 48

to puridy solids

Question 49

How is drying carried out?

Answer 49

- using a Buchner funnel and Buchner flask (suction filtration) - patting with filter paper

Question 50

How is melting/boiling point determination carried out?

Answer 50

- sample is loaded into a capillary tube and placed in apparatus - temp is slowly increased and sample is observed - mp range is from when liquid begins to appear to when no solid is left - mp can be compared to known values

Question 51

What is the purpose of refluxing?

Answer 51

allows continuous heating so all of the reactants react

Question 52

What is the purpose of distillation?

Answer 52

separates liquids based on boiling points

Question 53

How is solvent extraction carried out?

Answer 53

- impure product is dissolved in water/organic solvent and shaken - two layers (organic and aqueous layers) form in separating funnel and they can be run off

Question 54

How is purification by washing carried out?

Answer 54

impurity in organic product can be removed by adding another liquid which impurities react with