Topic 18: Organic Chemistry III

Question 1

What is the formula of benzene?

Answer 1

• C₆H₆

Question 2

Describe the Kekulé model and why it is not completely accurate

Answer 2

• it was proposed that benzene was made of a planar ring of carbon atoms with alternating single and double bonds between them (basically cyclohex-1, 3, 5-triene)
• each carbon also bonded to one H atom
• we would expect benzene to have three bonds with the length of a C-C bond (154pm) and three with C=C bonod length (134pm)
• but X-ray diffraction show that all benzene carbon-carbon bonds have the same length of 140pm

Question 3

Describe the delocalised model

Answer 3

• each carbon atom forms three σ-bonds (one to H atom, one to each neighbouring carbon atom)
• these bonds form due to head-on overlap of their atomic orbitals
• each carbon atom has one remaining p-orbital, containing one elctron, which sticks out above and below the plane of the ring
• these p-orbitals one each of the carbon atoms overlap sideway to form a ring of π-bonds that are delocalised around the carbon ring
• the delocalised π-bonds are made up of two ring-shaped clouds of electrons, one above, one below
• all bonds are the same, so have the same length

Question 4

How does the enthalpy changes of hydrogenation of benzene give evidence of the delocalised structure?

Answer 4

• when an alkene reacts with H gas, two H atoms add across the double bonds
• cyclohexene has one double bone and when hydrogenated, the enthalpy change is -120kJ/mol
• so you’d expect benzene to be -360kJ/mol using Kekule’s structure
• but experimentally, hydrogenation of benzene is -208kJ/mol, far less exothermic
• this means more energy is needed to break bonds in benzene because more energy had to be put in to break the bonds and than the energy released when bonds are made
• the extra stability to though to be due to the delocalised ring of electrons

Question 5

Does benzene undergo addition reactions under room temp like other alkenes?

Answer 5

• alkenes react easily with bromine water, decolourising it
• but benzene needs to be hot and have UV light for it to occur
• this is due to the delocalised π-bonds in benzene
• they spread out the negative charge and made the benzene ring very stable
• this supports the delocalised model
• furthermore, alkenes strongly attracts electrophiles due to the π-bond in C=C double bond being an area of localised high electron density
• in benzene, this attraction is reduced to the negative charge being spread out

Question 6

How does benzene burn?

Answer 6

• you get a very smoky flame
• there is too little oxygen to combust the benzene completely

Question 7

What happens when arenes undergo electrophilic substitutions?

Answer 7

1. two of the delocalised electrons are donated to the electrophile, forming a new covalent bond
2. • very unstable intermediate is formed
     - C-H bond breaks heterolytically
     - two electrons are returned to delocalised ring
3. H+ ion is formed as a byproduct

Question 8

Why is a halogen carrier needed for electrophilic substitution reactions?

Answer 8

• the delocalised π-bonds in benzene means that charge density is spread out across the ring, requiring a strong positive charge to be able to attack the benzene ring
• but most compounds aren’t polarised enough so a hydrogen carrier is needed
• a halogen carrier accepts a lone pair of electrons from a halogen atom on an electrophle
• as a lone pair of electrons in pulled away, the polarisation in the molecule increases, making the electrophile stronger

Question 9

Describe halogenation of benzene

Answer 9

Question 10

Describe the nitration of benzene to create nitrobenzene

Answer 10

Question 11

Describe Friedel-Crafts Alkylation

Answer 11

• uses halogenoalkanes
• also uses other electrophiles containing AlCl₃ to create alcohols

Question 12

Describe Friedel-Crafts Acylation

Answer 12

• substitutes an acyl group for an H atom on benzene
• reflux benzene with an acyl chloride to produce phenylketones
• reactants are heated under reflux in a non-aqueous solvent

Question 13

Describe the addition reaction of benzene of hydrogen

Answer 13

• Benzene + 3H₂ → cyclohexane
• raney nickel catalyst
• 200C

Question 14

Reaction of benzene and acid anhydride

Answer 14

Question 15

What is the formula of phenol

Answer 15

• C₆H₅OH

Question 16

What is the structure of phenol?

Answer 16

• one of the lone pairs of electrons in the p-orbital of the oxygen atom overlaps with the delocalised π-bonds iin the benzene ring
• so the lone pair of electrons from the oxygen aotm is partially delocalised
• high electron density in the ring

Question 17

What are the propeorties of phenols?

Answer 17

• at RT, it is a pink crystalline solid
• slightly soluble in water
• a weak acid
• more reactive than benzene

Question 18

Show the mechanism for the substitution with halogens in phenols

Answer 18

• substituent always goes in the 2, 4, 6 positions

Question 19

Show the reaction of phenols reacting with nitric acid

Answer 19

Question 20

What are amines?

Answer 20

• a nitrogen compound where one or more of the hydrogens in NH3 have been replaced by a carbon chin (aliphatic) or a carbon ring (aromatic)
• primary e.g. methylamine
• secondary e.g. diimethylamine
• tertiary e.g. trimethylamine

Question 21

What are the properties of amines?

Answer 21

• short chain amines, such as methylamine and ethylamine are gases at room temperature
• smell like ammonia
• dissolve easily in water because they can H-bond

Question 22

What are the two ways aliphatic amines can be made?

Answer 22

• by nucleophilic substitution of a halogenoalkane
• reductiion of a nitrile

Question 23

Describe the preparation of an aliphatic amine using nucleophilic substitution of a halogenoalkane

Answer 23

• problem is you get a mixture of primary, secondary and tertiary amines
• major product is primary amine if ammonia is in excess
• major product is tertiary amine if halogenoalkane is in excess

Question 24

Describe the preparation of aliphatic amiines by reducing a nitrile

Answer 24

1. heat a mixture of a nitro compound, tin metal and concentrated HCl under reflux to make a salt
2. Add NaOH
   - too expensive for industrial use because LiAlH₄ is too expensive
   - in industry, nitriles are reduced using H gas with a metal catalyst e.g. nickel
   - produces a single product, primary amine

Question 25

Describw how to make aromatic amines

Answer 25

1. Nitration to make a nitro compound
2. Reduction of the aromatic nitro-compound by boiling under redlux with tin and concentrated HCl
3. Free amine can be liberated from solution by adding NaOH solution
4. It is then separated by steam distillation

Question 26

Why do amines act as weak bases?

Answer 26

• they accept protons because there is a lone pair of electrons on the nitrogen atom that can form a dative covalent bond with an H⁺ ion

Question 27

Describe the solubility of amines

Answer 27

• small amines are soluble in water as they can form H bonds with the water molecules
• the bigger the amine, the greater the London forces between amine molecules and the more energy it takes to overcome the London forces
• larger carbon chains in larger amines can also disrupt H bonding in water
• large amines are less soluble in water than smaller ones
• when they dissolve, amines form alkaline solutions
• some amine molecules may take a hydorgen ion to form alkyl ammonium ions and hydroxide ions

Question 28

Describe amines reacting with copper(II) ions

Answer 28

• in copper(II) sulfate solution, the Cu²⁺ ions form [Cu(H₂O)₆]²⁺ complexes with water
• this solution is blue
• if you add a small amount of butylamine solution, you get a pale blue precipitate
• the amine acts as a base and takes two H⁺ ions from the complex, leaving copper hydroxide [Cu(OH)₂(H₂O)₄], insoluble
• add more butylamine and the precipitate dissolves to form a deep blue solution
• some of the ligands are replaced by butylamine molecules

Question 29

Describe the reaction where amines are acylated to form N-substituted amides

Answer 29

• an H atom on the amine is swapped for an acyl group to produce an N-substituted amide and HCl
• HCl reacrts with another molecule of the amine to produce a salt
• ethanoyl chloride is added to a concentrated aqueous solution of the amiine
• violent reaction occurs, producing a solid, white mixture of products

Question 30

What are amides?

Answer 30

• derived from carboxylic acids
• contains the functional group -CONH₂

_- the carbonyl group pulls electrons away from the rest of the CONH₂ group, so amides behave differently from amines

Question 31

How do you make amides?

Answer 31

• nucleophilic substitution using NH₃ as a nucleophile
• ethanol used as solvent because H₂O can compete

Two methods:

• acyl chloride + ammonia → amide + NH₄⁺ + Cl^-
• carboxylic acid + ammonia → amide + OH^- + NH₄⁺ (industrial method)

Question 32

What are the pros and cons of using acyl chlorides?

Answer 32

• they are more reactive
• but they are really difficult to store

Question 34

What are amino acids?

Answer 34

• an amino acid has a basic amine group and an acidic carboxyl group
• they are amphoteric: both acidic and basic properties
• there are 20 amino acids in the body
• they are chiral except glycine

Question 35

What is a zwitterion?

Answer 35

• an overall neutral molecule that has both a positive and a negative charge in different parts of the molecule
• an amino acid can only exist as a zwitterion near its isoelectric point

Question 36

What is the isoelectric point?

Answer 36

• the PH value where the overall charge on the amino acid is zero
• this depends on the R group, so is different for each amino acid

Question 37

How do you make peptides?

Answer 37

• a condensation reaction
• when a peptide chain has more than 50 amino acids, we call it a protein

Question 38

What are the two types of hydrolysis of polypeptides?

Answer 38

Question 39

What is condensation polymerisation?

Answer 39

• each monomer at least has two functional groups, reacting with the functional group on anotehr to form a link
• each time a link is formed, a small molecule, often water, is lost

Question 40

Describe the condensation polymerisation reaction between two monomers (that are the same)

Answer 40

• e.g. monomer with a carboxylic and an amine group

Question 41

Describe the reaction between dicarboxylic acids adn diamines

Answer 41

• carboxyl groups react with amino