Organics 3 - Benzene, Amino Acids etc.

Question 1

Is Benzene Aliphatic or Aromatic

Answer 1

Aromatic

Question 2

Give the formula for Benzene

Answer 2

C6H6

Question 3

Describe the bonding in Benzene

Answer 3

Each carbon atom has 3 sigma bonds The fourth valence electron is in a p orbital and is delocalised around the ring Creates a delocalised pi ring system of 6 electrons above and below the ring

Question 4

What is the bond angle in Benzene

Answer 4

120 deg

Question 5

What are the three pieces of evidence against the Kekule structure

Answer 5

1) Does not undergo electrophilic addition e.g. decolourise bromine water 2) It is more thermodynamically stable than expected 3) Bond length is shorter than usual C-C but longer C=C

Question 6

Describe the differences between Benzene and cyclohexene

Answer 6

* Benzene undergoes electrophilic substitution, cyclohexene undergoes electrophilic addition * Benzene has delocalised electrons, cyclohexene has localised electrons * Benzene has lower electron density * Benzene is more stable * Benzene does not react with bromine water, cyclohexene does

Question 7

Describe the halogenation of Benzene

Answer 7

1) Electrophile generated (Cl⁺/Br⁺)

Cl₂ + AlCl₃ -> [AlCl₄]^- + Cl⁺

Br₂ + FeBr₃ -> [FeBr⁴]^- + Br⁺

2) Mechanism - see photo
3) Regeneration

H⁺ + [AlCl₄]^- -> AlCl₃ + HCl

H⁺ + [FeBr⁴]^- -> FeBr₃ + HBr

Question 8

Describe the Acylation of Benzene with halogen alkanes

Answer 8

Electrophilic substitution

Friedel Crafts reaction

Generation of electrophile:

CH3Cl + AlCl3 -> [AlCl4]- + +CH3

Mechanism (see photo)

Regeneration

H⁺ + [AlCl₄]^- -> AlCl3 + HCl

Question 9

Describe the acylation of benzene with acyl chlorides

Answer 9

Electrophilic substitution

Friedel Crafts reaction

Generation of electrophile:

H3C-COCl + AlCl3 -> [AlCl₄]- + H3C-C⁺=O

Mechanism (see photo)

Regeneration

H+ + [AlCl4]- -> AlCl3 + HCl

Question 10

Naming substituted benzene compounds

Answer 10

When benzene in the main group use ____benzene

When benzene is the side group use phenyl prefix

Question 11

Why is phenol more easily substituted than benzene

Answer 11

The lone pair of electrons of oxygen atom overlaps with delocalised pi system

It is incorporated into the pi system so it is now activated

Electron density of pi system is increased so it can now induce dipoles and attracts electrophiles more

Question 12

Give the structure of primary, second and tertiary amines

Answer 12

Primary: R- N-H

|

H

Secondary: R- N-R

|

H

Tertiary: R- N-R

|

R

Question 13

Naming Amines

Answer 13

• Use suffix amines
• If more than one carbon chain

N-(short chain),N-(short chain)(long chain)amine

Question 14

Describe the melting and boling points of amines

Answer 14

Relatively high for primary & secondary amines due to H bonding

Increases with Chain Length

Lower for tertiary amines due to no H bonding

Question 15

Describe the solubility of amines

Answer 15

All small amines are soluble - decreases with chain length

Question 16

Describe the acidity of amines

Answer 16

Weak bases (lone pair on nitrogen atom accepts a proton)

Question 17

Describe the production of aliphatic amines from halogenoalkanes

Answer 17

Nucleophilic substitution

Primary Amine

* Use conc. HCl in alcoholic solution

R-X + 2NH₃ -> R-NH₂ + NH₂ + NH₄⁺X^-

Secondary amines

* Use primary amines in alcoholic solution

Tertiary amines

* Use secondary amines in alcoholic solution

Question 18

Describe the production of amines from nitriles

Answer 18

* Reduction

* Use LiAlH₄ OR H₂ with Ni/Pt catalyst

R-CN + 4[H] -> R-CH₂-NH₂

Question 19

Describe the production of phenylamine

Answer 19

* Reduction of nitrobenzene

* conc. HCl and Sn catalysts

Question 20

Describe and explain the relative base strength of amines

Answer 20

Primary Amines > Ammonia > Phenylamine

Primary amines are stronger because the alkyl groups have inductive effect increasing electron density on the nitrogen atom

Ammonia does not have an inductive effect as there are no alkyl groups

Phenylamine is weaker because the lone of pair electrons from the Nitrogen atom are drawn into the delocalised pi system

Question 21

Describe the reaction of amines with water and acid

Answer 21

Amines are weak bases

Dissociate in water to produce OH- ions

React with acid to form salts

Salts are named (carbon chain)ammonium(chloride/sulfate/nitrate)

Question 22

Describe the reaction between amines and copper complexes

Answer 22

Cu²⁺ exists as [Cu(H₂O)₆]²⁺ (light blue solution)

Add a little butylamine

[Cu(H₂O)₆]²⁺ + 2C₄H₉NH₂ <=> [Cu(OH)₂(H₂O)₄] + 2C₄H₉NH₃⁺

[Cu(OH)₂(H₂O)₄] is a light blue precipitate

Add excess butylamine

[Cu(H₂O)₆]²⁺ + 4C₄H₉NH₂ <=> [Cu(C₄H₉NH₂)₂(H₂O)2]²⁺ + 4H₂O

Dark blue solution

Question 23

Give the general formula for amino acids

Answer 23

R

|

H₂N- C -COOH

|

H

Question 24

Do amino acids display optical isomerism

Answer 24

All but glycine

Question 25

Why do amino acids form Zwitterions

Answer 25

The H from the COOH group is donated and is accepted by N in the NH2 group

Question 26

Describe the physical properties of amino acids

Answer 26

Very soluble due to zwitterions (two oppositely charged poles) Very high melting/boiling points due to electrostatic attraction

Question 27

Describe the polymerisation of amino acids

Answer 27

Condensation Polymerisation NH2 and COOH groups react to form amide linkages/peptide bonds Produces dipeptides When there are two or more different amino acids joining together there are different possible products (see photo)

Question 28

Describe polypeptides

Answer 28

Multiple amino acids come together to form long chains Primary structure of proteins

Question 29

Describe the acid and alkaline hydrolysis of amino acids

Answer 29

Breaks apart peptide bonds Acid Hydrolysis NH₂ group is protonated to NH₃⁺ Alkaline Hydrolysis COOH group becomes COO^- Amino acids produced from this can be separated by chromatography

Question 30

Describe Amino Acids in different pHs

Answer 30

Le Chatlelier's Principle NH₂ + H⁺ \<=\> NH₃⁺ COOH \<=\> COO^- + H⁺ In Acidic conditions [H⁺] increased so NH₃⁺ formed In alkali conditions [H⁺] decreased so COO^- formed If R group contains COOH or NH₂ groups then they'll also be affected

Question 31

Describe the formation of Polyamides

Answer 31

Condensation polymerisation Two different monomers used: - Dicarboxylic Acids - Diamines 350 degrees Alternatively diacyl chlorides can be used at room temp

Question 32

Describe the structure of proteins

Answer 32

Proteins are made up of amino acids chains (polypeptides) First Level Structure - Amino Acid Chains with Amide/Peptide bonds Second Level Structure - Folding of amino acid chains - H bonding between amide/peptide bonds Third Level Structure - Further folding - Ionic bonds or disulfide covalent bridges in R group Fourth level structure Multiple amino acid chains Possibly a prosthetic group