18 - Organics III

Question 1

what is the Kekulé model of benzene

Answer 1

• 6 carbon ring with 3 double bonds

Question 2

what is the delocalised model of benzene

Answer 2

• overlap of 6 p orbitals to form a pi system

Question 3

evidence of the delocalised model

Answer 3

• through x ray diffraction, found to be a perfect hexagon so can’t be Kekulé as double bonds are shorter than single
• enthalpy of hydration of benzene is less exothermic than expected, due to its extra stability due to the delocalised electrons increasing its entropy

Question 4

benzene and bromination

Answer 4

• resistant to electrophilic addition and instead does electrophilic substitution
• delocalised pi bonds in benzene vs dlocalised electron density in alkenes

Question 5

melting points of benzene compared to methylbenzene

Answer 5

methylbenzene much lower as the methyl group prevents good packing

Question 6

reaction of benzene with oxygen in air

Answer 6

combustion, gives a very sooty flame as the C:H ratio is very high

Question 7

mechanism of benzene with bromine

Answer 7

• check mechanism
• AlBr3 catalyst
• eventually goes from orange to colourless

Question 8

why is AlBr3 a good catalyst

Answer 8

aluminium is electron deficient as it has only 6 valence electrons

Question 9

mechanism of benzene with nitric acid and sulfuric acid

Answer 9

• check mechanism

- T < 50°C prevents too many substitutions and TNT is explosive

Question 10

mechanism of Friedel-Crafts alkylation

Answer 10

• check mechanism

- bromoalkane and AlBr3 catalyst

Question 11

mechanism of Friedel-Crafts acylation

Answer 11

• check mechanism

- acyl chloride and AlCl3 catalyst

Question 12

why does benzene undergo electrophilic substitution

Answer 12

• benzene has high electron density above and within the ring
• can easily donate electrons from the pi system

Question 13

relative reactivity of phenol compared to benzene

Answer 13

• phenol more reactive as the two lone pairs from the Os are donated into the ring
• creates more electron repulsion in the ring so more likely to be donated

Question 14

relative stability of phenol compared to benzene

Answer 14

• phenol more stable as the lone pairs on the Os delocalise into the ring, spreading the charge

Question 15

phenol acid

Answer 15

• draw dissociation
• phenoxide ion more stable due to delocalisation of negative charge
• weak and only reacts with OH-

Question 16

mechanism of bromination of phenol

Answer 16

• check mechanism

- solid product as it’s a large molecule and breaks more hydrogen bonds than it makes

Question 17

bromination of phenol vs benzene

Answer 17

phenol:
- fast
- bromine WATER
- room temp
3 substitutions
benzene:
- slow
- need to heat and add AlBr3 catalyst
- one substitution

Question 18

what is the amine functional group

Answer 18

N in a carbon chain

Question 19

solubility and boiling points of primary, secondary and tertiary amines

Answer 19

1° and 2° have higher boiling points and solubility as they can hydrogen bond
3° still soluble as they have lone pair

Question 20

bad way to make an amine

Answer 20

R - Br + NH3 –> R-NH2 + HBr

• nucleophilic substitution
• check mechanism
• can’t prevent multiple substitutions

Question 21

good way to make an amine

Answer 21

• reduction of a nitrile

- LiAlH4 in dry ether

Question 22

reduction of nitrobenzene with equations

Answer 22

• conc, HCl (aq)
• Sn
• heat under reflux
• add NaOH
• check equation

Question 23

amine + water equation

Answer 23

• forms alkaline solution
• reversible reaction
• NH2 part + H+ –> NH3+ group
• very basic so reacts with any acid

Question 24

amine + HCl

Answer 24

check salt

Question 25

amine + water

Answer 25

check salt

Question 26

amine + propanoic acid

Answer 26

check salt

Question 27

how to make an amide

Answer 27

amine + acyl chloride --> amide + HCl

Question 28

amine + ethanoyl chloride

Answer 28

check reaction

Question 29

amine + halogenoalkane

Answer 29

check reaction

Question 30

amine + Cu(II) ions

Answer 30

check complex ion

Question 31

relative basicities of ammonia, primary amines and aminobenzene

Answer 31

- aminobenzene weakest as the lone pair delocalises into ring, making it less likely to donate to H+ - NH3 middle - primary amine strongest as R group pushes electrons towards the N lone pair, more repulsion so more likely to donate

Question 32

what is the amide functional group

Answer 32

- C = O | N

Question 33

melting and boiling points of amides

Answer 33

higher than carboxylic acids and very high, two lone pairs on O and lone pair on N can do hydrogen bonding

Question 34

chemistry of amides, bonds

Answer 34

lone pair on N delocalises into C = O, reducing δ+ on C and strengthening pi bond

Question 35

formation of polyamides

Answer 35

- condensation polymerisation | - always release HCl or H2O

Question 36

draw repeat unit of dicarboxylic acid and diamine

Answer 36

check

Question 37

draw repeat unit of diacyl chloride and diamine

Answer 37

check

Question 38

draw repeat unit of amino acid polyamide

Answer 38

- check | - form peptide bond

Question 39

functional group of an amino acid

Answer 39

craboxylic acid with an amino side group

Question 40

acidity and solubility of amino acids (draw)

Answer 40

- H from OH dissociates and joins NH2 to make NH3+ and OH- in solution (draw) - form zwitterions - gives them acidic and basic properties - very high aqueous solubility as water can hydrate the +ve and -ve charges - form ionic compounds, white crystalline solid with high melting point

Question 41

draw amino acid + NaOH and HCl

Answer 41

check

Question 42

charge of amino acids in solution

Answer 42

mixture of both ions, so never neutral

Question 43

chirality of amino acids

Answer 43

- all chiral unless R group is H | - rotate plane polarised light in equal angles in opposite directions

Question 44

hydrolysis of peptide bond (draw)

Answer 44

- heat under reflux - HCl (aq) or NaOH(aq) and water - amino acids produced which can be separated by chromotography - check products with HCl and NaOH