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AS Chemistry > 6.1 > Flashcards

6.1 Flashcards

1
Q

Evidence against Kekulé’s model

A
  1. unlike alkenes, benzene is resistant to addition reactions
  2. enthalpy of hydrogenation of benzene shows that benzene is much more stable than was predicted
  3. all 6 carbon bonds in benzene are the same length
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2
Q

what structure is benzene now thought to have?

A
  • a delocalised electron structure
  • each C atom can donate 1 electron from its p-orbital
  • these electrons combine to form ring of delocalised electrons above and below plane of molecule
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3
Q

describe benzene’s resistance to reaction

A

benzene tends to undergo substitution of a hydrogen atom rather than addition reactions, using Kekeulé’s model you would expect benzene to undergo similar reactions to alkenes

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4
Q

describe the hydrogenation of benzene

A

experimentally it was found that benzene is more energetically stable than predicted

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5
Q

describe the bonds lengths of benzene

A

Kekeulé’s structure suggests that there should be 3 shorter C=C bonds and 3 longer C-C bonds but X-ray diffraction techniques have shown all 6 C bonds are the same length

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6
Q

describe nitration of benzene

A

-an electrophilic substitution reaction where a H atom is exchanged for a nitro group (NO₂)

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7
Q

equation for nitration of benzene

A

C₆H₆ + HNO₃ ➡️ C₆H₅NO₂ + H₂O

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8
Q

equations for mechanism of nitration of benzene with catalyst

A

HNO₃ + H₂SO₄ ➡️ NO₂+ +HSO₄- + H₂O

H+ + HSO₄- ➡️ H₂SO₄

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9
Q

Describe halogenation of benzene

A
  • benzene does not directly react with halogens as aromatic ring is too stable.
  • a halogen carrier is used - will generate a positive halogen ion
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10
Q

example of equation for halogenation of benzene

A

Br₂ + FeBr₃ ➡️ Br+ + FeBr₄-

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11
Q

example of equations for mechanism of halogenation of benzene with catalyst (AlCl₃)

A

AlCl₃ + Cl₂ ➡️ AlCl₄- + Cl+

AlCl₄- + H+ ➡️ HCl + AlCl₃

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12
Q

Why is a halogen carrier important to synthesis + what is this called

A
  • breaks a C-H bond and forms a C-C bond to an aromatic ring

- this is called alkylation

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13
Q

what is a Friedel-Crafts reaction

A
  • a substitution reaction where a hydrogen is exchanged for an alkyl, or acyl chain
  • allows electrophilic substitution to occur on an aromatic ring
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14
Q

what is a Lewis acid

A

an electron-pair acceptor

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15
Q

example equation for Friedel-Crafts reaction

A

R-Cl + FeCl₃ ➡️ R+ + FeCl₄-

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16
Q

what do haloalkanes do in Friedel-Crafts reactions?

A
  • they are mixed with a halogen carrier e.g iron chloride
  • halogen carrier acts as a catalyst and is regenerated at end of reaction
  • reactive carbocation made which undergoes electrophilic substitution with the benzene ring
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17
Q

what functional group does an acyl chloride have and what can it be used for in a Friedel-Crafts reaction?

A

RCOCl

-can be used as a halogen carrier to substitute just 1 H atom

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18
Q

what reaction is acylation

A

-as the carbonyl group withdraws electrons from the aromatic ring, a less reactive ketone is made, so only 1 substitution can occur

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19
Q

comparison of benzene and alkene’s reaction with bromine water

A
  • benzene does not react with bromine water
  • surprising as electrons in benzene are delocalised π-bonds
  • so benzene must have a lower electron density between the carbon atoms than an alkene
  • alkenes have localised high electron density
20
Q

comparison of benzene and phenol’s reactions

A
  • Phenol will readily undergo electrophilic substitutions with a variety reagents without the presence of a catalyst
  • this enhanced reactivity is due to extra electrons from the oxygen p-orbital being donated to the π-system of the aromatic ring
21
Q

why is phenol considered a weak acid?

A
  • it partially dissociates in water
  • it will react with strong bases (not weak bases) to form a salt and water
  • does not react with carbonates
22
Q

equation for neutralisation reaction of phenol with NaOH

A

C₆H₅OH + NaOH ➡️ C₆H₅O-Na+ + H₂O

23
Q

phenol reaction with bromine + equation

A

-phenol will undergo a triple substitution reaction with Br water at room temp:
C₆H₅OH + 3Br₂ ➡️ C₆H₂Br₃OH + 3HBr
-resulting product is white precipitate, smells like disinfectant 🤢 called:
2,4,6-tribromophenol

24
Q

phenol reaction with nitric acid + equation

A

-phenol will undergo a single substitution reaction with dilute nitric acid (HNO₃) at room temp
-reaction foams mix of 2-nitrophenol and 4-nitrophenol
C₆H₅OH + HNO₃ ➡️ C₆H₄(NO₂)OH + H₂O
-does not require conc nitric acid/sulfuric acid (H₂SO₄) catalyst, but if conc HNO₃ is used, triple substitution occurs forming 2,4,6-trinitrophenol

25
Q

2- and 4- directing effect of electron donating groups

A
  • in phenol the OH group pushes additional electrons into the π-system
  • makes substitution reactions mainly occur on the 2nd and 4th positions of the aromatic ring
  • OH group activates these C atoms so their rate of substitution is faster than other positions
  • known as the 2- and 4-directing effect
  • effect is more pronounced in aromatic compounds with an NH ₂ group directly attached to the aromatic ring
26
Q

3-directing effect of electron withdrawing groups

A
  • when -NO₂ groups are directly attached to the aromatic ring, a 3-directing effect is seen
  • the nitro group withdraws electrons from the π-system and makes the rate of substitution highest on the 3rd C atom
27
Q

how do aldehydes form carboxylic acids

A
  • undergo oxidation
  • reagents: K₂Cr₂O₇ and H₂SO₄ (Cr₂O₇²⁻/H⁺)
  • oxidising agent can be shown as [O]
28
Q

nucleophilic addition reaction with carbonyl compounds with NaBH₄

A
  • NaBH₄ is a reducing agent, can be shown as [H]
  • forms 1° alcohol
  • the C and O each gain a H so its 2[H]
29
Q

nucleophilic addition reaction with carbonyl compounds with HCN

A
  • NaCN(aq)/H⁺(aq)

- forms hydroxynitriles

30
Q

how to test for carbonyl functional group

A
  • use 2,4-dinitrophenylhydrazine (2,4-DNP)

- when added to a aldehyde or ketone, a yellow/orange precipitate derivative of 2,4-DNP forms

31
Q

how to identify the aldehyde or ketone from the derivative

A

-can be identified by comparing the melting point of the 2,4-DNP derivative with a database of accurately measured melting points of 2,4-DNP derivatives

32
Q

how to use Tollens’ reagent to distinguish between an aldehyde and a ketone

A
  • weak oxidising agent and can react with the carbonyl group in an aldehyde but not in a ketone
  • doesn’t react with ketones as they can be oxidised further
  • when added to an aldehyde a silver mirror is formed because a redox reaction occurs, silver ions reduced to silver and aldehyde functional group is oxidised [O] forming a carboxylic acid
33
Q

why are carboxylic acids soluble in water

A
  • very soluble in polar solvents like water as H bonds can be formed between the carboxylic acid function group and water
  • solubility decreases as hydrocarbon chain length increases as only the polar COOH group can form H bonds with water
34
Q

carboxylic acids reaction with metal

A
  • salt + H₂

e. g. 2Na + 2CH₃COOH ➡️ (CH₃COO)₂Na + H₂

35
Q

carboxylic acids reaction with metal oxides (bases)

A
  • metal salt + water

e. g. MgO(s) +HCOOH(aq) ➡️ (HCOO)₂Mg(aq) + H₂O(l)

36
Q

carboxylic acids reaction with metal hydroxides

A
  • metal salt + water

e. g. KOH (aq) + CH₃CH₂COOH(l) ➡️ CH₃CH₂COOK(l) + H₂O(l)

37
Q

carboxylic acids reaction with metal carbonates

A
  • metal salt, water, carbon dioxide

e. g. Na₂CO₃(s) + 2HCOOH(aq) ➡️ 2HCOONa(aq) + H₂O(l) + CO₂(g)

38
Q

how to make an ester using acid catalyst

A

carboxylic acid + alcohol (with cid catalyst e.g. conc H₂SO₄)
-reversible reaction

39
Q

how to make an ester with acid anhydrides

A
  • acid anhydrides will react with alcohols to form esters

- not reversible

40
Q

hydrolysis of esters with hot aqueous acid

A

-reversible reaction that reforms and alcohol and carboxylic acid e.g. HCl(aq)

41
Q

hydrolysis of esters with hot aqueous alkali

A
  • forms alcohol and carboxylate salt

- not reversible

42
Q

how do you form acyl chlorides from carboxylic acids

A
  • carboxylic acid + SOCl₂ ➡️ acyl chloride + SO₂ + HCl

e. g. CH₃COOH + SOCl₂ ➡️ CH₃COCl + SO₂ + HCl

43
Q

acyl chloride + alcohol ➡️

A

ester + HCl

44
Q

acyl chloride + water ➡️

A

carboxylic acid + HCl

45
Q

acyl chloride + ammonia ➡️

A

primary amide + NH₄Cl

46
Q

acyl chloride + primary amine ➡️

A

secondary amide + HCl

AS Chemistry (8 decks)

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