Aromatic Compounds and Carbonyls Flashcards

1
Q

What are the main features of the delocalised model of benzene?

A
  1. Each carbon atom uses three of its 4 available electrons to bond to two other carbons and one hydrogen.
  2. The fourth electron one each carbon occupies a p-orbital at right angles to the plane of the ring.
  3. Adjacent p-orbitals overlap sideways, in both directions, above and below the plane of the ring, to form a ring of electron density which spreads across all 6 carbon atoms.
  4. The six electrons in this π system are delocalised.
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2
Q

How do the bond lengths in the benzene ring disprove Kekule’s model?

A

Single C-C bonds (0.154 nm) are longer than C=C double bonds (0.133 nm). However, all the C-C bonds in benzene are the same length (0.139 nm), suggesting that there are no alternating single and double bonds.

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3
Q

How does the hydrogenation enthalpy of benzene disprove the Kekule structure?

A

If benzene did have 3 double bonds, its enthalpy of hydrogenation should be 3x greater than that of cyclohexene which has only one double bond, and has a hyrogenation enthalpy of -120 kJmol-1. The actual hydrogenation enthalpy of benzene is -208 kJmol-1, 152 kJmol-1 less exothermic than it should be. Therefore, the actual structure of benzene is more stable than is suggested by the Kekule model.

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4
Q

How are aromatic compunds named?

A
  1. Alkyl groups with <7 carbons, halogens & nitro (NO2) groups are all used as prefixes.
  2. When the ring is attached to an alkyl group with 7 or more carbons or to an alkyl group with a functional group, it is considered a substituent and the prefix phenyl is used.
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5
Q

What happens when benzene or a derivative undergoes a substitution reaction and what is the typical mechanism?

A
  1. A hydrogen atom on the benzene ring is replaced by another atom/group.
  2. Electrophilic substitution.
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6
Q

What are the conditions and reagents in the nitration of benzene?

A
  • Conc. sulphuric acid catalyst, heat to 50ºC (no higher because further substitution can occur, creating explosive products)
  • Benzene + conc. nitric acid → nitrobenzene + H2O
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7
Q

What is the electrophile in the nitration of benzene?

A

NO2+ formed from the reaction of nitric acid and sulphuric acid.

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8
Q

What is the mechanism for nitrogenation of benzene?

A
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9
Q

What is the electrophile in the halogenation of benzene and how is it made?

A
  • X+
  • Reaction of pure halogen with a halogen carrier, to form X+ & MX4-
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10
Q

What is the mechanism for halogenation of benzene?

A
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11
Q

How is alkylation carried out?

A

Reacting benzene with a haloalkane in the precense of a halogen carrier, which generates the electrophile.

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12
Q

What is the mechanism for alkylation?

A
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13
Q

What is formed in an acylation reaction?

A

An acyl chloride reacts with benzene in the presence of AlCl3 catalyst to form an aromatic ketone where the oxygen is always bonded to the carbon joined to the ring. Hydrochloric acid is also formed.

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14
Q

What is the mechanism for an acylation reaction?

A
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15
Q

Why is benzene less reactive with halogens than alkenes?

A
  • The double bond in an alkene has a π-bond w/ localised electrons above and below the σ-bond.
  • Benzene has a system of delocalised π electrons, therefore the electron density around each σ-bond is lower.
  • The double bonds have a high enough electron denisty to polarise halogen molecules whereas benzene requires a halogen carrier to generate electrophiles.
  • Alkenes can do electrophilic addition by themselves but benzene requires a catalyst to do electrophilic substitution.
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16
Q

What is the evidence for the weak acidity of phenol?

A
  • Phenol is more acidic than normal alcohols, but less acidic than carboxylic acids.
  • Carboxylic acids & phenol react with solutions of strong bases such as NaOH, whereas ethanoldoesn’t react with strong or weak bases.
  • Phenol can’t react with a weak base like sodium carbonate however carboxylic acids can (used as a test to distinguish phenols and carboxylic acids: CO2 produced if carboxylic acid).
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17
Q

How soluble is phenol in water and what does it form when it does dissolve?

A
  1. Less soluble than normal alcohols due to the presence of the non-polar benzene ring.
  2. Partially dissociates into a phenoxide ion and a proton (H+).
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18
Q

Describe the reaction of phenol with sodium hydroxide.

A

Neutralisation reaction: sodium phenoxide and water formed.

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19
Q

Why does phenol undergo electrophilic substitution more readily than benzene?

A
  • Oxygen donates lone pair of electrons from p-orbital into the π-system, increasing the electron density of the benzene ring.
  • Phenol therefore attracts electrophiles more strongly than benzene therefore the aromatic ring is more susceptible to attack from electrophiles.
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20
Q

Describe the reaction of phenol with bromine water.

A
  • White ppt. of 2,4,6-tribromophenol formed (+ 3HBr), decolourising the bromine water.
  • Reaction is carried out at room temp.
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21
Q

Describe the nitration of phenol.

A
  • Reaction carried out at room temperature with no catalysts.
  • Dilute nitric acid reacts with phenol to form a mixture of 2-nitrophenol and 4-nitrophenol (+H2O).
22
Q

What are activating and deactivating directing groups?

A
  • Activating: pushes electron density into the π-system, increasing susceptibility to attack from electrophiles.
  • Deactivating: pull electron density out of the π-system, decreasing susceptibility to attack from electrophiles.
23
Q

What groups are 2,4,6 directing and activating?

A
  • -OH
  • CH3
  • NH3
  • [Br] (basically neutral)
24
Q

What groups are 2,4,6 directing and deactivating?

A

The halogens (including Br to some extent).

25
Q

What groups are 3,5 directing and deactivating?

A
  • NO2
  • -(C=O)-R
  • -COOH
  • -COO-R
  • -CN
  • -CH=O
26
Q

Explain the water solubility of carboxylic acids.

A

The C=O and O-H bonds in carboxylic acids are polar, allowing carboxylic acids to form H bonds with water molecules.

*not to do with London forces or PD-PD interactions*

27
Q

What is the maximum number of carbons a carboxylic acid can have to be soluble in water and why?

A

4

  • No. of carbons increases
  • Strength of dispersion forces increases to outweigh the potential H-bonding.
28
Q

What are the two types of reactions that carboxylic acids are involved in?

A
  1. Redox reactions with metals
  2. Neutralisation reactions with bases: alkalis, metal oxides & carbonates
29
Q

What do carboxylic acids form when they undergo a redox or neutralisation reaction?

A

Carboxylate salts: Carboxylate anion + metal cation

30
Q

How are carboxylate ions named?

A
  1. Take away -ic from carboxylic acid
  2. Add -ate
31
Q

Describe the reaction of carboxylic acids with metals.

A
  • Redox reaction: metal oxidised, hydrogen reduced
  • Observations: metal disappears & bubbles of hydrogen gas evolved.
  • General equation:

Carboxylic acid (aq) + Metal (s) → Carboxylate salt (aq) so ions dissociated + Hydrogen gas

32
Q

Describe the reaction between carboxylic acids and metal oxides.

A
  • Neutralisation reaction
  • Carboxylate salt and water formed
  • Solid metal oxide disappears
33
Q

Describe the reaction between carboxylic acids and alkalis.

A
  • Neutralisation reaction with alkalis such as aqueous sodium hydroxide
  • Carboxylate salt and water formed
  • No observable indicators because reaction between two solutions
  • Ionic equation: H+(aq) + OH-(aq) → H2O(l)
34
Q

Why do carboxylic acids act as acids (give equation)?

A

When dissolved in water, they partially dissociate.

HCOOH (aq) ⇌ H+(aq) + HCOO-(aq)

35
Q

Describe the reaction between carboxylic acids and carbonates.

A
  • Neutralisation reaction with carbonates such as Na2CO3
  • Both reactants in solution: if carboxylic acid is in excess the solid carbonate with be observed to disappear
  • Carboxylate salt + water + carbon dioxide formed (bubbles of CO2 evolved)
36
Q

What is the significance of the carboxylic acid + carbonate neutralisation reaction?

A
  • Used as a test to distinguish between phenols and carboxylic acids
  • CO2 gas produced can be bubbled through limewater to prove that reaction has taken place.
  • Phenols not acidic enough to react with carbonates therefore if CO2 produced compound must be carboxylic acid.
37
Q

What is a carboxylic acid derivative?

A

A compound that can be hydrolysed to form the parent carboxylic acid. They all include an acyl group attached to an electronegative atom/group of atoms (to make hydrolysis at that point possible):

39
Q

What are the three types of carboxylic acid derivative?

A
  1. Esters: carboxylic acid with alcoholic H replaced by R group
  2. Acyl chlorides: carboxylic acid with alcohol replaced by chlorine
  3. Acid anhydrides: two carboxylic acids joined by removing alcohol from one and alcoholic H from the other and then joining at remaining O
40
Q

How are acyl chlorides named?

A

Replace -oic acid with -oyl chloride

41
Q

How are acid anhydrides named?

A
  • Use alkyl prefix corresponding to total number of carbons.
  • End with -oic anhydride
42
Q

Describe the esterification of carboxylic acids.

A
  • Alcohol and carboxylic acid are warmed with a small amount of concentrated acid catalyst e.g. H2SO4
  • Ester and water are formed.
43
Q

Draw the mechanism for acid hydrolysis.

A
44
Q

Describe the acid hydrolysis of an ester.

A
  • The ester is heated under reflux with dilute aqueous acid catalyst.
  • The ester is broken down by water into a carboxylic acid and an alcohol.
45
Q

Draw the mechanism for alkaline hydrolysis.

A
47
Q

Describe the alkaline hydrolysis of an ester.

A
  • Known as saponification and is irreversible.
  • Ester is heated under reflux with aqueous hydroxide ions.
  • Forms a carboxylate ion and and alcohol.
49
Q

Describe the esterification of acid anhydrides.

A
  • Acid anhydride + alcohol → ester + carboxylic acid
  • Conditions: gentle heating
  • If alcohol is in excess & the reaction kixture is heated strongly and concentrated acid catalyst is added, the carboxylic acid produced can react with some of the alcohol to make another ester.
50
Q

Describe the formation of acyl chlorides.

A
  • Carboxylic acid + thionyl chloride (SOCl2) → acyl chloride + SO2 gas + HCl gas
  • Reaction should be carried out in fume cupboard as gases evolved are harmful.
51
Q

Describe the esterification of acyl chlorides with alcohols.

A
  • Acyl chloride + alcohol
  • Ester + hydrogen chloride formed
52
Q

Describe the reaction between acyl chlorides and ammonia/amines.

A
  • Ammonia/amines can act as nucleophiles by donating the lone pair of electrons on the nitrogen atom
  • Acyl chloride + ammonia → primary amide + ammonium chloride
    • In a primary amide, the nitrogen atom is attached to only one carbon.
  • Acyl chloride + primary amine → secondary amide + alkylammonium chloride
  • Primary amine: R-NH2
53
Q

Why are acyl chlorides useful in synthesis?

A
  • Very reactive and can easily be converted into carboxylic acids or carboxylic acid derivatives such as esters and amides.
  • They react with nucleophiles by loosing the chloride ion whilst retaining the C=O bond.
54
Q

Describe the esterification of acyl chlorides with phenols.

A
  • Ester + hydrogen chloride formed.
  • Carboxylic acids are not reactive enough to form esters with phenols, but acyl chlorides and acid anhydrides readily react to form phenyl esters, without acid catalysts.
  • A phenol is a benzene ring with an alcohol bonded to one of the carbons.
55
Q

Describe the reaction between acyl chlorides and water.

A
  • Carboxylic acid + HCl formed
  • Violent reaction with dense steamy hydrogen chloride fumes evolved