A2 Chemistry Term 2 Flashcards
Describe the differences between Kekule’s structure and benzene
- Kekule’s structure suggests 3 longer C-C bonds and 3 shorter C=C bonds. Benzene has a perfect, hexagonal arrangement.
- Would undergo addition reactions if Kekule’s structure. Does not undergo readily.
- enthalpy of hydrogenation is less exothermic meaning benzene is more stable than expected.
Explain why the benzene ring is more stable than expected
- Each carbon atom contributes an electron to a pi bond as the other three electrons form sigma bonds
- Pi bonds are not localised between pairs of carbon atoms but rather is spread over all six carbon atoms in the hexagonal ring.
- The electrons are delocalised either above or below the plane of the carbon atoms.
Describe the reaction of benzene/methylbenzene with chlorine or bromine
- Electrophilic substitution
- AlCl3 catalyst, anhydrous
- methylbenzene will only react if in the presence of UV light, undergoing free radical subsitution
Explain why the carbon-halogen bond in halogenoarenes is stronger than in halogenoalkanes
Lone pairs on the halogen atom overlaps slightly with pi bonding system. This means the bond has a partial double bond character.
Describe the nitration of benzene
- electrophilic substitution
- electrophile is NO2+
HNO3 + H2SO4 -> NO2+ HSO4 - + H2O
- reflux + 55°C
Describe the Friedel-Crafts reaction
- electrophilic substitution
- used to introduce alkyl or aryl groups into a benzene ring.
- AlCl3 catalyst
- halogenoalkane is used to form carbocation electrophile.
Describe the oxidation of side-chains in arenes
- forms carboxylic acid
- reflux with alkaline potassium manganate (VII) and then acidified with dilute sulfuric acid, or acidified potassium dichromate
Explain the relative acidities of phenol and ethanol
Phenol: Stronger acid as phenoxide ion formed when phenol dissociates has negative charge spread over whole ion as one of the lone pairs on the oxygen atom overlaps with delocalised pi system. Phenol is less likely to reform.
Ethanol: Weaker acid due to positive inductive effect of alkyl group meaning it is electron-donating. This concentrates more negative charge on the oxygen atom which more readily accepts H+ ion. This means it is less likely to dissociate.
Describe the reactions of phenol
Breaking of OH bond:
- phenol dissolves well in alkali and reacts to form a salt and water.
- it also reacts with Na to form sodium phenoxide and hydrogen gas.
Substitution into ring of phenol:
- Overlap of one of lone pairs of oxygen atom with pi bonding system increases electron density of ring meaning it reacts more readily with electrophiles.
- reacts readily with bromine water
- reacts readily with dilute nitric acid for nitration
Explain why carboxylic acids are more acidic than phenols and describe the effect of chlorine substituted into the acid on its acidity
- O-H bond is weakened due to electronegativity of O in carbonyl group
- the carboxylate ion formed is stabilised by the delocalisation of the negative charge on the carboxylate ion reducing its charge density, reducing the likelihood of it binding to H+.
- These effects are strengthened by the presence of further electron-withdrawing groups such as chlorine, causing the molecule to become more acidic.
Describe the oxidation of carboxylic acids
Methanoic acid:
- stronger reducing agent and will be oxidised by warming with mild oxidising agents such as Tollens and Fehling’s reagent.
- CO2 + H2O is formed
Ethanedioic acid:
- acidified potassium manganate is required
- CO2 + H2O is formed
Describe the different reactions of acyl chlorides
- hydrolysis: water is required and the reaction produces carboxylic acid and HCl
- alcohols and phenols: Esters and HCl form. Will react vigorously with ethanol. With phenols, the reaction must be warmed and a base must be present in order to form the phenoxide ion which acts as a nucleophile.
- amines: amide forms. Vigorous reaction.
Explain the relative eases of hydrolysis of acyl chlorides, chloroalkanes, aryl chlorides.
Acyl chlorides: Contains strongly electronegative atoms meaning carbon atom has partially positive charge and is open to attack from nucleophiles.
Chloroalkanes: Contains fewer electronegative groups compared to acyl chlorides and so less susceptible to attack from nucleophiles.
Aryl Chlorides: p orbitals from Cl atom tend to overlap with delocalised p electrons in benzene ring causing C-Cl bond to have some double bond character, making it stronger.
Describe the ways in which amines are formed
- Halogenoalkane undergoes nucleophilic substitution with ammonia using excess, hot ethanolic ammonia.
- Reduction of nitriles using hydrogen gas and nickel catalyst. LiAlH4 can be used in dry ether. Nitrile formed by heat+ reflux of halogenoalkane with ethanolic KCN
- LiAlH4 in dry ether to reduce amides to amines.
Describe the relative basicities of ethylamine, ammonia and phenylamine
- Basicity is based on availability of lone pair of electrons to which the H+ can bind to.
Ethylamine: positive inductive effect releases electrons to N atom making the lone pair more readily available.
Phenylamine: Lone pair of N atom is delocalised into the benzene ring making it less available to pair with H+
