Aromatic compounds Flashcards
(28 cards)
Describe the similarities and differences between the bonding in the Kekules model and delocalised model of benzene.
Similarities -
- sideways overlap of P orbitals
- pi bond above and below C atoms
Difference -
- Kekule has alternating pi bonds that overlap in one direction
- Delocalised has pi ring and all P orbitals overlap in both directions.
Enthalpy of hydrogenation
Expected = - 360 KJ mol-1
Actual = -208 Kj mol-1
less exothermic than expected
Reactivity of benzene
Benzene is less reactive than alkenesb
Bond length of benzene
All C-C bond lengths are the same
What did Pauling suggest structure of benzene was like?
Benzene is a ring structure where the spare P-electrons are delocalised around all 6 carbon atoms.
Rather than pairs of electrons localised in double bonds, like in alkanes
Bonding in benzene according to Pauling.
Cloud of delocalised e- above and below the plane of the ring.
Delocalisation gives more/ extra stability called “delocalisation energy”.
What is a phenyl group?
Benzene ring attached to other compounds.
Rules of substituted aromatics
- If alkyl groups with lesson than 1 carbons, nitro groups, and halogens are attached directly to a benzene ring, you use their prefix (put in alphabetic order) - use di, tri and number.
- If a benzene ring is attached to an alkyl group with a fictional group or an alkyl group w 7 or more carbons, use prefix phenyl .
Equation showing formation of catalyst for nitration of benzene.
H2SO4 + HNO3 -> <- + NO2+ + HSO4- + H2O
What happens if the temp rises above 50 degrees celsius during nitration of benzene?
Further substitution can occur, leading to production of dinitrobenzene.
Formation of catalyst used for halogenation of benzene.
Br2 + AlBr3 -> <- Br+ + AlBr4-
What do halogens need in order to react with benzene?
A halogen carrier catalyst
e.g AlCl3, FeCl3, AlBr3
- which can be generated in situ from metal and halogen.
Formation of electrophile used for alkylation of bromine.
CH3Br +AlBr3 -> <- CH3+ + AlBr4-
Formation of electrophile of acylation of benzene.
CH3COCl + AlCl3 -> <- CH3CO+ + AlCl4-
Reagents and conditions for
- chlorination
- bromination
- Nitration
- Alkylation
- Acylation
Cl2 with AlCl3 catalyst
Br2 with AlBr3 catalyst
Conc HNO3, conc H2SO4, HUR at 50 degrees celsius for mononitration
Alkyl chloride with AlCl3 catalyst
Acyl chloride with AlCl3 catalyst
Comparing reactivity of alkenes with arenas.
- Bromine adds across the double bond in cyclohexene.
- Pi bond in alkenes has localised electrons above and below the plane of the 2 carbon atoms in the double bond -> provides area of high electron density.
- Localised electrons in pi bond induce dipole in non-polar bromine - 1 atom in Br2 becomes slightly positive and other slightly negative.
- Delta positive Br enables Br2 molecule to act as electrophile.
What is an acid and a base?
An acid is a proton donor.
A base is a proton acceptor.
What enables phenol and carboxylic acid to react as acids?
The donation of a proton from the OH groups.
Examples of strong and weak acids and what do they do?
HCl, H2SO4, HNO3 -> strong acids -> fully dissociate in water.
Carboxylic acids -> weak acids -> partially dissociate in water
Phenol -> very weak acid
Do alcohols react as acids?
Usually don’t as OH bond is not easily broken.
Reactions of the OH groups in phenols and carboxylic acids as an acid.
Aliphatic alcohols -> very, very weak acids -> only react with reactive metals (Na,K)
Phenols -> very weak acids -> react with reactive metal and bases (NaOH)
Carboxylic acids -> weak -> react with reactive metals and bases and metal carbonates.
What happens during reaction of phenol?
Phenol -> phenoxide ion and H+
- Occurs due to delocalisation of the negative charge of O- around the delocalised pi bond system of the benzene ring.
- Making the “phenoxy” ion more stable.
Phenol and base (e.g NaOH)
phenol and NaOH -> salt (sodium phenoxide) and H20
Phenol and metal
2 Phenol and 2Na -> 2 sodium phenoxide snd H2
2CH3COOH + 2Na -> 2CH3COO-Na+ + H2
