Aromatic- Aromacity Flashcards
Describe the structure of benzene
- 6 sp2 hybridised carbon atoms that link together to form a planar ring
- Angles of 120 degrees
- Bond length 139 pm- in between length of C-C and C=C
- Each of 6 sp2 hybridised carbon atoms in benzene has an unhybridized p orbital at right angles to the plane of the ring, containing a single electron
- P orbitals are parallel and overlap side on with p-orbitals on adjacent carbon atoms- pi electrons
- Create 2 continuos rings of pi electrons above and below the plane of the benzene ring
- The electrons are delocalised
What is wrong with kekules structure
- 3 c=c bonds would be shorter than the 3 C-C bonds- but all are the same length
- C=C would be expected to react in electrophilic addition reactions like in alkenes- but actually undergoes substitution
- More stable than thought- heat of hydrogenation of benzene is lower than expected- P-electrons are delocalised
How can the stability of benzene be measured
- C=C can be reduced using H2 and Pd catalyst - hydrogenation
- This is exothermic as more thermodynamically stable product is formed
- Heat of hydrogenation can be directly measured and gives an indication of stability of unsaturated compounds
How is the heat of hydrogenation of benzene different to expected
- Despite having 3 double bonds it has a heat of hydrogenation less than cyclohexadienes
- More stable than would be expected
- Due to delocalisation of the electrons in the ring- resonance
Why does benzene not undergo bromination
- Activation energy must be overcome
- Too large for benzene
- Due to aromatic stabilisation energy which must be overcome for aromatics to react with electrophiles
- When forced a substituted product is formed instead of addition product
What is Huckel’s Rule
Requirements for aromaticity
- Molecule must be cyclic
- Must be planar- sp2 hybridised
- Fully conjugated
- Must contain 4n+2 pi-electrons- n is any positive integer e.g. 6, 10, 12, 14 etc
Can heteroatoms be part of the aromatic ring
- Yes
- In some compounds a lone pair of electrons on the heteroatom is part of the ring of pi electrons- pyrrole
- Sometimes not- pyridine
What are antiaromatic compounds
- Cyclic, planar and contain an uninterrupted ring of pi electrons
- Less stable than a similar compounds with localised electrons
- Contain 4n pi-electrons
- Very unstable and reactive
What is a non aromatic compounds
- Non-planar and have an interrupted or uninterrupted ring of pi electrons
- Similar stability to related compounds that are non cyclic
How many MOs are present in benzene
- 6 carbon p-orbitals in benzene combine to give 6 pi molecular orbitals
Describe bonding MOs in benzene
- Lowest energy MO occurs due to all 6 p-orbitals combining in phase - no nodes
- Next energy level contains 2 degenerate MOs with one node each- one node across bonds and other with a node across atoms
- All these three MOs are lower in energy than the contributing AOs- all bonding MOs
Descirbe the anti-bonding MOs in benzene
- 3rd energy level contains 2 degenerate MOs, with 2 nodes each
- Highest energy level MO occurs due to all 6 p-orbitals combining out of phase - 3 nodes
- These 3 MOs are higher in energy than the contributing AOs and are therefore all anti-bonding MOs
Where do the electrons stay in benzene
- The 6 p-electrons are all paired and fill all bonding MOs
2. Contribute to bonding effects across the entire ring- why aromatic compounds are so stable
How can you use a Frost circle
- Draw a circle and place a horizontal line through its centre
- Draw a regular polygon with same number of sides as molecule in question- apex needs to point down
- Draw horizontal lines where the polygon touches the circle- vertices- represent the relative energy of MO
- Fill MO with number of electrons p-orbitals of the molecule- following correct rules
- Bonding= below line, non-bonding= on line, antibonding= above line
- If all paired and in bonding or non-bonding MO then the molecule is aromatic
- If there are unpaired electrons in non-bonding or anti-bonding orbitals- anti-aromatic
What can aromatic compounds be classed as and what is the symbol
- Arenes
2. C6R5= aryl (Ar) groups
What are the different aryl groups called
- C6H5- Phenyl (Ph)
- Phenyl group linked through a methylene unit- Benzyl (Bn)
- Phenyl group linked through a carbonyl- benzoyl (Bz) group
What is the benzylic position
- The CH2 carbon in benzene groups is known as the benzylic position
Describe the substitution positions
- Aromatic carbon atom directly bonded to substituent= ipso
- Adjacent to this - ortho (o)
- Next position around - meta (m)
- Position on opposite side of the ring - para (p)
What are the basics of naming aromatics
- Find highest priority root name
2. Find lowest numbering system
What is the closest heterocycle relative to benzene
- pyridine
- 6-membered nitrogen-containing compounds
- C-H unit has been replaced with a N: unit
- It conforms to Huckel’s rules and so is aromatic
Describe the position of the lone pair of nitrogen in pyridine
- Nitrogen atom is sp2 hybridised
- Its lone pair of electrons is in an sp2 orbital perpendicular to the aromatic ring
- The lone pair is not delocalised around the aromatic ring and is isolated to the sp2 orbital
What do the properties of pyridine mean it is used for
- Commonly-used organic base
2. Nucleophilic organocatalyst- DMAP (derivative)
Can 5-membered heterocycles be aromatic and describe an example
- Yes, if an entire C=C unit is replaced with an N-H unit - pyrrole
- The nitrogen atom is sp2 hybridised but the lone pair is in the unhybridised p-orbital not sp2
- Therefore it is delocalised around the aromatic ring
- 6 pi-electrons delocalised across 5 cyclic, planar, fully conjugated sp2 hybridised orbitals
- Also electron rich
What happens if C-H atoms in pyrrole are replaced with further N atoms
- The extra nitrogen atom is said to be pyridine-like - lone pairs are in sp2 hybridised orbitals and are perpendicular to the aromatic ring
What can Nitrogen in pyrrole be replaced with and describe what happens
- Other heteroatoms- sulfur or oxygen
- Heteroatoms are also sp2 hybridised with one lone pair in a p-orbital delocalised amongst the aromatic ring and one lone pair in an sp2 hybridised orbital perpendicular to the ring
What happens if a benzene ring is fused to an aromatic heterocycle e.g. pyrrole
- Give extended aromatic heterocyclic molecules
- All the heavy atoms are sp2 hybridised with 10 pi-electrons delocalised across them - aromaticity is observed over the entire molecule
What is a PAH and how is it formed
- polycyclic aromatic hydrocarbons
2. Caused by fusion of benzene to itself multiple times
Why are PAHs important for scientific research
- Suspected carcinogens
- Constituents of tobacco smoke
- PAH detection and monitoring- estimate more than 20% of all carbon in universe can be found in PAHs
Why are helicenes interesting
- 6 benzene rings
- Display axial chirality
- Uses in asymmetric synthesis
Why are fullerenes useful
- Can take form of spheres or tubes
2. Used within various applications
Why is graphene useful
- Worlds first 2D material
- 200 times stronger than steel but extremely thin diameter
- Lightweight, flexible and electrically and thermally-conductive
- Application in electronics, energy and biomedical industries
What is the simplest PAH and describe its properties
- Napthalene- 2 benzene rings fused
- 10 pi-electrons
- Less aromatic than benzene
- Only one ring is fully aromatic at a given time
- Can be easily reduced under mild conditions
- Used in OLED and solar cells
What is Huckel’s rule limited to
- Monocyclic systems
What rule can be used to describe aromaticity in PAHs
- Clar’s sextet theory
What does Clar’s sextet theory state
- Resonance structure with the largest number of disjoint aromatic pi-sextets is the most important for a PAH’s characterisation
- PAHs with more pi-sextets are kinetically more stable than those with less
- Rings which have the most pi-sextets in resonance structures are more aromatic than other rings and therefore more stable/less reactive
What are acenes and which ones are more aromatic
- Linear structure of benzene rings fused
- Only 1 pi-sextet can exist in a resonance structure at 1 time due to linear structure
- Large acenes are more stable as more resonance structures present
What does it mean if something is fully benzenoid
- Alternate rings contain pi-sextets with no isolated or conjugated double bonds in other rings
- Similar stability and reactivity of benzene
