Aromatic- Aromacity

Question 1

Describe the structure of benzene

Answer 1

1. 6 sp2 hybridised carbon atoms that link together to form a planar ring
2. Angles of 120 degrees
3. Bond length 139 pm- in between length of C-C and C=C
4. 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
5. P orbitals are parallel and overlap side on with p-orbitals on adjacent carbon atoms- pi electrons
6. Create 2 continuos rings of pi electrons above and below the plane of the benzene ring
7. The electrons are delocalised

Question 2

What is wrong with kekules structure

Answer 2

1. 3 c=c bonds would be shorter than the 3 C-C bonds- but all are the same length
2. C=C would be expected to react in electrophilic addition reactions like in alkenes- but actually undergoes substitution
3. More stable than thought- heat of hydrogenation of benzene is lower than expected- P-electrons are delocalised

Question 3

How can the stability of benzene be measured

Answer 3

1. C=C can be reduced using H2 and Pd catalyst - hydrogenation
2. This is exothermic as more thermodynamically stable product is formed
3. Heat of hydrogenation can be directly measured and gives an indication of stability of unsaturated compounds

Question 4

How is the heat of hydrogenation of benzene different to expected

Answer 4

1. Despite having 3 double bonds it has a heat of hydrogenation less than cyclohexadienes
2. More stable than would be expected
3. Due to delocalisation of the electrons in the ring- resonance

Question 5

Why does benzene not undergo bromination

Answer 5

1. Activation energy must be overcome
2. Too large for benzene
3. Due to aromatic stabilisation energy which must be overcome for aromatics to react with electrophiles
4. When forced a substituted product is formed instead of addition product

Question 6

What is Huckel’s Rule

Answer 6

Requirements for aromaticity

1. Molecule must be cyclic
2. Must be planar- sp2 hybridised
3. Fully conjugated
4. Must contain 4n+2 pi-electrons- n is any positive integer e.g. 6, 10, 12, 14 etc

Question 7

Can heteroatoms be part of the aromatic ring

Answer 7

1. Yes
2. In some compounds a lone pair of electrons on the heteroatom is part of the ring of pi electrons- pyrrole
3. Sometimes not- pyridine

Question 8

What are antiaromatic compounds

Answer 8

1. Cyclic, planar and contain an uninterrupted ring of pi electrons
2. Less stable than a similar compounds with localised electrons
3. Contain 4n pi-electrons
4. Very unstable and reactive

Question 9

What is a non aromatic compounds

Answer 9

1. Non-planar and have an interrupted or uninterrupted ring of pi electrons
2. Similar stability to related compounds that are non cyclic

Question 10

How many MOs are present in benzene

Answer 10

1. 6 carbon p-orbitals in benzene combine to give 6 pi molecular orbitals

Question 11

Describe bonding MOs in benzene

Answer 11

2. Lowest energy MO occurs due to all 6 p-orbitals combining in phase - no nodes
3. Next energy level contains 2 degenerate MOs with one node each- one node across bonds and other with a node across atoms
4. All these three MOs are lower in energy than the contributing AOs- all bonding MOs

Question 12

Descirbe the anti-bonding MOs in benzene

Answer 12

1. 3rd energy level contains 2 degenerate MOs, with 2 nodes each
2. Highest energy level MO occurs due to all 6 p-orbitals combining out of phase - 3 nodes
3. These 3 MOs are higher in energy than the contributing AOs and are therefore all anti-bonding MOs

Question 13

Where do the electrons stay in benzene

Answer 13

1. 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

Question 14

How can you use a Frost circle

Answer 14

1. Draw a circle and place a horizontal line through its centre
2. Draw a regular polygon with same number of sides as molecule in question- apex needs to point down
3. Draw horizontal lines where the polygon touches the circle- vertices- represent the relative energy of MO
4. Fill MO with number of electrons p-orbitals of the molecule- following correct rules
5. Bonding= below line, non-bonding= on line, antibonding= above line
6. If all paired and in bonding or non-bonding MO then the molecule is aromatic
7. If there are unpaired electrons in non-bonding or anti-bonding orbitals- anti-aromatic

Question 15

What can aromatic compounds be classed as and what is the symbol

Answer 15

1. Arenes

2. C6R5= aryl (Ar) groups

Question 16

What are the different aryl groups called

Answer 16

1. C6H5- Phenyl (Ph)
2. Phenyl group linked through a methylene unit- Benzyl (Bn)
3. Phenyl group linked through a carbonyl- benzoyl (Bz) group

Question 17

What is the benzylic position

Answer 17

1. The CH2 carbon in benzene groups is known as the benzylic position

Question 18

Describe the substitution positions

Answer 18

1. Aromatic carbon atom directly bonded to substituent= ipso
2. Adjacent to this - ortho (o)
3. Next position around - meta (m)
4. Position on opposite side of the ring - para (p)

Question 19

What are the basics of naming aromatics

Answer 19

1. Find highest priority root name

2. Find lowest numbering system

Question 20

What is the closest heterocycle relative to benzene

Answer 20

1. pyridine
2. 6-membered nitrogen-containing compounds
3. C-H unit has been replaced with a N: unit
4. It conforms to Huckel’s rules and so is aromatic

Question 21

Describe the position of the lone pair of nitrogen in pyridine

Answer 21

1. Nitrogen atom is sp2 hybridised
2. Its lone pair of electrons is in an sp2 orbital perpendicular to the aromatic ring
3. The lone pair is not delocalised around the aromatic ring and is isolated to the sp2 orbital

Question 22

What do the properties of pyridine mean it is used for

Answer 22

1. Commonly-used organic base

2. Nucleophilic organocatalyst- DMAP (derivative)

Question 23

Can 5-membered heterocycles be aromatic and describe an example

Answer 23

1. Yes, if an entire C=C unit is replaced with an N-H unit - pyrrole
2. The nitrogen atom is sp2 hybridised but the lone pair is in the unhybridised p-orbital not sp2
3. Therefore it is delocalised around the aromatic ring
4. 6 pi-electrons delocalised across 5 cyclic, planar, fully conjugated sp2 hybridised orbitals
5. Also electron rich

Question 24

What happens if C-H atoms in pyrrole are replaced with further N atoms

Answer 24

1. The extra nitrogen atom is said to be pyridine-like - lone pairs are in sp2 hybridised orbitals and are perpendicular to the aromatic ring

Question 25

What can Nitrogen in pyrrole be replaced with and describe what happens

Answer 25

1. Other heteroatoms- sulfur or oxygen 2. 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

Question 26

What happens if a benzene ring is fused to an aromatic heterocycle e.g. pyrrole

Answer 26

1. Give extended aromatic heterocyclic molecules 2. All the heavy atoms are sp2 hybridised with 10 pi-electrons delocalised across them - aromaticity is observed over the entire molecule

Question 27

What is a PAH and how is it formed

Answer 27

1. polycyclic aromatic hydrocarbons | 2. Caused by fusion of benzene to itself multiple times

Question 28

Why are PAHs important for scientific research

Answer 28

1. Suspected carcinogens 2. Constituents of tobacco smoke 3. PAH detection and monitoring- estimate more than 20% of all carbon in universe can be found in PAHs

Question 29

Why are helicenes interesting

Answer 29

1. 6 benzene rings 2. Display axial chirality 3. Uses in asymmetric synthesis

Question 30

Why are fullerenes useful

Answer 30

1. Can take form of spheres or tubes | 2. Used within various applications

Question 31

Why is graphene useful

Answer 31

1. Worlds first 2D material 2. 200 times stronger than steel but extremely thin diameter 3. Lightweight, flexible and electrically and thermally-conductive 4. Application in electronics, energy and biomedical industries

Question 32

What is the simplest PAH and describe its properties

Answer 32

1. Napthalene- 2 benzene rings fused 2. 10 pi-electrons 3. Less aromatic than benzene 4. Only one ring is fully aromatic at a given time 5. Can be easily reduced under mild conditions 6. Used in OLED and solar cells

Question 33

What is Huckel's rule limited to

Answer 33

1. Monocyclic systems

Question 34

What rule can be used to describe aromaticity in PAHs

Answer 34

1. Clar's sextet theory

Question 35

What does Clar's sextet theory state

Answer 35

1. Resonance structure with the largest number of disjoint aromatic pi-sextets is the most important for a PAH's characterisation 2. PAHs with more pi-sextets are kinetically more stable than those with less 3. Rings which have the most pi-sextets in resonance structures are more aromatic than other rings and therefore more stable/less reactive

Question 36

What are acenes and which ones are more aromatic

Answer 36

1. Linear structure of benzene rings fused 2. Only 1 pi-sextet can exist in a resonance structure at 1 time due to linear structure 3. Large acenes are more stable as more resonance structures present

Question 37

What does it mean if something is fully benzenoid

Answer 37

1. Alternate rings contain pi-sextets with no isolated or conjugated double bonds in other rings 2. Similar stability and reactivity of benzene