aromatics

Question 1

name two types of structure for a benzene ring

Answer 1

• Kekule
• delocalised ring structure

Question 2

what are some featured of the kekule model?

Answer 2

• hexagonal ring structure of 6 carbon atoms
• each carbon was bonded to a single hydrogen
• there were alternating single and double bonds

Question 3

what was kekule’s theoretical model of benzene called?

Answer 3

cyclohexa-1,3,5-triene

Question 4

bond lengths: what did kekule propose? what have experiments shown?

Answer 4

Kekule proposed: Bond lengths in the carbon ring
should alternate (between those of single (C-C) and double (C=C) covalent bonds)

experiments have shown: All bond lengths in the carbon ring are the same and intermediate in length between single / double bonds.

Question 5

reactions: what did kekule propose? what have experiments shown?

Answer 5

Kekule proposed: It should readily undergo addition
reactions, just like alkenes do (e.g. bromine rapidly reacting to decolourise bromine water)

experiments have shown: Benzene can undergo substitution reactions but does not undergo addition reactions easily.

Question 6

enthalpies of hydrogenation: what did kekule propose? what have experiments shown?

Answer 6

Kekule proposed: The enthalpy of hydrogenation
(adding H atoms to all unsaturated bonds) would be -360kJmol-1.

experiments have shown: The actual enthalpy of
hydrogenation is -208kJmol-1.

Question 7

what is the accepted model for bromine today?

Answer 7

the delocalised ring structure

Question 8

what are some features of the delocalised ring structure?

Answer 8

• it is a planar hexagonal model
• it contains 6 carbon atoms
• All C-C bond lengths are intermediate in length between that of a single C-C + double C=C
• Each C uses 3 of its outer electrons to form 3 σ bonds to 2 other C atoms, and 1 H atom. This leaves each C atom with one electron in a p orbital.

Question 9

how is the delocalised electron ring formed?

Answer 9

The lobes of the p orbitals overlap sideways with the neighbouring p orbitals to form a π bond.
- The overall result is a ring of -ve charge (“electron
cloud”) above AND below the plane of the
molecule

• The 6 p electrons in the π system are delocalised (i.e. they are free to move throughout the π system). * This delocalised cloud of π electrons is present in substances called aromatic compounds; as such, benzene and its derivatives (the arenas) are aromatic.
• A circle is used to represent the ring of delocalised electronS

Question 10

which out of the two models is more stable?

Answer 10

benzene (delocalised ring strcture) is more stable than the kekule structure - due to the delocalised electron ring