3.10 The Structure of Benzene Flashcards
What is the bond angle in a benzene molecule?
120 degrees
Describe the stability of benzene.
Benzene is more stable than expected due to its ring of delocalised electrons.
Is benzene classified as an alkene?
NO
Why does benzene have a less exothermic enthalpy of hydrogenation than expected?
Benzene has a ring of delocalised electrons.
List the three pieces of evidence used to prove benzene is not an alkene
Reactivity of bromine, Bond length, Hydration enthalpy.
How does bromine water provide evidence that cyclohexa-1,3,5-triene is not the correct structure for benzene?
Benzene does not react with bromine water
Describe the shape of benzene
Planar hexagon. All bonds have equal lengths.
What is observed when benzene is added to bromine water?
No observation. Benzene does not react with bromine water because it is more stable than an alkene.
How many covalent bonds does each C atom in a benzene ring have?
3
What is the chemical test for an alkene?
Bromine water, room temperature and shake, Alkenes decolourise bromine water (solution goes from orange to colourless).
The enthalpy change for the hydrogenation of cyclohexene is -120 kJmol-1. What is the theoretical enthalpy change for cyclohexa-1,3,5-triene?
-360 kJmol-1 (3 x 120)
What can the enthalpy of hydrogenation be used to prove?
That benzene is not an alkene.
What feature of the bonding in benzene distinguishes it from an alkene?
It has a ring of delocalised electrons.
Which bond is longer, a C-C bond or a C=C bond?
C-C (less attraction)
How does bromine water provide evidence that benzene is not an alkene?
Benzene does not react with bromine water
What is the molecular formula of benzene?
C6H6
Why is cyclohexa-1,3,5-triene considered to be a theoretical molecule?
The p-orbitals of adjacent C atoms overlap to form a delocalised ring. The result is benzene.
Why was the theoretical enthalpy of hydrogenation of cyclohexa-1,3,5-triene predicted to be more exothermic than that of benzene?
Cyclohexa-1,3,5-triene does not account for the delocalised ring of electrons that make benzene more stable and less reactive.
Benzene has a regular hexagon shape. What length are it bonds?
All its bonds are equal in length. Benzenes bonds are longer than C=C and shorter an C-C bonds
What is the enthalpy of hydrogenation?
The enthalpy change when one mole of C=C bonds react with hydrogen.
How many C atoms does benzene have?
6
Given that benzene has a ring of delocalised electrons how are its properties different from an alkene?
It is more stable and therefore less reactive.
What type of molecule is benzene?
Aromatic
What is the empirical formula of benzene?
CH
Describe the bonding in benzene.
Each C has 3 single covalent bonds. P-orbitals overlap to form a ring of delocalised electrons.
Why is benzene less reactive than a molecule that contains C=C bonds?
It has a ring of delocalised electrons
Why is benzene not classified as an alkene?
It has a ring of delocalised electrons.
All benzenes bonds are equal in length. What does this suggest?
That benzene does not have alternating single and double bonds. It is not an alkene.
What is another way of referring to the theoretical molecule cyclohexa-1,3,5-triene?
Kekule benzene.
Kekule incorrectly thought that benzene had alternating single and double bonds.
Why is benzene more stable than cyclohexa-1,3,5-triene?
It has a ring of delocalised electrons
