CD 2 - Organic Functional Groups

Question 1

What do fats and oils mostly consist of?

Answer 1

Fats and oils mostly consist of a mixed combination of esters (with varying degrees of unsaturation) of propane-1,2,3-triol

Question 2

What is an arene?

Answer 2

• An aromatic hydrocarbon
• Aromatic generally means that the compound contains a benzene ring

Question 3

How do electrons behave in arenes?

Answer 3

Electrons are delocalised i.e. not belonging to a particular atom or covalent bond

Question 4

What properties arise from arenes having delocalised electrons?

Answer 4

• Electrophiles are attracted to the aromatic ring system as it is a region of high electron density
• The aromatic ring system is very stable - therefore arenes are unable to undergo addition reactions but can undergo slow substitution reactions

Question 5

What are the two common representations of benzene?

Answer 5

The Kekule model and the delocalised model

Question 6

What does the Kekule model of benzene say about its bonding?

Answer 6

• Molecular formula C6H6
• 6 carbon atoms arranged in a hexagonal ring joined by alternative single and double bonds
• Each carbon atom is bonded to a hydrogen atom via a single sigma bond

Question 7

What is the bonding like in the delocalised model of benzene?

Answer 7

• Molecular formula C6H6
• 6 carbon atoms arranged in a hexagonal ring
• Each carbon atom uses an electron to bond with a hydrogen atom, and then two more electrons form sigma bonds with the carbon atoms either side of it
• Each carbon has one spare electron in its p-orbital. These p-orbitals overlap sideways, above and below the plane of the carbon atoms
• This forms a system of pi bonds that is spread cross the entire structure - a ring of delocalisation/electron density

Question 8

What does Kekule’s model of benzene suggest about its enthalpy change of hydrogenation?

Answer 8

• If the Kekule model of benzene was correct then its enthalpy change of hydrogenation would be -360kJmol-1 (3 times that of cyclohexane)
• However the enthalpy change of hydrogenation of benzene is actually -208kJmol-1, it is less exothermic than expected
• This suggest the Kekule model is unlikely to be correct