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Question 1

What is E/Z isomerism?

Answer 1

A type of stereoisomerism where the molecules have C=C double bonds and both carbons are attached to different groups

Question 2

How do you name E/Z isomers? (Cahn-Ingold-Prelog rules)

Answer 2

1. Locate the C=C bond and redraw the molecule to show the substituents (the groups on each side of the carbons)
2. Assign the four substituents their priorities (atomic number of the first atom). If two have the same priority, move to the next atom along and use the first point of difference
3. If the two highest priority groups are on the same side of the C=C bond, the isomer is Z
   If they are on opposite sides, the isomer is E

Question 3

What is cis-trans isomerism?

Answer 3

A type of E/Z isomerism where two substituents on each carbon atom are the same. This means that the E isomer is also a trans isomer and the Z isomer is also a cis isomer

Question 4

How do we know that benzene is not a normal alkene?

Answer 4

It does not react with orange bromine water to form a colourless bromoalkane solution. This is because the delocalised ring of electrons spreads out the negative charge and makes it less susceptible to electrophilic attack and addition reactions. Instead, benzene reacts through electrophilic substitution

Question 5

How are the carbon-carbon bonds in benzene different to normal carbon-carbon bonds?

Answer 5

The C-C bonds in benzene are shorter than normal, meaning benzene is not a perfect hexagon

Question 6

Describe the bonding in benzene

Answer 6

Each carbon has 4 valence electrons, and 3 are used to form sigma bonds with the 2 neighboring carbons and the 1 neighboring hydrogen. The remaining electron is in a p orbital. The 6 p orbitals overlap above and below the plane, forming a pi orbital, meaning the electrons are delocalised

Question 7

How do you name benzene molecules?

Answer 7

Apply the normal prefixes and suffixes to the word “benzene”

Question 8

What is the prefix for the NO2 group?

Answer 8

Nitro-

Question 9

What is the prefix for the OH group?

Answer 9

Hydroxy-

Question 10

What is the prefix used when a benzene ring is part of another molecule?

Answer 10

phenyl-

Question 11

How is benzene different to a regular alkene?

Answer 11

It reacts with halogens through electrophilic substitution instead of addition, and this reaction requires high temperatures and UV light. This is because benzene does not have areas of high negative charge like a C=C bond, instead it has a low electron density ring

Question 12

Why is phenol more reactive than benzene?

Answer 12

The lone pair of electrons from the oxygen atom is partially delocalised into the delocalised ring, increasing the electron density. This means phenol reacts easily with orange bromine water, decolourising it and forming precipitate

Question 13

What casts doubt about the Kekule model for benzene?

Answer 13

Benzene reacts via electrophilic substitution, and all its bonds are the same length. Since the enthalpy change when you hydrogenate cyclohexene (one double bond) is -120 kJ/mol, you would expect it to be -360 kJ/mol for benzene, but in reality it’s -208 kJ/mol. This implies that more energy is required to break the bonds, showing that benzene is more stable than Kekule thought. This is due to the delocalised ring of electrons

Question 14

What are electron-donating groups and what are their effects on the position of substitution?

Answer 14

Groups that donate electrons into the delocalised system. These include -OH and NH2. They increase electron density at 2, 4 and 6, making these areas more vulnerable to attacks from electrophiles

Question 15

What are electron-withdrawing groups and what are their effects on the position of substitution?

Answer 15

Groups that are electronegative and withdraw electrons from the delocalised system. These include -NO2 and -COOH. They decrease electron density at 2, 4 and 6, making 3 and 5 more vulnerable to attacks from electrophiles