A-Level CAIE Chemistry A2 29.3 Shapes of aromatic organic molecules; σ and π bonds Question Bank

(a)

State the hybridisation of the carbon atoms and the C-C-H bond angle in benzene, $\mathrm{C}_{6} \mathrm{H}_{6}$ . Explain how orbital overlap leads to the formation of $\sigma$ and $\pi$ bonds in benzene.

[ 3 ]

(a)

(i)

When benzene undergoes nitration a nitro group substitutes at a carbon atom.
State the shape (geometry) around the substituted carbon atom
- in the benzene molecule,
- in the intermediate complex,
- in the nitrobenzene product.

[ 2 ]

(a)

The ligand bipyridine consists of two pyridine rings.

Pyridine, $\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{~N}$ , and benzene, $\mathrm{C}_{6} \mathrm{H}_{6}$ , have similar planar, cyclic structures.

Fig. 4.2

By reference to the hybridisation of the carbon atoms and the nitrogen atom, and orbital overlap, suggest how the $\sigma$ and $\pi$ bonds are formed in a pyridine molecule.

[ 3 ]

[Maximum number: 5]

$5 \quad L$ -DOPA is used in the treatment of Parkinson's disease. It can be prepared from vanillin.

L-DOPA

vanillin

(a)

L-DOPA and vanillin each contain an aromatic benzene ring.

Describe, with the aid of a diagram, the bonding and shape of a molecule of benzene, $\mathrm{C}_{6} \mathrm{H}_{6}$ .

[ 5 ]

[Maximum number: 3]

Boron forms many useful compounds.

(a)

Benzene, $\mathrm{C}_{6} \mathrm{H}_{6}$ , and borazine, $\mathrm{B}_{3} \mathrm{~N}_{3} \mathrm{H}_{6}$ , have planar, cyclic structures.

[ 3 ]

(i)

Describe the structure of and bonding in benzene, $\mathrm{C}_{6} \mathrm{H}_{6}$ .

[ 3 ]

(a)

(i)

Briefly explain why the benzene molecule is planar.

[ 2 ]

(ii)

Briefly explain why all the carbon-carbon bonds in benzene are the same length.

[ 2 ]

(a)

Describe the shape of delocalised benzene.

Include the geometry of each carbon, the C-C-H bond angle and the type of bond(s) between the carbon atoms and between the carbon and hydrogen atoms.

[ 2 ]

(b)

Suggest why Dewar benzene and Ladenburg benzene are unstable isomers of $\mathrm{C}_{6} \mathrm{H}_{6}$ .

[ 1 ]

[Maximum number: 2]

Ruthenium and osmium are transition metals below iron in Group 8 of the Periodic Table.

(a)

Fig. 4.1 shows another ruthenium complex.

Fig. 4.1

This complex contains the neutral ligand pyrazine.

[ 2 ]

(i)

Pyrazine is an aromatic compound. The bonding and structure of pyrazine is similar to that of benzene.

Describe and explain the shape of pyrazine.
In your answer, include:
- the hybridisation of the nitrogen and carbon atoms
- how orbital overlap forms $\pi$ bonds between the atoms in the ring.

[ 2 ]

(a)

Benzene can be converted into cyclohexane.

[ 2 ]

(i)

State the bond angles in benzene and cyclohexane. bond angle in benzene bond angle in cyclohexane Explain your answers.

[ 2 ]

(a)

By reference to the formation of $\sigma$ and $\pi$ bonds, describe and explain the shape of a benzene molecule, $\mathrm{C}_{6} \mathrm{H}_{6}$ .

[ 3 ]

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