6. (a) Define the octane number of a fuel - Leaving Cert Chemistry - Question 6 - 2013

Compound A: 2,2,4-trimethylpentane Compound B: 2-methylbutane

Isomerisation is a chemical process whereby a compound is transformed into its isomer. This involves rearranging the atoms within the compound to form a new structural configuration without changing the molecular formula. This process is often used to improve the octane rating of fuels.

Another isomer of A (2,2,4-trimethylpentane) and B (2-methylbutane) could be 3-methylpentane. The structural formula is:

        CH3
         |
    CH3--C--CH2--CH3
         |
        CH2--CH3

A (2,2,4-trimethylpentane) is expected to have a higher octane number than B (2-methylbutane). The larger and more branched structures of A reduce the likelihood of premature ignition compared to B, which has a more linear structure. The octane number tends to increase with the level of branching.

Using the provided heats of formation:

riangle H = [227.4 + (-985.2)] - [(-59.8) + 2 	imes (-285.8)]

riangle H = [-757.8] - [-631.4]

riangle H = -126.4 	ext{ kJ}

Benzene consists of a cyclic structure with six carbon atoms connected through alternating double bonds. Each carbon forms a sigma bond with two neighboring carbon atoms. Additionally, each carbon atom is bonded to one hydrogen atom via a sigma bond, resulting in a planar structure.

In benzene, each carbon atom participates in sigma bonding through sp² hybrid orbitals, forming strong σ bonds with adjacent carbon atoms. The remaining p orbitals of each carbon atom overlap to create a delocalized π system, which is responsible for the stability and unique reactivity of benzene.