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In borazine, each B and N is approximately trigonal planar. The alternating ring contains a sigma framework with perpendicular p orbitals; the nitrogen lone pairs can donate into the boron acceptor orbitals. Six pi electrons can therefore be distributed around the ring, giving partial B–N multiple-bond character rather than six ordinary isolated single bonds. Equal ring distances are compatible with this symmetric delocalization.

The electronegativity difference makes each B–N bond strongly polar, ${\mathrm{B}}^{\delta +}-{\mathrm{N}}^{\delta -}$. The occupied pi orbitals are weighted toward N, and the empty acceptors toward B. In benzene, equivalent carbon centers give much more even pi distribution and strong resistance to addition that destroys the aromatic system. borazine resembles benzene geometrically but has substantially more polarized, more reactive bonding. It presents electrophilic boron sites to nucleophiles and nitrogen donors to Lewis acids. Polar reagents can add across B–N, and water can cleave the ring; benzene ordinarily favors substitution under suitable conditions rather than facile polar addition or hydrolysis. The label “inorganic benzene” should therefore not be interpreted as identical aromatic stabilization or reactivity. nitrogen-bound Lewis-acid adducts and hydrogen-halide additions demonstrate this contrast experimentally.^[ref]

The two starting ester groups are adjacent on the benzene ring. Select one ester carbon as the site receiving both incoming methyl groups. A first methyl addition gives a tetrahedral intermediate; its collapse expels methoxide to give the ortho-substituted ketone $\mathrm{o}-!\mathrm{Me}{\mathrm{O}}_2\mathrm{C}-{\mathrm{C}}_6{\mathrm{H}}_4-\mathrm{C}(=\mathrm{O})\mathrm{Me}$. A second methyl addition produces $\mathrm{o}-!\mathrm{Me}{\mathrm{O}}_2\mathrm{C}-{\mathrm{C}}_6{\mathrm{H}}_4-\mathrm{C}({\mathrm{O}}^{-})\mathrm{M}{\mathrm{e}}_2$, with the alkoxide bound to magnesium.

The newly generated oxygen is now positioned to attack the remaining ester intramolecularly. Draw an arrow from its lone pair to that ester carbonyl and another from the C=O pi bond to its oxygen. Collapse of the resulting tetrahedral intermediate reforms the carbonyl and expels methoxide, closing a five-membered lactone. Protonation of magnesium-bound species in the work-up gives 3,3-Dimethylphthalide. Equivalently, protonation can first give the ortho hydroxy ester, followed by acid-assisted lactonization with loss of methanol; both descriptions show the same required bond formation.

The oxygen in the new ring is the oxygen of the first ester's original carbonyl; the surviving lactone carbonyl comes from the second ester. The product retains the six ring carbons and both original acyl carbons, loses the two ester methoxy carbons, and gains two carbons from methylmagnesium bromide. The favourable intramolecular five-membered closure accounts for the displayed product. It does not make the isolated lactone universally inert to a large excess of grignard reagent; stoichiometry and trapping/work-up conditions matter.

For ${\mathrm{C}}_6{\mathrm{H}}_6(\mathrm{l})\to 6C(\mathrm{graphite})+3{\mathrm{H}}_2(\mathrm{g})$, reversing the standard formation reaction gives ${\Delta }_r{G}^{\circ }=-124.3\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$. Thus decomposition is thermodynamically favoured under the corresponding standard conditions. benzene persists because it is kinetically stable on the laboratory timescale. Breaking and reorganizing its bonding framework requires access to high-activation energy pathways, and suitable reactive intermediates or catalysts are absent under ordinary storage conditions. A negative Gibbs free energy change specifies the favoured thermodynamic direction, not a rapid reaction rate; thermal energy does not automatically overcome a large barrier.

The planar benzene structure has a sixfold principal axis normal to its plane, six transverse twofold axes, the molecular mirror plane and an inversion centre. Its molecular point group is $D_{6h}$, the D6h molecular point group.

Purified coal gas is mainly molecular hydrogen, methane and carbon monoxide, with smaller quantities of other hydrocarbons and diluent gases. It is made by heating coal in retorts with little or no admitted air, not by completely burning the coal.

The hot products include permanent gases, condensable organic vapours and water vapour, leaving coke in the retort. Cooling removes much of the coal tar and condenses water, which captures Ammonia as ammoniacal liquor. Further washing and purification remove much of the remaining Ammonia, hydrogen sulfide and other objectionable constituents before the gas is distributed. A crude retort product and purified household coal gas therefore have different compositions.

Molecular hydrogen is commonly the largest component by volume. Methane is another major combustible constituent, while carbon monoxide contributes fuel value and serious toxicity. Ethylene and other unsaturated hydrocarbons, together with vapours such as benzene, contribute strongly to the luminous flame. Their thermal breakdown can produce incandescent soot, explaining why a small fraction of heavier hydrocarbons can disproportionately affect lighting quality. Smaller or variable quantities of carbon dioxide and molecular nitrogen dilute the combustible mixture. Molecular oxygen is not an intended major constituent; air ingress can change the mixture.

There is no single balanced chemical equation for making coal gas: coal is a complex feed and produces many products simultaneously. Nor is one exact percentage composition universal; it depends on the coal, retort temperature, operating conditions and purification. The main distinction is between the light combustible gases, the smaller illuminating hydrocarbon fraction, and contaminants or diluents. Modern natural gas, usually rich in methane, should not be used as a synonym for historical coal gas.

Phenol is benzene with one ring hydrogen replaced by a hydroxy group. That O–H group supplies the hydrogen bond donor in the experiment under hydrogen bond acceptor. ^[ref]

A six-membered oxygen heterocycle containing an exocyclic carbonyl. A charge-separated contributor with carbonyl oxygen negative and a pyrylium-like ring clarifies why aromatic character is weaker and more polarized than in benzene.

Interaction involving a quadrupolar charge distribution. benzene has no permanent dipole by symmetry but can still have orientation-dependent electrostatic interactions.

A radical ion is both a radical and an ion. Removing one electron from neutral benzene can produce ${{\mathrm{C}}_6{\mathrm{H}}_6}^{\bullet +}$, a radical cation; adding one can produce ${{\mathrm{C}}_6{\mathrm{H}}_6}^{\bullet -}$, a radical anion. These formulae specify net electric charge and an unpaired electron, without claiming that both reside on one chemical atom.^[ref]

A dibenzo-gamma-pyrone framework with two outer benzene rings, an endocyclic ether oxygen and an exocyclic ketone oxygen. The outer rings are strongly aromatic; the central ring has a more polarized, partial aromatic description.