Table 8.

PCET Thermochemistry of Hydroquinones and Semiquinones in H₂O^a

HQ•/H2Qb	E∘(HQ•/−)	E∘(H2Q•+/0)	pKa(H2Q•+)	pKa(H2Q)		E∘(V vs H2)	BDFE
1,2-H2Q	0.59	–	–	12309	–	1.30	82.8
1,4-H2Q	0.36c	1.1310	−1.1311	9.85312	–	0.94	74.6
2-methyl-1,4-H2Q	0.31c	–	–	10.1312	–	0.91	73.8
2,3-dimethyl-1,4-H2Q	0.28c	–	–	10.4313	–	0.90	73.6
2,5-dimethyl-1,4-H2Q	0.37c	–	–	10.4313	–	0.98	75.6
2,3,5-trimethyl-1,4-H2Q	0.20c	–	–	10.8313	–	0.84	72.2
tetramethyl-1,4-H2Q	0.10c	–	–	11.3313	–	0.77	70.6
2-methyl-1,4-H2NQ	–	–	–	–	–	0.77	70.6
2,3-dimethyl-1,4-H2NQ	–	–	–	–	–	0.67	68.1
Q/HQ•b	E∘(Q•/−)	E∘(HQ•/−)	pKa(HQ•+)	pKa(HQ•)	pKa(HQ−)	E∘(V vs H2)	BDFE
1,2-HQ•	0.043	–	–	5314		0.34	60.6
1,4-HQ•	0.099315	[0.76]	−7311	4.1316,317	11.4312	0.34	60.7
2-methyl-1,4-HQ•	0.023315	–	–	4.5318	12d	0.29	59.5
2,3-dimethyl-1,4-HQ•	−0.074315	–	–	4.7318	12.6313	0.20	57.5
2,5-dimethyl-1,4-HQ•	−0.067315	–	–	4.6318	12.5313	0.21	57.5
2,3,5-trimethyl-1,4-HQ•	−0.165315	–	–	5318	12.9313	0.13	55.8
Tetramethyl-1,4-HQ•	−0.235319	–	–	5318	13.2313	0.06	54.2
2-methyl-1,4-HNQ•	−0.203319	–	–	4.4318	–	0.06	54.1
2,3-dimethyl-1,4-HNQ•	−0.240315	–	–	4.3318	–	0.01	53.1

^aPotentials for 1e⁻ reductions are in V vs SHE. Values for E^∘(V vs H₂) are in V; – eE^∘ is the average free energy for $\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.{\text{H}}_{2(\text{g})}$ addition. Uncited values of E^∘(V vs H₂) were calculated from other values in the row using Scheme 2 oreq 18. BDFEs are in kcal mol⁻¹ and were calculated from E^∘(V vs H₂) usingeq 18. Values in [square brackets] have been calculated from the other values in the row using Hess’ law.

^bH₂Q = 1,4-dihydroxybenzene; H₂NQ = 1,4-dihydroxynaphthalene; HQ = semiquinone; HNQ = seminaphthoquinone.

^cE^∘(HQ^•/−) derived from the E^∘(V vs H₂) of H₂Q/Q (Table 7) and HQ^•/Q.

^dAverage pK_a(HQ⁻)from refs 312 and 320.