Why can't two 3° free radicals combine?

Question

In Kolbe Electrolysis, my teacher told that when a 1° or 2° free radical is used, we get alkanes through dimerisation. Why wont dimerisation occur for a 3° one??

Answer 1

Question is: Why can't two $3^\circ$ free radicals combine (during Kolbe Electrolysis)?

The simplest answer is there is no reason why not. Here is the counter example to show it is possible and achieved as far back as 1959 Ref.1). I attached the available PDF file for convenience. There are several trisubstituted succinic acid derivatives, which are achieved by the dimerization of corresponding disubstituted malonic acid derivatives after decarboxylation during Kolbe Electrolysis.

Although the intermediate in this reaction is not technically a $3^\circ$ free radical but $2^\circ$ free radical with really bulky group ($\ce{CH3O2C-CH^.-CH2Si(CH3)3}$), I think this paper (Ref.2) is worth considering. Yet, some examples in similar vain (e.g., $\ce{CH3O2C-CH^.-CH2C(CH3)3}$) are also discussed in Ref.1. Eberson has continue his work with Kolbe Electrolysis to present some other example of tetraalkylsuccinc acid derivatives (Ref.3).

These examples are thoroughly reviewed by Schäfer in 1990 (Ref.4).

Other than Kolbe Electrolysis, there are some other examples of dimerization of $3^\circ$ free radicals have been reported. For instance, synthesis of 2,2,3,3-tetramethysbutane has been undergone with similar mechanism. The compound can be obtained by reaction of Grignard reagent, tert-butylmagnesium bromide with ethyl bromide, or of ethylmagnesium bromide with tert-butyl bromide in the presence of manganese(II) ions (Ref.6). The mechanism of this transformation is believed to be undergoing through the dimerization of two tert-butyl radicals, which are generated by decomposition of the organomanganese compounds generated in situ such as: $$\ce{(CH3)3C-MgBr + MnCl2 -> (CH3)3C-MnCl + MgBrCl}$$

References:

1. Lennart Eberson, “Studies on Succinic Acids I. Preparation of $\alpha,\alpha'$-Dialkyl- and Tetraalkyl-Substituted Succinic Acids by Kolbe Electrolysis,” Acta Chem. Scand. 1959, 13, 40-49 (DOI: 10.3891/acta.chem.scand.13-0040) (PDF).
2. Lennart Eberson, “The Synthesis of Some Aliphatic Organosilicon Dicarboxylic Acids. III,” Acta Chem. Scand. 1956, 10, 629-632 (DOI: 10.3891/acta.chem.scand.10-0629) (PDF).
3. Lennart Eberson, “Studies on Succinic Acids V. The Preparation and Properties of Diastereoisomers of Tetraalkylsuccinic Acids,” Acta Chem. Scand. 1960, 14, 641-649 (DOI: 10.3891/acta.chem.scand.14-0641) (PDF).
4. Hans-Jürgen Schäfer, “Recent Contributions of Kolbe Electrolysis to Organic Synthesis,” Topics in Current Chemistry: Electrochemistry IV; Vol. 152, Eberhard Steckhan, Editor; Springer-Verlag: Berlin, Germany, 1990, pp. 91-159 (PDF).
5. M. S. Kharasch, J. W. Hancock, W. Nudenberg, P. O. Tawney, “Factors Influencing the Course and Mechanism of Grignard Reactions. XXII. The Reaction of Grignard Reagents with Alkyl Halides and Ketones in the Presence of Manganous Salts,” J. Org. Chem. 1956, 21(3), 322-327 (https://doi.org/10.1021/jo01109a016).