Solventless reactions
It is possible to do a Solventless reaction where no solvent is used.
One type of reaction is one where a liquid reactant is used neat, for instance the reaction of 1-bromonaphthalene with P4S10 is done with no added liquid solvent. But the 1-bromonaphthalene acts as a solvent.
A reaction which is closer to a true solventless reaction is Knoevenagel condensation of ketones with dicyanomethane (malononitrile) where a 1:1 mixture of the two reactants (and ammonium acetate) is irradated in a microwave oven [1].
Collin Raston’s research group have been responsible for a number of new solvent free reactions [2] and ‘Towards benign synthesis of pyridines involving sequential solvent free aldol and Michael addition reactions’, Gareth W. V. Cave and Colin L. Raston, Chem. Commun., 2000, 2119 - 2120 and Gareth W. V. Cave and Colin L. Raston and in Journal of the Chemical Society, Perkin Transactions 1, 2001, (24), 3258 - 3264.
In some of these reactions all the starting materials are solids, they are ground together with some sodium hydroxide to form a liquid, which turns into a paste which then hardens to a solid.