[Norovirus]

Good afternoon,

This may sound like a basic question but as someone with little training in Chemistry, I cannot come up with an answer.

Why are solvents typically required in an organic (or inorganic ) reaction? Suppose all the reagents and reactants are all liquids. Why can't we simply mix them all together? Solvents add additonal costs to a process.

Also, if all reagents and reactants are solids, why can't one simply mix the reagents and reactants in a vessel without a solvent?

And if the reagents and reactants comprises of liquids and solids and the solids have a great degree of solubility  in the liquids, why can't the liquid reagents and/or reactants assume the role of a solvent?

Why is being soluble necessary? Do solvent molecules enhance the rate of molecular collision of the solute molecules? How so?

Thank you for your taking the time to read my queries

[OCSaviour]

Here is a better question. What are reaction intermediates? Do they have an important role in the formation of the product?

 

[discodermolide]

Generally the less solvent you use the better the process. So you are correct in some of your assumptions.
Two solids will typically not react (assuming they are not super reactive). In that case you would have to melt them to get a reaction.
However that said assuming your product is a solid generated from two liquids you will have problems in stirring the mixture and removing any exotherm. So a minimum of solvent will be beneficial in that case.
Also solvents are very good for controlling exothermic reactions in that the heat generated causes the solvent to boil and keeps the reaction under control.
Having the correct solvent is good also for the case when your product is a solid. Correct choice will let the product crystallise out of the reaction mixture giving a substantial purification effect.
Process chemists spend a lot of time examining solvent choice for their reactions and I'm confident that they get it right 99.99% of the time.

[Corribus]

Reactions generally require (more or less) two reacting molecules to collide with each other. In the case where individual reaction events are fast, then the reaction rate essentially becomes diffusion limited. That is, the rate of reaction is based on how fast two reactants can move about through the reaction medium (whatever that is). Another way to put this is that a reaction cannot go faster than the rate at which reactants travel through the reaction mixture. Solid-solid reactions tend to be slow not because reaction events are thermodynamically unfavorable, but rather because molecules in solids are practically immobile. So, effectively only interfacial molecules can interact - and only when the interfaces are in direct contact, which even then is infrequent because of insulating air spaces. When two solids are dissolved in a solvent, two things happen: the effective surface area of the reactant is reduced to basically the molecular size (every molecule is at "the interface") and, probably more important, the diffusion rates of reactants increase substantially, which dramatically increases the probability of a collision between reactants. In a word, solvents facilitate fast and homogeneous mixing. As discodermolide has mentioned, you usually want a concentrated solution. Concentrated and dilute solutions have (to a first approximation) the same diffusion rates of reactants - the difference as far as reaction rates goes now becomes how far on average a reactant has to diffuse before a collision takes place. This is also why reactions slow down as they proceed - the reaction concentration decreases as reactants are consumed, which increases the time it takes for remaining reactants to find each other by diffusion through the solution medium.

[critzz]

As a matter of fact there are plenty reactions witch are done "neatly", which means no solvent.
I also think a problem problem with neat reactions is the exothermic heat formation, which results in a thermal runaway.

I did a neat reaction once in my life (solid into liquid), but somehow the reaction mixture cooled down a bit and it worked like a charm.

[OCSaviour]

I guess we can give a complete answer for this one, so I am going to present you my answer. I earlier asked you to consider a question regarding reaction intermediates.

Solvents are necessary to check the stability of each element involved in the reaction, they also help in maintaining the physical conditions, for example, pH balance.

In Organic Chemistry reactions, they are, mostly, used to stabilize the reaction intermediates. Depending on the type of reaction mechanism, solvents are used either to stabilize the charge on the nucleophile( or other agents) or to keeps it's strength intact.

Your question regarding the rate of the molecular collision has been answered brilliantly already by senior members.

[kriggy]

Also, some reactions require high dilution to work fine. For example, synthesis of large rings (like 12-15 membered or bigger) by condenstaion of two bi-functional reagents is usualy done at high dilution because it reduces the chance of intermolecular reaction.

[Norovirus]

Ah that makes sense. All of the answers are explained well to a non-specialist. Thanks, everyone!