[dastgir]

Dear All,
 
Generally in a chemical reaction, reactants are classified as substrate and reagent.
How to differentiate that which one is reagent and which one is substrate.

Regards,

Gulam Dastgir

[bingyucas]

Generally, the compounds we care are called substrate, and we will need  other chemicals which are called reagent to reach our aim.

For example, we may use oxygen to make alcohol change to aldehyde.
oxygen is reagent and alcohol is substrate.

In other cases, they may change.

Best,

Michael

[stewie griffin]

I would be more specific than bingyucas. A substrate is our of compound of interest and it has carbon atoms that end up in the product. A reagent is a compound that simply acts on our molecule of interest but the reagent's atoms don't end up in our product of interest. A reactant is a something that has carbons which end up in our product, but usually the reactant is something simple like a grignard compound, a simple electrophile (allyl iodide), so on. You can certainly call your substrate a reactant, but you wouldn't call all reactants a substrate. 
Examples 1: Let's say you want to add CH₃MgBr into a fancy ketone that has taken you 15 steps to synthesize thus far. The starting ketone is your substrate (which you can also call a reactant), and the Gringard compound is the reactant (because it has carbons that show up in our desired tertiary alcohol product). However, one would not call the Gringard compound a substrate just because our focus is really on the fancy alcohol.
Example 2: Let's say you want to oxidize a secondary alcohol to a ketone using Jones reagent (CrO₃). The secondary alcohol is again our substrate (which can also call a reactant). The CrO₃ now is a reactant b/c it is just helping us transform our substrate into product, but it's not actually adding any carbon atoms to our substrate.
Finally, it's not like I read these rules in a book somewhere... this is just the way most synthetic chemists use these terms most of the time.

[bingyucas]

I would be more specific than bingyucas. A substrate is our of compound of interest and it has carbon atoms that end up in the product. A reagent is a compound that simply acts on our molecule of interest but the reagent's atoms don't end up in our product of interest. A reactant is a something that has carbons which end up in our product, but usually the reactant is something simple like a grignard compound, a simple electrophile (allyl iodide), so on. You can certainly call your substrate a reactant, but you wouldn't call all reactants a substrate. 
Examples 1: Let's say you want to add CH₃MgBr into a fancy ketone that has taken you 15 steps to synthesize thus far. The starting ketone is your substrate (which you can also call a reactant), and the Gringard compound is the reactant (because it has carbons that show up in our desired tertiary alcohol product). However, one would not call the Gringard compound a substrate just because our focus is really on the fancy alcohol.
Example 2: Let's say you want to oxidize a secondary alcohol to a ketone using Jones reagent (CrO₃). The secondary alcohol is again our substrate (which can also call a reactant). The CrO₃ now is a reactant b/c it is just helping us transform our substrate into product, but it's not actually adding any carbon atoms to our substrate.
Finally, it's not like I read these rules in a book somewhere... this is just the way most synthetic chemists use these terms most of the time.

More professional!
thanks~