[harsh]

Sir,I can't figure out why do [Eu+] and [Nu-] even attack a substrate? And why does a neutral compound[The reagent I meant] on reaction with certain organic compound breaks into two and one gets reacted with the substrate while the other does not?
For example,here..
R-OH + HX----(ZnCl2)---> R-X + H2O
Why does X attacks the substrate and not H? And how does the bond cleavage takes place here?
How should I know if it will cleave heterolytically or homolytically? And when will the reaction proceed even further. Like here...
C2H5Br + NH3-----> C2H5NH2 + HBr
Go on forming Primary,secondary,tertiary amine? And why not just stop like other reaction?
 I can't get this in my head.Am I to learn or memorize the reaction without knowing how/why?
It would be really nice if you tell me how to learn the reactions.I always forget them easily and I am not able to reproduce the reactions in my tests. Please guide me in the right direction and any correct tutorial to guide me. I would be really grateful. Thank you.

[Honclbrif]

Are you being taught mechanisms to go along with these reactions, or are you just being asked to memorize products based on reactants and reagents? Mechanisms should explain most of your questions.

The ability of an electrophile or nucleophile to attack a neutral molecule is based on uneven charge distribution in the neutral molecule. Take for instance a carbonyl (C=O) compound. The oxygen is more electronegative than carbon, so the electron density is greater around the oxygen than the carbon. This results in the oxygen possessing a partially negative charge and the carbon possessing a partially positive charge. Therefore a nucleophile can attack the carbon and an electrophile can attack the oxygen.

The same principles are at play in about 99% of organic reactions. Look at how the charge is distributed and you will know if a nucleophile or an electrophile can attack. As long as you recognize that an electrophile is relatively electron poor, and a nucleophile is relatively electron rich you can work your way through most problems.

As for the heterolytic vs. homolytic question: bond homolysis really only ever occurs when a radical initiator is present, or radiation (light) is involved. Otherwise most reactions proceed heterolytically.

With the amination question, there are a number of questions you can ask yourself to help determine how the reaction will proceed: The direct product of the reaction is a primary amine, Br-, and H+. Which is a better base: an amine, or Br-? Which will the H+ be associated with? When you consider how these products look, what further reactions can they undergo?

[harsh]

Thank you sir for your response! 
Mechanisms to only a few reaction is being taught like - Halogentation,Markovnikof/anti-mar etc. But most of them are without mechanism.So I have to memorize it.But there are lots of reaction without mechanism, how will I be able to learn them? Pretty confused.

Take for instance a carbonyl (C=O) compound. The oxygen is more electronegative than carbon, so the electron density is greater around the oxygen than the carbon. This results in the oxygen possessing a partially negative charge and the carbon possessing a partially positive charge. Therefore a nucleophile can attack the carbon and an electrophile can attack the oxygen.

Most of the reagent that attacks are molecule itself and does not exist as independent [Eu+] or [Nu-].So does the bond cleavage of the substrate takes after the reagent attaches itself to it or it cleaves before attacking and then attaches itself?
And why do they even have the tendency to attack? How will I know if the reaction will occur or not? And in the example I quoted why will the "X"[The halogen] will let go of its H on attacking?
"Which is a better base: an amine, or Br-?" -- I think amine will be better base because HBr is strong acid,hence Br- weaker conjugate base? Am I correct Sir?
"Which will the H+ be associated with?"-- I don't get this question
And a question sir? A little off topic from this.Like I have questions to arrange the organic compounds in order of their acidity. So how should I solve them? First form the conjugate base and then look for it stability?
So I have to apply inductive, resonance, hyperconjugation . But what to apply when? Our teacher uses sometime inductive to explain it. What's the correct order? I mean what should be my solving algorthim for such questions? 

[Hand15]

Most of the reagent that attacks are molecule itself and does not exist as independent [Eu+] or [Nu-].So does the bond cleavage of the substrate takes after the reagent attaches itself to it or it cleaves before attacking and then attaches itself?
And why do they even have the tendency to attack? How will I know if the reaction will occur or not? And in the example I quoted why will the "X"[The halogen] will let go of its H on attacking?
"Which is a better base: an amine, or Br-?" -- I think amine will be better base because HBr is strong acid,hence Br- weaker conjugate base? Am I correct Sir?
"Which will the H+ be associated with?"-- I don't get this question
And a question sir? A little off topic from this.Like I have questions to arrange the organic compounds in order of their acidity. So how should I solve them? First form the conjugate base and then look for it stability?
So I have to apply inductive, resonance, hyperconjugation . But what to apply when? Our teacher uses sometime inductive to explain it. What's the correct order? I mean what should be my solving algorthim for such questions? 

What kind of organic chemistry course are you taking? Most of your questions should have already had the answers written very clearly in the undergraduate organic chemistry textbooks.

When you're dealing with organic reactions, there are several things you must identify correctly before you could solve and understand the reaction.

For example in the reaction of: C₂H₅Br + NH₃ -> C₂H₅NH₃⁺ Br^-
You should ask yourself:
1. Are there any nucleophile? Yes, NH₃
2. Are there any good leaving group? Yes, Br
3. What kind of reaction will it undergoes? Nucleophile attack electrophile
4. Where is the electrophile? At the carbon where Br is attached to
5. What mechanism could it possibly go? Most possible is SN2 reaction
(If you don't know SN2 reaction, this link will help: http://en.wikipedia.org/wiki/SN2_reaction)
6. After the nucleophile attack the electrophile, what will happen? C-NH₃⁺ bond formed and Br^- leave.
7. Where will the Br^- end? If no other base exists, the Br^- countered with C₂H₅NH₃⁺ to form C₂H₅NH₃⁺ Br^- salt as final product

In the case of forming secondary and tertiary amine, you should ask yourself:
8. What if there are other base? NH₃ is a stronger base than C₂H₅NH₂, it will abstract the proton of C₂H₅NH₃⁺, forming NH₄Br salt.
9. What will happen then? C₂H₅NH₂ will be generated
10. Are there any side reaction that will occur? Yes, C₂H₅NH₂ is nucleophile itself....

and so nucleophile attack electrophile....

[harsh]

Sorry I am not among the smart lot so its difficult for me to understand.
Okay I think I pretty much get this.
But still I am clueless about the acidity/basicity questions which ask you to arrange in order of their acidity or basicity. What procedure should I follow in solving such questions? Please tell me,I would be really grateful.