[Hijaz Aslam]

One fact that always irritated me in organic chemistry is that, it is hardly possible to work out a chemical reaction. But recently I realized that mechanisms can help a great lot in solving a chemical reactions.

Can I know what are the best ways to solve a chemical reaction ourselves or to know whether a chemical reaction I've memorized and wrote in the answer sheet is correct? What are the basic points for mastering organic chemistry.

I would also like to know how to understand how a compound cleaves. For instance how do we know HCl -> (H+)+ (Cl-)?

[kriggy]

Well HCl is an acid so that why it gives H⁺ away.

About your fitst question:
memorizing reactions in orgchem is huge mistake IMO (becaue there are thousands of them). Of course there are some that are good if you memorize them but basicaly always its that negative charge is atracted to positive or electrophile is atracted to nucleophile.

So I suggest understanding reaction mechanisms and basic concept is a key to predict the product. Then its practice and practice.

[orgopete]

You should look at my book. I take a different approach to this problem. It is much like you may see some of my answers. If you do a reaction in the lab, the results are data. They are always correct. Our job is to interpret the results. I prefer to learn what appears to be rules or sets of rules. In this way, you would write a different set of rules for a primary halide versus a tertiary. Then you might try to determine if a secondary halide is going to react more like a primary than a tertiary.

When I encounter a new reaction, I try to evaluate its reaction conditions with other reactions that I know.

Although I know there are others that will disagree with me on this, electronegativity theory was a poorly researched notion. It has spawned many derivative theories to try to support it. This does result in some of the contradictions students must deal with.

[For anyone who feels compelled to do so, please send me your best example showing flourine is more electron withdrawing that Cl, Br, or I. Or post it in one of my electronegativity threads, e.g.: http://www.chemicalforums.com/index.php?PHPSESSID=4129f4fe0f5b6b0ef4a5a4686e38648a&topic=47837.0.
Let's not high jack the poster's question.]

[Hijaz Aslam]

Well HCl is an acid so that why it gives H⁺ away.

About your fitst question:
memorizing reactions in orgchem is huge mistake IMO (becaue there are thousands of them). Of course there are some that are good if you memorize them but basicaly always its that negative charge is atracted to positive or electrophile is atracted to nucleophile.

So I suggest understanding reaction mechanisms and basic concept is a key to predict the product. Then its practice and practice.

kriggy - Thanks for your response. Can you point out the most basic theories (like hyperconjugation, inductive effect etc). I feel like I am missing certain concepts at certain points.

For a long period I've despised the subject. As I can't avoid the subject as a part of my academics, I've taken up a little interest in it and found that it isn't that chaotic or unpredictable as I thought. I would like to master it (at the basic level) about in a month. Please do help me.

I am all confused with how we predict a cleavage. If we have a compound, does it do heterolytic or homolytic cleavage. Which atom takes up the electron, which one doesn't. I am all confused  .

[kriggy]

I think acid/base, mezomeric/inductive efects, electrophility/nucleophility are the most important imho. If I take it to the extreme I think you just need to know that electrophile reacts with nucleophile.

Well the cleavage depends on conditons. Using UV light or heat? You get homolitic cleavage. If its something else, its heterolytic (most of the time)

[Altered State]

kriggy - Thanks for your response. Can you point out the most basic theories (like hyperconjugation, inductive effect etc). I feel like I am missing certain concepts at certain points.

For a long period I've despised the subject. As I can't avoid the subject as a part of my academics, I've taken up a little interest in it and found that it isn't that chaotic or unpredictable as I thought. I would like to master it (at the basic level) about in a month. Please do help me.

I am all confused with how we predict a cleavage. If we have a compound, does it do heterolytic or homolytic cleavage. Which atom takes up the electron, which one doesn't. I am all confused  .

I see that ypu have quite a few specific questions, but my guess is that your problem is your lack of general knowledge on organic chemistry.

I totally encourage you to get your hands on an organic chemistry textbook (either the one orgopete has suggested, which from the freely avaliable pages, seems very good, or either another one, like Vollhardt's or Wade's, etc.) and start with basic concepts. For example, hyperconjugation and inductive effects, will be very well defined in a textbook. I see no reason for pointing out here those concepts or theories, since, except in the case that those are the only ones who you have problems understanding with, 2-3 explanations would be quite useless if you don't understand all of the most important ones, and its context. A basic organic chemistry / mechanisms book will help.

Concerning your later more concrete questions:
HCl is an acid. Like every acid, it disociates in solution. The rate of disociation (I'm talking here only about heterolytic cleavage)  depends on a constant (aciditý constant), and there is an equilibrium between the disociated form and the non-disociated form. HCl is considered a "strong acid", in effect, it practically disociates in a 100%, this means, in solution, almost 100% of HCl will be cleaved and in the form H+ and Cl-

An homolytic cleavege can only occur through the formation of radicals. And radicals can only form in certain conditions:
- In the presence of light (generally UV or visible light, which is frequently represented in rxn schemes as "hv")
- In the pressence of a radical initiator, which is a compound that, due to his structure, can be readily cleaved in an homolytic way, this is, forming two radicals.
This radicals will initiate a chain radical reaction, so one of these could react with a HCl molecule producing a new radical, which can be H· or Cl·
An example of radical initiator can be certain peroxides, or for example, AIBN, like this:



This reactions is really favoured due to the realease of a nitrogen gas molecule.

Any way, the radical is formed and can react with HCl (or other molecules, organic or inorganic ones) to make them cleave homolyticaly.

- Heat-induced radical formation can also take place, but don't even bother about these cases.

Most compounds (and HCl is one of these) won't cleave homolytically with just sunlight or UV exposure to an apreciable extent to be considered.

So, to sum up, in a fairly-basic exercise organic chemistry, never expect homolytic cleavages except a radictal initiator (such as AIBN or ROOR') or light (AKA, hv) is indicated.




Hope this helps.
Any other specific questions will be answered, but you will need to make use of a textbook and/or your lecture notes to get out of this

[Hijaz Aslam]

Altered State - Thanks a lot for your support.

I've gone through my school textbook and has learnt the basics of the organic chemistry. But at certain points like the one for homolytic cleavage, my text never mentions the conditions for the occurrence of homolytic cleavage. Moreover the syllabus does not correspond to the mechanism often. It does not encourage a critical approach towards the subject rather than a rote one.

My basic question is, is it possible to derive a chemical equation (given the substrates and the reagents) and find out the products? I am clear with the basics like carbon tetra-valence, Lewis Structure etc. (pre-organic chemistry, though I am not completely thorough I am pretty strong at it).

To add an example. We know that certain compounds for instance lets say CH₃Cl, cleaves into CH₃⁺Cl^- as Chlorine is more electronegative than Carbon (I believe that is the mechanism). But if we take the case of RMgCl, which is a Grignard's Reagent, during a reaction RMgCl cleaves as R^-[Mg⁺Cl] but Chlorine which is more electronegative does not take the electron and cleave as RMg⁺Cl^- (or am I wrong?).

I believe there are a number of basic rules that govern the mechanisms of organic chemistry. I specifically would like to know them and how they are applied. For instance, I have no idea how hyperconjugation works though I know the theory.

I am in a bit of a worry, I would like to master chemistry in (class +2) level within a month at least. Further, I would like to know how to keep in mind certain reactions and use it in conversions. And where can I get lots of conversion sheets (class +2) level, particularly Alcohols, Phenols and Ethers and Aldehydes, Ketones and Carboxylic Acids.

Please Do Help Me, and thanks for all your support kirggy and Altered State 

[kriggy]

Well its possible to derive the chemical equation if you know reactants and reagents and stuff but the thing is, you might draw it on a paper but when you do the reaction in a lab, you might get something totaly different (that is case for more complex structures and "advanced" reactions tbh)

in case of grignard reagent, it cleaves to
R^- + [MgCl]^- (because its Mg³⁺ and Cl^-)

TBH I have no idea what is the theory behind hyperconjugation, but I know that the result is that hydrogen at carbons adjacent to groups like CO or NO2 (in non aromatic systems) are acidic (or at least we were told that its due to hyperconjugation) which than have some interesting consequences in some reactions:
http://en.wikipedia.org/wiki/Acetoacetic_ester_synthesis

[Altered State]

My basic question is, is it possible to derive a chemical equation (given the substrates and the reagents) and find out the products? I am clear with the basics like carbon tetra-valence, Lewis Structure etc. (pre-organic chemistry, though I am not completely thorough I am pretty strong at it).

Well its possible to derive the chemical equation if you know reactants and reagents and stuff but the thing is, you might draw it on a paper but when you do the reaction in a lab, you might get something totaly different (that is case for more complex structures and "advanced" reactions tbh)

This is pretty much it... You can make predictios based on analogies with other reactions taking into account reagents, conditions, and mechanisms elucidated for the reactions.
But the truth is that you can never know for sure. In practice, you can expect a reaction to work perfectly, but you don't even get your expected product at trace levels.
In general, the more complex the reagents are, the more difficult would be to make an actual prediction.
I suppose that at your level, you will be asked to be able to predict products accepting several premises, so you should stick with that, but always have in mind that what works on paper do not necesarily have to work on the lab.

I've gone through my school textbook and has learnt the basics of the organic chemistry. But at certain points like the one for homolytic cleavage, my text never mentions the conditions for the occurrence of homolytic cleavage. Moreover the syllabus does not correspond to the mechanism often. It does not encourage a critical approach towards the subject rather than a rote one.

Don't be so obsesive with "cleavages". Most organic reactions you will study at your level (which I don't know but I suppose is like organic chemistry but not from a pure Chemistry degree?) are just based on moving pairs of electrons. Focus on the understanding of what nuclephiles/electrophiles acids/bases oxidants/reducing agents are.

Moreover, if your book do not show the mechanism for a reaction, just Google it. You probably will find something.

To add an example. We know that certain compounds for instance lets say CH₃Cl, cleaves into CH₃⁺Cl^- as Chlorine is more electronegative than Carbon (I believe that is the mechanism).

That molecule will just not cleave in ions under no drastic conditions. Most organic molecules wont (exceptions: carbocations, like those formed on E1 or Sn1 reactions or carbanions, like carbenes)
That molecule can undergo a nucleophilic substitution so a nuclephile, like lets say, bromide, attacks the carbon, so the chlorine leaves and CH3Br is formed, in a concerted 1 step mechanism (no cleavages!). You can say that the molecule is POLARIZED towards the chlorine, ie, partial negative charge on chlorine and partial possitive charge on carbon. Not

You need to understand each type of basic mechanisms and its basis, then you can start to get serious.

But if we take the case of RMgCl, which is a Grignard's Reagent, during a reaction RMgCl cleaves as R^-[Mg⁺Cl] but Chlorine which is more electronegative does not take the electron and cleave as RMg⁺Cl^- (or am I wrong?).

You have an organometallic compoud there, still not a ionic compound! So again it will not cleave in ions under normal conditios. You have an STRONG negative charge on carbon and partial possitive charge on the metal. This makes Grignard reagents very good nucleophiles (and bases, of course)

in case of grignard reagent, it cleaves to
R^- + [MgCl]^- (because its Mg³⁺ and Cl^-)

You'd better fix that "3" just in case any mad inorganic chemist is watching us 

@Hijaz Aslam, it is Mg2+

For instance, I have no idea how hyperconjugation works though I know the theory.

Hyperconjugation is a tricky concept that is usually just briefly explained (or even just mentiones) in undergrad courses. In my opinion it shouldn't be even mentioned in basic courses, because to truly understand the concept, requires a bit of a quantum-chemistry molecular orbitals background.

In a nutshell, you can sawy that hyperconjugation is a kind of interaction (normally stabilizing, but that can be destabilizing) somehow similar to that present in pi conjugation (which is the interaction between p orbitals), but that occurs between sigma bonds and adjacent empty p orbitals.

Further reading: http://en.wikipedia.org/wiki/Hyperconjugation
https://www.princeton.edu/chemistry/macmillan/group-meetings/hyperconjugation.pdf

TBH I have no idea what is the theory behind hyperconjugation, but I know that the result is that hydrogen at carbons adjacent to groups like CO or NO2 (in non aromatic systems) are acidic (or at least we were told that its due to hyperconjugation) which than have some interesting consequences in some reactions:
http://en.wikipedia.org/wiki/Acetoacetic_ester_synthesis

I believe that hyperconjugation is not the best concept to explain acidic properties of α to carbonyl hydrogens.
Even though is some cases it can contribute to the stabilization of the enolate, (e.g., tetrasusituted enolates are formed as a more thermodynamically stable products, due to the stabilization produced by the hyperconjugation of sigma bonds between sp2 and sp3 carbons with the empty p orbitals of the alkene, http://upload.wikimedia.org/wikipedia/commons/thumb/7/7d/Enolate_regio_1.svg/315px-Enolate_regio_1.svg.png) the reason of this is ust that the negative charge is delocalized through the carbonyl groups by resonant effect.

I would like to know how to keep in mind certain reactions and use it in conversions. And where can I get lots of conversion sheets (class +2) level, particularly Alcohols, Phenols and Ethers and Aldehydes, Ketones and Carboxylic Acids.

Do not memorize reactions, understand basic mechanisms, and apply them to ANY reaction you find

Here are the most important:
SN2 Reaction: Bimolecular Subsitution - http://www.chemhelper.com/sn2.html
SN1 Reaction: Unimolecular Substitution - http://www.chemhelper.com/sn1.html
E1: First Order Elimination - http://www.chemhelper.com/e1.html
E2: Second Order Elimination - http://www.chemhelper.com/e2.html
Electrophilic Addition to Alkenes - http://www.chemhelper.com/elecadd.html
Nucleophilic Addition to Carbonyl Groups - http://www.chemhelper.com/nucadd.html
Free Radical Halogenation - http://www.chemhelper.com/frhalog.html


Those schemes?
In Volhardt's Organic chemistry you have some in the end of most chapters.

Also I believe those are good:
http://store.masterorganicchemistry.com/pages/catalog?home_button (not trying to spam, I found that blog good, so I suppose the material they provide could be handy)
But I did not purchase them, so I cannot tell you for sure.
You can probably find lots of them free on the internet too.

[Hijaz Aslam]

kriggy and Altered State, am extremely pleased for your support.

Do not memorize reactions, understand basic mechanisms, and apply them to ANY reaction you find

This is exactly what I am looking forward to. It is frustrating to memorize a subject (my teacher often quotes 'by heart' for most of the chemical equations and mechanisms, and this just exasperates me.)


 

SN2 Reaction: Bimolecular Subsitution - http://www.chemhelper.com/sn2.html
SN1 Reaction: Unimolecular Substitution - http://www.chemhelper.com/sn1.html
E1: First Order Elimination - http://www.chemhelper.com/e1.html
E2: Second Order Elimination - http://www.chemhelper.com/e2.html
Electrophilic Addition to Alkenes - http://www.chemhelper.com/elecadd.html
Nucleophilic Addition to Carbonyl Groups - http://www.chemhelper.com/nucadd.html
Free Radical Halogenation - http://www.chemhelper.com/frhalog.html

So are the above theories the important rules of Organic Mechanisms? Can I at least see whether an equation is right or wrong or predict them at some instance?

Please refer to the textbook my syllabus covers, so that you can estimate the level at which we are taught at school, and what my requirements are (I forgot that in the first place). Here are the links:

Class +1 Textbooks
Part 1- http://www.ncert.nic.in/NCERTS/textbook/textbook.htm?kech1=0-7 - (Discard the 'errata' and prelims)
Part 2- http://ncertbooks.prashanthellina.com/class_11.Chemistry.ChemistryPartII/index.html

Class +2 Textbooks
Part 1- http://ncertbooks.prashanthellina.com/class_12.Chemistry.ChemistryI/index.html
Part 2- http://ncertbooks.prashanthellina.com/class_12.Chemistry.ChemistryII/index.html

Thanks for your support kriggy and Altered State

[kriggy]

Altered state: Maybe some mad mathematicians too? Such aweful typo lol

Hijaz: those links are some basic and most common mechanisms, I suggest you understand those. It seems that they are also covered in your book. Its good start.
Im sorry I cant tell you more, I dont know what level you are on (high school?) and although im teaching some guys chemistry Im realy not sure how can I help you. I suggest you read your coursebook, make notes and do the exercises and then ask here if you dont understand something, because honestly, asking whether is something important or not doesnt help you at all.
After you go throught your book, you most likely will be able to see if the reaction is "right" or not but it all depends on conditions so then there migh be reactions that seem very obscure and incorrect at first glance and there are reactions you cant even imagine.