Hi Guys, I'm having a little bit of trouble understanding organic chemical reactions.. Like substitution, addition and condensation, I believe?
I don't understand how it works, when do i know what to use/identify what has been used etc... and how are they different? I've tried youtubing some videos but i just don't understand? could someone please explain
I'm going to go overkill here because I can.
For substitution, it occurs when carbon is bonded to something more electronegative than itself. Like, you'll never have normal substitution of a hydrogen atom via a polar reaction; halogenation occurs because the X-X halogen dissociates into VERY reactive halogen radicals.
Let's take methyl bromide. Now, bromine is more electronegative than carbon, so the carbon is positively charged (slightly). This means that the carbon will attract anything negatively charged, like hydroxide ion. When a hydroxide ion meets this methyl bromide molecule, the OH- is attracted to the positive carbon. As it begins to form a bond with the carbon (i.e. donates a pair of electrons to form a covalent bond), there are three things that can happen. 1, the carbon kicks out a hydrogen atom while giving all of its electrons to the hydrogen forming H-. That's not likely as H- is way too unstable. 2, the carbon kicks out the hydroxide ion again and no reaction happens. 3, the carbon kicks out the bromine giving its electrons to the bromine in order to form a bond with hydroxide.
Which of 2/3 is going to happen? Well, HBr is a MUCH stronger acid than HOH (also known as water
). This suggests that Br- is more stable than OH-, so we can reasonably expect that Br- will be kicked off preferentially to OH-. This is indeed what happens and OH- does normally substitute for bromine. Use similar reasoning for substituting OH- for Cl.
What about ammonia? Well, HBr is still a much stronger acid than NH4+, so Br- is more stable than ammonia. That's why ammonia will also substitute for bromine atoms. Look up SN2 (substitution nucleophilic bimolecular) if you want more information on this type of reaction.
Now let's look at addition. Generally addition means addition over a double bond. In the VCE course, addition is generally of a hydrogen halide or of water across a double bond (do remind me if there are any others). Let's see what happens with the addition of HX where X is a halogen.
The double bond is quite electron rich, so it is a nice target for something like a H+ cation. What happens is that in addition, one carbon forms a bond with the proton, leaving the other carbon positively charged (carbon has no lone pairs, so to form a bond with another atom, it must drag electrons from elsewhere; in this case, one carbon breaks the C=C double bond and converts it into a single bond). This positively charged carbon then attracts the X- ion and you've completed the addition of HX. Something similar happens with X2, although I won't go into the details.
Now condensation? This is a VEEEERRRRYYY broad class of reactions; I wouldn't even call it a class of reactions. In esterification, the simplified answer is, the alcohol oxygen in ROH bonds with the carboxylic acid carbon in R'COOH. The alcohol hydrogen migrates to the acid OH group, that group leaves as water and you're left with R'COOR (yes this is overly simplified but I don't want to complicate things any further xP)
Why does this happen? Well, the oxygen in ROH is quite negative and the carbon in RCOOH is VERY positive due to its C=O and its C-OH bonds. That's why the reaction would occur.
As for amide formation, directly mixing a carboxylic acid and an amine won't give you an amide (even with acid catalyst) because protonated amines and carboxylates are both not very reactive. However, for the sake of VCE, imagine the amine nitrogen (with its negative charge) attacking or forming a bond with the positive carbon in the RCOOH, and then imagine the amine nitrogen replacing the OH group. You'll need to give me more specific reactions here I'm afraid; I can come up with a dozen condensation reactions that are probably all irrelevant.
How to identify what's used? Substitution, just remember that alcohols can be prepared from alkyl halides (slightly more complicated than this but oh well) and that amines can be prepared from alcohols and alkyl halides. Condensation, well in VCE there's only one way of making esters and amides. For addition, you're generally told to start with an addition or from something like ethene. You literally break the C=C bond and add atoms to each side.
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