Actually, it's never correct to say that an electrophile attacks a nucleophile. The nucleophile always "attacks" the electrophile. As for the original question, let me try to give my own spin here. I just finished teaching PR Ochem this past week so this stuff is still pretty fresh.Originally posted by phatty925
i'm not sure about a "trick," but i guess it's easy to tell when you know if something is being attacked by a nucleophile (usually has extra electrons and attacks a positive charge, like NH3) or electrophile (has positive charge and is therefore attracted to negative charge).
however, you do need to remember that for carbonyls, ketones and aldehydes usually only undergo NUCLEOPHILIC ADDITION and esters, carboxylic acids, and amides only undergo NUCLEOPHILIC SUBSTITUTION. from what i've seen, most of the reactions on the mcat seem to be nucleophilic substitution or addition and hardly ever electrophilic....
Originally posted by Dr. Dodger Dog
To correct the previous post, carboxylic acids do not generally undergo nucleophilic addition reactions. Nucleophiles usually deprotonate the carboxylic acid before anything happens. Remember-- the only difference between nucleophiles and bases is how they react. If the species attacks an electrophile, it is called a nucleophile. If the species deprotonates another molecule, it is called a base. Derivatives of carboxylic acid tend to undergo nucleophilic acyl substitutions...
i never said carboxylic acids undergo nucleophlic addition...i said they usually undergo nucleophilic SUBSITUTION. for example, an alcohol reacts with carboxylic acids through nucl. subsitution to form esters. yes, you are right that the reaction is acid catalyzed-- the hydroxyl group is first protonated.
aldehydes and ketones undergo addition.
The overall reaction is substitution. One that can be catalyzed in either acid or base, and isn't really that complicated at all. It is a simple two-step mechanism in most cases. The tetrahedral intermediate is formed by a nucleophilic attack at the sp2 carbon of the carbonyl, but it is a short-lived intermediate and not a product. The intermediate kicks out a leaving group, which is quite similar to an elimination reaction. Because reactions are named for the overall change, and not their mechanistic steps, the overall reaction is substitution. An alkoxy group has been substituted for a hydroxyl group.Originally posted by Dr. Dodger Dog
Carboxylic acids do undergo esterification which yes, is a nuclephilic addition/elimination process.... a more complicated one at that. Carboxylic acids are, however, relatively unreactive otherwise.
I think I' missed a transition there. Your original statement is that carboxylic acids don't do very many reactions. Now you have restated that they don't nucelophilic type of reactions. The reality is that seven of the eight reactions listed are what you are referring to as "nucleophilic addition/substitution processes", just each has a varying Lewis acid if you will.Originally posted by Dr. Dodger Dog
I was referring only to carboxylic acids being unreactive in nucleophilic type reactions due to the weak leaving group (hydroxide). The reactions you mention are all very valid, but more specific than just ur everyday nucleophilic addition/substitution processes.
Actually, it is very simple, and there are only two key steps. As I assume is true where you went to grad school (I have found it to be norm with most chemists), common mechanisms are discussed by their key steps, with catalysis and work-up steps assumed (an thus unstated).As for the mechanism of esterification, it is NOT a simple two step process like other addition elimination reactions. It involves protonation/deprotonation steps, esp in acidic medium.
Before you jump all over me again, let me correct this statement: The reactions you mentioned are in fact covered on the MCAT... just not usually in gory mechanistic detail. Often, you simply need to know the products of such reactions.Originally posted by Dr. Dodger Dog
Many of the reactions that you listed are not covered on the MCAT. They tend to stress only basic addition/elimination reactions.
I reread my post and want to apologize. I sound like a jerk, and did not mean too. I'm a chemistry geek and get excited when talking chemistry. Right now I'm as anxious as the students I tutor, and I think I got really carried away. I seriously didn't mean to sound so agressive, so I do hope you accept my apology.Originally posted by Dr. Dodger Dog
Before you jump all over me again, let me correct this statement: The reactions you mentioned are in fact covered on the MCAT... just not usually in gory mechanistic detail. Often, you simply need to know the products of such reactions.