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Organic Chemistry help

Discussion in 'MCAT Discussions' started by Papi, Aug 3, 2002.

  1. Papi

    Papi Member
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    Hi everyone,

    Is there an easy trick to easily tell if a reaction is:

    Nucleophilic substitution or addition?

    Electriphilic substitution or addition?

    Thanks!
     
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  3. phatty925

    phatty925 Senior Member
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    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....
     
  4. Dr. Dodger Dog

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    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.

    With respect to addition or substitution reactions, just look to see if anything has been substituted. If all the original atoms etc are still there, then it is an addition reaction. If something has been replaced, then substitution.

    As for whether you call it a nucleophilic or electrophilic reaction, that really is a moot point. Every reaction involves a nucleophile and electrophile. Many times it's just based on convention. Here are some tips tho that might help.

    For example, a nucleophilc substitution reaction, the nucleophile is the thing that SUBSTITUTES... sounds easy, but very applicable. In electrophilic aromatic substitution, the electrophile is the doing the substituting, not the nucleophile.

    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...

    Electrophilic reactions you will see on the MCAT will probably just include additions to alkenes (acid catalyzed hydration, etc) and electrophilic aromatic substitution. It's pretty safe to say that anything else is "nucleophilic" although as I said before, all reactions have both nucleophile and electrophiles.

    Hope this helps.
     
  5. phatty925

    phatty925 Senior Member
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  6. Dr. Dodger Dog

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    Good Point... sorry I mistinterpreted your previous post. 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.
     
  7. Mudd

    Mudd Charlatan & Trouble Maker
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    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.

    I'm not sure where the line between unreactive and reactive is drawn, but consider that carboxylic acids do the following reactions in chemistry and biochemistry:

    • reduction with LiAlH4 to a primary alcohol
      reduction with Dibal to an aldehyde
      Conversion to an acid chloride with SOCl2
      Dehydration to from anhydrides
      Formation of a thio ester with a thiol (like CoASH)
      Esterification with glycerol to form triglycerides and phospholipids
      Decarboxylation of ?-oxo acids
      Amide formation with the right catalyst (or enzyme)

    And these are just the common reactions of carboxylic acids. I'd have a tendency to say that is a pretty good set of reactions for a compound that is unreactive.
     
  8. Dr. Dodger Dog

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    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.

    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.
     
  9. Mudd

    Mudd Charlatan & Trouble Maker
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    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.

    The SOCl2 reaction is a perfect example. The sulfonyl group is attacked by the nucleophilic carbonyl oxygen, which causes the carbonyl carbon to be electron poor. This is the same first step of the mechanism when a carboxylic acid is treated with a strong acid (Lewis or Bro-Lowry). The chloride then attacks the carbonyl carbon to form a tetrahedral intermediate, much like 90% of carbonyl substitution chemistry. The rest just follows standard reactivity.

    The fundamental mechanism is simplistic and has appeared on the MCAT on a few occassions. I sincerely hope people reviewing carbonyl chemistry see that the mechanism is similar in almost all carbonyl reactions and thus only study one basic pathway.

    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).

    For instance, an SN1 reaction is said to be a two-step mechanism, despite cases where the leaving group is protonated and/or the nucleophile must be neutralized with a work-up step. You look in most textbooks, and SN1 reactions are referred to as two-step. Likewise, SN2 reactions are said to be a concerted mechanism, despite cases where the nucleophile must be neutralized with a work-up step (amines substituting for a halide are a great example).

    Hence, whether a carbonyl substitution reaction is carried out in acid (making the carbonyl more electrophilic) or base (making the nucleophile more nucleophilic), the number of key steps is still two: (1) formation of the tetrahedral intermediate following attack by the nucleophile and (2) the loss of the leaving group. Thus, substitution reactions involving carbonyls are commonly referred to as two-step mechanisms.
     
  10. Dr. Dodger Dog

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    Grad school? nope... never went to grad school... hopin to go to med school tho!

    When I teach esterification, I do go over more than just the basic steps. I feel it's important to recognize that the reaction is more complicated than just a simple addition-elimination process such as formation of amides via acid chlorides, etc. I emphasize activation of the carbonyl carbon through protonation as well as the protonation of -OH to form a better leaving group. These are key steps that you often see on the MCAT. (What is the purpose of protonating the carbonyl oxygen? etc) I don't know how it's taught in grad school however.

    As for reactivity of carboxylic acids... I think it's just important for the MCAT that people realize that they are unreactive in nucleophilic acyl substitutions when compared to their derivatives such as the acid anhydride, acid chloride, ester...

    I do recognize that those reactions are nucleophilic based... in fact in one of my previous posts, I emphasized that most organic reactions (except radical mediated processes and concerted processes) involved a nucleophile and electrophile. Many of the reactions that you listed are not covered on the MCAT. They tend to stress only basic addition/elimination reactions. Out of the reactions you mentioned, while teaching the MCAT course, I only covered the mechanism of beta decarboxylation briefly and of course esterification in conjunction with lipid chemistry.

    Again, I apologize for making sweeping generalizations for the reactivity of carboxylic acids-- but keep in mind that I was not intending to confuse MCAT people, only make their studying a lil easier. Details such as SOCl2 mechanisms probably aren't pertinent at this point. I'm no expert on organic chemistry but I do think I can help someone ace the organic chemistry that is on the MCAT. Just tryin to help-- that's all. :)
     
  11. Dr. Dodger Dog

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    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.
     
  12. Mudd

    Mudd Charlatan & Trouble Maker
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    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.

    I didn't mean anything snide about grad school, because I assumed you were in grad school. I thought PR required that of teachers. I remember in their advertising, Hyperlearning says they only hire people with advanced degrees. I taught in Westwood and Irvine years ago for Hyperlearning just at the time PR bought them. It was a requirement for instructors.

    I think our teaching styles are different. I like to present an easy reaction followed by a challenging reaction and point out that they are basically the same thing. If you have a fundamental idea of the reaction, you can work your way through everything the MCAT can throw. For someone who learns the acid-catalyzed conversion of an acid into a ester (using glycerol or some other alcohol), then hopefully they make the connection that amide formation, reaction with SOCl2, and any other reaction with a Lewis acid goes by the same route.

    Also, I would hope they could make the connection between the protonation step (on the carbonyl oxygen) and hydrogen bonding in beta-pleated sheets (where the carbonyl oxygen is the electron pair donor). You are completely right that the SOCl2 mechanism is beyond the scope of the MCAT, but by comparing it to easier mechanisms, hopefully students see that one mechanism generally fits all carbonyl reactions, with a few variations (acid versus base for instance).
     
  13. Dr. Dodger Dog

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    Mudd... you are a classy guy :) we are just two people who love organic chemistry! Thanks for your apology... i really felt like i had to defend myself out there... but again, everything you said was right on target.
     

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