how to know what reagent to use in ochem

Organic chemistry may cover many concepts and topics, but it's all about the reactions and mechanisms.

Information technology's non and then scary at first, remember near the simple acid/base deprotonation, an alkene reaction here, some other there. Before yous know information technology, you lot're drowning in dozens upon dozens of reactions!

You're asked to 'memorize' each one, to know what every reactant and reagent will do.
And once you have it all downwards, don't forget the dozen or and so exceptions to the rule.

Groan!

And once y'all take all THAT down, permit'due south put them all together: when you're given molecule A and asked to come up with all 20 steps to produce product Z!

Ok, peradventure I'k exaggerating a bit…but really, just a tiny bit!

The boilerplate Organic Chemical science 1 or two exam synthesis question will range from two to five steps with intermediates.

How do y'all keep everything you learned direct?

And more importantly, how do you lot sift through the hundreds of information points in your head to produce the exact steps required to achieve the desired outcome?

As a kid, I was proud to call myself a nerd. Peradventure with a bit of OCD.
I ever looked for trends and plant patterns where they didn't be. Perhaps that's why I enjoy Orgo so much.

The Surreptitious to synthesis is uncomplicated:

A) Await for patterns, equally we'll explain below.
B)when yous go stuck, remember there's likely more than one way to achieve your desired product.

Coming up with a proper synthesis requires a combination of forrard and reverse thinking!

We'll embrace the reverse thinking in the Retrosynthesis tutorial.
For this tutorial, we'll focus on the shorter and simpler synthesis.

Let's start by looking for patterns.

The following questions will help you empathize what to pay close attention to.

Look out for the following:

What functional group is nowadays on the reactant?
What functional grouping is present on the product?
Which reactions practise I know to catechumen from one to the other?
Practise I know of a reaction that produces an intermediate to the above product?

For case:
If you lot're starting out with a four-carbon alkene and the product asks for a four-carbon alcohol, ask yourself,

"What do I have in the reactant?"
A reactive pi bond!

" What do I accept in the product?"
An booze.

"Which reaction do I know can assistance me convert from an alkene to an alcohol?"

Well, at that place are many if y'all think about it. Let's go in order from about obvious to less obvious:

Acid catalyzed hydration
Oxymercuration-demercuration
Hydroboration-oxidation

These are the direct obvious reactions with potentially differing results. We'll use 3-methyl-1-butene to highlight these difference.

Only what if you weren't given the alkene, or didn't call back of these alkene reactions?
What about putting a element of group vii leaving group on the alkene through hydrohalogenation?
Or a radical halogenation if starting with an paraffin?

So y'all tin deport out an SN2 reaction using NaOH in a polar protic solvent.

What if it wasn't that piece of cake?

What if you're asked to start with an alkyne?

We can't go from alkyne to booze direct, since the enol production would immediately tautomerize to a ketone or aldehyde.

This is where we introduce many options:
–> If you lot reduce the alkyne to an alkene, y'all may utilize one of the post-obit as already discussed higher up:

Acid catalyzed hydration
Oxymercuration-demercuration
Hydroboration-oxidation

–> If you go from the alkyne to a carbonyl (ketone or aldehyde) yous can follow up by reduction:

NaBH4
LiAlH4

Practice you lot see where we're going with this?

A) Identify the patterns
B) Enquire yourself, "What practise I know?" to recognize these patterns
C) Go from at that place

Allow's outset with a simple case for concept, then apply the same logic to something more than complicated.

Say y'all're asked to synthesize 2-butanone from propyne. Go through your questions:

What functional group is present on the reactant?
Reactive last alkyne
What functional group is present on the product?
Ketone on carbon #2
Which reactions do I know to convert from ane to the other?
Acid catalyzed hydration of alkynes volition yield a ketone following Markovnikov's Dominion.

Yet , in this example we're starting with a iii-carbon chain withal ending with a iv-carbon concatenation.

4. Do I know of a reaction that produces an intermediate to the to a higher place product?
Yes! Terminal alkynes easily undergo concatenation elongation via SN2.

Nosotros'll start with an acrid/base of operations reaction to deprotonate the concluding alkyne forming a skillful nucleophile.

Nosotros need to elongate the chain by just i carbon. Allow'southward give this methyl group a good leaving group to facilitate a quick SN2 reaction –my 'go-to' is Bromine, merely you can too use Chlorine or Iodine.

Now that nosotros have a carbon chain of desired length, permit'due south carry out the acrid catalyzed hydration.

But expect, the production is an enol, not a ketone!!

The next stride will happen automatically. And then while you don't have to testify a reagent simply describe 'KET' over the reaction pointer for Keto Enol Tautomerization.

And there nosotros have it!

Even scarier than a synthesis by itself is the following exam question/request:

"Advise a reasonable mechanism to conduct out the following synthesis."

This is where yous're given one reactant, 1 production, and 1 or Two sets of reagents.
In other words, you're given all the steps but asked to show how the unlike molecules work together.

While the question is completely dissimilar, the concept is the aforementioned.

Nosotros'll modify the initial questions slightly:

What functional grouping is present on the reactant?
What functional grouping is present on the product?
WHERE IS THE REACTIVITY ON THE STARTING MOLECULE? As in, where do I commencement the mechanism?
Which reactions do I know to convert from i to the other?
Practise I know of a reaction that produces an intermediate to the in a higher place production?

A student recently showed me this test question for which not a single student in his class got full credit.

Permit me preface this by saying Yep, this is a tough question.
BUT, if you retrieve through it logically, you'll realize that if you studied the private steps and recognize them, y'all should be able to follow forth.

In fact, I challenge you to give this a try and encounter how far yous get before reading on.

Propose a reasonable mechanism for the post-obit reaction. Show all intermediates and formal charges. Is this reaction SN1 SN2 E1 E2?

What exercise you call up? Did you lot go it?

Here's a video with a step by footstep solution:

No affair how well y'all set, yous'll still go caught off-guard by ane tricky step or another.

And when you go stuck?

Chances are, there's more than one way to derive your product!

When I took my start weekly Organic Chemistry 2 quiz, I inadvertently gear up a tendency of scoring top of the course, just it was a fluke. I didn't realize we had weekly quizzes…

We were asked to outline a step by step procedure to split 2 similar molecules with different functional groups. The procedure involved a series of reactions to prepare 1 molecule for extraction.

I tried!

I remembered that there were about six steps, merely I could only confidently answer four. I'd already bombed, then aced Orgo 1, and didn't desire to do that once more!

No matter what I did, I couldn't come up with the other steps. So instead, I crossed out my four and a half steps and wrote a detailed pace by step procedure for carrying out partial distillation.

My TA very reluctantly gave me full credit with an amused warning.
No ane else came shut.

Am I asking yous to outsmart the question?

Non quite.

But recognize that the more reactions y'all learn, the more options you accept for creating a single functional grouping.

If y'all're stuck on a certain pathway or can't fully describe the steps,
Ask yourself, "Is there another way to create the same functional group?"

Think dorsum to the many booze formation reactions nosotros discussed above. If y'all forgot one option, simply utilize some other.

Here are a few interesting patterns and alternates to consider.

–> Chain Elongation 'Go-To' reactions

Use alkynes in Orgo 1
Use grignards or condensation in Orgo ii

–> Adding carboxylic acids or carbonyls

Alcohol -> oxidation
Oxidative cleavage using KMnO4 or O3 (smaller concatenation)
Grignard and CO2 (longer chain)

–> 'Moving' Reactivity and then that you can start/react at a different portion of the molecule compared to the current location of the agile group.

Active groups include leaving groups, pi bonds, nucleophilic centers susceptible to assault and more.

Here are some of my favorite 'moving the reactive location' tricks:

Move the Pi bond intermediate then use a Markovnikov or Anti-Markovnikov addition.
No pi bond? Radical halogenation to introduce a leaving grouping and So eliminate.

These are just SOME of the tricks you can use.

Maximizing Partial Credit On Your Exam

Gaining bonus points on exams is one matter, but here'due south the all-time part:

The boilerplate synthesis question is worth anywhere from ten-xxx points.
And the boilerplate professor WILL give fractional credit.
So, if you can just remember four of v steps, DO NOT leave it blank to receive cypher points!

Instead, write the iv steps and add in as much relevant information as possible.
Then VERY CONFIDENTLY simulated the 5th footstep.

Practice not write " Magic! " (yes, I've seen this on a pupil exam).

Make up something that appears to be only a 'careless' mistake. Your professor volition be impressed past your work and hopefully requite you an 80% for the question.

This besides applies to reagents!

If y'all simply remember the steps, but don't remember which reagents will get you there, start by drawing out the molecules:

A –> B –> C

This has 2 benefits:

It helps y'all call up without distractions so you can clearly see how the molecule and functional groups change/evolve from footstep to step.
Now that you take a clear movie of where yous're doing, you should be able to retrace your steps and fill up in the required reagents!

Of course if Reagents are giving you problem with this, here's a video on 'Memorizing' Organic Chemistry Reagents.

And if you tin can't remember them, INVENT SOMETHING rather than go out it blank. Endeavour to use a reagent that has the groups you lot're calculation.

For example, if yous forget that an alkyne will react with HgSO4 in H2SO4 to yield a ketone,

but you KNOW yous're trying to become oxygen onto the molecule, put something else with oxygen. For example: O3 or KMnO4.

Are they correct?
Not quite!
Both will cleave the alkyne.

Merely, I've seen enough students employ this every bit a legitimate mistake that your professor may think the aforementioned and hopefully give you partial credit.

Once again, I'm non request y'all to invent on your exam.

Just, guessing logically on a multi-pace problem where you've already earned sufficient points will aid you get closer to full credit.

I've done this successfully on my orgo exams and come out height of the form, despite missing half a point hither and there on multiple questions. My Study Hall members and tutoring clients practice the same affair to squeeze in a few extra points, bringing them to the top.

Bottom line, make sure you learn and UNDERSTAND all of the required reactions!

But fifty-fifty the best of the states forget some things under force per unit area.

That's when you can employ the tips to a higher place to assistance you lot try your best and you lot'll have an advantage with nearly professors. They'll hopefully give you the benefit of the doubt when answers are questionable or not exactly what they were seeking.

Ready to start thinking backwards? That's where the more hard topic of Retrosynthetic Assay or simply retrosynthesis comes into play.

I'd love to hear from you

Do you feel better almost synthesis and knowing what to do when you are stuck? Let me know in the comments below

dealoult1980.blogspot.com

Source: https://leah4sci.com/how-to-tackle-organic-chemistry-synthesis-questions/