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Organic Chemistry: Electrophilic Additions to Alkenes

Hey all! I wanted to put together a few review guides for the reactions I learned in my organic chemistry 1 class. I’m starting off with the alkene-related reactions, specifically with electrophilic additions. Despite being super hard to fully conceptualize (I still have trouble with them!!), they’re very important to recognize, because the same patterns repeat with other reaction types, particularly with alkynes. I’ll go through each reaction type here with a quick mechanism and jot down important bits of info. But before I start, I used a lot of abbreviations, so here’s a key:

R+ is carbocation

R is any group with a carbon

IM is intermediate

X is halogen

E+ is electrophile

RDS is rate-determining step

HOMO is highest occupied molecular orbital

LUMO is lowest unoccupied molecular orbital

Squiggly lines indicate enantiomers formed due to alternating wedges and dashes

Addition of HX

Introducing Markovnikov’s rule: the formation of a more stable R+ is favored b/c it’s a lower energy process

Alkene additions proceed faster via a more stable R+ IM (kinetics)

Tri-substituted carbons are favored over mono-substituted carbons

X is added to the more substituted carbon

Always watch for stereochemistry! Enantiomers can be formed as products

Carbocation Rearrangements 

Carbocation rearrangements will occur so the R+ is as stable as possible

That’s why the Markovnikov product won’t always be the major product

Rearrangements usually occur through hydride shifts, but alkyl shifts can also happen if that’s the only possible route

Remember: always check to see where the positive charge is at− if you can shift a hydrogen or methyl group over to make that carbon extra substituted, then it’s probably right

With the orbital alignment (aka alkyl shift), there’s a filled-empty orbital overlap, where the migrating bond is aligned with the empty 2p orbital

Ring Expansions

Some questions are gonna ask you to do ring expansions, so always think alkyl shifts (again, this is so we form the most stable R+)

NUMBER. YOUR. CARBONS. This will get messy

Note that even though the di-substituted carbon doesn’t change after the first arrow, the shape changes from cyclopentane to cyclohexane, so it’s thermodynamically more stable

Hydration

This follows Markovnikov’s rule− the OH group is always on the more substituted carbon of the alkene

Also this is pretty much a repeat of HX addition, but with some acid-base action

Pro-tip: your catalyst, H3O+, is always gonna be the electrophile, and your nucleophile is always gonna be the double bond, so START THERE!

There’s gotta be water after that first step (hydration, duh)

Your water is now a nucleophile, and it’s gonna start attacking the R+

Remember to follow through with any possible R+ rearrangements− we won’t need it here b/c the R+ is already tri-substituted

Your catalyst MUST be regenerated!! *insert acid-base reaction*

Side note: alkene dehydration is the reverse of hydration and is an E1 process 

DON’T ADD H2O/H3O+ UNDER DEHYDRATION CONDITIONS

Instead, use a strong acid, like concentrated H2SO4

Oxymercuration-Demercuration

This is essentially an alkene “hydration” w/o the rearrangement part

We don’t have rearrangement b/c the mercurinium ion doesn’t have a carbon

When the mercurinium ion bridge breaks, the nucleophile (in the second example, water), attacks the MORE substituted carbon, whereas the HgOAc attacks onto the LESS substituted carbon

The OH and HgOAc add ANTI to each other, but then the H from BH4- changes orientation on the LESS substituted carbon (forming diastereomers)

Side note: as a variation, you can also use an alcohol (ROH) instead of H2O in the oxymercuration step

Hydroboration/ Oxidation

The double bond nucleophile attacks the empty orbital in BH3

The BH3 attaches onto the LESS substituted carbon, so the positive charge builds on the MORE substituted carbon

I like drawing the third H in BH3 as an extension of BH2, so I can visually see the hydride shift

The OH from the peroxide basically replaces the BH2

The specific steps with BH3 get messy, so I was taught a revised method− basically, a concerted addition of BH3 w/ an internal H- transfer

There is regioselectivity in the sense that the partial positive charge follows Markovnikov’s rule

There is stereospecificity due to the syn (same side) addition of H and BH2 in the hydroboration product

Alkene Halogenation

Note that I2 doesn’t work here as a possible halogen

Markovnikov regioselectivity is present

There is ANTI stereospecificity, as in the two individual X’s from X2 attach themselves as opposite wedges and dashes

Alkene Halohydrin Formation

This is very similar to the previous alkene halogenation, except the second X in X2 doesn’t attach onto the final product

Rather, the nucleophile replaces the second X

Markovnikov regioselectivity is also present (this is SN1-like)

There is ANTI stereospecificity yet again (this is SN2-like)

A new “How To Study” series!! Part 1 of ???

see part 2: How To Study Physics

How to study BIOCHEMISTRY (from yours truly, a biochem major at ucla)

(but also note that i’ve only taken one biochem specific class so far but this class was general biochemistry so hopefully the tips and tricks i learned from taking this class will apply to higher levels of biochemistry!)

studygram: @markiveelle

Biochemistry is a VERY dense subject, it requires a good background in general chemistry (such as acids and bases, thermodynamics) and a strong biology background (hence BIO cheMISTRY) (who would have ever guessed)

Anyways I like to think that biochem is not exactly a strenuous subject in that you won’t need to rack your brain to solve application problems, but it DOES require a LOT of memorization which is arguably less painful, but is still scary.

General Memorization Tips

Start early! The earlier you start your studying, the more time your brain has to process the information which means it will get more comfortable with it and you’ll probably be able to remember it more easily later.

BABY STEPS, don’t try to memorize like 600 terms in one sitting. You’ll probably get them mixed up. The brain doesn’t work that way (for most of us!). Instead, start with the first ten things, get them down really well, then move on to the next ten, then try to get all twenty together. And then the next ten, and then a cumulative self check, and so on. I think this was the best way to start. You have to find how much information you can retain in one sitting!

Make use of free time, try to recall things when you’re waiting in line for food, for the bus, for the shower to warm up, etc. Jogging your memory at all times helps make it more solid.

Make whatever you’re studying your phone/laptop background so it’s of easy access to you! I made the amino acids my iPad lockscreen when I was trying to memorize all of those!

Seriously flashcards help

Memorizing the 20 Amino Acids (and their structures…) in one week

My first quiz in my biochemistry class was to be able to remember all 20 amino acid names, three letter abbreviations, one letter abbreviations, and structures at physiological pH. It was scary lol I had to memorize all that in oNE WEEK (evil)

Grouping them into their respective types can help you in the long run when you may be asked which amino acids are present in polar/nonpolar environments. 

I started memorizing the negative and positive amino acids first (there were the least of them lol). I used the cumulative batches method where I memorized the negative ones completely and then moved onto positive ones, and then did a cumulative self check. When I felt comfortable with those I moved onto nonpolar, then aromatic, then polar (idk why i did that order tho)

Most of the one letter abbreviations are the first letter of the name of the amino acid, especially in the nonpolar cases. I remembered the names and abbreviations of the nonpolar ones by just remembering GAVLIMP (read down the column!)

Glutamic Acid is GLU, E (which just spells glue)

Arginine is ARG ARRR ® like a pirate (lol)

Glutamine is Q and that’s weird so that resonated with me

Glutamine and Asparagine are similar to Glutamic Acid and Aspartic Acid in structure!

The nonpolar amino acid structures kind of build upon the previous one, which is also how I remembered those structures

I mainly took advantage of photographic memory in this case because whenever I was asked to recall a certain amino acid, in my head I would think about where that amino acid was on this poster I made. That helped my brain keep everything organized!

Memorizing Fatty Acids/Certain Carbohydrates

FLASH CARDS

Whenever you have to identify or name things, flashcards are the best!! I remember I made flashcards for everything on this page and me and my friends quizzed each other while waiting to be seated at korean bbq lol

Photographic memory again!! Write out all the stuff you gotta know onto one piece of paper (try to limit it to just one side) and study from that. It’s like organizing how your brain will remember stuff (kinda like you knowing exactly where something is in your drawer because you organized it that way)

Memorizing Pathways or Cycles

Glycolysis was the first pathway I had to remember, this one took me the longest because we were required to remember the structures, the names of the enzymes, the cofactors, LITERALLY EVERYTHING in one week it was a STRESS MESS

On this poster I have the detailed version that includes structures on the top and then a brief overview on the bottom that just has the names of stuff. I thought this helped (not sure how)

If you have taken organic chemistry in the past, then memorizing the structures may come a little easier to you because the names of the structures are super close to IUPAC naming so you can deduce from there!

It’s important to run through this IN ORDER over and over and over. Write it out so your hand gets a feel for it. 

GOING IN ORDER IS JUST SO IMPORTANT because it conditions your brain to follow that order and you’ll notice that everything builds upon the other (the brain loves patterns and order so this will expedite memorization)

We also had to track carbons, so color coding would have helped in this case too! Color code each carbon so it’s clear where everything goes. 

I think biochemistry is also a subject that requires a lot of time and effort, so make sure you give yourself time to study for it. Make a good study group as well so that you have many other people to correct you if you make mistakes, and you can correct them on their mistakes! I also think that catching errors or learning from previous errors is the BEST way to remember the RIGHT information because it shakes you; no one likes to be wrong!

GOOD STUDYING

Use recall. When you look at a passage and try to study it,  look away and recall the main ideas. Try recalling concepts when you are walking to class or in a different room from where you originally learned it. An ability to recall—to generate the ideas from inside yourself—is one of the key indicators of good learning.

Test yourself. On everything. All the time. Flashcards are your best friend. Use quizlet if you don’t want to hand-make flashcards. Get somebody to test you on your notes.

Space your repetition. Spread out your learning in any subject a little every day, just like an athlete. Don’t sit and study one subject for 2 hours, do half an hour every day.

Take breaks. It is common to be unable to solve problems or figure out concepts in math or science the first time you encounter them. This is why a little study every day is much better than a lot of studying all at once. When you get frustrated, take a break so that another part of your mind can take over and work in the background. You need breaks in order for your brain to retain the information. Try the Pomodoro method if you have trouble timing breaks!

Use simple analogies. Whenever you are struggling with a concept, think to yourself, How can I explain this so that a ten-year-old could understand it? Using an analogy really helps. Say it out loud, like you’re teaching it, whether it’s to an imaginary class or your sister who couldn’t care less.  The additional effort of teaching out loud allows you to more deeply encode.

Focus. Turn off your phone / iPad / any distractions and clear your desk of everything you do not need. Use apps like Forest if you can’t stay off them!

Do the hardest thing earliest in the day, when you’re wide awake and less likely to push it aside.

BAD STUDYING

Avoid these techniques—they can waste your time even while they fool you into thinking you’re learning!

Passive rereading—sitting passively and running your eyes back over a page. This is a waste of time, frankly, and doesn’t do anything to help information pass into your brain without recall.

Over-highlighting. Colouring a passage of text in highlighter isn’t helpful at all. It’s good for flagging up key points to trigger concepts and information, but make sure what you highlight goes in.

Waiting until the last minute to study. DON’T CRAM!!!

Doing what you know. This isn’t studying! This is like learning how to juggle but only throwing one ball. 

Neglecting the textbook. Would you dive into a pool before you knew how to swim? The textbook is your swimming instructor—it guides you toward the answers. 

Not asking your teachers for help. They are used to lost students coming in for guidance—it’s their job to help you. 

My brain as soon as I open the book:

Alternatively titled: How to Use Your Planner or How Organization and Discipline Will Get You Better Grades

This is my full guide on how to use your planner effectively and make sure that you are never missing deadlines or falling behind in classwork and homework. This is definitely more about the university level and works best with a dated planner, rather than a bullet journal. Let’s get started! 

Get all of your syllabi together and write down dates. Go through one class at a time and mark down all of your major tests and assignment due dates. I recommend putting these dates into the monthly and weekly views, and perhaps coming up with a symbol or other indicator that tells you they are of utmost importance. 

Make a weekly schedule of when to complete readings and do a weekly review of notes. Instead of trying to randomly decide when to do these things, assign a date for each task for each class. If you have a tutorial on Tuesday, do the assigned readings for it every Wednesday. I recommend scheduling one to two weekly tasks per day, and to leave a few days open, whether it be weekends or days when you have a lot of classes.

Make a master list of assignments. I find that sometimes, even having due dates in the calendar view isn’t enough, and they can still sneak up on you. The master list will be a good place to double check if you have any deadlines approaching easily. 

In the week or so before a due date, create a checklist of smaller tasks needed to complete the assignment. Set individual due dates for each smaller task by working backwards from the due date. Smaller tasks may include finding sources, making an outline, writing a rough draft, and editing and adding references to create a final copy. Write the smaller tasks into your daily to-do list. 

You can also do this with studying for tests, but the checklist would look slightly different. You could either sort by study method (first do flashcards, then do essay outlines, etc.) or sort by the topics you need to study. 

Stick to the schedule you have made. Obviously, if something comes up and you need to move your to-do list around, do so! But if you aren’t doing anything and you see readings on your list of daily tasks.. do them. Having the plan set out like this makes it easier for you to remain disciplined. 

Why use this method? 

By creating a schedule for repeated weekly tasks like completing readings, you make sure that you can’t repeatedly push off smaller tasks until you are weeks behind. I don’t think it is very reliable to just will yourself to do readings, or to keep up with them without tracking it. 

By writing down all of your due dates, you will never be shocked to find out something is due the night before. You will know and you will be prepared. 

By creating smaller checklists of tasks to complete before a major test or assignment, you will never find yourself in a situation where you have an essay due in a few days and haven’t even started. You will be following a timeline and making sure you don’t have to rush.

How to Finish

You are doing so great!