Major Product: Reaction & Thermodynamic Stability

Chemical reactions exhibit a variety of potential outcomes, but predicting the major product requires a deep understanding of reaction mechanisms and thermodynamic stability. Reaction conditions, such as catalyst or solvent choice, affect the reaction pathway. Identifying the limiting reagent help determine the maximum possible yield of the major product in a chemical reaction.

Contents

The Crystal Ball of Chemistry: Predicting What Happens Next!

Ever wondered what happens when you mix two chemicals? It’s not always as simple as “A + B = C.” In fact, it’s more like A + B = a whole bunch of possibilities! That’s where the art and science of predicting chemical reactions comes in.

What exactly is a chemical reaction? Simply put, it’s a process that involves the rearrangement of atoms and molecules to form new substances. Think of it like LEGOs – you’re taking apart one structure and building something completely different. These reactions are the backbone of everything!
Why bother predicting the outcome? Well, imagine trying to develop a new life-saving drug or a super-strong material without knowing what your ingredients will actually do when combined. Chaos, right? That’s why product prediction is absolutely vital in fields like drug discovery, materials science, and many more.
The plot thickens: So many things can influence the final result of a chemical reaction – from temperature and pressure to the very nature of the chemicals themselves. It’s like trying to bake a cake in a hurricane!
Your guide to wizardry!: Don’t worry, you don’t need a magic wand. This article is your guide to understanding the key principles and practical strategies for predicting the products of chemical reactions. We aim to give you the tools and knowledge you need to make informed predictions, just like a seasoned chemist!

The Foundation: Reactants, Products, and Reaction Mechanisms

Alright, let’s get down to brass tacks! Predicting chemical reactions isn’t about pulling rabbits out of hats; it’s about understanding the fundamental building blocks. We’re talking about reactants, products, and the secret sauce that connects them all: reaction mechanisms. Think of it like baking a cake – you need the ingredients (reactants), you want a delicious cake (products), and you absolutely need a recipe (reaction mechanism) to guide you. Let’s break down each part, shall we?

Reactants: The Starting Materials

First up, the stars of our show: the reactants. These are the substances you start with, the ingredients that are going to mix and mingle to form something new. The nature of these reactants is absolutely crucial because they dictate the pathways a reaction can take. It’s like knowing if you have flour or sugar – you can’t make a cake without flour, and you can’t make cookies without sugar!

Now, let’s zoom in a bit. The structure and properties of these reactants are key to understanding what’s likely to happen. Are we dealing with an electrophile, a “lover” of electrons, or a nucleophile, a “lover” of nuclei (positive charges)? These different types of reactants have predictable behaviors. For example, a nucleophile, like ammonia (NH3), is just itching to donate its electron pair to a carbon atom that’s slightly positive. On the other hand, electrophiles love to accept electrons.

Products: Identifying the Possibilities

Once we know who our reactants are, we can start imagining what they might turn into – the products. In many reactions, you might get several products, a major product which is the one you get the most of, and then a few minor products. Think of it like baking cookies: you might get perfectly shaped cookies (major product) but also a few that are a bit burnt or broken (minor products).
Now, a big question to consider is: what makes a product favored? It all comes down to stability. The more stable a product is (lower energy), the more likely it is to form. This is what we call thermodynamic control. When a reaction is under thermodynamic control, the product that forms is the one that is more stable!
So, how do we figure out what products are even possible? By combining the structure of reactants with our knowledge of common reaction types, we can begin to draw out the possible products and decide which of them are most likely to form.

Reaction Mechanisms: The Step-by-Step Guide

And now, for the most crucial part: the reaction mechanism. This is the story of how the reactants become products. It’s a step-by-step breakdown, showing exactly which bonds break and which bonds form along the way. Understanding the mechanism is like having the blueprint for the reaction.

During a reaction, short-lived reactive intermediates such as carbocations, carbanions, and radicals form. These intermediates are usually unstable, and they immediately react to form more stable products. Transition states are also important. They are a fleeting moment during a reaction when bonds are partially broken and partially formed, representing the highest energy point in the reaction.

By meticulously tracing the movement of electrons and atoms, we can use reaction mechanisms to predict which products are most likely to arise. Knowing the mechanism gives us the power to not just guess, but actually understand why a reaction goes the way it does.

Thermodynamics: Stability is Key

Imagine a seesaw, but instead of kids, we have reactants and products. Thermodynamics tells us which side the seesaw wants to be on. It’s all about stability. Think of it like this: molecules, just like us, prefer to be in a relaxed, low-energy state. That’s where free energy (ΔG) comes in. It’s the ultimate decider of whether a reaction will happen spontaneously. A negative ΔG means the products are more stable than the reactants – a “go” signal for the reaction! To understand ΔG, we need to introduce its friends, enthalpy (ΔH) and entropy (ΔS). Enthalpy is all about heat – is the reaction releasing heat (exothermic, negative ΔH, feels warm) or absorbing heat (endothermic, positive ΔH, feels cold)? Entropy, on the other hand, is a measure of disorder or randomness. Nature loves disorder, so a reaction that increases entropy (positive ΔS) is also favored. Thermodynamics is the compass, it guides us towards product stability in a chemical reaction, however, kinetics is the map, showing how fast or slow we can get there.

Kinetics: The Path Matters

So, thermodynamics tells us if a reaction will happen, but kinetics tells us how fast. Even if a reaction is thermodynamically favorable, it might take forever if the kinetics are slow! The key concept here is the reaction rate, which depends on the activation energy. Think of activation energy as a hill that the reactants need to climb to transform into products. A high hill means a slow reaction; a low hill means a fast reaction. Ever heard of a rate-determining step? It’s the slowest step in a reaction mechanism – the one that controls the overall rate. Now, things get interesting when we talk about kinetic versus thermodynamic control. Sometimes, the fastest reaction doesn’t lead to the most stable product. If the reaction is run at a lower temperature, the kinetically favored product (formed faster) will be the major product. However, at a higher temperature, there’s enough energy to overcome the activation energy barrier, leading to the thermodynamically favored (more stable) product. The path really matters and it is important to understand the kinetics of a reaction to predict the major product

Equilibrium: The Balance of Reactants and Products

Chemical reactions rarely go to 100% completion. Instead, they reach a state of equilibrium, where the rate of the forward reaction equals the rate of the reverse reaction. This is where the equilibrium constant (K) comes in. K tells us the relative amounts of reactants and products at equilibrium. A large K means the equilibrium lies to the right, favoring product formation. A small K means the equilibrium lies to the left, favoring reactants. But equilibrium isn’t static – it can be shifted by changing conditions. Le Chatelier’s principle tells us how: if you disturb an equilibrium (by adding heat, changing concentration, or pressure), the system will shift to relieve the stress. For example, if you add more reactant, the equilibrium will shift to the right to produce more product. Equilibrium concentrations can be calculated using an ICE table (Initial, Change, Equilibrium) and the equilibrium constant expression. By understanding equilibrium, we can optimize reaction conditions to maximize product yield.

The Reaction Environment: It’s All About Location, Location, Location!

Imagine you’re throwing a party. The ingredients (reactants) might be fantastic, and you’ve got a killer recipe (reaction mechanism), but the party’s location (reaction environment) can make or break the event! Chemistry is much the same. The conditions in which a reaction takes place can dramatically change what you end up with. Let’s check the different types of locations that will affect the reaction:

Temperature: Heating Things Up (or Cooling Them Down)

Got a slow reaction? Crank up the heat! Temperature is like the volume knob on your reaction’s stereo. The Arrhenius equation explains it nicely: higher temperature, more molecules have enough energy to clear the activation energy hurdle, speeding things up! Also, changing temperature can influence what products you get, shifting the equilibrium toward what’s more stable at that temp. For instance, some reactions only yield the desired product when kept ice-cold, while others need a hot bath to even get started.

Concentration: The More, The Merrier (Sometimes)

Ever tried baking a cake with too little flour? Disaster! Concentration is all about how much reactant you have floating around. Generally, more reactants mean more collisions, leading to a faster reaction and more product. Le Chatelier’s principle comes into play here: if you increase the concentration of reactants, the reaction will shift to the product side to restore balance. Think of it like a seesaw. However, it’s not always a straight shot – reaction order tells you exactly how concentration affects the rate, and sometimes, too much of something can actually slow things down!

Solvent Effects: The Medium Matters

The solvent is the reaction’s background music – it sets the vibe. Polar solvents work well with charged intermediates and reactants by stabilizing them and nonpolar solvents do the opposite. Solvents interact with reactants (solvation), changing their reactivity. For example, SN1 reactions love polar protic solvents (like water or alcohols) which stabilize carbocations, while SN2 reactions often prefer polar aprotic solvents (like acetone or DMSO) because protic solvents can interfere with the nucleophile.

Catalysis: Speeding Up the Process

Think of catalysts as your reaction’s personal trainers – they whip it into shape without being consumed themselves! Catalysts lower the activation energy, making it easier for the reaction to happen. There are a few types:

Homogeneous catalysts are in the same phase as the reactants (e.g., everything’s dissolved in the same solvent).
Heterogeneous catalysts are in a different phase (e.g., a solid catalyst in a liquid reaction).
Enzymes are biological catalysts, nature’s amazing reaction-boosters!

Common catalysts include acids, bases, and transition metals, each with specific applications to accelerate reactions.

Selectivity: Guiding Reactions Towards Specific Products

Okay, so you’ve got your reactants all lined up, ready to party and transform into something new. But here’s the thing: chemical reactions can be a bit like toddlers – they need guidance! That’s where selectivity comes in. Think of it as the reaction’s GPS, steering it towards making exactly what you want, and nothing else (or at least, mostly!). We’re talking about regioselectivity, stereoselectivity, and chemoselectivity. It’s all about control, precision, and getting that perfect product.

Regioselectivity: Location, Location, Location

Imagine you’re building a house (out of molecules, naturally). You wouldn’t just slap a window on any old wall, right? You’d want it in exactly the right spot. That’s regioselectivity in a nutshell. It’s all about directing a reaction to a specific site within a molecule. This is crucial because where something attaches totally changes the product!

Markovnikov’s Rule: Think of this as the popular kid in school. In addition reactions of alkenes, the hydrogen atom wants to attach to the carbon that already has more hydrogens! “The rich get richer,” as they say (in hydrogen atoms, at least!).
Real-World Impact: Imagine synthesizing a complex pharmaceutical. You want that crucial functional group to attach to exactly the right carbon. Mess it up, and you might end up with something totally useless or, worse, toxic!

Stereoselectivity: Achieving the Right 3D Structure

Now, let’s get spatial! Molecules aren’t flat; they exist in three glorious dimensions. Stereoselectivity is about controlling the 3D arrangement of atoms in your product. It’s like making sure your Lego castle has the perfect layout.

Enantioselectivity: This is all about making a single enantiomer (a non-superimposable mirror image) over the other. Think of it as choosing between a right-handed glove and a left-handed glove.
Diastereoselectivity: Here, you’re favoring the formation of one diastereomer (stereoisomers that aren’t mirror images) over others. It’s like choosing the right arrangement of chairs in your dining room!
The Power of Chirality: Chiral catalysts and reagents are your secret weapons here. They’re like tiny, molecular “hands” that can grab the reactants and force them to react in a specific orientation.
Drugs and Stereochemistry: Think about thalidomide. One enantiomer was a drug that helped with morning sickness; the other caused severe birth defects. Scary, right? That’s why stereoselectivity is so important in drug development!

Chemoselectivity: Reacting with the Right Functional Group

Molecules can be like crowded cities, full of different “neighborhoods” (functional groups). Chemoselectivity is about reacting with one specific neighborhood while leaving all the others untouched. It’s like going to a specific store in the mall without getting distracted by all the other shiny things!

Protecting Groups to the Rescue: Protecting groups are like temporary shields. They block off certain functional groups, preventing them from reacting while you work on another part of the molecule. Once you’re done, you simply remove the protecting group, and voilà!
Strategic Strikes Only: The goal is to be able to selectively react with, say, an alcohol in the presence of an amine. The alcohol reacts, the amine stays put. That’s chemoselectivity, baby!
Example scenario: Imagine you’re trying to add a bulky group to an alcohol but you also have an amine in the same molecule. The amine might get in the way or react first if you’re not careful. So, you protect the amine with a BOC protecting group. This makes the alcohol the most reactive spot on the molecule and you can easily get it to react. Then, you simply remove the protecting group on the amine, and voilà, you have the product you want!

Electronic and Steric Effects: Fine-Tuning Reactivity

Alright, buckle up, because we’re about to dive into the nitty-gritty of why some reactions are zippier than others, and why certain products pop up more often than their shy counterparts. Think of electronic and steric effects as the reaction’s personal trainers and interior decorators. They whip the molecules into shape and arrange them just so, to achieve the most favorable outcome. These effects can dictate whether a reaction sprints to completion or crawls at a snail’s pace.

Electronic Effects: The Push and Pull of Electrons

Imagine electrons as tiny, opinionated socialites, constantly gossiping and influencing their neighbors. This “gossiping” manifests as electronic effects, which are all about how electrons distribute themselves within a molecule and how that distribution impacts reactivity.

Inductive Effects

Think of sigma bonds as electron conduits, channeling the flow of electron density. Inductive effects occur through these sigma bonds, with electron-donating groups (like alkyl groups) pushing electron density towards a carbon atom, and electron-withdrawing groups (like halogens) pulling it away. This push and pull can dramatically affect reaction rates and where a reaction decides to happen on a molecule (selectivity). For example, adding a bunch of electron-donating groups to a carbocation can stabilize it, making reactions that form that carbocation more likely.

Resonance Effects (Mesomeric effects)

Now, let’s bring in the big guns: pi bonds! Resonance effects, also called mesomeric effects, are electron movements through pi systems, where electrons aren’t stuck in one place. It is much like a flowing river of electron density. When electrons can spread out (delocalize) over multiple atoms, the molecule becomes more stable. The different ways electrons can be arranged are represented by resonance structures. By drawing these structures, we can predict which parts of a molecule are electron-rich or electron-poor, and thus, where a reaction will preferentially occur. For example, carbonyl compounds are prone to have nucleophilic attack at the carbonyl carbon due to the resonance structures.

Steric Effects: Size Matters

Ever tried squeezing into a crowded elevator? That’s steric hindrance in action! Steric effects are all about how bulky groups get in the way of reactions. If a molecule has a lot of large, clunky substituents, it can physically block the approach of a reactant, slowing down or even stopping the reaction.

When large groups get close, they cause something called A-strain, which is like molecular claustrophobia. This strain can strongly influence which conformation a molecule prefers (the most comfortable way for it to twist and bend). For example, a bulky tert-butyl group attached to a cyclohexane ring will strongly prefer to be in the equatorial position to minimize A-strain with axial substituents. Reactions involving those positions are thus affected.

Hyperconjugation: More Stability Than You Thought

We’re not done yet! Let’s throw another curveball: hyperconjugation. It’s a stabilizing effect that arises from the interaction of electrons in a sigma bond (usually a C-H bond) with an adjacent empty or partially filled p orbital or a pi orbital. The electrons in the sigma bond “slop over” into the empty or pi orbital, which acts like a molecular hug, stabilizing the molecule.

This might sound esoteric, but hyperconjugation is key to understanding the stability of carbocations. A carbocation with more alkyl groups attached to the positively charged carbon is more stable because the adjacent C-H sigma bonds can donate electron density into the empty p orbital of the carbocation.

Also, hyperconjugation plays a role in the stability of alkenes. More substituted alkenes are generally more stable because of hyperconjugation, as more alkyl groups attached to the alkene carbons provide more C-H sigma bonds that can stabilize the pi system.

Leaving Group Ability: Who Leaves and How Well?

Ever wondered why some atoms or groups are itching to leave a molecule, while others are more like that one guest who just won’t take the hint? Well, in the drama of chemical reactions, leaving groups are those atoms or groups that bail out, taking their bonding electrons with them. Their exit strategy hugely impacts how fast and in what way a reaction proceeds, particularly in substitution and elimination scenarios.

So, what makes a good leaver? The key is stability. Imagine a reality TV show – the leaving group is like a contestant voted off the island. They’re much happier (more stable) being on their own than clinging onto the carbon atom they were once attached to. This happiness translates to a good leaving group being able to stabilize the negative charge it picks up when it departs with those bonding electrons. Think of it like this: a good leaving group is like a celebrity who thrives in the spotlight, perfectly comfortable with all the attention (electrons) on them.

The Acidity Connection: Strong Acids Make Good Leavers

Here’s a neat trick: If the conjugate acid of a leaving group is a strong acid, then the leaving group is generally a good one. It’s all about stability again! A strong acid readily donates its proton (H+), meaning its conjugate base (the leaving group) is super stable on its own. It doesn’t need that proton; it’s happy as a clam without it.

Meet the Leavers: A Rogues’ Gallery

Let’s introduce some common culprits:

Halides (Cl-, Br-, I-): These are your classic leaving groups. Iodine (I-) is generally better than bromine (Br-), which is better than chlorine (Cl-). Fluoride (F-), however, is a terrible leaving group (it’s way too clingy!).
Tosylates (TsO-): These are sulfonate esters and are fantastic leaving groups. They’re like the VIPs of leaving groups – always welcome to leave and highly sought after.
Water (H2O): Yes, even water can be a leaving group, especially when it’s protonated to form H3O+ (a good leaving group because it’s the conjugate acid of water, a weak base). It’s often seen when alcohols undergo reactions.
Alcohols (OH-): Like fluoride, is a terrible leaving group.

The relative ability of these leaving groups to depart is a crucial factor to consider when trying to predict reaction outcomes.

Leaving Groups in Action: SN1, SN2, E1, and E2

Leaving group ability plays a starring role in these key reaction types:

SN1 Reactions: The rate of an SN1 reaction is directly influenced by the leaving group ability. The better the leaving group, the faster the reaction proceeds because the departure of the leaving group is the rate-determining step. Think of it as a celebrity making a grand exit, clearing the way for the next act.
SN2 Reactions: Similar to SN1, a good leaving group speeds up an SN2 reaction. However, SN2 reactions are more sensitive to steric hindrance, so the leaving group’s departure needs to be coordinated with the incoming nucleophile.
E1 Reactions: Like SN1, the rate of E1 reactions depends on the leaving group’s ability to leave. A good leaving group leads to a faster E1 reaction.
E2 Reactions: While the base is very important in E2 reactions, a good leaving group also helps, as its departure is concerted (happening at the same time) with the proton abstraction and the formation of the double bond.

Reactive Intermediates: The Fleeting Participants

Ah, reactive intermediates! Think of them as the *drama queens* of the chemistry world – always popping up unexpectedly, causing a stir, and then vanishing almost as quickly as they appear. They’re not quite reactants and not quite products, but these fleeting species play a crucial role in many chemical reactions. Understanding these temporary characters is key to predicting what products will ultimately emerge from your reaction flask. Let’s dive into the world of carbocations, carbanions, free radicals and transition states!

Carbocations: Electron-Deficient Species

Ever feel like you’re missing something? That’s basically the life of a carbocation. These positively charged carbon atoms are formed when a covalent bond breaks heterolytically, meaning one atom takes both electrons.

Formation: Picture a molecule chilling out, then BAM! A leaving group takes off with a pair of electrons, leaving the carbon behind with a positive charge and an empty orbital.
Stability: Carbocations crave electron density! They’re stabilized by:
- Hyperconjugation: Neighboring C-H or C-C sigma bonds donate electron density into the empty p-orbital. More alkyl groups = more hyperconjugation = more stable carbocation.
- Inductive Effects: Alkyl groups are slightly electron-donating through sigma bonds, helping to offset the positive charge.
Reactions: Carbocations don’t stay lonely for long. They’re ready to react!
- Rearrangements: These are classic carbocation moves. A hydrogen or alkyl group can shift from a neighboring carbon to the carbocation, creating a more stable carbocation. This can completely change the product you get! For example, it will rearrange to form a tertiary carbocation from a secondary carbocation if possible.
- Addition to Alkenes: The carbocation, eager to find an electron, will attack the pi bond of an alkene.

Carbanions: Nucleophilic Anions

On the flip side, we have carbanions – carbon atoms with a negative charge and a pair of unshared electrons. Think of them as electron-rich and ready to share!

Formation: Carbanions form when a carbon acid (a C-H bond) is deprotonated by a strong base. The base snatches the proton, leaving the carbon with the electron pair from the C-H bond.
Stability: Carbanions are stabilized by anything that can spread out that negative charge:
- Inductive Effects: Electron-withdrawing groups near the carbanion help to delocalize the negative charge through sigma bonds.
- Resonance Stabilization: If the carbanion is next to a pi system (like a carbonyl), the negative charge can spread out over multiple atoms.
Reactions: Carbanions are fantastic nucleophiles!
- SN2 Reactions: They can attack alkyl halides, displacing the leaving group in a classic SN2 reaction.
- Addition to Carbonyls: Carbanions love to add to the electrophilic carbon of a carbonyl group (C=O). This is the foundation of many important carbon-carbon bond forming reactions.

Free Radicals: Unpaired Electrons

Free radicals are the rebels of the chemical world – atoms or molecules with an unpaired electron. This makes them highly reactive and prone to forming new bonds.

Formation: Radicals are born from homolytic bond cleavage, where a bond breaks and each atom gets one electron. This can be triggered by heat, light, or radical initiators.
Stability: Radicals, like carbocations, are stabilized by:
- Resonance Stabilization: Delocalizing the unpaired electron over multiple atoms makes the radical more stable.
Reactions: Radicals are known for their chain reactions:
- Initiation: A radical is formed.
- Propagation: The radical reacts with a stable molecule, creating a new radical. This new radical continues the chain.
- Termination: Two radicals combine, ending the chain reaction.
- Halogenation: A common example is the radical halogenation of alkanes, where a hydrogen atom is replaced by a halogen atom.

Transition States: The Highest Energy Point

Transition states aren’t really intermediates in the same way as carbocations, carbanions, and radicals. You can’t isolate them or directly observe them. Instead, they represent a very specific point: the highest energy point along the reaction pathway.

Definition: A transition state is the fleeting molecular structure at the peak of the energy barrier between reactants and products. Bonds are partially formed and partially broken.
Hammond’s Postulate: This useful “rule of thumb” tells us that the structure of the transition state resembles the species (reactant or product) that is closest to it in energy. This means:
- For endothermic reactions, the transition state will resemble the products. Stabilizing the products will also stabilize the transition state, and speed up the reaction.
- For exothermic reactions, the transition state will resemble the reactants.
Influence on Product Formation: The structure of the transition state dictates which product will form. By understanding the factors that stabilize the transition state (steric effects, electronic effects, etc.), we can predict which reaction pathway will be favored.

Reaction Types and Product Prediction: Putting It All Together

Alright, buckle up! Now that we’ve armed ourselves with a treasure trove of knowledge – from sneaky steric effects to the wild world of reactive intermediates – it’s time to put it all into action. Forget memorizing reactions; let’s predict them! We’ll be diving into the three musketeers of organic chemistry: substitution, addition, and elimination reactions. Think of it as becoming a reaction whisperer – you’ll start “hearing” what molecules want to do!

Substitution Reactions: Swapping Atoms

Think of substitution as a molecular dance-off where one atom or group kicks another off the stage. We’re talking about the classic SN1, SN2, and SNi mechanisms.

SN1: This is the unimolecular nucleophilic substitution – a two-step process. It’s like a drama queen leaving (the leaving group departs first, forming a carbocation – remember those guys?) then a new nucleophile waltzes in. It results in racemization because the carbocation intermediate is planar!
SN2: Ah, SN2. Bimolecular nucleophilic substitution. The power couple where the nucleophile attacks simultaneously as the leaving group exits. Think of it as a coordinated heist, and it’s all about steric hindrance. Oh, and get ready for inversion of configuration – the molecule flips like an umbrella in the wind.
SNi: This one’s a bit of a quirky character which means nucleophilic substitution internal. Here, the nucleophile comes from within the molecule, leading to retention of configuration. Sneaky, right?

What favors which pathway? It’s all about the substrate (methyl and primary love SN2, tertiary loves SN1), the nucleophile (strong ones prefer SN2), and the leaving group (the better, the faster, in pretty much all cases). Don’t forget to consider the solvent! (Polar protic solvents favor SN1, while polar aprotic solvents favor SN2).

Example: Let’s say we have tert-butyl bromide reacting with methanol. Which pathway would you predict, SN1 or SN2? Think about the carbocation stability of tert-butyl, and steric hindrance. And which product would you expect?

Addition Reactions: Adding Across a Multiple Bond

Double or triple bonds? “I’m about to end this molecule whole bond’s career.” That’s addition reactions for ya. We’re breaking those pi bonds and sticking new atoms or groups onto the carbons.

Electrophilic Addition: These reactions are all about electron-rich alkenes and alkynes getting attacked by electron-hungry electrophiles. Think of it like a high school bully finding the kid with the lunch money.
- Remember Markovnikov’s Rule: “The rich get richer!” The electrophile (usually hydrogen) goes to the carbon with more hydrogens already. The carbocation intermediate forms at the most stable, most substituted carbon.
Nucleophilic Addition: Now, this one’s the opposite! Think carbonyls! Nucleophiles are attracted to the partially positive carbon in a carbonyl, creating a new bond. It’s like moths to a flame, but less deadly for the moths (hopefully).
Radical Addition: With radicals in the mix, it’s a free-for-all. These reactions proceed via a chain mechanism and can lead to some wild products. And you absolutely cannot forget the Anti-Markovnikov addition when you have peroxides around.

Example: Hydrating propene (aka adding water across the double bond) with an acid catalyst. Markovnikov or anti-Markovnikov? Which carbon gets the H and which gets the OH?

Elimination Reactions: Forming Multiple Bonds

Time to make some alkenes and alkynes! Elimination reactions are all about kicking off atoms or groups and forming a double or triple bond. We have E1, E2, and E1cB, each with its own quirks.

E1: Just like SN1, this is a two-step dance. The leaving group leaves first, forming a carbocation. Then, a base swoops in to remove a proton, forming the double bond. Again, look out for carbocation rearrangements!
E2: The cool cousin of SN2. Everything happens simultaneously – the base removes a proton, the double bond forms, and the leaving group departs. It’s a concerted effort, and stereochemistry is key. Needs to be anti-periplanar.
E1cB: This one’s a bit rare. The base removes a proton first, forming a carbanion, then the leaving group leaves. You need a relatively stable carbanion and a poor leaving group for this to happen.

Zaitsev vs. Hoffmann: When it comes to elimination, which alkene do you make? Zaitsev says the more substituted alkene is the major product (thermodynamically favored), while Hoffmann says the less substituted alkene is favored when you have a bulky base or leaving group (sterically hindered).

Example: Dehydrohalogenating 2-bromobutane with a strong base. Zaitsev or Hoffmann product? What factors influence the outcome?

So, there you have it! While organic chemistry can sometimes feel like navigating a maze, understanding the reaction mechanism and considering factors like stability and steric hindrance can usually guide you to the most likely product. Keep practicing, and you’ll be predicting products like a pro in no time!