Difficulty: Intermediate | Prerequisites: Part 1 (alkene structure and stability), Chapter 3 (thermodynamics basics)
This half of Chapter 4 moves from "what alkenes look like" to "what they do." The central themes are regioselectivity (which product forms preferentially), carbocation intermediates (short-lived species that determine reaction outcomes), and Hammond's postulate (the bridge between intermediate stability and reaction speed). You will also meet catalysts and learn how they change reaction rates without changing the thermodynamics.
When hydrogen halides add to alkenes, Markovnikov's rule predicts which carbon gets the halogen (the more substituted one), and this selectivity is explained by the formation of the more stable carbocation intermediate. Hammond's postulate ties it together: a more stable intermediate means a lower-energy transition state, which means a faster reaction. Catalysts speed up both forward and reverse reactions equally without being consumed.
Regioselective Reaction
A reaction in which constitutional isomers could form as products, but one isomer is produced in a distinctly larger amount than the other. The favoured product is called the major product; the other is the minor product. In simple terms, the reaction "chooses" where to put the new atoms, and one placement wins.
Markovnikov's Rule
When a hydrogen halide (HX) adds to an alkene, the halogen attaches to the carbon of the double bond that has fewer hydrogens (and more carbon substituents). Equivalently, the hydrogen goes to the carbon that already has more hydrogens. Think of it as: the halogen bonds to the more substituted carbon, because that path goes through a more stable carbocation.
Carbocation
A carbon atom bearing a positive charge, with only three bonds and an empty p orbital. Carbocations are reactive intermediates: they form and react so quickly that they never accumulate and cannot be isolated. In simple terms, a carbocation is a carbon that desperately wants another electron pair.
Reactive Intermediate
A short-lived, high-energy species that forms during a reaction but is consumed before the reaction is complete. It cannot be isolated from the reaction mixture. Carbocations, carbanions, and free radicals are all reactive intermediates.
Catalyst
A substance that increases the rate of a reaction without being consumed. Key properties: it does not affect the energies of reactants or products (so it does not change the equilibrium position), and it accelerates both the forward and reverse reactions by the same factor.
Heterogeneous Catalyst
A catalyst that is in a different phase from the reactants. Example: Pd/C (palladium on carbon), a solid catalyst used with liquid or gaseous reactants.
Homogeneous Catalyst
A catalyst that is soluble in the reaction solution, in the same phase as the reactants.
Lindlar's Catalyst
A "poisoned" palladium catalyst (Pd on CaCO3, treated with lead acetate and quinoline) that hydrogenates alkynes selectively to cis alkenes. It stops at the double bond rather than reducing all the way to the alkane. In simple terms, Lindlar's catalyst is a gentle version of Pd that knows when to stop.
Transition State
The highest-energy point along a reaction pathway. It is a fleeting arrangement of atoms that cannot be isolated, even in principle. It sits at the top of the energy barrier between reactants and products (or between reactants and an intermediate).
Hammond's Postulate
In a reaction step where a relatively high-energy intermediate forms from much lower-energy reactants, the transition state resembles the intermediate in both structure and energy. A more stable intermediate leads to a lower-energy transition state, and therefore a faster reaction.
Activation Energy
The energy difference between the reactants (or intermediate) and the transition state. A catalyst lowers the activation energy without changing the overall energy difference between reactants and products.
Rate-Limiting Step
The slowest step in a multi-step reaction mechanism. It has the highest-energy transition state and determines the overall reaction rate.
Tags: regioselectivity, constitutional isomers, major product, minor product
A regioselective reaction is one where two or more constitutional isomers could form, but the reaction favours one over the other. The preferred product is the major product; the less-favoured one is the minor product.
This concept becomes concrete with hydrogen halide addition to alkenes. When HBr adds across an unsymmetrical double bond, two different products are possible depending on which carbon the H attaches to and which carbon the Br attaches to. The reaction does not produce them in equal amounts.
Tags: Markovnikov, HBr addition, HCl addition, HI addition, electrophilic addition, regioselectivity
The rule
When a hydrogen halide (HCl, HBr, HI) adds to an alkene, the halogen attaches to the carbon of the double bond that bears fewer hydrogen atoms and more carbon substituents. Put another way: hydrogen goes to the carbon with more hydrogens already on it.
Example
H2C=C(CH3)2CH2CH3 + HI:
The H adds to the terminal CH2 (which already has more hydrogens)
The I adds to the internal carbon (which has more alkyl substituents)
Product: H3C-C(CH3)2(I)-CH2CH3 (Markovnikov product, major)
Why it works this way
The rule is a consequence of carbocation stability. The reaction proceeds through a carbocation intermediate, and the pathway that forms the more stable (more substituted) carbocation is favoured. The halogen will bond to tertiary over secondary, and secondary over primary.
Practical shortcut
Look at the double bond. The halogen goes to the more substituted end. If one carbon has no hydrogens and three alkyl groups (tertiary), and the other has two hydrogens and one alkyl group (primary), the halogen goes to the tertiary carbon.
Tags: carbocation, reactive intermediate, electrophilic addition mechanism, empty p orbital
What they are
Carbocations are positively charged carbon species with three bonds and an empty p orbital. They are reactive intermediates: they form during a reaction, react almost immediately, and cannot be isolated or bottled.
How they form in HX addition
When HBr approaches an alkene, the pi electrons of the double bond attack the H of HBr (the alkene acts as a nucleophile). This breaks the H-Br bond heterolytically, producing a carbocation on one of the former double-bond carbons and a bromide ion.
The bromide ion then attacks the carbocation, forming the final product.
Stability ordering
Tertiary (3 degree) > secondary (2 degree) > primary (1 degree) > methyl
More alkyl groups stabilise the positive charge through hyperconjugation and inductive effects. This is the fundamental reason Markovnikov's rule works: the reaction preferentially forms the more stable carbocation.
Why they matter
Carbocations are central to many organic reactions beyond HX addition: SN1 substitutions, E1 eliminations, rearrangements. Understanding their stability ordering is one of the most broadly useful skills in organic chemistry.
Tags: catalyst, Pd/C, Lindlar's catalyst, hydrogenation, heterogeneous, homogeneous, activation energy
Four defining properties of a catalyst
It increases the reaction rate.
It is not consumed in the reaction.
It does not affect the energies of reactants or products (so it does not change the equilibrium constant or the overall deltaG).
It accelerates the forward and reverse reactions by the same factor.
Because a catalyst lowers the activation energy equally in both directions, it brings the system to equilibrium faster but does not shift where that equilibrium lies.
Heterogeneous vs homogeneous
Heterogeneous: catalyst and reactants are in different phases. Example: Pd/C (solid catalyst, liquid or gas-phase reactants). Most hydrogenation catalysts are heterogeneous.
Homogeneous: catalyst is dissolved in the reaction solution, same phase as reactants.
Hydrogenation catalysts
Pd/C (palladium on carbon) is the standard catalyst for reducing alkenes to alkanes with H2 gas. It fully reduces double bonds to single bonds.
Lindlar's catalyst
A specialised catalyst for partial hydrogenation. It reduces alkynes (triple bonds) to cis alkenes (double bonds) and stops there. It will not reduce the double bond further to a single bond.
This selectivity matters because a standard Pd/C catalyst would take the triple bond all the way to a single bond. Lindlar's catalyst gives you the cis (Z) alkene product specifically.
Ranking heat of hydrogenation
For ranking purposes: molecules with more pi bonds or fewer substituents have higher heats of hydrogenation. The least stable alkene (fewest substituents, most s-character) releases the most energy. Trans isomers release less energy than cis isomers of the same compound.
Tags: Hammond's postulate, transition state, intermediate, energy diagram, reaction coordinate
Two foundational ideas
Bonding releases energy (forming bonds is exothermic).
Separating opposite charges requires energy.
The postulate
In a reaction step where a relatively high-energy intermediate (such as a carbocation) forms from much lower-energy reactants, the transition state resembles the intermediate in both structure and energy.
Put plainly: the transition state for forming a carbocation looks a lot like the carbocation itself. So if you can work out which carbocation is more stable, you can predict which transition state is lower in energy, and therefore which pathway is faster.
The practical conclusion
More stable intermediate = lower-energy transition state = faster reaction.
This is the theoretical backbone of Markovnikov's rule. The Markovnikov product forms faster because it goes through the more stable (more substituted) carbocation intermediate.
Energy diagram interpretation
On a reaction-coordinate diagram, the transition state sits at the peak of the energy curve. A less stable intermediate corresponds to a higher-energy transition state (taller energy barrier), and therefore a slower reaction. The rate-limiting step is the step with the highest transition state.
Effect of a catalyst on the diagram
A catalyst lowers the transition-state energy (reduces deltaG-double-dagger) without changing the energies of reactants or products. On the diagram, both the forward and reverse activation barriers shrink by the same amount.
Tags: reaction rate, carbocation stability, substitution, HBr addition rate
The principle
For reactions that proceed through carbocation intermediates, the rate of reaction tracks carbocation stability. A more stable carbocation means a faster reaction (Hammond's postulate in action).
Ranking example: reaction with HBr
Given three alkenes A (least substituted, e.g. terminal alkene), B (moderately substituted), and C (most substituted), the rate of reaction with HBr follows:
C > B > A (approximately, with B roughly equal to A in some cases)
The most substituted alkene reacts fastest because it forms the most stable carbocation intermediate, giving the lowest-energy transition state.
The least stable carbocation corresponds to the slowest reaction. This is the single most important takeaway for predicting relative reaction rates in electrophilic addition.
Real-world connection
This stability-rate relationship is the reason industrial alkylation processes preferentially form branched products. The same carbocation stability principles that govern a textbook HBr addition govern large-scale petrochemical manufacturing.
Students often think Markovnikov's rule means "the hydrogen goes to the carbon with more hydrogens." While this gives the right answer, it misses the reason: the rule exists because the reaction forms the more stable carbocation. Learning the mechanism, not just the shortcut, is essential for the harder problems.
Students often believe a catalyst changes the equilibrium of a reaction. It does not. A catalyst changes the rate (speed) equally in both directions, bringing the system to the same equilibrium faster.
Students sometimes confuse the transition state with the intermediate. The transition state is at the energy maximum and cannot be isolated. The intermediate is a local energy minimum between two transition states and, while short-lived, is a real species.
Students assume Lindlar's catalyst fully reduces everything. Lindlar's catalyst specifically stops at the cis alkene when reducing alkynes. It will not reduce a double bond to a single bond.
True or false: Markovnikov's rule predicts that the halogen bonds to the less substituted carbon. ______
Fill in the blank: A catalyst lowers the ________ energy without changing the overall deltaG of the reaction.
True or false: A carbocation is a stable species that can be isolated from the reaction mixture. ______
Fill in the blank: Hammond's postulate says that a more stable intermediate leads to a ________ transition state.
True or false: Lindlar's catalyst converts alkynes directly to alkanes. ______
Answers: 1. False (the halogen bonds to the more substituted carbon). 2. Activation. 3. False (carbocations are reactive intermediates that cannot be isolated). 4. Lower-energy. 5. False (it stops at the cis alkene).
Q: When 2-methylpropene reacts with HBr, what is the major product? Explain using Markovnikov's rule.
A: The major product is 2-bromo-2-methylpropane (tert-butyl bromide). The H adds to the terminal CH2 (fewer substituents) and the Br adds to the central carbon (more substituents), because the tertiary carbocation intermediate is more stable than the primary alternative.
Q: A catalyst is added to a reaction at equilibrium. What happens to the equilibrium position?
A: Nothing. A catalyst accelerates both the forward and reverse reactions equally. It does not shift the equilibrium; it only affects how quickly equilibrium is reached.
Q: Using Hammond's postulate, explain why a tertiary carbocation forms faster than a secondary carbocation in HBr addition.
A: The tertiary carbocation is more stable (lower energy). By Hammond's postulate, the transition state leading to it resembles it in structure and energy, so the transition state for tertiary carbocation formation is lower in energy than the one for secondary formation. A lower transition state means a smaller activation energy barrier, so the reaction is faster.
Q: What product do you get when you treat an internal alkyne with H2 and Lindlar's catalyst?
A: A cis (Z) alkene. Lindlar's catalyst partially reduces the triple bond to a double bond and delivers both hydrogens from the same face (syn addition), giving the cis product.
Q: Rank these intermediates from most to least stable: CH3+, (CH3)2CH+, (CH3)3C+.
A: (CH3)3C+ (tertiary, most stable) > (CH3)2CH+ (secondary) > CH3+ (methyl, least stable).
Carbocation intermediates reappear throughout the course: in SN1 substitution and E1 elimination mechanisms, carbocations are the key intermediate, and rearrangements (hydride and methyl shifts) become important. The stability ordering you learn here applies directly.
Hammond's postulate is the general tool for predicting selectivity in any reaction that goes through a high-energy intermediate. It will surface again in radical reactions (Chapter 6 or equivalent) and in understanding why certain SN1/E1 substrates react faster than others.
Catalysis connects to biochemistry: enzymes are biological catalysts that follow the same principles (lower activation energy, unchanged equilibrium). The heterogeneous/homogeneous distinction matters in industrial chemistry and materials science.
regioselective, regioselectivity, Markovnikov's rule, Markownikoff, anti-Markovnikov, carbocation, carbenium ion, reactive intermediate, electrophilic addition, hydrogen halide addition, HBr addition, HCl addition, catalyst, palladium on carbon, Pd/C, Lindlar's catalyst, partial hydrogenation, cis alkene, heterogeneous catalyst, homogeneous catalyst, Hammond's postulate, transition state, activation energy, rate-limiting step, energy diagram, reaction coordinate, carbocation stability, tertiary carbocation, secondary carbocation, primary carbocation, organic chemistry chapter 4, HEA 0004, University of Washington, Prof. Forrest Michael