This cheat sheet covers the four major organic reaction mechanisms students meet in high school and early college chemistry: , , , and . These mechanisms explain how alkyl halides and similar compounds undergo substitution or elimination reactions. Students need this reference because mechanism choice depends on several factors at once, including substrate structure, nucleophile strength, base strength, solvent, and temperature.
A clear comparison helps students predict products, rates, and stereochemistry more confidently.
The most important ideas are the number of steps, the rate law, and whether a carbocation forms. and are unimolecular and usually follow , while and are bimolecular and follow or . gives backside attack and inversion, while requires an anti-periplanar -hydrogen and leaving group.
Strong nucleophiles favor substitution, strong bases favor elimination, and bulky bases often favor less substituted alkenes.
Key Facts
- is a two-step substitution mechanism with carbocation formation, and its rate law is .
- is a one-step substitution mechanism with backside attack, and its rate law is .
- is a two-step elimination mechanism with carbocation formation, and its rate law is .
- is a one-step elimination mechanism, and its rate law is .
- The usual substrate reactivity order for and is because more substituted carbocations are more stable.
- The usual substrate reactivity order for is because steric hindrance blocks backside attack.
- at a chiral center causes inversion of configuration, while often gives racemization because the carbocation is planar.
- For many reactions, the major alkene follows Zaitsev's rule, so the more substituted alkene is favored unless a bulky base favors the Hofmann product.
Vocabulary
- Nucleophile
- A nucleophile is an electron-rich species that donates an electron pair to form a new bond.
- Leaving group
- A leaving group is an atom or group that departs with an electron pair during substitution or elimination.
- Carbocation
- A carbocation is a positively charged carbon intermediate, usually written as , that is stabilized by alkyl substitution and resonance.
- Rate law
- A rate law shows how reaction rate depends on reactant concentrations, such as for .
- Anti-periplanar
- Anti-periplanar describes two bonds in the same plane but pointing in opposite directions, a geometry often required for elimination.
- Zaitsev product
- The Zaitsev product is the more substituted alkene that often forms as the major product in elimination reactions.
Common Mistakes to Avoid
- Using with a substrate is wrong because bulky alkyl groups block the backside attack needed for the one-step mechanism.
- Forgetting the nucleophile or base in a bimolecular rate law is wrong because and rates depend on both the substrate and the attacking species.
- Predicting rearrangements in or is wrong because these mechanisms are concerted and do not form carbocation intermediates.
- Assuming every strong nucleophile gives substitution is wrong because strong bulky bases often favor elimination instead of substitution.
- Ignoring stereochemistry in is wrong because backside attack inverts the configuration at the reacting chiral carbon.
Practice Questions
- 1 For an reaction with , what happens to the rate if is doubled and is tripled?
- 2 For an reaction with , the rate is when . What is the rate when ?
- 3 A alkyl bromide reacts with a strong bulky base such as . Which mechanism is most likely, , , , or ?
- 4 Explain why a polar protic solvent can favor but slow down .
Understanding Reaction Mechanisms (SN1, SN2, E1, E2)
A mechanism is a particle-level story of bond changes. Start by locating the carbon attached to the leaving group. Then inspect the atoms on neighboring carbons, called beta carbons.
Those beta carbons matter because they can supply a hydrogen when an alkene forms. A good leaving group makes every pathway easier because it can hold the electron pair after the carbon bond breaks.
Iodide and bromide are usually better leaving groups than chloride, while hydroxide often needs to be changed into water or another better leaving group first. This explains why the same carbon skeleton can react very differently after a small chemical change.
Solvents do more than dissolve reactants. Polar protic solvents, such as water and alcohols, surround negative ions with strong attractions. This slows a nucleophile because its electron pair is less available to attack.
These solvents can help reactions that pass through charged carbocations, so SN1 and E1 become more likely. Polar aprotic solvents, such as acetone or dimethyl sulfoxide, do not trap negative ions as strongly.
A nucleophile stays more reactive in them, which often helps SN2. When comparing reaction conditions, treat the solvent as an active part of the evidence rather than background information.
Carbocations create extra possibilities that are easy to miss. After a leaving group departs, a nearby hydrogen or carbon group can shift with its bonding electrons. This rearrangement may place the positive charge on a more stable carbon.
The final substitution or elimination product can then have a changed carbon skeleton. Rearrangements occur in SN1 and E1 because those pathways have a real carbocation intermediate.
They do not occur during ordinary SN2 or E2 reactions, since bond breaking and bond making happen in one continuous event. This difference is useful when a product seems to have an unexpected arrangement of atoms.
Elimination needs careful attention to shape, not just reagent labels. In an open-chain molecule, rotation around single bonds can often place a beta hydrogen opposite the leaving group for E2. In rings, especially cyclohexane rings, the required arrangement can be difficult.
The hydrogen and leaving group usually need axial positions on opposite sides of the ring. A compound may therefore react slowly or give only one alkene even when several beta hydrogens exist. Heat often increases elimination because forming separate alkene products can be favored at higher temperature.
In real organic synthesis, chemists use these choices to build alkenes for plastics, flavors, medicines, and many other carbon compounds. For school problems, first classify the substrate, then judge the reagent, solvent, and possible geometry before choosing a mechanism.