Organic Chemistry I reaction mechanisms explain how bonds break, bonds form, and products result from electron movement. This cheat sheet helps students choose between substitution, elimination, radical halogenation, and alkene addition pathways. It is useful for predicting major products, stereochemistry, regiochemistry, and rate laws.
A compact reference is especially helpful because many mechanisms use similar reagents but give different outcomes.
The core ideas are nucleophile strength, base strength, substrate structure, solvent effects, temperature, and intermediate stability. Substitution mechanisms include , which is concerted, and , which forms a carbocation. Elimination mechanisms include , which requires an anti-periplanar geometry, and , which also uses a carbocation intermediate.
Alkene additions are organized by whether they follow Markovnikov addition, anti-Markovnikov addition, syn addition, anti addition, or radical chain behavior.
Key Facts
- reactions are concerted and follow the rate law .
- reactions form a carbocation and follow the rate law .
- reactions are concerted and usually require the leaving group and beta hydrogen to be anti-periplanar.
- reactions form a carbocation, so rearrangements by hydride or alkyl shifts are possible.
- Strong bulky bases favor and often give the less substituted Hofmann alkene instead of the Zaitsev alkene.
- Free-radical halogenation uses initiation, propagation, and termination steps, commonly with or and light .
- Hydrohalogenation of an unsymmetrical alkene usually follows Markovnikov regiochemistry, placing on the carbon with more hydrogens.
- Hydroboration-oxidation gives anti-Markovnikov alcohols by adding then with syn addition.
Vocabulary
- Nucleophile
- A nucleophile is an electron-rich species that donates an electron pair to form a new bond.
- Electrophile
- An electrophile is an electron-poor species that accepts an electron pair during a reaction.
- Carbocation
- A carbocation is a positively charged carbon intermediate whose stability generally increases from methyl to primary to secondary to tertiary.
- Leaving group
- A leaving group is an atom or group that departs with an electron pair during substitution or elimination.
- Regiochemistry
- Regiochemistry describes which constitutional product forms when a reaction can occur at more than one position.
- Stereochemistry
- Stereochemistry describes the three-dimensional arrangement of atoms in reactants and products.
Common Mistakes to Avoid
- Treating every strong nucleophile as an reagent is wrong because strong bulky bases such as usually favor over backside attack.
- Ignoring substrate structure is wrong because methyl and primary substrates favor , while tertiary substrates usually block and favor , , or .
- Forgetting carbocation rearrangements is wrong because , , and some alkene additions can shift to form a more stable carbocation before product formation.
- Assigning Markovnikov products to hydroboration-oxidation is wrong because followed by places on the less substituted alkene carbon.
- Drawing products without checking geometry is wrong because the beta hydrogen and leaving group must be anti-periplanar for efficient elimination.
Practice Questions
- 1 For a reaction with rate law , what happens to the rate if both and are doubled?
- 2 A radical chlorination gives relative hydrogen reactivities . For propane, estimate the relative formation of primary versus secondary monochlorination products.
- 3 Predict the major product when reacts with in the absence of peroxides, and identify the regiochemical rule used.
- 4 A tertiary alkyl bromide reacts with ethanol at room temperature. Explain why or is more likely than .
Understanding Organic Chemistry I Reaction Mechanisms
Mechanisms become easier when every arrow has a job. A curved arrow begins at electrons, not at a positive charge. It may start from a lone pair, a bond, or a negative charge.
Its head points toward the atom or bond that receives those electrons. This rule helps students catch impossible steps before they finish a product. When a bond breaks unevenly, one atom takes both bonding electrons.
That process creates ions. When a bond breaks evenly, each atom takes one electron. That process creates radicals.
Charge and electron counting after every step are useful habits. Carbon normally has four bonds.
Oxygen usually has two bonds and two lone pairs when neutral. Nitrogen usually has three bonds and one lone pair when neutral.
Reaction choice is often a competition rather than a fixed rule. Start by identifying the carbon attached to the leaving group. A methyl or unhindered primary carbon usually gives easy backside attack.
Crowding makes that approach difficult, so a bulky reagent may remove a nearby hydrogen instead. Secondary substrates are the most mixed cases. Their outcome depends strongly on solvent, reagent size, and heat.
Polar protic solvents surround small negative ions with hydrogen bonding. This slows their attack and makes ion-forming pathways more likely when the substrate can support them. Polar aprotic solvents leave many negative nucleophiles more exposed.
This often makes them much more reactive. Higher temperature often increases elimination because forming an alkene plus a small molecule gives more possible arrangements of particles.
Three dimensional shape can decide the product even when the reagent choice seems clear. Backside attack flips the arrangement at the reacting carbon. If that carbon is chiral, the product has the opposite configuration at that center.
In elimination, the hydrogen removed must be positioned correctly relative to the leaving group. A line drawing can hide this requirement. Students should redraw open-chain molecules as sawhorse or Newman projections when needed.
For cyclohexanes, draw a chair. A common E2 elimination needs both the leaving group and the beta hydrogen in axial positions on opposite sides.
A molecule may need a chair flip before it can react. This explains why one stereoisomer can eliminate quickly while a closely related one reacts slowly or gives a different alkene.
Alkene reactions require separate attention to where groups add and which face they approach. Regiochemistry follows the stability of developing charge or radical character in many pathways. Stereochemistry follows the shape of the intermediate or transition state.
A free carbocation is flat, so attack can occur from either face. This often produces a mixture of stereoisomers. A bridged intermediate blocks one face and directs attack to the other side.
A concerted addition can place two new groups on the same face. Radical reactions have their own logic. A radical chain continues only if each propagation step creates a new radical.
Light starts many halogenations by splitting a halogen bond. Bromination is more selective than chlorination because it favors formation of the more stable carbon radical. In problems, write every plausible intermediate first, then compare its stability, geometry, and the conditions before choosing a major product.