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 .