Reacciones orgánicas

Understanding Organic Reaction Mechanisms

Key Concepts in Nucleophiles and Electrophiles

• To grasp the development of a reaction in organic chemistry, one must first understand nucleophiles, which are substances with an excess of electrons that are attracted to positively charged nuclei.

• Nucleophiles can vary in strength; for instance, cyanide is a strong nucleophile with a negative charge, while water acts as a neutral nucleophile. Not all strong nucleophiles carry a charge.

• Electrophiles are electron-deficient species that attract electrons. Charged electrophiles include carbocations (which can be tertiary or secondary), while uncharged examples include carbonyl groups.

Reaction Dynamics

• In organic reactions, it is always the nucleophile that attacks the electrophile, indicated by arrows in reaction diagrams.

• Organic compounds exhibit various physical and chemical properties leading to different types of reactions. The most common reactions include substitution reactions categorized into nucleophilic and electrophilic substitutions.

Types of Substitution Reactions

Nucleophilic Substitution

• Nucleophilic substitutions involve the attack of a nucleophile on an electrophilic carbon to replace a radical; for example, chlorination of alkanes via ultraviolet light replaces hydrogen with chlorine.

• There are two types: unimolecular substitution occurs in one step through a transition state where bonds break and form simultaneously; bimolecular substitution involves an intermediate carbocation allowing attack from two directions resulting in racemic mixtures.

Electrophilic Substitution

• In contrast to nucleophilic substitutions, electrophilic substitutions involve the organic molecule itself acting as the nucleophile attacking smaller electrophiles. This is often referred to as aromatic electrophilic substitution when involving aromatic compounds like benzene.

Addition and Elimination Reactions

Addition Reactions

• Addition reactions occur when groups or atoms are added to unsaturated organic molecules. An example includes hydrochloric acid adding to propene where chlorine predominantly attaches to the less substituted carbon due to Markovnikov's rule.

Elimination Reactions

• Elimination reactions result in the removal of groups or atoms from an organic molecule forming unsaturation. Two mechanisms exist: unimolecular and bimolecular elimination; dehydration of alcohol (e.g., 2-propanol converting into propene under heat) exemplifies this process.