Chapter 6: Reactions of Alkenes and Stereochemistry of Addition Reactions

Vocabulary flashcards covering the reaction mechanisms, carbocation stabilities, rearrangements, regioselectivity, and stereochemistry of alkene additions from Chapter 6.

Electrophilic Addition Reaction

An organic reaction characteristic of alkenes wherein the loosely held $\pi$ electrons attack an electrophile, adding it to one $sp^2$ carbon, followed by the addition of a nucleophile to the other $sp^2$ carbon to form two new $\sigma$ bonds.

Rate-Determining Step in Hydrogen Halide Addition

The slow, first step of the electrophilic addition of $\text{HX}$ to an alkene, which involves the addition of $\text{H}^+$ to an $sp^2$ carbon to form a carbocation intermediate.

Carbocation Stability Trend

The relative thermodynamic stability ordering of carbocations: tertiary ($3^\circ$) > secondary ($2^\circ$) > primary ($1^\circ$) > methyl cation, resulting from alkyl groups decreasing the concentration of positive charge on the carbon.

Hyperconjugation

The stabilizing interaction that delocalizes positive charge when the orbital of an adjacent $\sigma$ bond overlaps with an empty $p$ orbital of a carbocation, which occurs only if the orbitals possess the proper spatial orientation.

Hammond Postulate

The postulate stating that a transition state resembles the species (reactants or products) to which it is closer in energy: resembling the reactants in an exergonic reaction and the products in an endergonic reaction.

Regioselective Reaction

A reaction that preferentially forms one constitutional isomer over another; it can be classified as moderately, highly, or completely regioselective.

Regioselectivity Rule for Electrophilic Addition

In an electrophilic addition reaction to an unsymmetrical alkene, the electrophile adds preferentially to the $sp^2$ carbon bonded to the most hydrogen atoms to yield the more stable carbocation intermediate.

Hydration

The addition of water ($\text{H}_2\text{O}$) to a molecule; in alkenes, an acid catalyst is required because water is too weakly acidic to act as an electrophile on its own.

Acid Catalyst

A chemical species that increases the reaction rate by providing a lower-energy pathway without being consumed in the net chemical reaction.

1,2-Hydride Shift

A type of carbocation rearrangement in which a hydrogen atom migrates with its bonding pair of electrons from an adjacent carbon to the positively charged carbon, occurring only if a more stable carbocation is formed.

1,2-Methyl Shift

A carbocation rearrangement where a methyl group ($-\text{CH}_3$) shifts with its bonding pair of electrons to an adjacent positively charged carbon to convert a less stable carbocation into a more stable carbocation.

Hydroboration-Oxidation

A two-step synthetic pathway converting an alkene into an alcohol without carbocation rearrangements; it involves the concerted addition of $\text{BH}_3$ followed by oxidation with basic hydrogen peroxide ($\text{HO}^-$, $\text{H}_2\text{O}_2$, $\text{H}_2\text{O}$).

9-BBN

9-borabicyclo[3.3.1]nonane; a bulky organoborane reagent ($\text{R}_2\text{BH}$) with only one available hydride ion that shows high steric preference for hydroborating the less substituted $sp^2$ carbon of an alkene.

Vicinal Dihalide

A compound in which two halogen atoms are bonded to adjacent carbon atoms, typically formed by adding $\text{Br}_2$ or $\text{Cl}_2$ across an alkene.

Cyclic Bromonium Ion

A three-membered positively charged cyclic intermediate formed when electrophilic bromine adds across a double bond, preventing carbocation rearrangements and directing anti-addition.

Halohydrin

An organic compound containing a halogen atom and a hydroxyl group ($-\text{OH}$) on adjacent carbons, produced when an alkene reacts with a halogen ($\text{Br}_2$ or $\text{Cl}_2$) in aqueous solution.

Stereoselective Reaction

A chemical reaction in which a single reactant leads to the preferential formation of one stereoisomer over another.

Stereospecific Reaction

A reaction where the reactant can exist as stereoisomers and each individual stereoisomer of the reactant specifically yields a different stereoisomeric product.

Racemic Mixture Formation in Alkene Addition

The equal formation of enantiomeric pairs occurring when an achiral reactant (having no asymmetric centers) reacts to produce a product containing a single asymmetric center.

Syn Addition

An addition process wherein two incoming substituents attach to the same side or face of a double bond, such as in catalytic hydrogenation ($\text{H}_2/\text{Pd/C}$).

Anti Addition

An addition process wherein two incoming substituents attach to opposite faces of a double bond, as seen in halogenation proceeding through a cyclic halonium ion intermediate.

Meso Compound

An achiral compound that contains asymmetric centers but is superimposable on its mirror image due to internal symmetry, formed for example when syn addition occurs on a cis alkene with identical substituents.