CHM 2210 Chapter 8: Addition Reactions of Alkenes

Flashcards covering the vocabulary, rules, reagents, and mechanisms for alkene addition reactions including hydrohalogenation, hydration, dihydroxylation, and oxidative cleavage.

Addition Reaction

The opposite of elimination, where a $C=C$ $\pi$ bond is converted into two new $\sigma$ bonds as the $\pi$ bond acts as an electron-pair donor.

Propene (Propylene)

A naturally occurring, acyclic alkene critical in the chemical industry, with $70$ billion pounds made from cracking petroleum each year.

Ethene (Ethylene)

An industrial precursor made from petroleum cracking, with a production volume of $200$ billion pounds per year.

Enthalpy ($\Delta H$) in Addition

Addition reactions are favored by enthalpy because $\sigma$ bonds are stronger and more stable than $\pi$ bonds ($\Delta H ≈ -19\,kcal/mol$).

Entropy ($\Delta S$) in Addition

Addition reactions are not favored by entropy because two molecules combine to form one product, causing $\Delta S$ to decrease; therefore, lower temperatures are used to favor addition.

Hydrohalogenation

The addition of $H-X$ (where $X$ is $Cl$, $Br$, or $I$) across an alkene.

Regioselectivity

The preference of a reaction to form one constitutional isomer over another, such as when a halogen installs at the more substituted carbon versus the less substituted carbon.

Markovnikov's Rule

Observations from 1869 stating that in the addition of $H-X$ to an alkene, the $H$ atom adds to the carbon already bearing more $H$ atoms (the less substituted carbon).

Anti-Markovnikov Addition

Observation that when peroxides ($ROOR$) are used with $HBr$, the halogen is installed at the less substituted carbon.

Carbocation Rearrangement

A process (such as a $1,2$-hydride shift or $1,2$-methyl shift) where a less stable carbocation intermediate (e.g., $2^∘$) transforms into a more stable one (e.g., $3^∘$) during hydrohalogenation or hydration.

Acid-catalyzed Hydration

The addition of the components of water ($H$ and $OH$) across a $\pi$ bond using an acid catalyst, typically sulfuric acid ($H_2SO_4$), following Markovnikov regioselectivity.

Oxymercuration-Demercuration

An alternative to acid-catalyzed hydration that adds $H$ and $OH$ with Markovnikov regioselectivity using $Hg(OAc)_2$ and $NaBH_4$ without the risk of carbocation rearrangements.

Mercurinium Ion

A stabilized cationic intermediate formed when a $\pi$ bond attacks a mercuric cation ($Hg^{2+}$); it prevents carbocation rearrangements during oxymercuration.

Hydroboration-Oxidation

A two-reaction sequence using $BH_3 \times THF$ followed by $H_2O_2$ and $NaOH$ that adds $H$ and $OH$ with anti-Markovnikov regioselectivity and syn stereoselectivity.

Syn Addition

A stereospecific process where two groups are added to the same face of a $\pi$ bond, as seen in hydroboration-oxidation and catalytic hydrogenation.

Catalytic Hydrogenation

The addition of $H_2$ across a $C=C$ bond using a transition metal catalyst (such as $Pt$, $Pd$, or $Ni$) to convert an alkene into an alkane via syn addition.

Heterogeneous Catalyst

A catalyst that does not dissolve in the reaction medium, such as $Pt$ or $Pd$ metal surface.

Wilkinson’s Catalyst

A homogeneous catalyst soluble in the reaction medium, used for hydrogenation and capable of asymetric hydrogenation if chiral phosphine ligands are used.

Halogenation

The addition of two halogen atoms (usually $Cl_2$ or $Br_2$) across a $C=C$ double bond, proceeding via anti addition.

Bromonium Ion

A bridged intermediate formed during bromination that results in anti addition because the nucleophile must attack from the side opposite the leaving group.

Halohydrin Formation

A reaction conducted in water where a halogen and a hydroxyl group ($OH$) are added across a $\pi$ bond; the $OH$ adds to the more substituted carbon and the halide to the less substituted carbon.

Anti-dihydroxylation

A two-reaction process to add two $OH$ groups across a $\pi$ bond using a peroxy acid ($RCO_3H$) followed by aqueous acid ($H_3O^+$).

Syn-dihydroxylation

The addition of two $OH$ groups across a $\pi$ bond in a concerted syn fashion, achieved using catalytic $OsO_4$ with $NMO$ or cold, basic $KMnO_4$.

Ozonolysis

An oxidative cleavage reaction using ozone ($O_3$) followed by a reducing agent like dimethyl sulfide ($DMS$) or $Zn/H_2O$ to break a $C=C$ bond and form two carbonyl groups.