Alkene Reactions and Electrophilic Addition

The most substituted alkene is formed in an elimination reaction.

A reaction that forms one isomer preferentially over others.

A reaction that produces one constitutional isomer over others.

An alkene with substituents on the same side of the double bond.

An alkene with substituents on opposite sides of the double bond.

The process of removing a molecule of water (H2O) from an alcohol molecule.

A reaction where two molecules combine to form a single molecule.

A chemical reaction where an electrophile (electron-loving species) attacks an electron-rich alkene, adding atoms or groups to the double bond.

The addition of hydrogen halides (HX, where X is a halogen like Cl, Br, or I) to an alkene, resulting in the formation of an alkyl halide.

A type of addition reaction where the two new atoms or groups are added to the same side of the double bond.

A type of addition reaction where the two new atoms or groups are added to opposite sides of the double bond.

In the addition of HX to an unsymmetrical alkene, the hydrogen atom attaches to the carbon atom that already has more hydrogen atoms.

A positively charged species that acts as an electron acceptor in an electrophilic reaction.

A negatively charged species that acts as an electron donor in a reaction.

A carbon atom with a positive charge, often formed as an intermediate in electrophilic reactions.

The rate of both SN1 and SN2 reactions is increased by better leaving groups.

This is because better leaving groups can more easily accept the electron pair in the C-X bond, leading to a faster reaction.

Polar protic solvents are better for SN1 reactions. Their ability to solvate the leaving group and the carbocation promotes the formation of the carbocation intermediate.

Polar aprotic solvents, which are good at solvating cations, facilitate SN2 reactions by enhancing the nucleophile's reactivity and reducing its hindrance.

Vinyl and aryl halides, with a halogen atom attached to an sp2 hybridized carbon, do not undergo SN1 or SN2 reactions.

The instability of vinyl carbocations prevents SN1 reactions, and the steric hindrance of the sp2 carbon restricts SN2 reactions.

Elimination reactions involve the removal elements from a starting material to form a new pi bond in the product.

A common example is the formation of an alkene through the removal of HX.

In a beta-elimination reaction, the base removes a proton from the beta carbon. Simultaneously, the electron pair in the C-H bond forms a new pi bond between the alpha and beta carbons.

Beta-elimination reactions are facilitated by strong bases and involve the removal of a proton from a carbon atom adjacent to the carbon bearing the leaving group. The reaction produces a new pi bond, typically an alkene.

A type of chemical reaction where a hydrogen halide (HX, like HCl or HBr) adds across a carbon-carbon triple bond in an alkyne. The halogen (X) attaches to the carbon with more hydrogen atoms.

The addition of HX to an alkyne, where the hydrogen atom of HX attaches to the carbon atom at the end of the alkyne chain.

The intermediate formed during the addition of HX to an alkyne, where a carbon bears a positive charge.

A type of carbocation where the positively charged carbon atom is sp hybridized, making it less stable than a typical sp2 hybridized carbocation.

A type of organic compound containing a carbon-carbon triple bond.

The process of adding a halogen (like Cl or Br) across a carbon-carbon triple bond in an alkyne.

A cyclic intermediate formed during the halogenation of alkenes and alkynes, where the halogen atoms are bonded to two adjacent carbon atoms.

A compound where two halogen atoms are attached to a carbon atom, formed by the addition of two moles of halogen.

The speed of a reaction is influenced by the size of the groups attached to the central carbon atom. Larger groups, like alkyl groups, create steric hindrance, making it harder for nucleophiles to attack from the backside. This effect slows down the reaction rate.

The SN2 reaction involves a one-step process where a nucleophile attacks the carbon atom bearing the leaving group, simultaneously pushing out the leaving group. This mechanism is favored when the carbon atom is not excessively hindered.

A carbocation is a species where a carbon atom possesses a positive charge. It is formed in the first step of an SN1 reaction, due to the departure of a leaving group. This carbocation is susceptible to attacks by nucleophiles in the second step.

An SN1 reaction proceeds in two steps: Step 1: The leaving group departs, generating a carbocation. Step 2: The nucleophile attacks the carbocation, forming the final product. This mechanism is influenced by the stability of the carbocation.

The SN1 reaction is a two-step process, and the rate determining step is the first step, the formation of the carbocation. This is why the rate of an SN1 reaction is independent of the concentration of the nucleophile.

In an SN1 reaction, a carbocation intermediate has planar geometry. Since the nucleophile can attack from either side, the product is a racemic mixture with equal amounts of both enantiomers.

The formation of equal amounts of both enantiomers from a single starting material is known as racemization. This occurs in SN1 reactions because the carbocation intermediate lacks any chirality, meaning it does not have a preferred orientation for nucleophilic attack.

The SN1 reaction is a multi-step process, and the rate is determined by the slowest step. This slowest step is typically the formation of the carbocation. Therefore, the stability of the carbocation plays a crucial role in the rate of the SN1 reaction.

In hydroboration, the boron atom preferentially bonds to the less substituted carbon atom of an alkene, due to steric hindrance.

The B-H bond in hydroboration is polarized, with boron having a partial positive charge. This makes BH2 the electrophile that attacks the alkene.

The transition state in hydroboration is stabilized when the more substituted carbon bears a partial positive charge because alkyl groups stabilize positive charges.

When an alkylborane is oxidized, the C-B bond is replaced with a C-O bond, forming an alcohol. This reaction occurs with retention of configuration.

Hydroboration occurs with syn addition, meaning both the H and B attach to the same side of the alkene.

The oxidation step in hydroboration-oxidation also occurs with retention of configuration, meaning the stereochemistry at the carbon is preserved.

The overall outcome of the hydroboration-oxidation sequence is syn addition of H and OH to the double bond. The OH group ends up on the less substituted carbon.

Hydroboration-oxidation of an achiral alkene produces a racemic mixture of alcohols, because the product has a chiral center with equal probability of forming either enantiomer.