Alkenes and Electrophilic Addition Reactions

Alkenes are hydrocarbons containing a carbon-carbon double bond, making them electron-rich and susceptible to attack by electron-deficient species called electrophiles.

Electrophilic addition is a reaction where an electrophile attacks an alkene, breaking the double bond and forming a new single bond. This leads to the addition of two new atoms or groups to the carbons that were originally double-bonded.

Markovnikov's Rule states that in the addition of a protic acid (like HX) to an unsymmetrical alkene, the hydrogen atom of the acid will attach to the carbon atom in the double bond that already has more hydrogen atoms attached. In simpler terms, the hydrogen atom goes to the carbon with more hydrogens.

A carbocation is a positively charged carbon atom with only three bonds. It is an intermediate that forms during electrophilic addition reactions. These intermediates can be stabilized by the presence of electron-donating groups, which can distribute the positive charge and make the carbocation more stable.

The stability of the carbocation intermediate formed during the reaction determines the major product. More stable carbocations are formed more easily, leading to the formation of the major product. The stability of carbocations is determined by the number of alkyl groups attached to the positively charged carbon. Tertiary carbocations are the most stable, followed by secondary, and then primary carbocations.

Isomers with the same molecular formula but different arrangements of atoms within the carbon chain. For example, but-1-ene and 2-methylpropene both have the formula C4H8 but differ in the arrangement of their carbon atoms.

Isomers with the same molecular formula but different positions of a functional group within the molecule. For example, but-1-ene and but-2-ene both have the formula C4H8 but differ in the position of the double bond.

Isomers that have the same connectivity of atoms but differ in the three-dimensional arrangement of atoms in space. The most common type is cis-trans isomerism.

A type of stereoisomerism where the atoms are arranged differently around a double bond. Two groups bonded to one carbon atom are on the same side of the double bond (cis) or on opposite sides (trans).

A stereoisomer where two identical groups are bonded to the same carbon atom involved in the double bond. This prevents cis-trans isomerism because there's no difference in the arrangement around the double bond.

The restricted rotation of the carbon-carbon double bond due to the presence of the pi (π) bond. This restriction is what allows for cis-trans isomerism.

Two different groups of atoms must be bonded to each of the carbon atoms in the C=C bond. This ensures there's a difference in the arrangement of groups around the double bond.

The inability of a molecule to exhibit cis-trans isomerism because it only has one type of group on each side of the double bond. For example, but-1-ene has two hydrogens on one carbon and two different groups on the other carbon.

A chemical process where a molecule loses atoms or groups to form a double or triple bond. This is often triggered by heat and catalysts like concentrated sulfuric acid, aluminum oxide, or potassium hydroxide in ethanol.

A reaction where a halogenoalkane loses hydrogen halide (HX) in the presence of ethanolic potassium hydroxide (KOH) to form an alkene.

A reaction where an alcohol loses water in the presence of concentrated sulfuric acid (H2SO4) or aluminum oxide (Al2O3) to form an alkene. This process requires heat.

The degree of substitution of an alkene describes the number of alkyl (R-) groups directly attached to the carbon atoms involved in the double bond. A more substituted alkene has more R-groups attached.

A rule that states the major product in an elimination reaction where multiple alkene products are possible is the more stable, more highly substituted alkene. This means the alkene with more alkyl groups attached to the double bond is the main product.

A positively charged carbon atom with only three bonds, which commonly forms as an unstable intermediate during electrophilic addition reactions.

The stability of a carbocation directly influences which alkene product forms in an elimination reaction. More stable carbocations are favored and lead to the formation of the major alkene product. Tertiary carbocations are the most stable, followed by secondary and then primary carbocations.

A specific type of reaction where an electrophile, an electron-seeking species, attacks a double bond, leading to the addition of two new atoms or groups across the original double bond.

The addition of a hydrogen atom to the carbon with more hydrogen atoms in an unsymmetrical alkene.

Carbocations can be categorized into primary, secondary, or tertiary based on the number of alkyl groups attached to the carbon bearing the positive charge.

The more stable the carbocation intermediate, the more likely it will form, leading to a major product.

A molecule containing two or more enantiomers in equal proportions, resulting in a mixture that does not rotate plane-polarized light.

An intermediate formed when an alkene reacts with an electrophile, where the carbon atom bearing the positive charge is trigonal planar.

The driving force of a chemical reaction, leading to the formation of a stable product.

A reaction where alkenes are oxidized by cold potassium permanganate (KMnO4) to form diols. The double bond is partially cleaved, and two hydroxyl (OH) groups are added to the carbons that were part of the double bond.

The reaction involves treating alkenes with hot potassium permanganate (KMnO4) to break the double bond completely and form carbonyl compounds (ketones or carboxylic acids).

This reaction is classified as an oxidation reaction where the alkene loses electrons, leading to the formation of new products with a higher oxidation state.

When the carbon atom involved in the double bond is bonded to two alkyl or aryl groups (R), the product formed is ketone.

When the carbon atom involved in the double bond is bonded to one alkyl or aryl group and one hydrogen atom, the product is carboxylic acid.

When the carbon atom involved in the double bond is attached to two hydrogen atoms (H), the products are carbon dioxide gas and water.

Potassium dichromate (K2Cr2O7) is a weaker oxidizing agent compared to potassium permanganate (KMnO4). It cannot oxidize alkenes, even under mild or strong conditions.

Oxidative cleavage reactions can be used to identify the structure of unknown alkenes by analyzing the products formed. The type of carbonyl compounds (ketones or carboxylic acids) provides clues about the original alkene's structure.

The stability of a carbocation is influenced by the electron-donating or electron-withdrawing nature of groups attached to the positively charged carbon. Alkyl groups, being electron-donating, stabilize carbocations, while groups like halogens or –COOH (electron-withdrawing) destabilize them.

A carbocation is considered more stable when the positive charge on the carbon is better dispersed. This dispersal occurs due to the electron-donating effect of alkyl groups, which effectively 'spread' the positive charge across the molecule.

Tertiary carbocations, with three alkyl groups attached to the positively charged carbon, are the most stable. Secondary carbocations have two alkyl groups, and primary carbocations have only one, making them less stable. The more alkyl groups, the better the dispersal of the positive charge, leading to greater stability.

Alkyl groups are electron-donating groups (EDGs), meaning they push electron density towards the positively charged carbon, lessening the positive charge. This stabilizes the carbocation.

Halogens, –COOH, and –NO2 groups act as electron-withdrawing groups (EWGs). They pull electron density away from the positively charged carbon, intensifying the positive charge and making the carbocation less stable.

A carbocation that forms more easily due to its higher stability will be the primary intermediate in a reaction. Therefore, reactions tend to favor the formation of more stable carbocations, ultimately influencing the major product formed.

In an electrophilic addition reaction, the addition of a reagent (like HCl) to an alkene occurs in a way that ultimately leads to the formation of the most stable carbocation intermediate. This stability determines the major product formed.

Distinguishing tests are simple chemical tests used to differentiate between organic compounds based on their functional groups. They are commonly carried out in test tubes.