Chemistry | Organic Chemistry | Reactions (Substitution, Addition and Elimination)

The video script begins with an introduction to organic reactions, following a comprehensive coverage of organic compound naming. The instructor invites viewers to subscribe and reach out with questions or to discuss past papers. The first topic of discussion is the combustion reaction, which involves hydrocarbons reacting with oxygen to produce carbon dioxide and water, with complete combustion yielding these products in the presence of excess oxygen, and incomplete combustion resulting in carbon monoxide. An example of balancing the combustion reaction of ethane (C2H6) is provided to demonstrate the process of balancing chemical equations.

This paragraph delves deeper into the process of balancing combustion reactions, using the example of propane (C3H8) to illustrate the steps. The instructor explains how to account for the correct number of atoms on both sides of the equation by adjusting coefficients, resulting in a balanced chemical equation. The concept of stoichiometric ratios is introduced, emphasizing the preference for whole numbers over fractions or decimals in chemical equations.

The script highlights that combustion reactions are exothermic, meaning they release energy. The instructor uses the example of petrol (C8H18) combustion, a reaction that occurs in car engines, to explain this concept. The release of energy is represented by a negative change in enthalpy (ΔH < 0), a fundamental principle in thermochemistry.

The instructor contrasts the reactions of saturated and unsaturated hydrocarbons. Saturated hydrocarbons, such as alkanes, typically undergo substitution reactions with halogens, facilitated by UV energy, leading to the formation of haloalkanes and hydrogen halides. Unsaturated hydrocarbons, including alkenes and alkynes, are introduced, with their distinct reactions to be discussed in subsequent paragraphs.

This section explains the difference between substitution and addition reactions in hydrocarbons. Alkanes, with only single bonds, mainly undergo substitution reactions where hydrogen atoms are replaced by halogen atoms. In contrast, alkenes, containing double bonds, primarily undergo addition reactions where the double bond breaks and the components of the reactant add across the former double bond, leading to the formation of new products.

The instructor introduces Markovnikov's rule, which predicts the major product in the addition of a proton (H+) and a nucleophile (such as OH-) to an alkene. According to the rule, the hydrogen atom will preferentially add to the carbon with the greater number of hydrogen atoms, resulting in the formation of the major product. This principle is illustrated with the example of an alkene reacting with water (hydration) and hydrogen chloride (hydrohalogenation).

The script moves on to discuss elimination reactions, where a molecule loses one or more small molecules (like H2O or HCl), forming an unsaturated compound such as an alkene. The process of cracking in petroleum refining is mentioned as a real-world application of elimination reactions, where large hydrocarbon molecules are broken down into smaller, more useful ones like gasoline and jet fuel.

Zaitsev's rule is introduced, which states that in an elimination reaction, the hydrogen atom will be preferentially removed from the carbon with the fewest hydrogen atoms, leading to the formation of the more stable alkene. This rule is applied to predict the major product of an elimination reaction involving an alkane with a halogen substituent.

The final paragraph covers hydrolysis and dehydrohalogenation reactions. Hydrolysis involves the reaction of a compound with water, often catalyzed by an acid or base. Dehydrohalogenation is a specific type of elimination reaction where a hydrogen and a halogen are removed from a molecule, forming a double bond and a hydrogen halide. The conditions that favor each reaction type are discussed, highlighting the importance of understanding reaction mechanisms in organic chemistry.