Introduction to Organic Chemistry: Reactions, Hydrocarbons, and Petrochemicals
Enthalpy of a Reaction
Enthalpy Change
When a process takes place at constant pressure, the heat of reaction is called enthalpy change (ΔH). Under standard conditions (1 atm pressure and 25°C), it is written as ΔH°. Enthalpy is a relative measure of the energy of the initial or final state of a system. Therefore, it quantifies the change in enthalpy.
Enthalpy of Formation
The enthalpy of formation (ΔH0f) is the heat absorbed or released when one mole of a compound is formed from its constituent elements at a pressure of 1 atm and a temperature of 25°C.
Enthalpy of Reaction
The enthalpy of reaction is calculated by subtracting the sum of the enthalpies of formation of the reactants (multiplied by their respective coefficients) from the sum of the enthalpies of formation of the products (multiplied by their respective coefficients).
How Chemical Reactions Occur
Collision Theory
A reaction is initiated by the collision of reactive molecules. For example, if molecules A and B react to form AB, they must collide. Let’s assume that effective collisions are strong enough that the result of an encounter of particles with adequate orientation, moving at high speed, leads to a reaction. Therefore, for a reaction to occur, the molecules must possess a certain kinetic energy.
Critical Kinetic Energy
The critical kinetic energy (Ec) is the minimum kinetic energy required for a collision to be effective. Any collision between particles with kinetic energy greater than Ec is considered effective.
Activation Energy
A thermodynamically favorable reaction may take place only at high temperatures or occur abruptly. This implies the existence of an energy barrier that must be overcome to move from reactants to products. A reaction can be activated by increasing the temperature.
The activation energy (Ea) is the difference between the energy of the activated complex (the highest energy state along the reaction pathway) and the energy of the reactants. The activation energy can be considered the energy necessary to weaken the bonds in the reactants.
Requirements for a Reaction
The requirements for a reaction to occur are:
- A favorable geometric arrangement of the colliding molecules.
- A collision with kinetic energy greater than the critical kinetic energy (Ec), thus exceeding the activation energy (Ea).
Speed of Reaction
The speed of reaction, also known as the reaction rate, is the change in the concentration of reactants or products over time. The unit of reaction rate is typically moles per liter per second (mol/L/s).
Factors Influencing the Speed of Reaction
Nature of the Reagents
Generally, there are no strict rules regarding the influence of the nature of the reagents on the reaction rate. Reactions in ionic solutions are often very fast because the bonds are already broken, and the reaction involves a simple rearrangement of ions. Reactions involving the exchange of electrons between ions are also fast. Exothermic reactions between gases can also proceed at high speeds.
Concentration of Reagents
The number of collisions between reactive molecules is proportional to the concentration of the reactants. If the concentration increases, the reaction rate also increases.
Contact Area
In heterogeneous reactions, where reactants are in different phases, the reaction occurs only at the interface between the phases. Therefore, the reaction rate depends directly on the contact surface area. Crushing or grinding solid reactants can increase the contact area and thus the reaction rate. Reactions between gases typically occur very fast due to the high degree of mixing and contact between the molecules.
Temperature
The rate of reaction is strongly influenced by temperature. Many reactions have a slow rate at low temperatures but become much faster when the temperature is increased. Increasing the temperature increases the average kinetic energy of the molecules, and consequently, the number of molecules that possess enough energy to overcome the activation energy barrier.
Catalysts
A catalyst is a substance that affects the rate of a reaction without being consumed in the reaction itself. The action of a catalyst is called catalysis. A catalyst changes the reaction pathway, providing an alternative route with a lower activation energy. Catalysts that increase the reaction rate are called positive catalysts, while those that decrease the reaction rate are called negative catalysts or inhibitors.
Catalysts can significantly alter the reaction rate even when present in very small amounts. Enzymes are an important class of biocatalysts that regulate biological processes in living organisms.
Alkanes
The first four alkanes (methane, ethane, propane, and butane) are gases at room temperature. Alkanes with 5 to 17 carbon atoms are liquids, and those with 18 or more carbon atoms are solids. The melting point and boiling point of alkanes increase with increasing molecular weight. Branched alkanes have lower boiling points than their straight-chain isomers.
Alkanes are nonpolar compounds and are therefore soluble in nonpolar solvents but insoluble in water. The density of alkanes increases with the number of carbon atoms, approaching 0.8 g/cm3.
Sources and Reactivity
The main sources of alkanes are oil and natural gas. Alkanes are relatively unreactive and do not readily react with acids or bases. They are commonly used as fuels.
Alkenes
Alkenes with fewer than four carbon atoms are obtained in pure form from the oil industry. More complex alkenes are prepared by removing atoms from larger molecules.
Reactivity
Alkenes are more reactive than alkanes due to the presence of the double bond. They are characterized by addition reactions, in which atoms or groups of atoms are added to the carbon atoms of the double bond.
Alkynes
Alkynes are insoluble in water but soluble in organic solvents. They are less dense than water. The boiling points of alkynes increase with increasing molecular weight and decrease with increasing branching of the carbon chain.
Alkynes are very reactive and burn more readily than alkanes and alkenes. They undergo addition reactions, similar to alkenes.
Oil
Oil is a fossil fuel formed from the anaerobic decomposition of organic matter, primarily marine organisms. The decomposition process converts fats into fatty acids, which over millions of years are transformed into hydrocarbons. Oil is typically found in underground reservoirs.
Crude oil is a complex mixture of hydrocarbons (typically around 90%), along with varying amounts of impurities. The composition of crude oil varies depending on its origin.
Refining
Crude oil is refined to separate it into its various components, which have different uses. The first step in refining is to remove impurities by decantation or filtration. The oil is then subjected to fractional distillation, where it is heated to about 400°C and passed through a series of condensing towers. Each tower collects a fraction of the oil with a specific boiling point range. The different fractions are then further processed to produce various petroleum products.
Gasoline
Gasoline is the most economically important product derived from oil refining. However, only about 12% of crude oil is directly converted into gasoline. To increase the yield of gasoline, heavier fractions of crude oil are subjected to cracking, a process that breaks down larger hydrocarbon molecules into smaller ones. Cracking can be done thermally or catalytically.
Gasoline Requirements
Gasoline must meet certain requirements to function properly in internal combustion engines. It should not be corrosive, which is achieved through a process called sweetening. It should have good carburetion properties, meaning it should readily vaporize and mix with air. It should not contain double bonds, which can lead to the formation of gums and deposits. It should not be explosive, which is addressed by using antiknock agents such as tetraethyl lead (now largely phased out) or other additives.
Petrochemicals
Petrochemicals are chemicals derived from petroleum or natural gas. They are used as raw materials for the production of a wide variety of products, including plastics, synthetic fibers, detergents, and solvents.
Pyrolysis
Pyrolysis is a process in which hydrocarbons are thermally decomposed at high temperatures in the absence of oxygen. Pyrolysis of heavier oil fractions produces olefins (alkenes), which are important building blocks for the production of plastics.
Reforming
Reforming is a process that converts paraffinic hydrocarbons (alkanes) into aromatic hydrocarbons, such as benzene, toluene, and xylenes. Aromatic hydrocarbons are used as feedstocks for the production of various chemicals and materials.
Biofuels
Bioethanol
Bioethanol is an alcohol produced by the fermentation of sugars derived from biomass, such as corn, sugarcane, or barley. Bioethanol can be blended with gasoline to reduce reliance on fossil fuels and decrease greenhouse gas emissions.
Biodiesel
Biodiesel is a fuel made from vegetable oils or animal fats. It is produced by transesterification, a process that converts triglycerides in the oils or fats into fatty acid methyl esters (FAMEs). Biodiesel can be used as a substitute for or blended with conventional diesel fuel. It produces fewer emissions than diesel fuel and is biodegradable.