CHE 176 Lecture 4 PDF: Kinetics and Mechanism of Organic Reactions

KINETICS AND MECHANISM OF ORGANIC REACTIONS Chemical kinetics Chemical kinetics is the study of how fast chemical reactions occur. The speed of the reaction is measured by the change in concentration of either the reactants or products with time. Chemical kinetics also sheds light on the reaction me...

KINETICS AND MECHANISM OF ORGANIC REACTIONS Chemical kinetics Chemical kinetics is the study of how fast chemical reactions occur. The speed of the reaction is measured by the change in concentration of either the reactants or products with time. Chemical kinetics also sheds light on the reaction mechanisms (how the reaction occurs) Chemical reactions can be classified into three types based on the speed of the reaction: Instantaneous reaction: Reactions that occur within a short period of time (10-13 to 10-16 sec). E.g ionic reactions AgNO3 + NaCl AgCl + NaNO3 Moderate reactions: Reactios which occur not too fast or slow (some minutes to some hours). E.g. Hydrolysis of ester Very slow reaction: Reactions which occur over a long period of time (some months or years). E.g rusting of iron 4Fe + 3O2 + 6H2O → 4Fe(OH)3 Rate of Chemical Reaction Rate of chemical reaction is the ratio of change of concentration (Δc) of either reactant or product with time (Δt). So, the rate of reactions can be defined as decrease in concentration of reactant per unit time or increase in concentration of product per unit time Factors affecting reaction rate Temperature: Generally, as temperature increases, so does the reaction rate. At higher temperature, reaction molecules have more kinetcs energy, move faster and collide more often with greater energy. Concentration of reactants: As the concentration of reactants increases, the rates of collision of the reactant molecules and reaction increase. Catalyst: Catalyst will speed up reaction rate by lowering the activation energy Surface area of solid reactant: Increasing the surface area of a solid reactants will expose more of its particles to attack. This results in an increase in chances of collisions and rate of reaction. Pressure of gaseous reactants or products: Increase in pressure will results in increase in the number of collosions and thus increase in reaction rate. Rate laws - Rate laws are always determined experimentally - Reaction order is always defined in terms of reactant (not product) concentrations - The overall concentration dependence of reaction rate is given in a rate law - A general rate law for a reaction will look like For a reaction: n AB → A + B Rate α [AB] Rate = K [AB] The order of reaction of the equation R = K [AB] is known as 1st Order reaction For a reaction RX + OH- → ROH + X- Rate = K [RX][OH] Order = 2; i.e. this is a 2nd order reaction because the sum of the powers of the reactant concentration is equal to two. REACTION MECHANISM A balanced equation for a chemical reaction indicates what is reacting and what is produced , but reveals nothing about how the reaction actually takes place. Reaction mechanism is the actual sequence of events by which reactants are converted to products. Organic reactions could be one step or multi–step. In actual fact, most reactions involve many steps. The rate determining step is the slowest step in the sequence of steps leading to product formation The N2O2 in the above equation is known as reaction intermediate. Reaction intermediates are formed in one step and then consumed in a later step of the reaction mechanism. Reaction intermediate: Reaction intermediates are chemical species, often unstable and short-lived (however sometimes can be isolated), which are not reactants or products of the overall chemical reaction, but are temporary products and/or reactants in the mechanism's reaction steps. Reaction intermediates are often free radicals or ions. Organic chemical reactions normally involve breaking and forming of bonds: i.e. [AB + CD → AC + BD]. The bonds could be broken in two different ways depending on the electronegativity of the atoms involved. The two methods of bond breaking are referred to as Homolytic and Heterolytic Fission HOMOLYTIC FISSION Homolytic fission takes place between atoms of similar electronegativity. This is fission in which the electron pair [which constitutes the covalent bond] is shared equally between the two atoms sharing the bond. Thus, each atom carries an unpaired electron and is known as a radical. i.e. A–B = A :B [An electron pair = a covalent bond] e.g. Cl2 = Cl–Cl A–B → A. + B. [Radicals] Radical are very reactive species. Homolytic fission is also known as Radical Fission. Chemical reactions involving homolytic fission are known as free radical reactions. Homolytic fission also requires a large amount of energy, thus it takes place at very high temperature or under UV light. HETEROLYTIC FISSION Heterolytic fission takes place between atoms of different electronegativities. It involves the transfer of the electron pair to the more electronegative atom, leading to the formation of charged species or ions. Hence, heterolytic fission is also known as Ionic or Polar Fission. i.e. C – D → C+ + D- [D is more electronegative than C] D- is said to be electron rich while C+ is said to be electron deficient. e.g H+─Cl- → H+ + Cl- NUCLEOPHILES AND ELECTROPHILES Nucleophiles are atoms or group of atoms with a negative (-ve) charge or a lone pair of electrons. They are always electron rich. They are called nucleophiles because they are nucleus loving/seeking (i.e they seek the electron deficient centres). e.g. :NH3, H2O:, -OH, Halide ions [X-] e.g. Cl-, R3N:, e.g. (CH3N:), ROH, HS-, RO-, CH3CO2- etc. Electrophiles: These are atoms or group of atoms which are electron deficient or positively charged. They are called electrophiles because they are electron loving (seeking) i.e. they seek electron rich centres. e.g NO2, H3O+,Br+, FeCl3, RN2+, BF3, CH3C+O, R+  Types of Organic Reactions There are 5 main types of organic reactions: Substitution, Addition, Elimination, Condensation and Rearrangement. 1. SUBSTITUTION OR DISPLACEMENT REACTION This involves direct displacement of an atom or a group of atoms by another atom or group of atoms. e.g., (i) Chlorination of methane (ii) Alkylation of benzene. 2. ADDITION REACTION: This reaction is typical of unsaturated compounds and leads to a decrease in degree of unsaturation. It involves addition across an unsaturated bond and gives rise to a less unsaturated or fully saturated product. In most of these reactions, unsaturated compounds function as a source of electrons i.e. act as nucleophiles. Consequently, they are susceptible to attack by electron deficient species i.e. electrophiles. (3) ELIMINATION REACTION: This reaction involves the removal of atoms or group of atom from two adjacent C atoms to form a multiple bond. e.g (4) CONDENSATION REACTION: This is the reaction in which two or more compounds react to form another compound with the elimination of simple substances like water. e.g 5. REARRANGEMENT REACTION This involves migration of an atom or group of atoms from one site to another within the same molecule. It could involve migration of functional group from one point to the other. FACTORS AFFECTING ORGANIC REACTIONS An organic reaction can be facilitated or inhibited by certain factors. These include: (i) Distribution of electrons around the reaction site (ii) The nature and/or size of the atoms or group of atoms surrounding the reaction site. 1. INDUCTIVE EFFECT This is a permanent polarizing effect in single bonds. The effect is caused initially by a covalent bond between atoms of different electronegativities. The effect which is brought about by the unequally sharing of electrons is transmitted along a chain of sigma bonds. Cδ+−Xδ− If the carbon atom attached to polarizing atom or group is itself attached to other C-atom, the inductive effect is transmitted along the chain, although it tends to be insignificant beyond the two carbon atom. When the group (polarizing) attached withdraw electron with respect to carbon atom, the effect is term NEGATIVE INDUCTIVE EFFFECT (–I). When the group is electron donating, the effect is called a POSITIVE INDUCTIVE EFFECT (+I). E.g the alkyl groups have a +I effect. -I effect: The -I effect is seen around a more electronegative atom or group, and electron density is higher there than elsewhere in the molecule. Electron- withdrawing groups include halogen (X), nitro (−NO2), cyano (−CN), carboxy (−COOH), ester (−COOR) and aryloxy (−OAr) +I effect: The +I effect is observed among the less electronegative atoms of the molecule by electron-releasing (or electron-donating) groups. The alkyl groups ® are usually considered electron-releasing (or electron-donating) groups. 2. MESOMERIC EFFECT The shift of π electrons in multiple bonds towards the more electronegative atom is referred to as mesomeric effect. It is similar to inductive effect in single bonds. it is most common in carbonyl compounds (-C=O) where there is a shift of π electrons towards oxygen. The effect can be transmitted along the chain. The difference between the inductive and mesomeric is that there is no visible shift of electron in inductive but there is a visible shift of electrons in mesomeric effect. 3. STERIC EFFECT This is the effect due to the size of the groups surrounding the reaction site of the molecule. This occurs when certain atoms or group of atom alter or in some cases completely prevent a reaction from taking place, despite favourable electronic conditions. This effect is known steric hindrance and particularly effective when the reaction site is crowded with the large bulky groups. E.g (a) CH3COOH + CH3CH2OH CH3COOCH2CH3 + H2O Note: In (b) above, the –OH is in between the two carbon atom, this makes it less assesible to the ethanioc acids.This hinders the rate of reaction or sometimes prevents the reaction from taking place. In (a) above, the –OH group is at the end, this makes it easy for the CH3COOH to react.