Chapter 2

CHAPTER 2ORGANIC REACTION TYPES

• Why and how chemical reactions take place– What kind of reaction occur– How reaction occur

Kinds of organic reaction

• Addition reaction

• Elimination reaction

• Substitution reaction

• Rearrangement reaction

Addition reaction

• Two reactants add together to form single product with no atoms left over

• Characteristic of compounds containing double and triple bond

• Less energy to break p than s bond (app. 15kJ/mole weaker)

• Electrophilic addition reaction (Br2 + =)• Nucleophilic addition reaction (C=O +

HCN)

Elimination reaction

• Opposite of addition reactions.• Occur when single reactant splits into two

products, often with formation of a small molecule such as water or HBr.

• Ex. Acid-catalyzed reaction of an alcohol to yield water and an alkene

• E1, E2, E1cB

Substitution reaction

• Two reactants exchange parts to give two new products

• Replacement or substitution of one or more atoms or groups of compound by other atoms or groups

• Free radical substitution• Ionic Substitution

– Electrophilic substitution (benzene + NO2+)

– Nucleophilic substitution (SN1 and SN2)

Rearrangement reaction

• Occur when a single reactant undergoes a reorganization of bonds and atoms to yield isomeric product

• Atoms/groups shift from one position to another within the substrate molecule itself giving a product with a new structure

Mechanism

• Reaction mechanism: overall description of how a reaction occurs

• Describes in detail exactly what takes place at each stage of chemical transformation-which bonds are broken/formed and in what order. Also account for all reactants used and all products formed

• Two ways in which a covalent two-electron bond can break/form– Symmetrically/homolytic ( ) radical

reaction– Unsymmetrically/heterolytic ( ) polar

reaction

Radical reaction

• Not as common as polar reactions• Radicals react to complete electron octet of valence

shell– A radical can break a bond in another molecule and

abstract a partner with an electron, giving substitution in the original molecule

– A radical can add to an alkene to give a new radical, causing an addition reaction

Radical reaction • Initiated by free radicals• Example: methane chlorination• Initiation: irradiation with UV light begins

the reaction by breaking the relatively weak Cl-Cl bond to form chlorine radical

• Propagation: Chlorine radical collides with methane, abstract hydrogen to give HCl and methyl radical (*CH3). Which reacts with Cl2

Radical reaction

• Termination: two radicals collide and combine to form stable product

Polar reactions• Molecules can contain local unsymmetrical

electron distributions due to differences in electronegativities

• This causes a partial negative charge on an atom and a compensating partial positive charge on an adjacent atom

• The more electronegative atom has the greater electron density

• Elements such as O, F, N, Cl more electronegative than carbon

Polar reactions• An electrophile, an electron-poor species,

(neutral or +ve) combines with a nucleophile, an electron-rich species (neutral or –ve)

• The combination is indicate with a curved arrow from nucleophile to electrophile

• Example: addition of HBr to Ethylene

Polar reactions

• Using curved arrows – Rule 1: electrons move from a nucleophilic

source(-ve or neutral) to an electrophilic sink (+ve or neutral)

– Rule 2: octet rule must be followed

Describing a Reaction: Equilibria, Rates, and Energy Changes

• Reactions can go either forward or backward to reach equilibrium– The multiplied concentrations of the products

divided by the multiplied concentrations of the reactant is the equilibrium constant, Keq

– Each concentration is raised to the power of its coefficient in the balanced equation.

aA + bB cC + dD

• If the value of Keq is greater than 1, this indicates that at equilibrium most of the material is present as products

– If Keq is 10, then the concentration of the product is ten times that of the reactant

• A value of Keq less than one indicates that at equilibrium most of the material is present as the reactant

– If Keq is 0.10, then the concentration of the reactant is ten times that of the product

Free Energy and Equilibrium

• The ratio of products to reactants is controlled by their relative Gibbs free energy

• This energy is released on the favored side of an equilibrium reaction

• The change in Gibbs free energy between products and reacts is written as “DG”

• If Keq > 1, energy is released to the surroundings: DGo negative (exergonic reaction)

• If Keq < 1, energy is absorbed from the surroundings : DGo positive (endergonic reaction)

• The standard free energy change at 1 atm pressure and 298 K is DGº

• The relationship between free energy change and an equilibrium constant is:

DGº = - RT ln Keq where

– R = 1.987 cal/(K x mol)– T = temperature in Kelvin

– ln Keq = natural logarithm of Keq

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Describing a Reaction: Energy Diagrams and Transition States

• The highest energy point in a reaction step is called the transition state

• The energy needed to go from reactant to transition state is the activation energy (DG‡)

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Describing a Reaction: Intermediates

• If a reaction occurs in more than one step, it must involve species that are neither the reactant nor the final product

• These are called reaction intermediates or simply “intermediates”

• Each step has its own free energy of activation

• The complete diagram for the reaction shows the free energy changes associated with an intermediate

Hammond postulate• The structure of a transition state resembles the

structure of the nearest stable species. • Transition states for endergonic steps

structurally resemble products• Transition state for exergonic steps structurally

resemble reactants• More stable product/intermediate form faster due

to greater stability reflected in lower energy transition state leading to them