Chapter 2
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Transcript of 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