Topic 3 Reaction Supplement • As we begin to study how organic reactions occur in more detail, we must first have a general understanding of four important concepts: • types of organic reactions (2) carbocations • (3) reaction energy diagrams(4) the Hammond Postulate • These two concepts are important in studying many organic reactions so we will examine them here before we proceed to study electrophilic addition reactions of alkenes.
( 1 ) TYPES OF ORGANIC REACTIONS • ADDITION REACTIONS: • 2 reactants add together to form a single product • A + B C These two reactants Add to give this one product • ELIMINATION REACTIONS: • A single reactant splits into two products • A B + C This one reactant Gives these two products
( 1 ) TYPES OF ORGANIC REACTIONS • SUBSTITUTION REACTIONS: • 2 reactants exchange parts to give two new products • A - B + C - D A - C + B - D These two reactants Add to give these two products • REARRANGEMENT REACTIONS: • A single reactant undergoes rearrangement of atoms and bonds to yield an isomeric product This single reactant Gives this isomeric product
( 2 ) CARBOCATIONS • CARBOCATION: (carbo-cat-ion) • A trivalent carbon atom w/ 6 e- • Has a positive charge • CARBOCATION STABILITY: • More substituted carbocations are more stable than less highly substituted ones • Major Reason Why: inductive effects • Electrons from alkyl groups can shift towards the + charge thereby stabilizing it • - Order of stability: (R = alkyl group) > > > 3° (tertiary) 2° (secondary) 1° (primary) methyl
( 3 ) REACTION ENERGY DIAGRAMS • REACTION ENERGY DIAGRAM: • A graphical representation of the energy changes that occur • during a reaction • y-axis: energy • x-axis: reaction progress (reactants products) Here are some key concepts represented on reaction energy diagrams: • TRANSITION STATE: • Highest energy species formed in a reaction (very unstable) • ACTIVATION ENERGY (∆G‡): • Difference in energy between reactants and transition state • Determines how quickly a reaction occurs @ a given temp. • Larger ∆G‡ = slower reaction • Smaller ∆G‡ = faster reaction
( 3 ) REACTION ENERGY DIAGRAMS Parts of a reaction energy diagram: activation energy
( 3 ) REACTION ENERGY DIAGRAMS SOME POSSIBLE REACTION ENERGY DIAGRAMS: A.) fast exergonic reactionB.) slow exergonic reaction Energy Energy - small ∆G‡ , negative ∆G° - large ∆G‡ , negative ∆G° ∆G‡ ∆G‡ Reaction progress Reaction progress
( 3 ) REACTION ENERGY DIAGRAMS SOME POSSIBLE REACTION ENERGY DIAGRAMS: C.) fast endergonic reactionD.) slow endergonic reaction - small ∆G‡ , positive ∆G° - large ∆G‡ , positive ∆G° Energy Energy ∆G‡ ∆G‡ Reaction progress Reaction progress
( 3 ) REACTION ENERGY DIAGRAMS • REACTION INTERMEDIATE: • The carbocation which exists momentarily in a multi-step reaction • As soon as the intermediate forms, it reacts further to form the product • The intermediate cannot be isolated Intermediate This reaction has 2 ∆G‡ - It is a 2 step reaction
( 4 ) THE HAMMOND POSTULATE • HAMMOND POSTULATE: • The structure of the transition state (TS) resembles the structure of the nearest stable species • TS for endergonic steps structurally resemble products • TS for exergonic steps structurally resemble reactants Energy Energy Exergonic Endergonic transition state transition state reactant product product reactant Reaction progress Reaction progress
( 4 ) THE HAMMOND POSTULATE • The importance of the Hammond Postulate: • We can determine the structure of an intermediate (which is • key to determining how a reaction occurs) by knowing whether a reaction/ step of a reaction is endergonic or exergonic
Class Assignment: • Prepare a study guide of at least 8 questions on 1 sheet of paper and an answer key on a second sheet of paper for content for tomorrow’s quiz. Be sure that your study guide and answer key are neat! Give it to a neighbor when you are done! • Be effective! Make sure your classmates know what they need to know!
Extra Credit Opportunity • Haiku is an ancient form of Japanese poetry, best know to most Westerners as three-line poems with a 5-7-5 syllable count. Here is your chance to combine your poetic and organic chemistry skills by writing your very own organic chemistry haiku for extra credit! • Present the poem on a piece of paper and provide an illustration around the poem by hand! NO CLIP ART!
Exergonic Energy Reaction progress H R R R C+ C+ C+ C+ R R H H R H H H