T OPIC 3 R EACTION S UPPLEMENT As we begin to study how organic reactions occur in more detail, we must first have a general understanding of four important concepts: (1)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

 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 ( 2 ) CARBOCATIONS

 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 a given temp. -Larger ∆G ‡ = slower reaction -Smaller ∆G ‡ = faster reaction ( 3 ) REACTION ENERGY DIAGRAMS

Parts of a reaction energy diagram: ( 3 ) REACTION ENERGY DIAGRAMS activation energy

SOME POSSIBLE REACTION ENERGY DIAGRAMS : Energy Reaction progress  Energy Reaction progress  A.) fast exergonic reaction B.) slow exergonic reaction - small ∆G ‡, negative ∆G° - large ∆G ‡, negative ∆G° ∆G ‡ ( 3 ) REACTION ENERGY DIAGRAMS

SOME POSSIBLE REACTION ENERGY DIAGRAMS : C.) fast endergonic reaction D.) slow endergonic reaction - small ∆G ‡, positive ∆G° - large ∆G ‡, positive ∆G° Energy Reaction progress  Energy Reaction progress  ∆G ‡ ( 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 ( 3 ) REACTION ENERGY DIAGRAMS

 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 Reaction progress  Energy Reaction progress  reactant Endergonic transition state reactant transition state product Exergonic ( 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 ( 4 ) THE HAMMOND POSTULATE

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 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!

C+C+ R RR C+C+ R HR C+C+ R HH C+C+ H HH Energy Reaction progress  Exergonic