Chapter 7- Alkenes: Structure and Reactivity Ashley Piekarski, Ph.D. Alkene What is an alkene func<onal group? hydrocarbon with carbon-carbon double bond occurs in many natural materials (flavors, fragrances, vitamins) 1
Why am I learning this, Dr. P? C- C double bonds are present in most organic and biological molecules Alkene stereochemistry Focus on a general alkene reac<on: Electrophilic addi4on Use of alkenes Ethylene and propylene are the most important organic chemicals produced 2
Degree of Unsaturation Are alkenes saturated or unsaturated? Formula for saturated acyclic compound is C n H 2n+2 Degree of unsatura<on: number of mul<ple bonds or rings Each ring or multiple bond replaces 2H s Learning check How many degrees of unsaturated are in C 6 H 10? What are some possible structure for this molecular formula? 3
With other elements Organohalogens (X: F, Cl, Br, I) Halogen replaces a hydrogen For example, C 4 H 6 Br 2 and C 4 H 8 have one degree of unsatura<on Organoxygen compounds (C, H, O) These don t affect the total count of H s Organonitrogen compounds Nitrogen has three bonds So if it connects where H was, it adds a connection point Subtract one H for equivalent degree of unsaturation in hydrocarbon 4
Summary Count pairs of H s below C n H 2n+2 Add number of halogens to number of H s Ignore oxygens Subtract N s- they have two connec<ons Learning check Calculate the degree of unsatura<on for C 10 H 12 N 2 O 3 5
Nomenclature Name the parent hydrocarbon Number carbons in chain so the double- bond carbons have the lowest possible numbers Rings have a cyclo prefix Common names 6
Learning check Name the following alkenes: (E)-4-methylhept-2-e Stereochemistry Make a model of 1,2- dichloroethene. Can you rotate the double- bond, like an alkane? This creates two possible isomers for alkenes! 7
Cis- and Trans- Isomers The presence of a carbon- carbon double bond can create two possible structures cis isomer- 2 similar groups on the same side of the double bond trans isomer- similar groups on opposite sides Each carbon must have two different groups for these isomers to occur Now, draw the cis- and trans- isomers for 1,2- dichloroethene Cis- and Trans- Isomers 8
Sequence Rules: The E, Z Designation Neither compound is clearly cis or trans Substituents on C1 are different than those on C2 We need to define similarity in a precise way to distinguish the two stereoisomers Cis, trans nomenclature only works for disubs<tuted double bonds E, Z Stereochemistry Compare where higher priority groups are with respect to bond and designate as prefix E entgegen, opposite sides Z zusammen, together on the same side 9
Ranking Priority Rules! Rule 1 Rank atoms that are connected at comparison point Higher atomic number gets higher priority Ranking Priority Rules! Rule 2 If atomic numbers are the same, compare at next connection point at same distance Compare until something has higher atomic number Do not combine- always compare 10
Ranking Priority Rules! Rule 3 Substituent is drawn with connections shown and no double or triple bonds Added atoms are valued with 0 ligands themselves Learning check Name the following alkenes using E, Z configura<on: Cl 11
Stability of Alkenes Cis alkenes are less stable than trans alkenes Compare heat given off on hydrogena<on Less stable isomer is higher in energy Comparing Stabilities of Alkenes Evaluate heat given off when C=C is converted to C- C bond More stable alkene gives off less heat 12
Hyperconjugation Hyperconjuga3on is the stabilizing interac<on between filled pi orbital and a neighboring filled C- H sigma bond on a subs<tuent. The more subs<tuents there are, the greater the stabiliza<on of the alkene Electrophilic Addition of Alkenes- Mechanism 13
Electrophilic Addition Energy Diagram Two step process First transi<on state is high energy point Electrophilic Addition Examples 14
Markovnikov s Rule Addi<on is regiospecific Halide will attack the carbon that is more substituted Markovnikov s Rule Why? What is the intermediate of an electrophilic addition reaction? 15
Learning check Predict the product for the following reac<on: + HBr Learning check Retrosynthesis: predict the reactants to make the following product Cl 16
Carbocation Structure and Stability Carboca<ons are planar and the tricoordinate carbon is surrounded by only 6 electrons in sp 2 orbitals The fourth orbital on carbon is a vacant p- orbital Carbocation Structure and Stability The stability of the carboca<on (measured by energy needed to form it from R- X) is increased by the presence of alkyl subs<tuents 17
Inductive stabilization The Hammond Postulate If carboca<on intermediate is more stable than another, why is the reac<on through the more stable one faster? The relative stability of the intermediate is related to an equilibrium constant (ΔGº) The relative stability of the transition state (which describes the size of the rate constant) is the activation energy (ΔG ) The transition state is transient and cannot be examined 18
Transition State Structures A transi3on state is the highest energy species in a reac<on step By defini<on, its structure is not stable enough to exist for one vibra<on But the structure controls the rate of reac<on So we need to be able to guess about its proper<es in an informed way We classify them in general ways and look for trends in reac<vity the conclusions are in the Hammond Postulate Hammond Postulate A transi<on state should be similar to an intermediate that is close in energy Sequen4al states on a reac4on path that are close in energy are likely to be close in structure - G. S. Hammond 19
Competing Reactions Normal Expecta<on: Faster reac<on gives more stable intermediate Intermediate resembles transi<on state Rearrangements of Carbocations Carboca<ons undergo structural rearrangements following set pajerns 1,2- H and 1,2- alkyl shiks occur Goes to give more stable carboca<on Can go through less stable ions as intermediates 20