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FSU CHM 2210 - Study Guide

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CHM2210 – Test 3 – Dr. Hilinski – Study GuidePotential Energy Diagrams:** The point of maximum potential energy encountered by the reactants as the proceed to products is called the transition step**The difference between the reactants and the transition state is known as the activation energy.-A plot of potential energy vs. reaction coordinate-Draw the transition state for each of the reactions in the mechanism. Use dashed lines to show bonds that are breaking and forming in the reaction. Include partial + and partial – where they belong. -When carbocations are formed they have a high energy level so place them at the top of the graph. If the reaction in the mechanism is endo thermic, it will be product like and placed high on the graphIf the reaction in the mechanism is exo thermic, it will be reactant likeAlkyl groups attached to the carbocation make it more stable. The electro(-) atom pulls electrons from the Carbon so if you have to choose the most stable compounds, choose the one with less electronegativity The most stable: Tertiary carbocation>Secondary>Primary>methylOnly electrons that are Beta to the positively charged Carbon can stabilize it by hyper conjugation. Beta ones are the ones that surround the positive carbon directly. Rate Laws:The rate of reaction of alcohols is: Tertiary carbocation>Secondary>Primary>methylSN1/E1 rate law: for secondary and tertiary--Rate= k [RO+H2]SN2/E2 rate law: for secondary and tertiary--Rate= k [RO+H2][x-]Arrhenius equation for activation energy: (determines how fast the reaction will happen)k=Ae-Ea/RT**OH groups are bad leaving groups**H2O groups are good leaving groupsNaming AlkenesHighest Priority: -OH C=C R, XCount the longest Carbon chain through the double bondIn a ring, you start at the double bond and count giving the substituents the lowest number [put the –ene in the name of the ring]Free Radical Substitution Reaction*Don’t use Fluorine or Iodine because they are too unreactive*Bromine always needs heat and light, Chlorine can use either heat or lightMechanism:Initiation: the Cl2 or Br2 goes through bond hemolysis and splits in halfPropagation:a) The Cl/Br takes a H+ from the most likely spot (tertiary>secondary>primary) and forms acid and a radical (a molecule with 1 free electron)b) The radical and the remaining Cl/Br combine to form the target moleculeTermination:a) Cl2 or Br2 is reformedb) The 2 radicals join togetherc) The radical and the Cl/Br bond together –most of this was formed in propagation**Phenols are often free-radical inhibitors*tert-butyl cation has an Sp2 Carbon and the anion has an Sp3 Carbon because the anion has an electron pair forming another sigma orbitalChlorination and BrominationWhen giving all possible mono chlorination/bromination products be careful to not give a repeat!To calculate the % of each product formation, you divide the number of possible hydrogen’s it can replace by the total number of spots the Cl or Br can bind to. Chapter 5E- and Z- LabelingE- > transZ- > cisWhen adding E or Z to an alkene you have to determine the highest priority and lowest priority group on each side of the double bond. To determine the priority you label what the first carbon is attached to and if it is tied you progress outward until the first point of difference. Rules:1) Higher atomic number takes precedence over lower2) When two atoms attached to the same carbon of the double bond are identical, compare the atoms attached to these two on the basis of their atomic numbers3) Work outward from the point of attachment4) If an atom has a double bond, you count what ever it is bonded to twice.*Cis- and Trans- is not possible when a carbon in the double bond of the alkene has two identical substituents* NEVER use E or Z to describe substituents on a cyclic compoundPhysical Properties A polar molecule with a dipole moment has a higher boiling point then a non polar molecule with no dipole moment The more substituents on the double bond, the more stable it is giving it a lower bp. and heat of combustion Alkenes < 8 carbons are more stable in the cis- form Alkenes between 8 and 11 Carbons are more stable in the trans- form Alkenes > 11 carbons are equally stable in each form*In cyclohexene there is a half-chair confirmation and the substituents go from equatorial and axial to pseudoaxial and pseudoequatorial Preparation of Alkenes: Elimination Reactions**Zaitsev’s rule: elimination reactions, the major reaction product is the alkene with the more highly substituted (more stable) double bond. This most-substituted alkene is also the most stableRegioselective- when a reaction can proceed in more than one direction but one of the directions is preferred Stereoselective- a reaction with a single starting material that yields 2 or more stereoisomeric products but one of them is produced in greater amounts. Dehydration of Alcohols (H2SO4 is the common acid used)*Tertiary alcohols dehydrate at the lowest temperaturesMechanism for E1:1. The OH on the alcohol grabs an H+ from H3O+ to form an oxonium ion and water2. Water breaks off from the oxonium ion to form a carbocation3. A hydrogen from the carbon without the positive charge is taken by water and its electrons go to forming the double bond > to make an alkene and H3O+Mechanism for E2:1. The OH on the alcohol grabs an H+ from H3O+ to form an oxonium ion and water2. Water steals a proton from the alcohol allowing its electrons to form a double bond and water also breaks off of the alcohol making an alkene, H3O+, and water* If the carbon skeleton of the product looks different then the reactant then a rearrangement could have occurred. The carbocation in a rearrangement can either rearrange or give an alkene. Rearrangements occur to create a more stable carbocation (secondary is usually made into tertiary).Dehydrohalogenation*The loss of a hydrogen and a halogen from an alkane to form an alkene*The preparation happens with a strong base*Beta elimination predominates in the direction that leads to the more highly substituted alkene*It is regioselective and sterioselective forming the more stable stereoisomer (cis or trans)*When a ring is less than 10 carbons the reaction produces cis- In an E2 mechanism the main things taking place are:1. Base-Hydrogen bond forming2. C-H bond breaking3. C=C formation from the H’s electrons it leaves behind4. C-X bond breakingStereoelectronic effect when one special


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