Orgo Chem Study Notes: Chapter 1
Orgo Chem Study Notes: Chapter 1
Popular in Course
verified elite notetaker
Popular in Department
This 8 page Class Notes was uploaded by Rebecah Watts on Saturday January 2, 2016. The Class Notes belongs to a course at a university taught by a professor in Fall. Since its upload, it has received 30 views.
Reviews for Orgo Chem Study Notes: Chapter 1
Report this Material
What is Karma?
Karma is the currency of StudySoup.
You can buy or earn more Karma at anytime and redeem it for class notes, study guides, flashcards, and more!
Date Created: 01/02/16
CHAPTER 7 STUDY NOTES ALKYL HALIDES AND NUCLEOPHILIC SUBSTITUTION - Section 7.1: Introduction to Alkyl Halides o Alkyl halides are organic molecules containing a halogen atom, X, bonded to an sp3 hybridized carbon atom. o Allylic halides have X bonded to the carbon atom adjacent to a carbon-carbon double bond - Section 7.3: Physical Properties of Alkyl Halides o Typically polar so they have small dipole-dipole interactions because of the polar C-X bond. - Section 7.4: Interesting Alkyl Halides o CHCl3 Chloroform or trichloromethane o CCl4 Carbon tetrachloride or tetrachloromethane o Teflon o PVC Poly(vinyl chloride) o Chlorofluorocarbons - Section 7.5: The Polar Carbon-Halogen Bond o The properties of alkyl halides is what dictates how they react in a reaction o The characteristic reactions of alkyl halides are substitution and elimination Undergo substitution with nucleophiles Undergo elimination with Bronsted-Lowry bases - Section 7.6: General Features of Nucleophilic Substitution o Three components are necessary R: an alkyl R group containing an sp3 hybridized carbon bonded to X X: an atom called a leaving group which is able to accept the electron density in the C-X bond Most common leaving groups o Water o N2 o Halide anions Nu: a nucleophile, which contains a lone pair of a pi bond but not necessarily a negative charge Negatively charged nucleophiles are used as salts o –OH o –SH Neutral nucleophiles produce a positive product How to carry out a nucleophilic substitution Find the sp3 hybridized carbon with the leaving group Identify the nucleophile, which is normally the species with the lone pair or pi bond Substitute the nucleophile for the leaving group and assign charges, if necessary, to any atom that’s involved in bond breaking or bond formation - Section 7.7: The Leaving Group o What makes a good leaving group? It must be able to easily accept electrons Weak Lewis bases make really good leaving groups The weaker base will always be the better leaving group The more stable the leaving group, the better able it is to accept an electron pair o Trends for good leaving groups on the periodic table Leaving group ability increases from left to right Leaving group ability increases from top to bottom o All good leaving groups are weak bases with strong conjugate acids having low pKa values o Equilibrium favors the products of nucleophilic substitution when the leaving group is a weaker base than the nucleophile - Section 7.8: The Nucleophile o Nucleophiles and bases are structurally similar: both have a lone pair or a pi bond o Bases attack protons whereas nucleophiles attack other electron-deficient atoms which are usually carbons - Section 7.8A: Nucleophilicity vs. Basicity o Nucleophilicity is the nucleophile strength o Strong base is a strong nucleophile o For two nucleophiles with the same nucleophilic atom, the strong base is the stronger nucleophile o A negatively charged nucleophile is always stronger than its conjugate acid o Right to left across a row of the periodic table, nucelophilicity increases as basicity increases - Section 7.8B: Steric Effects and Nucelophilicity o Steric hindrance is a decrease in reactivity resulting from the presence of bulky groups at the site of the reaction o Steric hindrance decreases nucleophilicity but not basicity This is because bases pull off small, easily accessible protons, they are unaffected by steric hindrance whereas nucleophiles, on the other hand, must attach a crowed tetrahedral carbon, so bulky groups decrease reactivity o Sterically hindered bases that are poor nucleophiles are called nonnucleophilic bases - Section 7.8C: Comparing Nucleophiles of Different Size- Solvent Effects o Nucleophilicity depends on the solvent used in the substitution reaction Substitution reactions involve polar starting materials o Polar protic solvents Capable of intermolecular hydrogen bonding Dissolve both cations and anions well Nucleophilicity increases down a column of the periodic table as the size of the anion increases This is opposite to basicity o Polar aprotic solvents Incapable of hydrogen bonding, therefore, they only solvate cations, not anions Nucleophilicity parallels basicity in polar aprotic solvents, meaning that it decreasies down a column and the stronger nucleophile is the stronger base - Section 7.8D: Summary o It is generally true that the stronger base is the stronger nucleophile o In polar aprotic solvents, however, nucleophilicity increases with the size of the anion o Steric hindrance decreases nucleophilicity without decreasing basicity - Section 7.9: Possible Mechanisms for Nucleophilic Substitution o Nucleophilic substitution involves two sigma bonds The bond to the leaving group, which is broken The bond to the nucleophile which is formed o Three possibilities of when bond breaking and making occurs Bond breaking and bond making occur at the same time If the C-X bond is broken as the C-Nu bond is formed, the mechanism is one step The rate of this reaction depends on the concentration of both reactants o Second order rate equation Bond breaking occurs before bond making If the C-X bond is broken first, the mechanism is two steps and a carbocation is formed as an intermediate Because the first step is rate-determining, the rate depends on the concentration of RX only o The rate of this reaction is first order Bond making occurs before bond breaking If the C-Nu bond is formed first, the mechanism has two steps but this mechanism has a problem because the intermediate generated in the first step has ten electrons around carbon, violating the octet rule which ELIMINATES THIS POSSIBILITY - Section 7.10: Two Mechanisms for Nucleophilic Substitution o Second order rate equation suggests a bimolecular reaction with a one-step mechanism Sn2 mechanism o First order rate equation suggest a two-step mechanism in which the rate-determining step involves the alkyl halide only Sn1 mechanism - Section 7.11A: Kinetics o A Sn2 reactions exhibits second-order kinetics; the reaction is bimolecular Changing the concentration of either reactant will affect the rate of the reaction - Section 7.11B: One Step Mechanism o Concerted reaction: bond breaking and bond making occur at the same time - Section 7.11C: Stereochemistry of the Sn2 Reaction o Frontside attack: the nucleophile approaches from the same side as the leaving group The nucleophile replaces the leaving group on the same side o Backside attack: the nucleophile approaches from the opposite side of the leaving group The nucleophile replaces the leaving group on the opposite side Leads to inversion of configuration around the stereogenic center o The products of frontside and backside attack are different compounds They are stereoisomers that are nonsuperimposable They are enantiomers o All Sn2 reactions that proceed with backside attach of the nucleophile, resulting in inversion of configuration at a stereogenic center Inversion of configuration Dashed to wedged Wedged to dashed - Section 7.11D: The Identity of the R Group o As the number of R groups on the carbon with the leaving group increases, the rate of an Sn2 reaction decreases More substituents, slower the reaction Sn2 reactions are fastest with methyl o 3 degree alkyl halides do not undergo Sn2 reactions o Steric hindrance caused by bulky R groups makes nucleophilic attack from the back side more difficult o Increasing the number of R groups on the carbon with the leaving group increases crowding in the transition state, decreasing the rate of an Sn2 reaction o The Sn2 reaction is fastest with unhindered halides - Section 7.13: Sn1 Mechanism o The Sn1 mechanism is first-order kinetics o Unimolecular, only affecting one alkyl halide o More than one step occurs o The identity and concentration of the nucleophile have no effect on the reaction rate - Section 7.13B: A Two-Step Mechanism o Two-step mechanism is when the bond breaking occurs BEFORE the bond making o The key features to this mechanism: The mechanism has two steps Carbocations are formed as reactive intermediates - Section 7.13C: Stereochemistry of the Sn1 Reaction o A carbocation (with three groups around C) is sp2 hybridizied and trigonal planar, and contains a vacant p orbital extending above and below the plane. o Front and backside can also occur and it will produce two different stereoisomers: enantiomers Because there is no preference for nucleophilic attack form either direction, an equal number of the two enantiomers is formed: a racemic mixture We say that racemization has occurs Racemization is the formation of equal amounts of two enantiomeric product from a single starting material Sn1 reactions proceed with racemization at a single stereogenic center - Section 7.13D: The Identity of the R Group o As the number of R groups on the carbon with the leaving group increases, the rate of an Sn1 reaction increases Methyl and primary alkyl halides don’t undergo Sn1 reactions o The trend is exactly opposite the Sn2 reaction - Section 7.14: Carbocation Stability o Carbocations are classified as primary, secondary, or tertiary by the number of R groups bonded to the charged carbon atom. As the number of R groups on the positively charged carbon atom increases, the stability of the carbocation increases - Section 7.14A: Inductive Effects o Inductive effects are electronic effects that occur through sigma bonds o Electron-withdrawing inductive effects stabilize a negative charge o Electron-donating groups are used to stabilize a positive charge - Section 7.14B: Hyperconjugation o Hyperconjugation is the spreading out of the charge by the overlap of an empty p orbital with an adjacent sigma bond to stabilize the carbocation intermediate o Used to observe the trend of carbocation stability A tertiary carbocation is more stable than a secondary, primary, or methyl carbocation because the positive charge is delocalized over more than one atom o The larger the number of alkyl groups on the adjacent carbons, the greater the possibility for hyperconjugation, and the larger the stabilization o Hyperconjugation thus provides an alternate way of explaining why carbocations with a larger number of R groups are more stabilized - Section 7.15: The Hammond Postulate o The rate of an Sn1 reaction depends on the rate of formation of the carbocation The rate of Sn1 reaction increases as the number of R groups on the carbon with the leaving group increases The stability of a carbocation increases as the number of R groups on the positively charged carbon increases o The rate of an Sn1 reaction increases as the stability of the carbocation increases - Section 17.15A: The General Features of the Hammond Postulate o The Hammond postulate provides a qualitative estimate of the energy of the transition state o According to the Hammond Postulate, the transition state of a reaction resembles the structure of the species (reactant or product) to which it is closer in energy. Endothermic reaction: the transition state is closer in energy to the products Exothermic reaction: the transition state is closer in energy to the reactants o Lowering the energy of the transition state decreases the energy of activation, which increases the reaction rate o According to the Hammond postulate, the transition state to form the more stable product is lower in energy, so this reaction should occur faster. o Conclusions: In an endothermic reaction, the more stable product forms faster In an exothermic reaction, the more stable product mayor may not form faster because the activation energy is similar for both products - Section 7.15B: The Hammond Postulate and the Sn1 Reaction o According to the Hammond postulate, the stability of the carbocation determines the rate of its formation The more stable the carbocation, the faster the reaction - Section 7.17: When is the Mechanism Sn1 or Sn2? o Four factors are examined when determining this The alkyl halide The nucleophile: strong or weak The leaving group: good or poor The solvent: protic or aprotic - Section 7.17A: The Alkyl Halide-The Most Important Factor o The most important fact in determining whether a reaction follows an Sn1 reaction or an Sn2 reaction is the identity of the alkyl halide Increasing alkyl substitution favors Sn1 Decreasing alkyl substitution favors Sn2 o Methyl and primary halides undergo Sn2 reactions only o Tertiary alkyl halides undergo Sn1 reactions only o Secondary alkyl halides undergo both Sn1 and Sn2 reactions - Section 7.17B: The Nucleophile o Strong nucleophiles present in high concentration favor Sn2 reactions o Weak nucleophiles favor Sn1 reactions by decreasing the rate of any competing Sn2 reactions o The most common nucleophiles in an Sn2 reaction bears a negative charge. o The most common nucleophiles in Sn1 reactions are weak nucleophiles H 2 and ROH - Section 7.17C: The Leaving Group o A better leaving group increases the rate of both Sn1 and Sn2 reactions o The better the leaving group, the more willing it is to accept the electron pair in the C-X bonds - Section 7.17D: The Solvent o Polar protic solvents are especially good for Sn1 reactions o Polar aprotic solvents are especially good for Sn2 reactions - Section 7.18: Vinyl Halides and Aryl Halides o Sn1 and Sn2 reactions occur only at sp3 hybridized carbon atoms - Section 7.19: Organic Synthesis o Organic synthesis is the systematic preparation of a compound from a readily available starting material by one or many steps
Are you sure you want to buy this material for
You're already Subscribed!
Looks like you've already subscribed to StudySoup, you won't need to purchase another subscription to get this material. To access this material simply click 'View Full Document'