Chem 241a-Ist Edition Lecture11 Outline of Last Lecture I. Substitution reactionsa. Four Factors that Influence Substitution Reactionsb. Substratesc. Leaving Groupsd. Nucleophilesi. Steric HindranceII. SolventsOutline of Current Lecture I. Solvents ContinuedII. Substitution Reaction Mechanismsa. SN2 Reactionsi. Four factors affect’s on SN2 rxnsii. Stereochemical consequences of SN2b. SN1 Reactionsi. Stereochemical consequences of SN1Current LectureI. Solvents ContinuedThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.a. Polar protic solvents solvate anions well; the nucleophilic trends for reactions using polar protic solvents are: going up the periodic table, the nucleophilicity decreases i. This is because small ions are trapped within the solvent cage that is surrounding the compound and cannot react with other compoundsii. I-- > Br- > Cl- > F-. This is the opposite of basicity. iii. NOTE: F is so bad, that it doesn’t leave at all when in protic solventsb. Polar aprotic solvents solvate cations well, such as acetone. They dissolve the cations really well but cannot really get into the anions, thus letting the anions befree to leavei. A naked anion in aprotic solvents are those anions that aren’t fully dissolved ii. The nucleophilic trend for aprotic solvents are opposite protic solvents, and follow basicity: decrease as you up down the periodic table)iii. F- < Cl- < Br- < I-II. Substitution Rxn – Mechanismsa. An SN2 reaction is a substitution, nucleophilic, and bimoleculari. Bimolecular means that two molecules in the rate determining step)ii. When showing the mechanisms for a reaction, we draw curved arrows to show the breaking and formation of a bondiii. In an SN2 reaction, these two parts happen AT THE SAME TIME; there are no intermediate steps (1 step reactions). This is called a concerted processiv. Ex: One of the curvedarrows goes from thebond connecting the LGto the actual LG since ittakes its lone pair. Thesecond arrow goes fromthe lone pair on thenucleophile to thesubstrate, since it bonds to itv. We can also draw an energy diagram for this:vi. Note that the rate equation would be: rate =k[R-X][Nuc:-], because it is bimolecular andhas two factors affecting ratevii. So how do we raise the transition state? (TS) – by adding more groups to the substrate and making the products more crowded, viii. How do these nucleophiles bind to the substrate? They take what’s called the backside approach, where they attach to the side of the molecule that is opposite of the LGb. Stereochemical consequences of SN2i. When the carbon attached to the leaving group is a chiral center, the product is “inverted stereochemistry”, where the R inverts to an S and vice versa.c. How do th four factors affect SN2 rxnsi. Substrates – SN2 rxns favor unhindered substrates1. CH3 > 1° > 2° > 3°2. Tertiary substrates are too hindered that it doesn’t reactii. Leaving Groups – better LG the faster the reactiond. Nucleophiles – the better the Nu the faster the reactione. Solvent – polar aprotic solvents favors speedy nucleophiles via the SN2 ruleIII. SN1 reactions – unimolecule, meaning there is an intermediate step, and the rate determining step is only by one moleculea.i. In the first step, the bond breaks between the substrate and the LG, giving the LG lone pair of electrons to form a carbocation and the LG. The step leading to the formation of a carbocation is ALWAYS the rate determining stepii. The second step is the lone pair leaving the Nu and binding to the substrate to form the product while the LG remains unnattachediii. This can also be represented inan energy diagram, where therate limiting step is the firststep because it creates thecarbocationb. So are there stereochemicalconsequences with SN1 rxns? No. Thecarbon’s that are connected to the LGare not chiral and thus will be the samesterochmically reactants and products,R/R or