CHEM 3331 1nd Edition Lecture 13Outline of Last Lecture I. DiastereomersII. Meso compoundsIII. Absolute and relative configurationsIV. Physical propertiesV. Separation of enantiomersOutline of Current Lecture I. HaloalkanesII. UsesIII. Physical propertiesIV. PreparationV. Substitution reactions (Sn2)Current LectureI. HaloalkanesThere are three main types of haloalkanes. Alkyl halides, where a carbon is sp3 hybridized to halogen molecule(s). vinyl halides, where a carbon is sp2 hybridized to halogen molecule(s) due to a double bond between carbon atoms. Aryl halides, where the carbon is sp2 hybridized in an aromatic compound. Sp hybridized carbons do exist, but are rarely used so they will not be covered.Examples of these three types are in order: (CHCl3, CHClF2), (CH2CHCl, C2F4), and finally an aromatic like benzene with halogens attached to any carbon.We uses the prefixes flouro-, chloro-, bromo-, and iodo- when naming compounds with these attached. For example, 1-Flouropentane or trans-1-bromo-2-chloro-cyclopentane. These 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 primary halogen is a halogen attached to a carbon that is bonded with only one other carbon. A secondary halogen is a halogen attached to a carbon that is bonded to two other carbons. A tertiary halogen is a halogen attached to a carbon that is bonded with three other carbons. II. UsesThese are used as industrial compounds. They are used as solvents for dry cleaning or to make de-caf coffee. They are used as anaesthetics, such as halothane (CF3CHClBr) and CH3CH3Cl. They are used as freons. Freon-12 (CF2Cl2) was replaced by Freon-20 (CHClF2) by the montreal protocol. They are also used as pesticides, such as the infamous DDT. III. Physical propertiesThey are polarized depending on the case. Halogens are more electronegative than carbon.Electronegativity: F(4)>Cl(3.5)>Br(3)>I(2.7)Bond length: I(2.14)>Br(1.94)>Cl(1.78)>F(1.38)C-I=1.29D C-Br=1.48D C-F=1.51D C-Cl=1.56DAs a generalized claim, CX4 μ=0D, CHX3 μ=1D, CH2X2 μ=1.5D, and CH3X μ=1.8D. X= halogen atom.If a fluorine is added to butane the boiling point is 3˚C, chlorine added is 47˚C, bromine added is71˚C, and iodine added is 102˚C. The boiling point also depends on the configuration. For example linear pentane with a bromine has a boiling point of 102˚C. its sp3 hybridized configuration with a bromine has a boiling point of 73˚C.Halogens are heavier atoms, their density is greater.Alkyl bromides and iodides are more dense than water. Alkyl flourides and alkyl chloride are lessdense than water. Alkyl dichloride to tetrachloride are more dense than water. IV. PreparationRadical halogenation only used for bromine and chlorine, fluorine and iodine are too reactive. When drawing a step by step analysis always propose the most selective reagent in synthesis. Which is going to be bromine.Allylic bromination is when a bromine atom always chooses an allylic. An allylic is the sp3 hybridized carbon adjacent to a double bond.Most times we don’t have straight bromine so it must be prepared we do this using NBS and hydrobromic acid. This yields the Br2 we need for bromination. NBS is most acceptable response to molecule used to get from a molecule with no bromine to a molecule with a bromine attached.V. Substitution reactionsWe have our substrate for example, CH3Cl. Carbon is an electrophile and chlorine is our leaving group. When nucleophile such as OH- is introduced, it pushes the chlorine off and bonds to the electrophile. In this case our reaction is CH3Cl + OH-  CH3OH + Cl-. This is termed nucleophilic substitution because we substitute our chorine nucleophile for the hydroxide nucleophile. To dothis we need to have a partially positive carbon which is doen by being bonded to a halogen. There can be non negative charged nucleophiles. The nucleophile has to have available lone pairs. This substitution is a single step reaction. Rate= K[nucleophile][substrate].this is an Sn2 reaction.Sn2= S(substitution) n(nucleophilic) 2(bimolecular).Lastly, when this substitution is underway the nucleophile cannot insert itself where the halogen is it must insert itself as far away from the halogen in the back of carbon. This causes aninversion of the molecule. This is called an umbrella inversion. With Iodine attached the carbon will have an R configuration. As OH- attempts to bond with the carbon, the molecule flattens out and the bond with iodine begins to break. Once the OH- is bonded the molecule now has anS configuration. Due to the way the OH- was bonded to the back rather than taking the place of the halogen. Nucleophiles include HO-, RO-, CN-, I-, Br-, Cl-, NH3, H20, and PR3.As a final note since iodine is so reactive we can’t use radical halogenation. Instead we can create a molecule with bromine using NBS through halogenation then add a compound such as K+I- to create a product with iodine