CHEM 3331 1nd Edition Lecture 8Outline of Last Lecture I. CycloalkanesII. NomenclatureIII. StabilityIV. Conformational analysisOutline of Current Lecture I. Chlorination of Alkanes/ HalogenationII. Free radicalsIII. Thermodynamics IV. KineticsV. Transition statesCurrent LectureI. Chlorination of Alkanes/ HalogenationFirst let us begin with simple mechanisms. ABCD, here B and C are intermediates in the transformation of A to D. How A becomes be is called a reaction mechanism. One such reaction mechanism is the chlorination of methane.CH4 + Cl2CH3Cl + HCl (only in the presence of light). CH3Cl is chloromethane. If we react chloromethane with more Cl2 we gain dichloromethane. We do the reaction again and gain trichloromethane or chloroform. We do the reaction one last time and obtain tetrachloromethane. We will only focus on the first mechanism outlined above.CH4 +Cl2CH3Cl + HCl. There is no chlorination in the absence of light. If we expose both reactants independently to light only chlorine will react not methane. We only need a small amount of light to begin the reaction. There is no linear correlation between the amount of lightThese 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.present and the amount of product created. There are three parts to the chlorination of methane: initiation, propagation, and termination. Initiation: light is absorbed by chlorine. Initiation only involves chlorine. During initiation the bond between the two chlorines is broken and each chlorine receives one electron from the broken bond. This creates two chlorine atoms with seven electrons. They are not charged. II. Free radicalsOur two chlorine atoms just created are called free radicals. They contain a half-filled orbital. They have unpaired electrons and do not bond with each other. They are not stable because they do not possess an octet.I. Continued:The energy needed to complete initiation is 58 kcal/mol. Breaking bonds requires energy. Initiation generates free radicals also termed reactive intermediates. Propagation: propagation contains two steps. The first is the introduction of one chlorine atom to a molecule of methane. (the dot represents an unpaired electron).Cl˚ + CH4 HCl + ˚CH3The second is the introduction of a chlorine molecule to our ˚CH3 molecule.˚CH3 + Cl2CH3Cl + Cl˚ this chlorine product is then used at the beginning of the first propagation. It self generates free radical chlorine through propagation after initiation. Termination: this has two options. ˚CH3 + Cl˚CH3Cl or Cl˚+Cl˚Cl2Termination destroys free radicals, initiation creates free radicals, and propagation continues the manufacture of free radicals.A radical chain reaction propagates until it runs out of free radicals. III. ThermodynamicsA+BC+D this has an equilibrium constant or Keq. Which equals [C][D]/[A][B]. this is about 10^19 for CH3Cl formation.ΔG= -RTln(Keq). ΔG is free energy or spontaneity. When it is negative then the reaction is spontaneous if it is positive it is not spontaneous. When G is negative Keq is greater than one. When G is positive Keq is less than one.ΔG=ΔH-TΔSIn most cases ΔG=ΔH. ΔH tells us whether a reaction is endothermic or exothermic.ΔH= sum of bonds broken – sum of bonds formed. Here we can break molecules up into individual contributions of bonds.A-B A˚ +B˚ this is bond dissociation energy (BDE). This is an example of homolytic BDE.A-B A:(-) + B(+) this is heterolytic BDE.ΔH= ΣBDE broken – ΣBDE formed.Now lets look at the thermodynamics of our chlorination of methane. Initiation has a BDE of 58 kcal/mol because bonds are being broken.Our first step of propagation has a BDE of 1 kcal/molThe second step has a BDE of -26kcal/mol. Giving propagation a total BDE of -25kcal/mol.Termination is the opposite of initiation and has a BDE of -58kcal/mol.IV. KineticsA+BC+D rate= d[C]/dt= K[A]n[B]m. for this we have to measure concentration. N and m= order of reaction. K= Ae^-Ea/RT. This is the Arrhenius equation. A is a constant. Ea is activation energy. When T increases K increases. This is why reactions speed up 2 or 3 times with a temperature increase of 10˚. V. Transition statesTransition states are the most unstable specimens.For example, CH4 + Cl˚[CH3--H--Cl]҂ ˚CH3 + HCl. What is in brackets is a transition