Chapter 7 Chemical Reactions Bond Energy and Kinetics A chem rxn involves making new chem bonds and breaking old chem bond Each bond has inherent E which is released when the bond is broken o Likewise it will cost that amount of E to make the bond Hydrolysis an acid base rxn b w the alkoxide base and the H3O as the acid All rxns involve modification of the original molecule starting molecule and formation of a new molecule the product w the gain of 1 atoms or groups loss of 1 atoms or groups or transformation of 1 functional group into another Alipathic Nucleophile Substitution Reaction when one group or atom replace another the first group substitutes for the 2nd Nucleophile Acyl Substitution when a nucleophile attacks a carbonyl the acyl C the pi bond is broken and a new covalent bond is formed b w the nucleophile and the acyl C Addition Reaction the sp2 C s are transformed to sp3 C s o An acid base rxn Elimination Reaction sp3 C s are transformed into sp2 C s in a new pi bond Carbon Carbon Bond Forming Reaction a C C bond is formed Functional Group Transformation or Functional Group Interchange Reaction if no C C bond is formed but the functional group is simply changed or modified The E released when a sigma bond is cleaved bond breaking is called bond dissociation E o The amount of E released when the bond is broken and the amount required to make the bond o The exact E of a bond depends on the bond length function of the size of the atoms electronegativity considerations solvent in some case and other bonds that are connected to the atom of interest o One parameter used to predict whether or not a chem rxn is likely to proceed to give a particular product o Change in E during a rxn H means that E is released exothermic exergonic Said to be spontaneous if G is b c it produces enough E during the course of the rxn to be self sustaining Releases E in the form of work from the system to the surroundings means E is put in endothermic endergonic Usually not spontaneous b c less E is produced during the rxn than is required to keep it going Absorbs E in the form of work from the surroundings into the system o A spontaneous rxn should continue to produce a product once it has started o A nonspontaneous rxn requires more E than is supplied by breaking and making bonds in order to continue and will likely require more vigorous rxn conditions Transition State the point in the rxn where the starting materials begin to react and begin the transition to the product o There s no formal bond yet but the bonding process as begun and is represented by a o Midpoint of the rxn and is an E associated w making and breaking bonds as the rxn dashed line A B proceeds Synchronous a rxn w only one step to get to the final product Intermediates transient products that are formed in a rxn prior to the formation of the final product o Brackets indicate a transient species to set it apart from the rest of the rxn o A rxn that s not isolated but reacts to give another more stable product o W an intermediate there are 2 rxns o Can be a carbocation carboanion or a radical Mechanism a rxn sequence that includes each reactive intermediate produced on the way to Cations electron deficient species that bear a charge and quickly react w species that can the final product donate electrons o Carbocations or carbenium ions have a charge on a C atom FC of C 1 Formed when a covalent bond to C is broken s such a way that 2 electrons are transferred to one atom and the C receives no electrons during the transfer Called a heterolytic cleavage breaking a bond in such a manner that both electrons are transferred to one atom and no electrons to the other The C of a carbocation is sp2 hybridized and must have trigonal planar geometry Will react w another species that can donate the 2 missing electrons to give it 8 satisfy the valence requirements of C and form the 4th bond to make C tetravalent High E intermediates and relatively high activation E s are required for Once formed carbocations have a low activation E for rxn w the their formation nucleophile Anions electron rich species that easily donates electrons to an electron defiecient species An empty p orbital that s localized on an atom in this case C o Carboanion a charge resides on the C FC of C 1 When a carboanion reacts w another C atom it s classified as a nucleophile Formed by breaking a covalent bond in such a way that 2 electrons are transferred to the C involved in the bond and the 2nd atoms receives no electrons during the transfer a heterolytic cleavage The pair of electrons reside in a p orbital A filled p orbital that s localized on an atom in this case C Radicals a species w one unshared electron in a p orbital which makes it very reactive and it will react to form a bond in a manner diff of that of a cation or an anion o Only has 1 electron and it will react w another radical to form a covalent bond where each species donates 1 electron 3 covalent bonds and 1 unpaired electron o Also react w neutral species to form a new s covalent bond and a new radical o o A high E species and a very reactive intermediate o The single electron in the orbital will slightly repel the electrons in the covalent bonds so one expect a squashed tetrahedron however there s evidence that a planar structure is probably the lowest E structure o One way to form a C radical is by chem rxn b w a neutral species and a preformed radical o Can be formed by breaking a covalent bond in a way that one electron is transferred to each of the 2 atoms involved in that bond Homolytic Cleavage Chem rxns are driven by changes in E o Heating imparts E to the molecules in a rxn which will increase the E of the system o Molecules absorb E from their environment by collision w side of rxn vessel flask or w another atom or molecule This E is utilized in making and breaking bonds G is the E of the entire system free E Whether or not a chem rxn will proceed spontaneously is determined by G o If the of particles for a rxn remains the same or decreases the magnitude of the entropy is small o o If the of particles greatly increases during the course of a rxn then entropy increases If entropy is assumed to be 0 the G H However ignoring the entropy term is an assumption and sometimes leaving it out can lead to significant error Exothermic spontaneous Endothermic nonspontaneous …