Table 12.5 page 359, SchwarzenbachLecture 2 Organic ReactionsWe will look at organic reactions in the context of:- hydrolysis- Acid / Base1. definition of acids and bases2. nucleophilic and electrophilic compounds3. nucleophilic substitution reactions4. Elimination Reactions 5. acid and base catalyzed hydrolysis rates and life times in the environment6. Hammett and Taft rate constant observationshalomethanesDDT and DDEcarboxylic acid esterscarboxylic acid amidescarbamates1Bronstead-Lowry acid - basesacid is defined as a species that can give up a proton, [H+]a base is a species that will accept a proton.HNO3---> H+ NO3-CH3COOH ---> H+ CH3OO-conjugate bases C6H5OH ---> H+ +C6H5O-2Lewis Acids and Bases (1923)Acids have less than a full octet of electrons and are “electron deficient or electron poor” (electrophiles)What the Lewis system does for us is allow species besides the hydrogen ion to display acidic behavior page 116, Holt3Gould, Mechanism and Structure in Organic Chemistry, Holt, Rinehart and Winston, NY,1993 p 115, 1963Nucleophilic compounds:From a reaction perspective nucleophilic species carry a negative charge and are usually polar in character.They may have an electron rich bond or unbonded electron pair which is often the site of attack.In the environment the majority of the nucleophiles that react with organics are inorganic4Table 12.5 page 359, SchwarzenbachBecause of its great abundance, water plays a pivotal role among the nucleophiles in the environmentA reaction in which water (or hydroxide) substitutes for another atom or group is called hydrolysis.The resulting organic products of hydrolysis are typically more polar than their parent compounds and generally of less environmental concernCH3-Br + H2O ---> CH3-OH + H+ + Br-Chlorinated organics often find their way into the environmentand hence their reaction with nucleophiles is of interest5Relative Nucleophilicty of inorganic NucleophilesSwain and Scot (see page363, Fig. 12.5) observed that for different nucleophiles, x, attacking different methyl halides that X- + CH3-Br  -X--CH3--Br - CH3-X + Br -log (kx/kH2O) = s x nWhere n is an indicator of the attacking ability of x, thenucleophile, and s is the sensitivity of the organic to nucleophilic attack; n is really important because it represents the ability of the nucleophile to donate electronsAs a standard for methyl bromide, s is set equal to 1; we can now ask what does the concentration of a given nucleophile have to be to compete with water; ie when is its rate similar?log (kx/kH2O) = 1 x n; so kH2O/kx = 10-n6To be similar, the nucleophilic rate must equal the water, d(CH3BrH2O /dt) or d CH3Brx/dt = d(CH3BrH2O) for the nucleophile CN- as an example,d(CH3BrCN-)/dt = kCN [CN- ] and d CH3BrH2O /dt = kH2O [H2O ]so kCN [CN- ]50% = kH2O [H2O ] and using the ratio of the rate constants above[CN- ]50% = [H2O ] X 10-n using values for n (page 359) for differentnucleophiles and [H2O ]= 55 mol/Lsee Table 12.6 page 364 of Schwarznebach for [x-]50%NO3-= 6 molar Br- = 7x10-3SO42-= 2x10-1OH-= 4X10-3Cl-= 6x10-2I-= 6X10-4In fresh water Cl-= 10-3; SO42-= 2x10-4; OH-=10-6 mol/L;so what do we conclude??7SN2 substitution (Substitution, nucleophilic bimolecular)A nucleophile attacks a carbon from the opposite side of the leaving group. An intermediate is theorized in which the nucleophile is partially bonded to the molecule, while the leaving group is partially dissociated. The nucleophile donates two electrons and the leaving grouptakes two electronsblow up page 109 of Richard Larson’s book on Environmental Organic Mechanisms 8The free energy of activation -G‡ and the rate of reaction will depend on the:nucelophlicity of Y: steric factorsHydrolysis t1/2 of chloromethanes (years)CH3Cl CH2Cl2CHCl3CCl4 0.93 704 3,500 7,0009Sterioisomers,dieldrin and endrin, are two examples of insecticides that contain epoxide moieties. Both hydrolyze bySN2 reaction with H2O and OH-, resulting in diolsCarbon skeleton sterically impedes nucleophilic attack by H2O and OH-. As a result persistence in aquatic eco-systemsare long and they have been banned in the US, but still used in other countries.10Richard A. Larson and Eric J. Weber. Reaction Mechanisms in Environmental Organic Chemistry, Lewis Publishers, Ann Arbor, 1994Epichlorlhydrin is used for the manufacture of glycerol and expoy resins. Its calculated half-life in distilled water at 20oC is 8 days 11Page 122 LarsonThe SN1 MechanismThis mechanism (nucleophilic substitution, monomolecular) differs from the SN2 in that a dissociation of the organic molecule 1st takes place to form a carbonium ion (carbocation). The carbonium ions is then attacked by a nucleophile- exhibits 1st order behavior- factors that stabilize the carbonium ionwill increase reactivity, such as resonance or inductive effects12Figure 12.4 page 36213SN1 vs. SN2For mono and di halomethanes, an increase in the # of halogen substituents on carbon increases the hydrolysis half-life. Why?R-Cl+H2O --> R-OH + HCl Cl Cl H-C-Cl Cl-C-ClH ClSN2By contrast, as the steric bulk in the form of methyl addition to the central carbon bearing the halogen occurs, a significant INCREASE in reactivity can be observed. Why? Cl ClCH3-C-CH3CH3-C- CH3 H CH338 days 23 seconds14As halogen electronegativity decreases (F>Cl>Br) hydrolysis rates increasePage 366 Table 12.715Explaining mechanismsUnder neutral or basic conditions nucleophilic attack on the primary carbon occurs by SN2; the epoxide opens up and the deuterated oxygen appears at the primary carbon siteUnder acidic conditions, the high conc. of [H]+ attacks the epoxide oxygen and water attacks the primary carbon.16Write analogous SN1 and SN2 mechanisms for the neutral hydrolysis of a substituted epoxide.COCHR2R2R317to be SN1 a bond breaks and water addsto be SN2 water adds and the OH leavesElimination Reactions- sterically hindered nucleophilic substitution- when acidic protons are present next to the carbon of the leaving group- presence of strong bases ---elimination-C-C- --------------------> C=C H X -HXexample of such a reaction is the conversion of 1,1,2,2-tetrachlorethane to trichloroethylene. This can be viewed as an SN2 reaction followed by elimination18DDT conversion to the more environmentally stable DDE via elimination as a function of increasing pH or increasing strength of nucleophilic OH-19Carboxylic Acid Esters: Esters are important because they are