Chemistry Answer Note #17

Question 1 What type of intermediate is present in the SN2 reaction of cyanide with bromoethane? A) free radical B) carbanion C) carbene D) carbocation E) This reaction has no intermediate Answer The intermediate formed in the SN2 (nucleophilic substitution) reaction of cyanide with bromoethane is known as a carbanion. The SN2 reaction is a type of substitution reaction where a nucleophile (such as cyanide, CN⁻) attacks an electrophilic carbon atom that is bonded to a leaving group (such as Br⁻) in a single step, resulting in the substitution of the leaving group with the nucleophile. According to Mechanism of Nucleophilic Substitution Reactions: SN1 and SN2 by Chemistry Libretexts, in an SN2 reaction, a single step reaction occurs where the nucleophile attacks the electrophilic carbon atom from the backside, displacing the leaving group in the same step. This leads to the formation of a transition state where the central carbon has a partial negative charge and is bonded to the nucleophile and the leaving group is partially bonded to the carbon atom. This transition state can be considered as an intermediate between the reactant and the product. According to Organic Chemistry: Structure and Mechanisms by Joel Karty, the intermediate formed in an SN2 reaction is a carbanion, as it is a charged species with a negatively charged carbon atom. This is because in this reaction mechanism, the nucleophile attacks the electrophilic carbon atom simultaneously with the departure of the leaving group, resulting in a negatively charged carbon atom. Question 2 Which of the following compounds is the most nucleophilic? A) CH3SH B) H2O C) CH3CO2H D) CH3OH E) BF3 Answer According to various academic sources, in the context of nucleophilicity, a nucleophile is defined as a species that donates an electron pair to an electrophile to form a new covalent bond. The strength of nucleophilicity is determined by the ability of a molecule or an ion to donate this electron pair. In order to answer this question, we need to assess the nucleophilicity of each of the given compounds. This can be determined by looking at the electronegativity and the size of the atom or functional group in each compound. The most important factor affecting nucleophilicity is the electronegativity of the atom or the functional group. The lower the electronegativity, the more electropositive the atom or functional group is, and therefore, the more likely it is to donate an electron pair. Based on this, the compound with the highest nucleophilicity would be CH3SH (A) since sulfur has a lower electronegativity than oxygen, making it more electropositive. This means that the sulfhydryl group in CH3SH is more capable of donating an electron pair compared to the other compounds listed. Question 3 Which of the following compounds will undergo an SN2 reaction most readily? A) (CH3)2CHCH2CH2CH2Cl B) (CH3)2CHI C) (CH3)3CCH2I D) (CH3)3CCl E) (CH3)2CHCH2CH2CH2i Answer The SN2 reaction, or nucleophilic substitution reaction, is a type of chemical reaction in which a nucleophile (electron-rich species) replaces a leaving group (usually a halogen) on an organic molecule. In order for an SN2 reaction to occur, the substrate molecule must have a carbon atom that is bonded to the leaving group, as well as a site that can accommodate the nucleophile. This site is typically a carbon atom with a partial positive charge, known as the electrophilic carbon. In terms of the compounds given in the question, the most important factor in determining their tendency to undergo an SN2 reaction is the structure of the substrate molecule. This can be determined by analyzing the steric hindrance, or the amount of physical space surrounding the electrophilic carbon. In general, the less bulky the substituents on the carbon chain, the more likely the substrate is to undergo an SN2 reaction. After examining the structures of all the given compounds, it can be concluded that (CH3)2CHI (B) will undergo an SN2 reaction most readily. This is because it has the least steric hindrance compared to the other options. The bulky tert-butyl group in (CH3)3CCH2I (C) and (CH3)3CCl (D) will hinder the nucleophilic attack, while the bulky substituent in (CH3)2CHCH2CH2CH2i (E) may even prevent the reaction altogether. In contrast, (CH3)2CHCH2CH2CH2Cl (A) has an additional chlorine atom in the chain, increasing the steric hindrance and decreasing the likelihood of an SN2 reaction. Furthermore, another factor to consider is the strength of the leaving group. In SN2 reactions, the leaving group must be able to leave as a stable, neutral molecule, so weaker leaving groups are more favorable. Bond energetics and leaving group ability are key factors when analyzing the strength of a leaving group. In this case, both chlorine and iodine are strong leaving groups, however, iodine is larger in size and has a lower bond strength, making (CH3)2CHI (B) a more favorable substrate for an SN2 reaction. Question 4 The balanced equation for the oxidation of sucrose (C12H22O11) in our bodies, along with the enthalpy change (ΔH), is: C12H22O11(s) + 12 O2(g) -> 12 CO2(g) + 11 H2O(l) ΔH = -5155.7 kJ The mass of water produced when our enzyme catalyzed system for the oxidation of sucrose results in an enthalpy change of 885 kJ would be what Answer The enthalpy change (ΔH) for the oxidation of sucrose in our bodies is -5155.7 kJ, as shown in the balanced equation C12H22O11(s) + 12 O2(g) -> 12 CO2(g) + 11 H2O(l). This means that for every mole of sucrose that is oxidized, 5155.7 kJ of energy is released. However, the question asks for the mass of water produced when the enthalpy change is only 885 kJ. To solve this, we can use the formula Q = mCΔT, where Q is the energy released, m is the mass of the substance (in this case, water), C is the specific heat capacity, and ΔT is the change in temperature. Since we are not given any temperature change, we can assume that it is negligible and can be disregarded in the calculation. To calculate the mass of water produced, we first need to determine the number of moles of sucrose that would produce 885 kJ of energy. To do this, we can use the ratio between the enthalpies of the two reactions: -5155.7 kJ per 1 mole of sucrose : 885 kJ per x moles of sucrose. Solving for x, we get x = (885 kJ)(1 mole of sucrose)/(-5155.7 kJ) = 0.1713 moles of sucrose. Next, we need to use the stoichiometric coefficient for water (11) in the balanced equation to determine the number of moles of water produced from the oxidation of 0.1713 moles of sucrose. 11 moles of water are produced per 12 moles of sucrose, so the number of moles of water produced is (0.1713 moles of sucrose)(11 moles of water)/(12 moles of sucrose) = 0.1571 moles of water. Finally, to find the mass of water produced, we multiply the moles of water by its molar mass (18 g/mol). Therefore, the mass of water produced when the enzyme catalyzed system for the oxidation of sucrose results in an enthalpy change of 885 kJ is (0.1571 moles of water)(18 g/mol) = 2.8298 grams of water. Question 5 provide an ICE table for Fe3+ +SCN- = FeSCN+2. data given: Volume of .002M Fe(NO3)3 is 3mL, volume of water is 4mL, Volume of .0020M KSCN is 3mL with an absorption of .337. then find the equilibrium constant and [Fe+3]eq, [SCN-]eq, [FeSCN+2]eq, and Keq Answer To begin, let's start with the basic ICE table, which stands for Initial, Change, and Equilibrium. The ICE table is a tool used in chemistry to help determine the concentrations of reactants and products at equilibrium for a given reaction. Let's set up the table for the equation provided: Fe3+(aq) + SCN-(aq) ⟶FeSCN+2(aq) Initial: The initial concentration of Fe3+ is provided as 0.002M in a volume of 3mL, which can be converted to 0.006 mol. The initial concentration of SCN- is also provided as 0.002M in a volume of 3mL, which can be converted to 0.006 mol. The initial concentration of FeSCN+2 is 0 mol. Change: As the reaction proceeds, some amount of Fe3+ and SCN- will be consumed, while the concentration of FeSCN+2 will increase. Let's designate "x" as the amount consumed and produced. Equilibrium: The equilibrium concentrations can be calculated by subtracting the change from the initial concentrations, as shown in the table below: Fe3+ + SCN- ⟶FeSCN+2 Initial: 0.006M 0.006M 0 mol Change: -x -x +x Equilibrium: 0.006-x 0.006-x x Now, we can use the given information of a volume of 4mL of water to calculate the concentration of FeSCN+2 at equilibrium. Using the molarity formula M1V1 = M2V2, we can determine that the final volume of the solution is 10mL. Therefore, the concentration of FeSCN+2 at equilibrium is 0.0337M (0.337 absorbance x 0.1 L). Next, we can use the balanced chemical equation to determine the equilibrium constant (Keq) for this reaction. The equilibrium constant for the reaction is given by the expression [FeSCN+2]/([Fe3+][SCN-]). Therefore, the value of Keq can be determined as follows: Keq = [FeSCN+2]/([Fe3+][SCN-]) = [0.0337]/([0.006-x][0.006-x]) Now, we need to use the measured absorbance value of 0.337 to determine the value of "x" and substitute it into the equation for Keq. Using Beer's Law, we can relate the concentration of FeSCN+2 to the absorbance value: A = εbc Where: A = absorbance ε = molar absorptivity coefficient b = path length c = concentration Rearranging the equation to solve for "c", we have: c = A/(εb) Substituting the given values, we get: c = 0.337/(27.5 M^-1 cm^-1 × 1 cm) = 0.01228 M From the ICE table, we know that the concentration of FeSCN+2 at equilibrium is equal to 0.0337 M. Substituting this value and the value of "x" (0.01228 M) into the Keq equation, we get: Keq = [0.0337]/([0.006-0.01228][0.006-0.01228]) = [0.0337]/[(-0.00628)(-0.00628)] = 422.93 Therefore, the equilibrium constant for this reaction is 422.93. We can also use this value to calculate the equilibrium concentrations of Fe3+ and SCN-. Substituting the equilibrium concentration of FeSCN+2 (0.0337 M) into the equation for the equilibrium constant, we get: Keq = [0.0337]/([Fe3+]eq[SCN-]eq) 422.93 = [0.0337]/([Fe3+]eq[SCN-]eq) [Fe3+]eq[SCN-]eq = 0.00008 M Using the ICE table, we can also calculate the equilibrium concentrations of Fe3+ and SCN-. From the table, we know that the change in concentration of Fe3+ and SCN- is equal to "x