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Tutor profile: Joshua G.

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Joshua G.
Licensed Chemist; Former College Chemistry Instructor
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Questions

Subject: Organic Chemistry

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Question:

Which of the following molecules reacts the fastest in an SN1 reaction? A. 1-bromobutane B. 2-bromobutane C. 2-bromo-2-methylpropane

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Joshua G.
Answer:

The rate-determining step of an SN1 mechanism is the explusion of the leaving group, which forms the carbocation intermediate. For the three molecules, the leaving group is bromide ion or Br(-). The speed of this step is dictated primarily by the stability of the carbocation that forms. The more stable the carbocation is, the faster is the reaction. When the leaving group is expelled, 1-bromobutane forms a primary carbocation; 2-bromobutane forms a secondary carbocation; and 2-bromo-2-methylpropane forms a tertiary carbocation. A carbocation is a positively-charged species. In order to stabilize a carbocation, the electron-deficient atom must be given electron density. Alkyl groups are known to be electron-donating substituents via inductive effect, which means they are capable of stabilizing the positive charge in a carbocation. The more alkyl groups attached to the carbon bearing the positive charge, the more stable the carbocation becomes. Therefore, tertiary carbocation is more stable than secondary carbocation, and secondary carbocation is more stable than a primary carbocation. Answer: Since 2-bromo-2-methylpropane forms a tertiary carbocation, it reacts the fastest in an SN1 reaction.

Subject: Chemistry

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Question:

Which of the following has a lower freezing point, 1.0 molal aqueous solution of glucose of 0.50 molal aqueous solution of NaCl?

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Joshua G.
Answer:

First we need to differentiate electrolytes from non electrolytes. Electrolytes are substances that when dissolved in a solvent produces a solution that conducts electricity. These substances are either ionic substances that dissociate into ions, such as NaCl dissociating into Na(+) and Cl(-), or polar covalent substances such as HF that ionizes into H(+) and F(-). An electrolyte can be classified as either a strong electrolyte, which dissociates completely in solution, or a weak electrolyte, which undergoes partial dissociation only. On the other hand, nonelectrolytes are substances that do not dissociate or ionize in the solvent to produce an electrically-conducting solution. An example of a nonelectrolyte is glucose. Freezing point depression is given by the following equation: FPsolvent-FPsolution=(i)(Kf)(m) where: FPsolvent = freezing point of pure solvent (0 deg C for water); FPsolution= freezing point of solution; i = van't Hoff factor; Kf = cryoscopic constant of the solvent (1.86 deg C/molal for water); m = molality of the solution The equation can be rearranged to: FPsolution = -[(i)(Kf)(m)-FPsolvent] 1.0 molal aqueous solution of glucose FPsolution = -[(1)(1.86 degC/molal)(1.0molal)-0 degC] FP solution = -1.86 deg C NOTE: The van't Hoff factor of glucose is 1 since glucose is a nonelectrolyte, which does not dissociate or ionize into ions in solution. 0.50 molal aqueous solution of NaCl FPsolution = -[(2)(1.86 degC/molal)(0.50molal)-0 degC] FP solution = -1.86 deg C NOTE: The van't Hoff factor of glucose is 2 since NaCl is a strong electrolyte, which completely dissociates in solution. 1 mole of NaCl gives 1 mole of Na(+) and 1 mole of Cl(-), so van't Hoff factor is equal to 2. Answer: Both solutions have the same freezing points.

Subject: Basic Chemistry

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Question:

What is a polar and a non-polar covalent bond?

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Joshua G.
Answer:

When we talk about bond polarity, we must first be familiar with the concept of electronegativity. When two atoms form a covalent bond, they share two electrons within that bond. In reality, the sharing is not actually equal all the time. There are instances where the electron density is greater around one atom than the other. Electronegativity is the tendency of an atom to pull the bonding electrons towards itself. The more electronegative the atom is, the stronger is its pull for bonding electrons. Now, when electrons are not equally shared by two atoms, the more electronegative atom will bear a partially negative charge (since electron density is greater), while the less electronegative atom bears a partially positive charge (since electron density is lesser). Since the two ends of the bond bear different charges, the bond is known to be a polar bond (with negative and positive poles). An example of a polar covalent bond is H-F. Fluorine is way more electronegative than hydrogen, so electron density is greater around fluorine than in hydrogen. Therefore, fluorine bears the partial negative charge while hydrogen bears the partial positive charge. On the other hand, such unequal sharing of electrons is absent in a nonpolar bond. a nonpolar bond may be due to the following: 1. The atoms involved in bonding are the same element. For example, H-H is a nonpolar bond. This is because there is no difference in electronegativities, so both hydrogen atoms have the same electron density. 2. The atoms involved in bonding have almost the same electronegativities. For example, C-H bond is considered a nonpolar bond. Carbon and hydrogen have almost the same electronegativities, so the bond is essentially nonpolar.

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