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Tucker H.
Research Chemist
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Organic Chemistry
TutorMe
Question:

How can I tell if a reaction will be an Sn1 or an Sn2?

Tucker H.
Answer:

There are 4 major factors that help determine whether a reaction will go by an Sn1 or Sn2 mechanism: the substrate, the nucleophile, the leaving group, and the solvent. Substrate: Steric hindrance in the substrate disfavors the Sn2, and improved carbocation stability will favor the Sn1 (e.g. adjacent C=C double bond or aromatic group that can resonance stabilize the +). Tertiary carbons will never react by Sn2, and methyl-halides or primary carbons will never react by Sn1 (unless highly resonance stabilized). Secondary carbons will depend on the other factors. Nucleophile: Weak nucleophiles (e.g. H20) will slow down Sn2 reactions, allowing Sn1 to dominate. Nucleophile strength doesn't affect Sn1 rate (unless it is also the solvent, see below). Leaving group: Sn1 reactions prefer very good leaving groups. Although leaving group strength accelerates both Sn1 and Sn2 reactions, Sn1's are a bit more sensitive to leaving group and bias the reaction coordinate towards carbocation formation. Solvent: Polar protic solvents will accelerate Sn1 reactions, wherase polar aprotic solvents will accelerate Sn2 reactions. With charged nucleophiles, an apolar solvent can actually accelerate the Sn2 by destabilizing the charged ground state relative to the less polarized transition state.

Chemistry
TutorMe
Question:

Is O2 diamagnetic or paramagnetic and how can I tell?

Tucker H.
Answer:

A paramagnetic substance is one that is attracted the poles of a magnet, while diamagnetic substances are not. For a chemical to be paramagnetic, it must have some number of unpaired electrons. To determine whether or not O2 is paramagnetic, we need to look at it's molecular orbitals and see if any of them are singly occupied. O2 will have a total of 12 valence electrons and filling the molecular orbitals from the bottom to the top will result in the last two going into the two degenerate pi* antibonding orbitals. According to Hund's rule, these 2 electrons will singly occupy the two pi* orbitals, having the same spin and resulting in a net spin and thus a paramagnetic molecule. Therefore, O2 is paramagnetic.

Biochemistry
TutorMe
Question:

How do I know what the net charge of a peptide will be at a given pH?

Tucker H.
Answer:

The net charge of a peptide is going to be the sum of the charges on all of the amino acid R groups + the charges on the N and C-termini. At neutral pH, the acidic amino acids (Asp, Glu) and the C-terminus will be negatively charged, while Lys, Arg, and the N-terminus will be positively charged. At other pH's, it is necessary to compare the R group and termini pKa's to the pH; if the pH is higher than the pKa, the group will be deprotonated, and if the pH is lower than the pKa, the group will be protonated. This will determine whether the group is charged or neutral but depends on the R group/terminus in question (protonating increases charge by 1 and deprotonating decreases charge by 1). Going through each group and then adding the charges together will give you the net charge on the peptide.

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