Describe why an Sn2 reaction requires a polar aprotic solvent to react quickly.
An Sn2 reaction involves a nucleophile attacking an electrophile so reaction rate depends on how easily the nucleophile can reach the electrophile so solvation of the nucleophile is crucial. Solvation refers to how well a solvent interacts with dissolved molecules in solution, in this case the nucleophile, so having a highly solvated nucleophile would mean that the nucleophile does not interact with the electrophile easily. In polar protic solvents such as methanol, the solvent will interact with a negative nucleophile like Cl- because of hydrogen bonds that form between the Cl- and the methanol therefore slowing the rate of reaction down. On the other hand, polar aprotic solvents such as dimethylformamide do not form hydrogen bonds so they do not interact with nucleophiles like Cl- allowing the Cl- to reach the electrophile much faster.
Why is hydrofluoric acid considered a weaker acid than hydrochloric acid?
Acidity is determined by the H+(proton) concentration of an acid in an aqueous solution, the higher the proton concentration, the more acidic it is. Fluorine is a highly electronegative atom and tightly bonds to the hydrogen so it will not dissociate into H+ and F- ions easily in an aqueous solution so the proton concentration is not very high. On the other hand, chlorine is not as electronegative as fluorine so it will dissociate into H+ and Cl- ions in water. In fact, HCl is a strong acid meaning it will completely dissociate in water therefore the proton concentration will be very high, raising the acidity of the solution more than HF.
Epigenetics describes the expression of genes without changing the DNA sequence. An organism contains the same DNA all across its body but an arm expresses different genes than the stomach and this is because gene expression is being controlled by epigenetic factors. Processes such as histone acetylation which increases gene expression and DNA methylation which slows down gene expression are all epigenetic factors because they do not change the DNA sequence in any way. Histone acetylation will unbind DNA from histones allowing them to be expressed whereas DNA methylation will bind methyl groups to DNA repressing their expression. Both of these factors do not change the genetic code in any way therefore they fall under epigenetics.