Tutor profile: Aaron W.
Subject: Organic Chemistry
Why does the hydrogen on the hydroxide group of carboxylic acid have a significantly lower pKa than any hydrogen on methane?
For a certain hydrogen to have a low pKa (a.k.a be more acidic), when it leaves the molecule and forces a negative charge on the molecule it left from, that molecule has to be able to effectively stabilize the negative charge through one or more methods. These methods include delocalization of charge through resonance, inductive effects, and nearby opposite charges. In the case of carboxylic acid, the main effect will be resonance stabilization as the negative charge is delocalized between the two oxygens. This allows the acidic proton on carboxylic acid to be quite acidic with an approximate pKa of 5. Methane has no way to form any resonance structures and contains no highly electronegative atoms that can perform the induction effect because methane is a single carbon with 4 hydrogens. A negative charge on the carbon of methane is not anywhere as close as stable as a delocalized negative charge on carboxylic acid.
Subject: Environmental Science
Around the country, geochemists are observing through sampling and data collection that a signifiant number of the freshwater streams, lakes, and rivers are becoming saltier as the years go on. These same scientists have noticed that this trend is tightly correlated to the increase in the concentration of dissolved metals (for example, Pb, Mn, and Hg) in our tap water. These two trends are especially true for suburban and urban areas in the Northern regions of the country. What would you suggest is one major reason for the increase in salt content in our freshwater sources? Also, how does this relate to the increase of heavy metals in our tap water?
One major source of salt is the road salt used in the winter time to try and make the roads safer to drive on during freezing conditions. Due to the fact that roads are an impervious surface, when the snow/ice melts and/or when it rains, all of that water and salt is washed into freshwater sources. The added salt has the ability to drastically change the chemistry of the water. Salt causes accelerated weathering of the pipes in our infrastructure. The concrete and other materials that make up our water infrastructure contain easily weathered minerals. As the salt moves through the underground pipes, it can mobilize metals as it interacts with different ion exchange sites in the material that makes up the pipes. A lot of road salt is made of NaCl, commonly known as sodium chloride or halite. Sodium has the capacity to overwhelm these exchange sites and release positive ions into solution. If a certain city or town has very old pipes, there is great potential to leach a lot of metals out of the pipes. For example, a lot of old pipes contain lead.
In the laboratory, you have removed cells from the liver of a mouse from the same region on two separate days. You perform a procedure that freezes the cells in their current state so you can examine them using microscopy at a later date. When examining the cells from the two different sampling events, you notice that in one sample, the cells have a prominent nucleolus, rough endoplasmic reticulum, and Golgi apparatus. In the other sample, you notice that the cells have a prominent smooth endoplasmic reticulum and relatively less prominent nucleolus, rough endoplasmic reticulum, and Golgi apparatus. Being that these two cell samples are of the same cell type and from the same region of the liver, why would they have such different characteristics regarding their organelles when sampled on two different days?
The hepatocytes (liver cells) serve many functions. The hepatocytes will have a prominent nucleolus, rough endoplasmic reticulum, and Golgi apparatus when they are involved in the extracellular secretion of proteins. The nucleolus is the site of rRNA production and ribosomal subunit synthesis. The rough endoplasmic reticulum is the site where ribosomes bind to the endoplasmic reticulum to synthesize proteins bound for secretion. The Golgi apparatus modifies, and tags proteins based on their final destination in the secretory pathway. On the other hand, the smooth endoplasmic reticulum is prominent in the hepatocytes when they are involved in lipid metabolism, like creating steroid molecules.