Tutor profile: Acacia S.
Subject: Organic Chemistry
What type of leaving group makes an SN2 reaction most favorable? Which molecule is commonly mistaken as a good leaving group and why?
Weak bases, like halides, are the best leaving groups because they are stable as an anion. In an SN2 reaction the nucleophile attacks the central atom which causes the bond to leaving group to break. Hydroxide is a strong base even though its negative charge makes it look like a good leaving group.
How can hormones be classified by their structure? Compare and contrast these classes.
Classifying hormones by their structure is essential to understanding how they function. The three classes are peptide, steroid, and tyrosine-derived hormones. Peptide hormones are composed of amino acids and are excreted from cells. They are typically charged, which makes them water-soluble but incapable of passing through the membranes of their target cells. Therefore, peptide hormone receptors exist on the cell surface where they can initiate a secondary messenger cascade. Second, steroids come from lipids, usually cholesterol. These hydrophobic hormones can easily pass through target cells' membranes as act as a primary messenger at cytoplasmic receptors. Lastly, tyrosine-derived hormones are unique in that they are made up of amino acids, but those like T3 and T4 can act like steroid hormones.
How can both the sympathetic and parasympathetic systems influence heart rate?
If the heart rate was not influenced by parasympathetic or sympathetic output then the sinoatrial node (SAN) would cause the heart rate to be 100bpm. At rest, parasympathetic input causes the heart rate to be about 60bpm and the brain reduces this input to cause any initial increase in heart rate. Parasympathetic input consists of the vagus nerve releasing acetylcholine onto postganglionic cells in the heart. When the heart needs to beat above 100bpm sympathetic postganglionic fibers from the sympathetic trunk release catecholamines into the synapses with the SAN cells.
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