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Kiran G.
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Organic Chemistry
TutorMe
Question:

We have to determine which kind of reaction can occur based on the type of substrate. How can I tell which substrates are good for nucleophilic substitution, and which are good for elimination?

Kiran G.
Answer:

One good way to start is to categorize your substrates, and break down the reaction types. There can be three types of substrates: primary, secondary, and tertiary. I like to think of the 4 main reaction types: SN1, SN2, E1, and E2. Both SN1 and E1 have a unimolecular rate determining step, and require the formation of a carbocation. Carbocations like to be tertiary, and will tolerate being secondary, but not primary. So, the electronics here determine that primary substrates can never do SN1 and E1. SN2, on the other hand, requires as little steric interference as possible, because the incoming group (nucleophile) attacks the carbon from the back while the leaving group leaves. So, the sterics determine that tertiary substrates cannot do SN2. By looking at the steric and electronic requirements of the reaction mechanisms, we can begin to narrow down our options. All three types of substrates can do E2, and secondary substrates can do all 4 reactions. Thus, we need more information, and after classifying the substrate, you should also look at the bond strength between carbon and the leaving group; solvent characteristics (protic or aprotic); and nucleophile characteristics (the less electronegative atom is more willing to give up its negative charge and bond to the carbon). Eventually, you can make a table for each reaction type that includes all of the matching characteristics.

Writing
TutorMe
Question:

I'm having trouble writing the body paragraphs for my argumentative essay. I know what stance I want to take, and I know my arguments too, but I don't know what order to put them in.

Kiran G.
Answer:

A simple bullet outline would be great for this! Think about each argument you want to make in broad terms, and make that a bullet (without thinking about the order). Think about how you want to back up each argument, using at least 2 or 3 different facts, examples, or pieces of evidence. Make each of those a sub-bullet under the argument. Do this for each argument, and when you're done, you will have a rough outline for each of your body paragraphs. Next, try to categorize the arguments, see which go together and think of transitions between them. Some arguments might need to be divided into more, while others might need to be combined. The outline will help you rearrange and reorder things for better logic and flow. It's important to start writing out your ideas first without worrying about the order. Everything can be arranged and fleshed out later!

Biology
TutorMe
Question:

Do different cells in the body have separate genes? Would a cone or rod cell in the eye have different genes from a liver cell?

Kiran G.
Answer:

In order to answer this question, we need to know a bit about cell/developmental biology and genetics. We all start out as a single cell, a fertilized zygote, which then goes on to divide repeatedly until we have a full organism with distinct body parts. Every time the cells divide, just as in mitosis, the genetic material is fully duplicated and evenly divided between the daughter cells. This remains true for all non-reproductive (somatic) cells. During early development, when embryonic cells are dividing and being fine-tuned for specific tasks (called cell differentiation), the amount or type of genes does not change between the cells. What does change is the overall molecular makeup, and the regulatory elements that determine which genes are expressed, and which are not. Each cell can have its own profile, or a subset of genes in the complete genome that are activated and expressed. For analogy, consider this: a company can have an overall employee handbook or manual, and all employees can have access to it. But depending on each person's specific role, different employees will bookmark and access separate chapters of the manual that are relevant to their function. Their functions are determined by their own educational background and experience. So, cells in the eye and liver have the same exact genetic material! But very different subsets of genes are actually being expressed in them, and the inactive genes are just coiled up and "locked away".

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