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Haley V.
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Organic Chemistry
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Question:

Tamoxifen is a medication commonly used to prevent and treat estrogen-receptor-positive breast cancer. In one of the key steps to synthesize the drug, 1,2-diphenyl-1-butanone and 4-methoxyphenyl magnesium bromide are reacted in THF. What reaction is this? Can you draw the mechanism of action? (Hint: I recommend drawing out the structures of both starting materials. This makes the mechanism much easier to visualize)

Haley V.
Answer:

This is a Grignard reaction, where an alkyl-magnesium halide is reacting with a carbonyl group. This results in the addition of the alkyl group on the magnesium halide to the carbonyl, forming an alcohol. The mechanism is as follows: the group attached to the magnesium bromide acts as a nucleophile, attacking the electrophilic carbon atom in the carbonyl. The suffix "-phile" means to love, but you can also think about it as what the atom needs to be happy. carbons on carbonyls are sharing four electrons with an oxygen. Since oxygen atoms are very electronegative, that means the carbon isn't getting its fair share of electrons. On the other hand, the carbon interacting with the magnesium bromide has plenty of electrons. In fact, it has so many that it's looking to share some with another nucleus. Hence, it's called the nucleophile of the reaction. So the nucleophile attacks the carbonyl in 1,2-diphenyl-1-butanone, which causes two of the electrons shared between the carbonyl carbon and oxygen to go to the oxygen. To keep happy and have two bonds, the oxygen then grabs a hydrogen, and we have an alcohol. This is one of the most important tools in an organic chemist's "tool box" for forming carbon-carbon bonds. Reactions can seem tricky in organic chemistry, and there are certainly a lot of terms to learn, but by understanding what different atoms need to be "happy" and how they get there, you can skip the memorization and understand a lot more about how molecules interact.

Trigonometry
TutorMe
Question:

Suppose we have a triangle ABC with sides a ,b, and c. Side a is of length 5, side b is of length 10, and they include an angle of 60 degrees. What is the length of side c?

Haley V.
Answer:

This is a classic Law of Cosines problem. The question doesn't state that triangle ABC is a right triangle, so we cannot assume that the Pythagorean theorem will apply in this instance. The Law of Cosines says as follows: c^2 = a^2 + b^2 - 2ab cos(gamma) where a,b, and c are the sides of the triangle and gamma is the angle between sides a and b. This is an extension of a Pythagorean theorem, because if gamma were 90 degrees, cos(gamma) would equal 0. Now, to solve this problem! Either by using our knowledge of the unit circle or a calculator, we know that cos(60) = 0.5. Plugging in sides a and b to the Law of Cosines equation, we get: c^2 = 25 + 100 - 2 * 5 * 10 * 0.5 which simplifies to: c^2 = 25 + 100 - 50 c^2 = 75 so side c is equal to the square root of 75, approximately 8.6.

Biochemistry
TutorMe
Question:

What are the one letter abbreviations for the following amino acids? (Bonus: Can you draw out the two polypeptide chains?) 1. Serine, Threonine, Selenocysteine, Aspartate, Tyrosine 2. Histidine, Alanine, Arginine Aspartate

Haley V.
Answer:

1. S-T-U-D-Y 2. H-A-R-D And about the bonus: conventionally, polypeptides are drawn with the amino-terminus on the left and the carboxyl-terminus on the right. It helps to draw the backbone of the amino acid straight across the page! That way, functional groups alternate pointing up and down from the main chain. Don't forget to consider pKa's when drawing polypeptide or protein chains; physiological pH (your body's acidity or basicity) is about 7.4, so your amino-terminus should be NH3 + and your carboxyl-terminus should be a carbanion. Some functional groups will be affected by the pH too!

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