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John F.
CRLA Certified Tutor, Chemistry and Biology
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Organic Chemistry
TutorMe
Question:

You are performing an alpha halogenation reaction with 3'-chloropropiophenone and Br2. in a round bottom flask. The procedure requires you to gently warm the the flask to start the reaction, and then place it on ice. Why?

John F.
Answer:

3'-chloropropiophenone is a ketone, but the starting material for reaction with the bromine is this compound's enol tautomer; to expedite the appearance of an enol tautomer, the reaction mixture is heated. The round bottom flask can then be placed in an ice-bath because the reaction is autocatalytic; one equivalent of hydrobromic acid (HBr) is formed for every equivalent of the 2-bromo-3’chloropropiophenone product. The acid protonates the ketone and catalyzes the enolization process. The reduced temperature allows the reaction to proceed in a controlled manner.

Biology
TutorMe
Question:

How does overuse of antibiotics lead to antibiotic resistance?

John F.
Answer:

If a randomly occurring mutation codes for production of an enzyme that breaks down an antibiotic (or any other change that provides resistance), exposure to that antibiotic increases the relative fitness of bacteria with those genes Repeated use of the antibiotics provides selective pressure that increases the incidence of those mutations in future generations of the bacteria. Proliferation of bacteria with genes coding for these enzymes is a good indicator that antibiotics are used to the extent that the fitness of these bacteria is increased. There is a metabolic cost to producing these enzymes, so without the presence of the antibiotic they would actually lower the fitness of the bacteria producing them.

Biology
TutorMe
Question:

The capacity to taste phenylthiocarbamide (PTC) is an autosomal dominant phenotype and the inability to taste it is recessive. If a taster woman with a nontaster father marries a taster man who, in a previous marriage, had a nontaster daughter, what is the probability that their first child will be a non taster girl?

John F.
Answer:

Draw a pedigree or punnet square to help. Let T represent the dominant allele and t represent the recessive allele. With an autosomal dominant inheritance pattern, only one T allele is necessary for the 'taster" phenotype. For a non-taster "both alleles must be the recessive t. The woman must have at least one T allele because she is a taster. Her non-taster father must have given her a t allele, because his phenotype was homozygous recessive ("tt"). This gives her the genotype Tt. The father is a taster man who had a nontaster daughter in a previous marriage. In order for this to happen, he must have passed down a t allele to his daughter This makes his genotype Tt. Their first child is the result of a Tt X Tt monohybrid cross. Given this inheritance pattern, we would expect a 1:2:1 TT:Tt:tt pattern in the offspring. There is a 1/4 chance that the child will be a non-taster We then use Baye's theorem to multiply the probability of the daughter being a girl (1/2) by the probability that the child will be a non-taster (1/4). (1/4) * (1/2) = 1/8 = 0.125 There is a 12.5% chance that their first child is a non-taster daughter.

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