Stereochemistry

Question 1

What are essential amino acids?

Answer 1

They are amino acids that humans cannot produce on their own so they must take it through diet

Question 2

Why is only 25% of synthesized isoleucine be used biologically?

Answer 2

Because there are two chiral centers, this makes four stereoisomers being formed. Because of the specificity of enzymes, only one of the four can be used. This is true for all amino acids with two chiral centers in their side chain.

Question 3

Most naturally occurring amino acids are what stereochemistry?

Answer 3

S (or L)

Question 4

Which one has the higher boiling point, the S-enantiomer or the R-enantiomer?

Answer 4

It’s the same! It’s diastereomers that have different boiling points. Enantiomers have the same BP, MP, density, absolute optical rotation (one is + the other is -)

Question 5

Degrees of Unsaturation equation with N, X, and O

Answer 5

DOU = [ 2(C) + 1(N) + 2 - 1(H) - 1(X) ] / 2

Oxygen not in this equation

Question 6

What is a gem diol?

Answer 6

Two -OH on the same carbon

hehe…..gemini

Question 7

What is vicinal diol?

Answer 7

Two -OH on neighboring carbons

Question 8

When counting how many chiral centers a compound has, do you count the sp2 carbons (double bond, carbonyl)?

Answer 8

Nope, only the sp3 carbons bonded to four different things

Question 9

How is optical rotation measured?

Answer 9

Using a polarimeter device, they rotate one polarimetry disk until they get the highest intensity of light and that’s the optical rotation value

Question 10

Why does a R/S enantiometric mixture have a higher MP, BP, and density than pure samples of either the R or S enantiomers?

Answer 10

The enantiomers pack more tightly together and has stronger intermolecular bonds. It’s like how holding hands can take up less space than putting your hand on top of another hand. This is usually true, but if the experimental values show otherwise, then that just means R and S don’t pack together as well as by themselves.

Question 11

How do you favor a substitution reaction over E1 by altering experimental conditions?

Answer 11

Lower the temperature and increase pH

Question 12

Is Br- or Cl- the better leaving group?

Answer 12

Br- because the carbon-bromine bond is weaker so the LG can come off easier.

Question 13

Is I- or Br- or Cl- or F- the best leaving group?

Answer 13

I- is the best leaving group because HI is the strongest conjugate acid.

Question 14

Why is fluoride a better nucleophile than iodide in ether solution but a worse nucleophile in alcohol solvent.

Answer 14

In protic solvents, flouride is hindered by hydrogen bonding and iodide is not.

Question 15

What is a favorable condition for elimination reactions?

Answer 15

High temperatures because elimination is an endothermic reaction so it has a greater activation energy usually than substitution reactions.

Question 16

When does E1 compete with SN1?

Answer 16

When there is a good leaving group and it is on a tertiary carbon.

Question 17

What is a good laboratory technique to get pure enantiomer form racemic mixtures?

Answer 17

Adding mixture to a chromatography column filled with gel with just one of the enantiomers bond to it. Or you can add chiral specific enzymes.

Question 18

How do you determine R and S in a Fischer projection?

Answer 18

because H is usually one left or right, that means it is pointing out, so whatever stereochemistry you get from just looking at the structure, you must reverse it because hydrogen needs to be in the back.