Structures: Isomerism, Spectroscopy, etc.

Question 1

Should deuterium atoms be placed axial or equatorial position for most stable chair conformation?

Answer 1

Axial

Question 2

Is diequatorial vs. diaxial intereactions more important or gauche vs. anti in chair conformation?

Answer 2

Overall, diequatorial preference over diaxial is a more important factor than a preference for anti over gauche.

For 1,2-disubstituted compound
1,2-diaxial orientation results in anti orientation, while
1,2-dieqatorial orientation results in gauche orientation

In order for substituents to be gauche or anti to one another, they must be bonded to carbons
that are connected to one another. In the case of 1-4-dimethylcyclohexane, there is no gauche or anti between the methyls

Question 3

Can equilibrium constant ever be zero?

Answer 3

Nope

Question 4

What does l,3-diaxial interactions cause?

Answer 4

Steric repulsion for methyl groups and hydrogen atoms.

Allows hydrogen bonding with polar groups though.

Question 5

Is hydrogen bonding strongest for gauche or anti orientation?

Answer 5

Gauche because they are closer together

Question 6

Is gauche or anti better for methyl groups?

Answer 6

Anti because of steric hindrance they want to be as far apart as possible.

Question 7

The only single bond about which rotation is not possible is?

Answer 7

a single bond between two atoms

in a cyclic compound

Question 8

Should ethyl or methyl be placed on equatorial position?

Answer 8

Ethyl because it is bigger

Question 9

What has a broad IR signal at around 2850 cm^-1 ?

Answer 9

A broad signal near 2850 cm^-1 indicates the hydroxyl group of a carboxylic acid. Because the O—H
bond of a carboxylic acid is weak, its absorbance is lower than that of standard hydroxyl groups.

Question 10

The more hydrogen bonding present in a hydroxyl, the broader or narrower of the peak?

Answer 10

Broader

Question 11

What proton NMR shift does aldehyde have?

Answer 11

9.7 ppm

Question 12

What IR signal does alkenes have?

Answer 12

1620-1660 cm^-1

Being that a C=C bond is slightly weaker than a C=0 bond (they are both double bonds, but a carbonyl bond is
slightly shorter), it takes slightly less energy to stretch the C=C bond than the C=0 bond.

Question 13

Coupling in C13 NMR is from carbons attached to carbon, or hydrogens attached to carbon?

Answer 13

Hydrogens attached to carbon

Question 14

The proton NMR peak for ester is at 3.5-4.0 ppm, does the peak at this spot tell you the substituents attached to the ester oxygen or the substituents attached to the carbonyl side?

Answer 14

Substituents on ester oxygen

Question 15

What is the IR signal for alkynes?

Answer 15

About 2220 cm^-1

Question 16

A shift value between 170-220 ppm on C13-NMR indicates what?

Answer 16

Carbonyl carbon

Question 17

For carbons of equal hybridization, the greater the electronegativity of the atom bonded directly to the carbon, the higher or lower shift value in the C13-NMR?

Answer 17

Higher shift value

Question 18

Does cis or trans have a larger coupling constant? This means greater distance between the peaks for the double bond

Answer 18

Trans

Question 19

As strength of a bond increases, does the absorbance value in IR increase or decrease?

Answer 19

Increase

Question 20

As the atoms in the bond become heavier, does the absorbance in IR increase or decrease?

Answer 20

Increase

This explains why C=O bonds (1700) have higher absorbances than C=C bonds (1600)

Question 21

What is the signal in C-NMR for carbonyls?

Answer 21

180 ppm

Question 22

What is the signal for conjugated diene in UV spectroscopy?

Answer 22

UV absorbance above 200 nm for the n to n* transition.

Question 23

What is the signal for carbonyl group in UV spectroscopy?

Answer 23

190 nm and 260 nm

Question 24

What functional group(s) shows a broad peak in H-NMR?

Answer 24

Alcohol or carboxylic acids

Question 25

What does integration show in H-NMR?

Answer 25

Integration is used to determine the relative quantity of hydrogens within a signal by looking at the area of the signal.