Ranking & Comparing Molecules

Question 1

Rank ketones, aldehydes, carboxylic acids, and alcohols based on boiling points from highest to lowest.

Answer 1

Carboxylic acids, alcohols, ketones, aldehydes

Question 2

How do you determine which proton is the more acidic one (get deprotonated first) when comparing two compounds? You look at the following factors:

Answer 2

1. The proton is more acidic when it’s next to the more electronegative atom. When comparing two highly electronegative atoms in the same row, the atom that is a bigger size would constitute having the more acidic proton.
2. The proton is more acidic when it participates in resonance structures
3. Induction: The proton is more acidic when there are other highly electronegative atoms next to it such as Cl, F, S, I (the more in number, the more acidic the proton is).
4. When there is triple bond on the atom with the proton, this makes it more acidic. Higher # of bonds constitute more acidic protons attached. (sp hybridized proton is most acidic)

Question 3

Based on which factors do you rank resonance structures?

Answer 3

1. The most important factor is having a full octet. Or more covalent bonds.
2. The second most important is no formal charges.
   (In ideal situations, the significant contributor to resonance will have a full octet & no formal charges.)

Question 4

Does SH or OH have the more acidic proton?

Answer 4

SH because Sulfur is bigger in size than O, which means it stabilizes the negative charge better when deprotonating it.

Question 5

How do you determine R or S for a chiral compound?

Answer 5

1. First if there is a H put it as #4.
2. Then rank the rest of the compounds from highest to lowest atomic # (S > O > N > C >H)
3. Now, trace an arrow from 1 to 4. If it points clockwise it is R, counterclockwise it is S.
4. Lastly, make sure the lowest priority group is a dashed line (going to back of page).
5. IF lowest priority group is a WEDGE LINE, then you take the opposite of the R/S configuration you got.

Question 6

Question: Determining R/S and enantiomer/diastereomer relationship

Answer 6

They are both S, so they have same configuration.
If one compound was R & the other S, they would be enantiomers. THEY CAN’T be diastereomers because, there is only one stereocenter, there has to be more than 1 for them to be able to be diastereomers.

Question 7

How do you determine E and Z configurations? What are they generally used for?

Answer 7

They are used for tri and tetrasubstituted alkenes. You rank groups based on priority (higher atomic number). The group with highest atomic # is given top priority by looking at each atom one by one and comparing just like R and S configuration.
If the two highest priority groups are on the same side the configuration will be Z. If they are on opposite sides, configuration is E.

Question 8

When assigning Z or E, what if you have a group that is an alkene (double bond)?

Answer 8

You treat the double bond as C — C. So after comparing C, the next group would be another C (this forms the double bond) and the alkene would have priority over a carbon atom that has only a single bond because it’s connected to H.

Question 9

How do you determine if a compound is cis or trans?

Answer 9

Make sure the compound you are looking at is disubstituted. Then, look at whether the two groups are on the same side or on opposite sides. Keep in mind, they have to be on separate carbons.

Question 10

If two molecules are cis/trans, what is their relationship to each other?

Answer 10

Geometric isomers, configurational isomers

Question 11

Can double bonded alkenes have a stereocenter?

Answer 11

No, alkenes can’t have stereocenters because they are bonded to two of the same groups (two C’s) on each side

Question 12

How do determine which molecule is a stronger acid based on given pKa values of each molecule?

Answer 12

The stronger acid will have a lower/smaller pKa value.

Question 13

Trends to look for when determining which acid is stronger when comparing an acid and conjugate acid in a reaction.

Answer 13

Looking out for:
1. The more acidic protons next to electronegative atoms (O, Cl, S) the better.
2. Acidic protons that participate in resonance. More resonance structures!!
3. Acidic protons on atoms that have a triple bond.

Question 14

Which carbocation is most stable? (Tertiary, primary, secondary)

Answer 14

Tertiary

Question 15

Rank single, double, and triple bonds in order of strongest to weakest.

Answer 15

Triple > double > single

Question 16

Which bond is stronger, a sigma or pi bond? Why?

Answer 16

A sigma bond is stronger because it is lower in energy and contains more overlap between the two electrons density orbitals.

Question 17

Describe the overlap of electrons in a pi bond.

Answer 17

The electrons are located in orbitals that overlap above and below the plane of the bond.

Question 18

Rank single, double, and triple bonds from longest length to shortest length.

Answer 18

Single > double > triple

Question 19

Which bond has the most potential energy? A triple bond or a single bond?

Answer 19

A single bond has the most potential energy because it is the least stable.

Question 20

Molecules that are stable, have a high or low heat of combustion?

Answer 20

Stable molecules have a low heat of combustion.

Question 21

Rank the rings starting from highest to lowest heat of combustion.

Answer 21

Cyclopropane > cyclobutane > cycloheptane > cyclooctane > cyclopentane > cyclohexane

Question 22

Why does cyclohexane have the least amount of ring strain?

Answer 22

Cyclohexane is able to adopt a more stable chair conformation. it also has ideal bond angles at 109.5 degrees.

Question 23

What is the criteria for a cyclic structure to be antiaromatic?

Answer 23

It should follow all the same criteria: (planar, cyclic, each atom containing a pi orbital). Except contains a total number of electrons in increments of 4n (4,8,12,16, etc).

Question 24

Rank the following functional groups highest to lowest BP.

Answer 24

Amide > Carboxylic acid > alcohol > ketone > aldehyde > amine > ether > haloalkane > alkyne > alkene > alkane

Question 25

Rank the carboxylic acid groups from most stable to least stable.

Answer 25

carboxylic acid > amide > ester >thioester > acid anhydride > acid halide

Question 26

Rank the acidity of the following compounds:
Amine
Alcohol
Carboxylic acid
Phenol
Alkane

Answer 26

Highest acidity to lowest:
Carboxylic acid
Phenol
Alcohol
Amine
Alkane

Question 27

In a chair conformation, how do you determine what substituents are on a wedge vs. dash.

Answer 27

Anything that is pointing down is dash; substituents pointing up are wedges.
THE EQUATORIAL OR AXIAL POSITION DOESN’T MATTER WHEN DETERMINING DASH/WEDGE.

Question 28

Is it true that molecules with strong IMF’s have high boiling points AND high melting points?

Answer 28

Yes, high boiling points correlate with high melting points. For example, carboxylic acids which have the highest boiling and melting points because of H bonding & ability to form dimers.

Question 29

In Haworth project, how do you assign R/S for a carbon?

Answer 29

Up = wedge, down = dash
but remember that #2 priority is the closer carbon that connects to the hemiacetal