The Covalent Bond

Question 1

Number of valence electrons and bonds for Hydrogen

Answer 1

1 valence electron & 1 bond

Question 2

Number of valence electrons and bonds for Carbon

Answer 2

4 valence electrons & 4 bonds

Question 3

Number of valence electrons and bonds for Nitrogen

Answer 3

5 valence electrons and 3 bonds (four bonds makes Nitrogen positive)

Question 4

Number of valence electrons and bonds for Oxygen

Answer 4

6 valence electrons and 2 bonds (3 bonds makes Oxygen positive)

Question 5

Number of valence electrons and bonds for Flourine

Answer 5

7 valence electrons and 1 bond

Question 6

Number of valence electrons and bonds for Sulfur

Answer 6

6 valence electrons and either 2 or 6 bonds.

Question 7

Number of valence electrons and bonds for Phosphor

Answer 7

5 valence electrons and either 3 or 5 bonds

Question 8

Number of valence electrons and bonds for Silicon

Answer 8

4 valence electrons and 4 bonds

Question 9

Number of valence electrons and bonds for Chlorine

Answer 9

7 valence electrons and 1 bond

Question 10

What do resonance structures result from?

Answer 10

Resonance structures result from electrons not being fixed in position (that’s why you “push” electrons when drawing resonance structures).
When electrons are not fixed in position, they are referred to as “delocalized electrons.”

Question 11

What is the resonance structure for this ion?

Answer 11

Question 12

What is the resonance structure for this aromatic ring?

Answer 12

Question 13

What is the resonance structure for this conjugated double bond?

Answer 13

Question 14

What is the effect of multiple bonding on bond energy and bond length?

Answer 14

Multiple bonding decreases bond length.

Multiple bonding increases bond energy

Question 15

What is the effect of multiple bonding on rigidity in a molecular structure?

Answer 15

Multiple bonding increases rigidity in molecular structure.

Single bonds can rotate, but double and triple bonds can’t.

Even partial double bonds like those found in the peptide bond prevents free rotation.

Question 16

What are structural/constitutional isomers?

Answer 16

Structural (constitutional) isomers have the same molecular formula, but different connectivity

Question 17

What are geometric isomers?

Answer 17

Geometric isomers have the same molecular formula, same connectivity, but have different orientation across a double bond.

When both sides of the double bond contains the same 2 groups, then cis and trans is used. When different groups are on either side, “E” and “Z” are used.

Question 18

Classify the following four molecules are cis, trans, E, and Z.

Question 19

What are stereoisomers?

Answer 19

Stereoisomers have the same molecular formula, same connectivity, but have different 3-D arrangements across one or more asymmetric (chiral) centers.

Question 20

What is a chiral atom?

Answer 20

Chiral center is any atom with 4 different entities attached to it.

Question 21

How do you calculate the number of stereoisomers in a molecules?

Answer 21

A compound will have a total of 2^{#chiralcenters}stereoisomers if it is not meso. If there is a meso stereoisomer, you divide the number of meso stereoisomers by 2.

Question 22

How many stereoisomers does this molecule have?

Answer 22

This molecule has 2 chiral centers, so technically 4 stereoisomers. BUT because two of those stereoisomers are meso, there are only 3 stereoisomers.

Question 23

What are enantiomers?

Answer 23

Enantiomers are mirror images of each other. That means ALL chiral centers in one enantiomer is reversed in the other.
(each R will become an S and each S will become an R).

Question 24

What are diastereomers?

Answer 24

Diastereomers - more than one chiral center, inversion of stereochemistry on some but not all of its chiral centers. For examples, diastereomers would have stereochemistries of (R)-(R) vs (R)-(S). Another example of diastereomers would be (R)-(R)-(S)-(R) vs (R)-(R)-(R)-(R).

Question 25

What does a “D” biological molecule and a “L” biological molecule correspond to in terms of absolute configuration?

Answer 25

Usually a “D” biological molecule will be a “R” molecule and a “L” biological molecule will be a “S” molecule

Question 26

Do enantiomers or diastereomers have the same physical properties as each other?

Answer 26

Enantiomers have the same physical properties.

Question 27

What is a conformational isomer (a conformer)?

Answer 27

Conformational isomers have the same molecular formula, same connectivity, same stereochemistry, but can rotate about a single bond to switch between different conformations.

Question 28

Rank the following conformers from least stable to most stable:

1. Staggered, Gauche
2. Ecliped, Syn
3. Ecliped, Anti
4. Staggered, Anti

Answer 28

Eclipsed Syn is the most unstable. It has the highest torsional strain and the unstable, bulky groups eclipse each other.

Eclipsed Anti is unstable. It has high torsional strain, and the unstable bulky groups eclipse hydrogens.

Staggered Gauche is stable. It has low torsional strain, and the bulky groups are 60 degrees staggered.

Staggered Anti is the most stable. It has the lowest torsional strain and the bulky groups are 180 degrees staggered.

Question 29

Of the three conformers of cyclohexose, rank from least stable to most stable:

1. Chair
2. Twist boat
3. Boat

Answer 29

Boat is the least stable as everything is eclipsed. Twist boat is less stable, as some things are not completely eclipsed. Chair is the most stable as everything is staggered.

Question 30

Between axial and equatorial, which is the most stable?

Answer 30

Axial: most unstable because the axial groups are orientated with a high degree of clashing.

Equatorial: most stable because the equatorial groups are orientated away from one another.

Question 31

Which chair conformer is more stable?

Answer 31

The less axial, the better.

Question 32

What is an optically active molecule?

Answer 32

chiral molecules containing a single enantiomer will rotate polarized light (to varying degrees) either to the left or to the right. This is why chiral molecules are said to be “optically active”.

Question 33

How can you tell whether a sugar is (+) or (-) optically active?

Answer 33

You can only know through a polarimeter.

Question 34

What is a racemic mixture?

Answer 34

Racemic mixtures contain equal amounts of both enantiomers. Another name for racemic mixtures is racemate.

Racemic mixtures do not rotate polarized light, so they are optically inactive.

Question 35

Biologically, how are enantiomers seperated?

Answer 35

Enzymes are highly specific and can differentiate between enantiomers. For example, if an enzyme digests or modifies all L-amino acids, then you’d be able to use that enzyme to separate a D/L racemic mixture.

Question 36

Are most proteins made up of L or D amino acids?

Answer 36

L amino acids.