Alkanes

Question 1

Classification of carbons

Answer 1

Depends on the number of carbons attached to the specific carbon we are looking at; primary, secondary, tertiary, quaternary

Question 2

Primary carbon

Answer 2

1 carbon attached

Question 3

Secondary carbon

Answer 3

2 carbons attached

Question 4

Tertiary carbon

Answer 4

3 carbons attached

Question 5

quaternary carbon

Answer 5

4 carbons attached

Question 6

Conformations

Answer 6

help us understand bond angles and distances between atoms

Question 7

Strains

Answer 7

make molecule less stable–>affects structure of molecule

Question 8

Torsional strain

Answer 8

strain where non-bonded atoms separated by three bonds are eclipsing

Question 9

Angle strain

Answer 9

has a bond angle different than what it wants

Question 10

Steric strain

Answer 10

An increase in molecular potential energy (strain) caused when atoms or groups separated by at least four covalent bonds are forced closer

Question 11

Newman Projections

Answer 11

given molecules that you look down a bond btwn atoms (carbons)

Question 12

Staggard conformation

Answer 12

1 Y up, 1 Y down

Question 13

Eclipsed Conformation

Answer 13

Both Ys down/up

Question 14

Dihedral angle

Answer 14

angle between the C-H on the front carbon with the C-H on the back (60 deg)

Question 15

What is the dihedral angle in the eclipsed conformation?

Answer 15

0 degrees

Question 16

When switching between staggard and eclipsed conformations, what do you do?

Answer 16

Keep 1 carbon in the same position and rotate by 60 degrees clockwise

Question 17

What is the preferred conformation for molecules? Why?

Answer 17

Staggard because it is lower in energy; therefore, more stable

Question 17

What is the preferred conformation for molecules? Why?

Answer 17

Staggard because it is lower in energy; therefore, more stable

Question 18

Why is the eclipse conformation high in energy?

Answer 18

hydrogens of the molecule are close together=torsional strain

Question 19

Steps to drawing a Newman projection

Answer 19

1. Draw in Hydrogens
2. Split the bond I’m looking at down the middle (vertically)
3. Draw Ys, putting dashes on the same side as the Newman “Dasher” and wedges on the opposite side of the Dasher

Question 20

Rotational Itinerary

Answer 20

Relative energy values for each conformation

Question 21

Anti-conformation

Answer 21

lowest in EN (most stable), the biggest groups are 180 degrees apart

Question 22

Gauche interaction

Answer 22

2 non-hydrogens are 60 degrees from each other, an increase in gauche interactions–>increase in EN

Question 23

Syn “sin” conformation

Answer 23

biggest substituents are eclipsed; highest in EN (least stable)

Question 24

Ask yourself

Answer 24

How far apart are your biggest groups? How many gauche interactions are there?

Question 25

Look for relationship between two Newman projections

Answer 25

Do they have the same molecular formula? Are the groups in the front and back connected the same in both molecules?

Question 26

Cyclopropane

Answer 26

triangle shaped molecule with3 carbons; hybridization= sp3 so wants 109.5 but actually has 60 degree bond angle; lots of angle strain; all hydrogens are eclipsing so has torsional strain

Question 27

Cyclobutane

Answer 27

Sqaure-shaped molecule; hybridization=sp3 so wants 109.5 degree angle but has 90 degree bond angle=angle strain, has torsional strain because 4 groups of hydrogen are eclipsing

Question 28

Cyclopentane

Answer 28

If planar; very little angle strain present (wants 109.5 but has 108); however has lots of torsional strain because 5 pairs of hydrogens are eclipsing ; if it bends, the torsional strain will decrease and the angle strain will increase slightly (wants 109.5 but has 105)–> molecule will make this sacrifice most of the time.

Question 29

Ranking the cyclopropanes from most stable to least stable

Answer 29

1. C5 (bent), C5 (planar)
2. C4
3. C3

Question 30

Cyclohexane

Answer 30

If planar, all Hydrogens are eclipsing so there would be a lot of torsional strain, BUT if bent to look like chair conformation–>all Hs are staggard= no torsional strain; very very little angle strain (109.5 but has 110)

Question 31

Chair Conformation of a Cyclohexane

Answer 31

Every carbon has an axial position (think y axis) and an equitorial position (think x-axis), just remember each carbon has both; lowest EN conformation

Question 32

Chair flip

Answer 32

Hydrogens or groups that are axial become equitorial; hydrogens or group that are axial become equitorial

Question 33

How to do a chair flip

Answer 33

1. Stay consistent with the numbering (if you number clockwise, then number clockwise once flipped)
2. Wedges go up, dashes go down
3. Make all axial hydrogens/groups equitorial and all equitorial hydrogens/groups axial
4. Check which chair has more axial groups–>higher in EN (less stable)

Question 34

Strain associated with chairs and why?

Answer 34

Steric strain is when big groups are near each other; e.g. 1,3- diaxial interactions

Question 35

1,3 diaxial interactions

Answer 35

interaction between axial substituents and hydrogen or non hydrogen group; cis= same side (both wedges/dashes); trans=opposite side (one wedge and one dash)

Question 36

Dashes

Answer 36

pointing away from me

Question 37

Wedges

Answer 37

pointing towards me