CH 4: Alkanes

Question 1

what are alkanes?

Answer 1

Saturated hydrocarbons (CH) that lack pi bonds.

Question 2

What is the nomenclature for Alkanes?

Answer 2

1 C: Methane
2 C: Ethane
3 C: Propane
4 C: Butane
5 C and onwards: latin numeric -ane

Question 3

How do you compare the stability of constitutional isomers of alkanes?

Answer 3

Change in enthalpy associated with complete combustion of 1 mol of alkane = -∆Hº = heat of combustion.

Question 4

How does heat of combustion predict the stability of a compound?

Answer 4

The more heat released, the more potential energy the molecule stored, and the less stable it is.

The less heat released, the less potential energy the molecule stored, and the more stable it is.

Question 5

What is the difference between a staggered conformation and an eclipsed conformation in a newman projection?

Answer 5

Staggered: 60º angle between substituents, lowest energy conformation due to favourable interaction between occupied MO and unoccupied antibonding MO

Eclipsed: 0º angle between substituents, highest energy conformation due to torsional strain

Question 6

What are the energetic costs of torsional strain in eclipsed confomrations?

Answer 6

4kJ/mol per pair of eclipsing H’s

6kJ/mol per eclipsing H and CH3

11kJ/mol per pair of eclipsing CH3

H/H eclipsed: 4kJ/mol

H/CH3 eclipsed: 6 kJ/mol

CH3/CH3 eclipsed: 11kJ/mol

CH3/CH3 gauche: 3.8 kJ/mol

Question 7

What is the anti-conformation?

Answer 7

Staggered conformation with a dihedral angle of 180º resulting in maximum separation. 3.8kJ/mol less than staggered conformations with gauche interactions

Question 8

What is the gauche interaction?

Answer 8

A type of steric interaction where electron clouds of two methyl groups in a newman projection are repelling each other. 3.8kJ/mol higher in energy than an anti conformation.

Question 9

What is the difference between torsional strain and steric strain?

Answer 9

Torsional strain: electric repulsion due to eclipsing, may be minimized to staggered via bond rotation

Steric strain: electric repulsion due to size of electron groups and cannot be minimized.

Question 10

What is the angle and energy in cyclopropane?

Answer 10

60º, high energy due to angle strain (small bond angles) and torsional strain (3 sets of eclipsing H’s)

Question 11

What is the angle and energy in cyclobutane?

Answer 11

88º, higher energy than cyclopropane due to 4 sets of eclipsing H’s. Avoids torsional strain by puckering.

Question 12

What are the bond angles in cyciohexane?

Answer 12

Around 109.5º

Question 13

What is the difference between the boat and chair conformations of cyclohexane?

Answer 13

Chair: all H’s are staggered

Boat: H’s on either side of the ring experience flagpole interactions, puckers to “Twist Boat” instead

Question 14

Where are substituents placed in the lowest energy state of cyclohexane?

Answer 14

Equatorial is favoured rather than axial, as axial positions will have steric interactions with the axial H’s on the ring.

Question 15

Whaat is cis and trans isomers?

Answer 15

Cis: two groups are on the same side of the ring

Trans: two groups are on opposite sides of the ring

Question 16

What are isomers?

Answer 16

Compounds constructed from the same atoms but still differ from each other.

Question 17

What is the difference between constitutional isomers and stereoisomers?

Answer 17

Constitutional: Same molecular formula but different order of connectivity of atoms

Stereoisomers: same molecular formula and constitution but different spatial arrangement

Question 18

What does it mean when something is superimposable and non superimposable?

Answer 18

Superimposable: The mirror image is identical to the object

Non-superimposable: The mirror image is NOT identical to the object

Question 19

What are chiral molecules?

Answer 19

Molecules that are NOT superimposable on their mirror images.

Question 20

Whta is a chiral center/stereocenter?

Answer 20

Tetrahedral carbon bearing FOUR different groups.

Question 21

What are enantiomers?

Answer 21

A pair of chiral molecules that are non superimposable mirror images of each other

Question 22

How to designate configuration in alkanes?

Answer 22

1. Assign priorities to each of the groups attached to the chiral centre
2. Rotate the molecule so that lowest priority is in the back
3. If the sequence 1-2-3 is counterclockwise, it is designated S. If the sequence 1-2-3 is clockwise, it is designated R

Question 23

How do you assign priorities?

Answer 23

• Higher atomic number = higher priority
• If same atomic number, look at bonded atoms for higher atomic number
• in a double bond, the atom with the double bond will be counted twice

Question 24

How to convert from S to R? How does this help us put priority 4 in the back?

Answer 24

Switching positions of a pair of priorities. If you switch the positions of priority 4 and what ever is on the dash, you’ve changed configurations. To undo this, switch the other two groups and you will return to the same configuration.

Question 25

What is the difference between an enantioenriched mixture and a racemic mixture?

Answer 25

enantioenriched: One enantiomer will be present at a higher concentration than another Racemic: Both enantiomers will be present in the same concentration

Question 26

How do you distinguish enantioenriched or racemic mixtures?

Answer 26

chiral molecules will be optically active, achiral compounds are not optically active. enantioenriched mixtures will bend the light, but racemic will have each enantiomer bending at different directions at the same time so light will not bend

Question 27

How do you calculate specific rotation?

Answer 27

observed rotation / concentration (g/mL)* path length. (dm)

Question 28

What are diastereomers?

Answer 28

Stereoisomers that are non superimposable NON mirror images with at least two stereocenters

Question 29

How do you find the number of stereoisomers possible in a molecule?

Answer 29

2^ n, where n is the number of stereocenters

Question 30

What is the difference between a plane of symmetry and an axis of symmettry?

Answer 30

plane: line through a molecule that exhibits reflectional symmetry axis: line through a molecule that exhibits rotational symmetry

Question 31

what is the relationship between symmetry and chirality?

Answer 31

1. presence or absence of rotational symmetry is irrelevant to chirality 2. compound that has a plane of symmetry will be achiral 3. A compound that lacks a plane of symmetry will most likely be chiral (unless it can be reflected about a point at the center of the compound.

Question 32

What are meso compounds?

Answer 32

Compound that exhibits reflectional symmetry with chiral centers. Superimposable on its own mirror image.

Question 33

What are fischer projections?

Answer 33

Horizontal lines come out of the page, vertical lines go behind the page.

Question 34

How to assign configuration in alkenes?

Answer 34

1. Split through middle of double bond into L group and R group 2. Assign priorities to each end of the branch in both groups 3. If both priorities are on the same side, it is Z. If the priorities are on different sides, it is E.

Question 35

What makes alkenes stable?

Answer 35

Alkenes are more stable when they are highly substituted due to hyperconjugation between the pi bond orbitals and the p orbitals of the substituents

Question 36

What makes something aromatic?

Answer 36

1. Cyclic 2. Fully conjugated (sp2 or sp hybrid) 3. Huckel's rule 4. Planar

Question 37

What is Huckel's rule?

Answer 37

The number of pi electrons in the ring can be expressed as 4n + 2, with n being a whole number (0, 1, 2)

Question 38

What is the difference between antiaromatic and non aromatuc

Answer 38

Anti-aromatic: molecule is cyclic, fully conjugated and planar BUT violates huckel's rule Non-aromatic: molecule is not planar of fully conjugated or cyclic

Question 39

How to draw the energy levels for a ring of overlapping p orbitals?

Answer 39

1. Draw circle 2. Inscribe a polygon based on the shape of the ring 3. Draw horizontal lines at each vertex 4. Erase circle and polygon and draw dotted horizontal line through 5. Identify antiboding and bonding MO's (based on whether or not the molecule has 3 or 5 BMO's)